Synthesis of Polyhydroxylated

Surfactants

Comparison of Surfactant Stereoisomers and Investigation of Haemolytic Activity

Kristina Neimert-Andersson

Doctoral Thesis

Stockholm 2005

Akademisk avhandling som med tillstånd av Kungl Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie doktorsexamen i kemi med inriktning mot organisk kemi, fredagen den 4 november, 2005 kl 10.00 i Salongen, KTHB, Osquars backe 31 Stockholm. Opponent är Professor Krister Holmberg, Chalmers Tekniska Högskola. Avhandlingen försvaras på svenska.

ISBN 91-7178-162-5 ISRN KTH/IOK/FR--05/98--SE ISSN 1100-7974 TRITA-IOK Forskningsrapport 2005:98 © Kristina Neimert-Andersson 2005

Populärvetenskaplig sammanfattning I den här avhandlingen har vi studerat hur man kan göra nya tensider. En tensid är en speciell molekyl som har förmågan att lösa sig i både vatten och olja.

Man kan göra följande experiment hemma: Fyll en glasburk till hälften med vatten och tillsätt en droppe matolja. Oljan bildar en droppe ovanpå vattnet, därför att vatten och olja inte är blandbara. Vatten är polärt och olja är opolärt. Om man rör om med en sked kommer oljedroppen förvisso att dela upp sig i mindre droppar, men så snart man slutar att röra kommer dessa att lägga sig på vattenytan igen. Sätt nu en droppe diskmedel till blandningen och rör om. Nu sprider sig oljedropparna mycket bättre i vattnet, och de lägger sig heller inte på vattenytan lika fort när man slutar att röra. Det här beror på att diskmedel innehåller en tensid, som har en polär och en opolär del. Den polära delen passar ihop med det polära vattnet, medan den opolära delen passar ihop med den opolära oljan. På så vis kan tensiden hjälpa till att lösa upp opolära ämnen i polära vätskor.

Den aktiva delen av ett läkemedel består ofta av opolära ämnen, vilka inte löser sig i polära vätskor såsom vatten. Eftersom kroppen består till stor del av vatten måste man ändå försöka få läkemedlet vattenlösligt, för att möjliggöra transport via blodet till problemområdet. Det kan man uppnå genom att tillsätta tensider. Om läkemedel-tensidblandningen ska ges till djur eller människor får inte tensiden orsaka någon skada i kroppen.

Vi har försökt framställa tensider som ska kunna användas för att just lösa läkemedel i vatten. För att kunna framställa nya tensider måste man ha kunskap i organisk syntes. Det betyder att man måste veta hur man från små intermediat (”byggstenar”) successivt kan bygga upp nya molekyler som har de önskvärda egenskaperna. Genom olika typer av organisk syntes har vi byggt upp tre nya tensidtyper, vars egenskaper vi studerat med olika mätningar. Ingen av dessa tensider lämpade sig som tillsats till läkemedel, men vårt arbete har givit mycket ny kunskap om hur framtida tensidmolkyler kan tillverkas och vilka egenskaper de får.

Abstract

This thesis deals with the synthesis and characterization of new polyhydroxy surfactants. The first part describes the synthesis of three new surfactant classes, and the second part concerns the surface chemical characterization of the synthesized surfactants.

A stereodivergent route for preparation of hydrophilic head groups was developed, that featured consecutive stereoselective dihydroxylations of a diene. This method provided in total four different polyhydroxylated head groups. These surfactant head groups were natural and unnatural sugar analogues, and were used for the coupling with two different hydrophobic tail groups. Another approach took advantage of a metathesis reaction and provided a polyhydroxylated compound that was coupled to 12-hydroxy stearic acid. The third class of surfactants contained an amide linkage between the hydrophilic and the hydrophobic parts. The hydrophilic part consisted of two glucose units, and 12-hydroxy stearic acid was used as the hydrophobic part. The hydroxy moiety in the tail group was further functionalized as aliphatic esters, which provided in total four different surfactants. A selection of the surfactants was used to investigate the chiral discrimination in Langmuir monolayers at an air-water interface. The isotherms showed a remarkable difference in compressibility between diastereomeric surfactants and also a pronounced chiral discrimination between racemic and enantiomerically pure surfactants, favoring heterochiral discrimination. The monolayers were also investigated with Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD). It was not possible to observe any chirality dependent features from the BAM images, but the GIXD measurement supported the conclusion that heterochiral discrimination governed the intermolecular forces within the racemic monolayer.

The third class of surfactants, containing an amide linkage between the glucose units and 12-hydroxy stearic acid was evaluated with respect to the CMC and the haemolytic activity. These surfactants were all haemolytic close to their respective CMC.

Kristina Neimert-Andersson; Synthesis of Polyhydroxylated Surfactants. Comparison of Surfactant Stereoisomers and Investigation of Haemolytic Activity. Doctoral Thesis written in English. Organic Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden.

Keywords: Polyhydroxy surfactants, Sugar surfactants, Dihydroxylation, Metathesis, Langmuir monolayers, Chiral discrimination, Haemolysis

Table of Contents Populärvetenskaplig sammanfattning Abstract List of Publications Contribution Report 1 Introduction.......................................................................................................1

1.1 Aim of the Thesis ....................................................................................1 1.2 Surfactants in General .................................................................................2 1.3 Surfactants in Society..................................................................................3

1.3.1 Polyhydroxylated Surfactants ..............................................................4 1.3.2 Surfactants in Drug Delivery................................................................5

1.4 Polyhydroxylated Compounds in Organic Synthesis ..................................6 2 Synthesis of Polyhydroxylated Surfactants ......................................................9

2.1 General Considerations in Surfactant Synthesis .........................................9 2.2 Stereodivergent Approach Towards Polyhydroxy Surfactants ..................12

2.2.1 Introduction and Synthetic Outline ....................................................12 2.2.2 Method Evaluation Using an Alkene..................................................13 2.2.3 Preparation of Head Groups from a Diene .........................................15 2.2.4 Alkylation of Dimethyl Malonate.......................................................18 2.2.5 Completion of the Surfactants............................................................19 2.2.6 Conclusions........................................................................................20

2.3 Synthesis of a Surfactant Head Group by a Metathesis Approach............21 2.3.1 Introduction and Synthetic Outline ....................................................21 2.3.2 Synthesis of the Polyhydroxylated Head Groups...............................23 2.3.3 Completion of the Surfactants............................................................27 2.3.4 Conclusions........................................................................................30

2.4 Synthesis of Surfactants from Glucose and 12-Hydroxy stearic acid .......31 2.4.1 Strategy ..............................................................................................31 2.4.2 Synthesis of the Amine Head Group ..................................................32 2.4.3 Completion of the Surfactants............................................................33 2.4.4 Conclusions........................................................................................34

3 Surface Chemical Characterization ...............................................................35 3.1 Introduction...............................................................................................35 3.2 Solubility...................................................................................................35 3.3 Stereochemistry.........................................................................................37

3.3.1 Background ........................................................................................37 3.3.2 Introduction to Langmuir Monolayers ...............................................37 3.3.3 Strategy and Experimental Methods ..................................................39 3.3.4 Results................................................................................................39 3.3.5 Chiral Discrimination.........................................................................47 3.3.6 Conclusions........................................................................................47

3.4 CMC..........................................................................................................48 3.4.1 Introduction........................................................................................48 3.4.2 Results................................................................................................49

3.4.3 Conclusions........................................................................................51 3.5 Haemolysis................................................................................................51

3.5.1 Introduction........................................................................................51 3.5.2 Results................................................................................................52 3.5.3 Conclusions........................................................................................53

4 Conclusions and Future Outlook ...................................................................55 Acknowledgements.............................................................................................57

List of Publications This thesis is based on the following papers, referred to in the text by their roman numerals.

I. Stereoselective Synthesis of Polyhydroxyl Surfactants. Stereochemical Influence on Langmuir Monolayers. Kristina Neimert-Andersson, Eva Blomberg and Peter Somfai

J. Org. Chem. 2004, 69, 3746-3752

II. A Metathesis Approach for the Preparation of Polyhydroxylated Compounds as Head Groups in Surfactant Synthesis. Kristina Neimert-Andersson and Peter Somfai

Submitted

III. Synthesis of New Sugar-Based Surfactants and Evaluation of their Haemolytic Activities Kristina Neimert-Andersson, Sven Sauer, Olaf Panknin, Tessie Borg, Erik Söderlind and Peter Somfai

Submitted

IV. Investigation of the Influence of Surfactant Stereochemistry on Intermolecular Forces in Langmuir Monolayers. Kristina Neimert-Andersson, Eva Blomberg, Dieter Vollhardt and Peter Somfai

Manuscript

Contribution Report The following is a description of my contribution to papers and I-IV:

I. I performed all lab work and wrote the manuscript.

II. I performed all lab work and wrote the manuscript.

III. I supervised the synthetic part and shared the work with the CMC and haemolytic measurements with Erik Söderlind. I wrote the manuscript.

IV. I made the π–A isotherms and relaxation studies together with Eva Blomberg. Eva Blomberg made the BAM experiments and Dieter Vollhardt made the GIXD experiments. I wrote the manuscript.

Abbreviations Abbreviations and acronyms are used in agreement with the standards of the subject.1 Listed here are non-standard and unconventional abbreviations that appear in the thesis. AD-mix Asymmetric dihydroxylation mixture, see ref. 55.

BAM Brewster angle microscopy

C12G2 Dodecylmaltoside

CMC Critical micelle concentration

CPP Critical packing parameter

DMT-MM

4-(4,6-Dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride

EDC 1-(3-diethylaminopropyl)-3-ethyl-carbodiimide

ee Enantiomeric excess

equiv. Equivalents

GIXD Grazing incidence X-ray diffraction

hfc Europium tris[3-heptafluoropropylhydroxymethylene)-(–)-camphorate]

KHMDS Potassium hexamethyldisilazane

LC Liquid condensed

LE Liquid expanded

PMB p-Methoxybenzyl

PMBz p-Methoxybenzoyl

PTFE Polytetrafluoroethane

TBAF Tetrabutyl ammonium fluoride

TBS Tertbutyldimetylsilyl

γ Surface tension

π Surface pressure

∆ Heat

(1) https://paragon.acs.org/paragon/ShowDocServlet?contentId=paragon/menu_content/authorchecklist/jo_authguide.pdf

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1

Introduction 1.1 Aim of the Thesis

Synthetic organic chemistry and surface chemistry are two different areas within the field of chemistry, the former dealing with preparation of carbon-containing compounds from small simple building blocks, and the latter describing the physical chemistry at interfaces. Nevertheless, in this project we have tried to merge these two fields, by synthesizing compounds with a preference to adsorb at interfaces, i.e. surfactants. Surfactant synthesis offers a challenge to a synthetic organic chemist for several reasons. Most commercial surfactants are obtained as regioisomeric mixtures, and together with unreacted starting material. The synthesis of pure and well-characterized surfactant systems therefore requires other synthetic pathways than those utilized for commercial production. The synthetic plan might also be obstructed by the different polarities of the components building the surfactant, and the final product carries properties making purification and handling difficult.

We have had three main interests during this project that will be discussed throughout this thesis:

• To design and synthesize polyhydroxylated compounds that can be used in surfactant synthesis.

• To design and synthesize surfactants with a possible use as drug solubilizers.

• To evaluate the influence of stereochemistry on surfactant behavior. This thesis is divided into three parts: A general introduction to surfactants and polyhydroxylated compounds is given in Sections 1.2–1.4. The industrial applications of surfactants, and especially the use of surfactants in pharmaceutical industry, are also discussed in this part. In Chapter 2 are the synthetic efforts towards a number of nonionic, polyhydroxylated surfactants presented. In the third chapter, the synthesized surfactants are described with respect to their physicochemical properties.

2

1.2 Surfactants in General

Surfactants are a class of compounds featuring both a hydrophilic and a hydrophobic part within the same molecule. The hydrophilic part is often referred to as the head group and the hydrophobic part as the tail group (Figure 1).

Headgroup Tail group

Figure 1. A schematic representation of a surfactant molecule.

The amphiphilic2 character of surfactants enables the molecules to adsorb at interfaces between hydrophilic and hydrophobic media. The adsorption is spontaneous and the driving force is the lowering of the interfacial energy.3

At the critical micelle concentration (CMC), the surfactant monomers spontaneously aggregate into superstructures (e.g. micelles). The driving force for this aggregation is the increase in entropy when the system goes from the highly organized state illustrated in Figure 2a to the state described in Figure 2b. In Figure 2a the water molecules encapsulate the tail group in a cavity-like fashion, while in Figure 2b the hydrophilic core of the micelle shields the hydrophobic groups. The release of the water molecules causes an increase in entropy, which facilitates micelle formation. This entropically driven process is referred to as the hydrophobic effect.4

OH

HOHH

OH

H

OHH

OH

H

OHH

OH

H

OH HOHH

OH

H

OHHO

H

H

OH

H

a b

OH

HO

HO

H

HO

HO

H

HO

H

HO

H

H

H

H

OH

HO

HO

H

HO

HO

H

HO

H

HO

H

H

H

H

Figure 2. (a) Encapsulation of a surfactant monomer. (b) Arrangement of surfactants in a micelle.

(2) "Amphiphilic" comes from Greek meaning "ambivalent of what to like". Compare with "hydrophilic" that means "to like water", and "lipophilic" that means "to like lipids".

(3) Hunter, R. J. Introduction to Modern Colloid Science; Oxford University Press: Oxford, 1994.

(4) Evans, D. F.; Wennerström, H. The colloidal domain. Where physics, chemistry, biology and technology meet; VCH: New York, 1994.

3

Spherical micelles are not the only possible structures that can form. The shape of a micelle depends on the head- and tail groups, and can be described using the critical packing parameter, CPP (equation 1).

(1)

In equation 1, a represents the effective area of the head group, whereas v and l denote the volume and the length of the tail group, respectively. For a spherical micelle CPP≈1/3, and for a cylindrical micelle CPP≈1/2. Lamellar phases have CPP≈1 and reversed micelles have CPP>1 (Figure 3).5

Figure 3. Different aggregation structures.

1.3 Surfactants in Society

The surfactant characteristics discussed in the previous section have significant values for industry and society. Surfactants are used in cleaning applications,3,6 in tablets and solutions in the pharmaceutical industry,7 as well as in food applications and in paper industry, to mention a few. These different applications have individual requirements on the surfactants employed (toxicity, biodegradability, CMC, viscosity, solubilizing capacity etc.), and the need for target-designed surfactants is therefore ever increasing. The individual combination of head and tail groups influence physicochemical properties such as solubility, CMC, foaming properties and phase behavior. Extensive studies of surfactant properties as well as surfactant synthesis are required to obtain better, cheaper and more environmentally friendly surfactants.

CPP = val

Lamellar phase

Reversed micelle

Spherical micelle

Cylindrical micelle

(5) Jönsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley: Chichester, 1998.

(6) Hellsten, M. Tenside Surf. Det. 1986, 23, 337-341. (7) Lawrence, M. J. Chem. Soc. Rev. 1994, 23, 417-424.

4

1.3.1 Polyhydroxylated Surfactants Surfactants can be divided into two main classes: ionic and nonionic, depending on the electrostatic nature of the head group.3 The ionic surfactants constitute the major surfactant class, and can be further divided into anionic, cationic and zwitterionic surfactants. This class will not be further discussed here.8-10 Nonionic surfactants contain an uncharged head group. Two main subclasses are the ethylene oxide-based surfactants, having a polyethylene glycol head group, and the polyhydroxylated surfactants, with a polyhydroxy-based head group. With the increasing awareness of environmental issues the chemical industry and the society must limit their use of non-renewable materials. In this sense, the study of polyhydroxy-based surfactants has become a field of growing interest, as the polyhydroxylated head group can be derived from a naturally occurring carbohydrate. Sugars are cheap renewable starting materials and are available in different forms (monomers, oligomers and polymers). Many polyhydroxylated surfactants present favorable dermatological properties, which is why they are often used in personal care products and pharmaceutical applications, as discussed further below.11-13

(8) Schmalstieg, A.; Wasow, G. W. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: Chichester, 2002; Vol. 1, p 271-292.

(9) Steichen, D. S. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: Chichester, 2002; Vol. 1, p 309-348.

(10) Floyd, D. T.; Schunicht, C.; Gruening, B. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: Chichester, 2002; Vol. 1, p 349-372.

(11) Bevinakatti, H. S.; Mishra, B. K. Annual Surfactants Review 1999, 2, 1-5. (12) Holmberg, K. Curr. Opin. Colloid Interface Sci. 2001, 6, 148-159. (13) Garcia, M. T.; Ribosa, I.; Campos, E.; Sanchez Leal, J. Chemosphere 1997, 35, 545-

556.

5

1.3.2 Surfactants in Drug Delivery The active components of drugs often suffer from low aqueous solubility. Although the solid state is the preferred dosage form for administration, aqueous formulations are needed at some stages of drug development, especially in early toxicity, bioavailability, and metabolic studies.7,14 A key to this problem is therefore to use surfactants as solubility enhancers.15 Polyethylene oxide derivatives of fatty acids or castor oil are commonly used in commercial surfactant systems for pharmaceutical use.14 However, as a result of the manufacturing process these surfactants are often complex mixtures of different components.16,17 Although surfactant mixtures might have beneficial properties, several drawbacks exist including complicated chemical analysis, product specification issues, and large batch-to-batch variations. They may also cause severe side effects, among which histamine release might be the most acute.18,19 Consequently, there is a need for new surfactants with defined composition and low toxicity. Sugar-based surfactants are interesting candidates for pharmaceutical applications as they are available from cheap starting materials, and are regarded to be readily biodegradable.13 However, one of the most restricting properties of surfactants in pharmaceutical applications is their ability to cause haemolysis20. Known sugar-based surfactants cause more severe haemolysis than traditional ethylene oxide-based surfactants, giving room for improvements. Still, studies have shown that surfactants containing open chain sugar head groups are less haemolytic compared to surfactants with ring-closed sugar head groups.21 Furthermore, surfactants with a larger, oligosaccharide head group are often less haemolytic than surfactants with a smaller, monosaccharide head group.21-24 The literature covering the use of sugar-based surfactants in pharmaceutical applications is rather limited, and more research is needed.21,25-27

(14) Attwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry, Pharmacy and Biology; Chapman and Hall: London, 1982.

(15) Yalkowsky, S. H. Solubility and Solubilization in Aqueous Media; Oxford University Press: New York, 1999.

(16) Strickley, R. G. Pharm. Res. 2004, 21, 201-230. (17) Kosswig, K. In Nonionic Surfactants Organic Chemistry; van Os, N. M., Ed.; Marcel

Dekker: New York, 1998; Vol. 72, p 123-146. (18) Singla, A. K.; Garg, A.; Aggarwal, D. Int. J. Pharm. 2002, 235, 179-192. (19) Gelderblom, H.; Verweij, J.; Nooter, K.; Sparreboom, A. Eur. J. Cancer 2001, 37,

1590-1598. (20) Lysis of erythrocytes, causing cell leakage and death. (21) Söderlind, E.; Wollbratt, M.; von Corswant, C. Int. J. Pharm. 2003, 252, 61-71. (22) von Corswant, C.; Hult, K.; Söderlind, E.; Viklund, F. PCT Int. Appl. WO 2004089869,

2004. (23) Vinardell, M. P.; Infante, M. R. Comp. Biochem. Physiol. C. 1999, 124, 117-120. (24) Ohnishi, M.; Sagitani, H. J. Am. Oil Chem. Soc. 1993, 70, 679-684. (25) Isomaa, B.; Hägerstrand, H.; Paatero, G. Biochim. Biophys. Acta 1987, 899, 93-103. (26) Isomaa, B.; Hägerstrand, H.; Paatero, G.; Engblom, A. C. Biochim. Biophys. Acta

1986, 860, 510-524. (27) Reinhart, T.; Bauer, K. H. Pharmazie 1995, 50, 403-407.

6

1.4 Polyhydroxylated Compounds in Organic Synthesis

Carbohydrates belong to the most simple and versatile building blocks for the preparation of polyhydroxylated compounds, as they are available in different chain lengths and with different stereochemistry, as well as in ring-closed and ring-opened forms (Figure 4a). Monosaccharides can via (1,4)- or (1,6) α- or β- glycosidic bonds be combined to oligosaccharides (Figure 4b), and derivatives thereof play important roles in biological systems.

CHOOHHHHOOHHOHH

CH2OH

Pyranose form Fisher projection

OHOHO

OH OHO

OH

OH

OOHO

OH

OH

O

On

1

4α

Oβ

a

b

O

OH

OH

HO

OH

OH

3

6

Figure 4. (a) D-Glucose in its pyranose form and as a Fisher projection. (b) An oligosaccharide demonstrating the 1,4-α-, and 1,6-β-glycosidic linkages.

The chemical modification of carbohydrates has interested chemists for more than a hundred years. Consequently, carbohydrates have served as starting materials in many synthetic sequences towards natural products,28,29 and they have also been used as chiral auxiliaries in stereoselective synthesis.30 Despite the diverse stereochemical building blocks available from monosaccharides, there are still sugars not very common in nature, such as the L-sugars. Moreover, carbohydrate chemistry is often accompanied with tedious protection/deprotection strategies to selectively functionalize the many hydroxy groups available in the molecule. Much effort has been put into the development of simple and stereodivergent approaches for the preparation of unnatural or rare sugars, including asymmetric dihydroxylation strategies,31-33 chemoenzymatic approaches34 as well as stereoselective aldol35,36 and nucleophilic addition37,38 reactions.

7

The synthesis of highly polyhydroxylated carbon frameworks containing 7–11 hydroxymethylene units, so called “higher sugars”, from small building blocks or hexoses, has been given much attention, as these molecules often possess important biological functions.39 Various methods for the chain elongation have been utilized, including Wittig reactions,39 Horner-Wadsworth-Emmons olefination followed by subsequent osmolylation,40-42 epoxidation,43 and coupling via allyl tin reagents.44 Sharpless and coworkers have also demonstrated how highly polyhydroxylated compounds can be accessed by asymmetric dihydroxylations of polyenes.45,46

A highly efficient way of combining organic chemistry and surface chemistry would be to utilize known divergent synthetic methods, and to develop new methods for the preparation of highly hydroxylated compounds, and to use these molecules in the construction of new surfactants.

(28) Nicolaou, K. C.; Mitchell, H. J. Angew. Chem. Int. Ed. Engl. 2001, 40, 1576-1624. (29) Hanessian, S. Total Synthesis of Natural Produts: The 'Chiron' Approach; Pergamon:

Oxford, 1983. (30) Kunz, H.; Rück, K. Angew. Chem. Int. Ed. Engl. 1993, 32, 336-358. (31) Henderson, I.; Sharpless, K. B.; Wong, C.-H. J. Am. Chem. Soc. 1994, 116, 558-561. (32) Honzumi, M.; Taniguchi, T.; Ogasawara, K. Org. Lett. 2001, 3, 1355-1358. (33) Ko, S. Y.; Lee, A. W. M.; Masamune, S.; Reed, L. A., III; Sharpless, K. B.; Walker, F.

J. Tetrahedron 1990, 46, 245-264. (34) Gijsen, H. J. M.; Qiao, L.; Fitz, W.; Wong, C.-H. Chem. Rev. 1996, 96, 443-473. (35) Davies, S. G.; Nicholson, R. L.; Smith, A. D. Org. Biomol. Chem. 2005, 3, 348-359. (36) Northrup, A. B.; MacMillan, D. W. C. Science 2004, 305, 1752-1755. (37) Dondoni, A.; Marra, A.; Massi, A. J. Org. Chem. 1997, 62, 6261-6267. (38) Roush, W. R. In Trends in Synthetic Carbohydrate Chemistry, ACS Symposium

Series 386; Horton, D., Hawkins, L. D., McGarvey, G. J., Eds. Washington DC, 1989, p 242-277.

(39) Secrist, J. A. I.; Barnes, K. D.; Wu, S.-R. In Trends in Synthetic Carbohydrate Chemistry, ACS Symposium Series 386; Horton, D., Hawkins, L. D., McGarvey, G. J., Eds. Washington DC, 1989, p 93-106.

(40) Brimacombe, J. S.; Hanna, R.; Kabir, A. K. M. S. J. Chem. Soc., Perkin Trans. 1 1986, 823-828.

(41) Ikemoto, N.; Schreiber, S. L. J. Am. Chem. Soc. 1992, 114, 2524-2536. (42) Jørgensen, M.; Iversen, E. H.; Madsen, R. J. Org. Chem. 2001, 66, 4625-4629. (43) Brimacombe, J. S.; Hanna, R.; Kabir, A. K. M. S. J. Chem. Soc., Perkin Trans. 1 1987,

2421-2426. (44) Jarosz, S.; Fraser-Reid, B. J. Org. Chem. 1989, 54, 4011-4013. (45) Crispino, G. A.; Ho, P. T.; Sharpless, K. B. Science 1993, 259, 64-66. (46) Crispino, G. A.; Sharpless, K. B. Tetrahedron Lett. 1992, 33, 4273-4274.

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2

Synthesis of Polyhydroxylated Surfactants

2.1 General Considerations in Surfactant Synthesis

The synthesis of selected commercially important sugar-based surfactants is presented in Scheme 1 and 2. Sucrose esters 1 are commonly synthesized by heating sucrose together with fatty acids in a melt or in polar aprotic solvents.11,47 The sorbitan esters 2 were one of the first types of commercially synthesized sugar surfactants. Sorbitan esters can be obtained by heating sorbitol with a fatty acid, but unfortunately 2 is commonly obtained together with several isomers.11

Sucrose

RCOOMe

1

CH2OHOHHHHOOHHOHH

CH2OHSorbitol

RCOOMe

HO OH

O OH

OO

R2

+ isomers

+ isomers

OHOHO

OOH

OH

O

OHOH

HO

OH OHOHO

OOH

O

O

OHOH

HO

OHR

O

Scheme 1. Synthesis of sucrose esters (1), and sorbitan ester (2). R=C6–C20 aliphatic chain.

(47) Drummond, C. J.; Fong, C.; Krodkiewska, I.; Boyd, B. J.; Baker, I. J. A. In Novel Surfactants. Preparation, Applications, and Biodegradability; 2 ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; Vol. 114, p 95-127.

10

Alkyl polyglucosides 3 are formed if D-glucose is heated together with a fatty alcohol (Scheme 2). The reaction is non-selective, and a mixture of isomers is often obtained.11,48 Alkyl glucamides 4 and alkyl aldonamides 5 can generally be accessed with less formation of isomers. In the synthesis of 4, glucose is first converted to alkyl glucamine 6 by reductive amination. The intermediate glucamine 6 is then treated with a fatty acid ester at elevated temperatures with or without solvents to give amide 4. Glucolactones, which can be obtained by oxidation of glucose, can be treated with a fatty amine to form alkylaldonamide 5.11,49

OHO

HOOH OH

OH

1) NH2R1OHHHHOOHHOHH

CH2OH

R1HN

2) H2, Pd/C

OHHHHOOHHOHH

CH2OH

NR1

R

O

D-glucose 3

6

ROH

H+O

OHO

OH O

OH

4

OHO

HOOH

OH

glucolactoneO

RNH2NH

RHOO

OH

OH

OH

OH

5

RCOOMe

O

HO HO

OHOR

n

Scheme 2. Commercial synthesis of alkyl polyglucosides (3), alkyl glucamides (4), and alkyl aldonamides (5). R=C6–C20 aliphatic chain. R1=C1–C4 aliphatic chain.

For certain applications, one of the most severe problems in the synthesis of the commercial surfactants above is the lack of selectivity, giving rise to product mixtures. On a laboratory scale, these problems can be overcome by the use of protecting groups, however at the expense of an increased number of reaction steps. The different polarities of the fatty acid or alcohol used as tail group and the hydrophilic compound used as head group make it difficult to find reaction solvents equally suitable for both parts. Surfactant purification might also be troublesome because of the amphiphilic character of the molecules.

(48) von Rybinski, W.; Hill, K. In Novel Surfactants. Preparation, Applications, and Biodegradability; 2 ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; Vol. 114, p 35-93.

(49) Burczyk, B. In Novel Surfactants. Preparation, Applications, and Biodegradability; 2 ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; Vol. 114, p 129-192.

11

With careful synthetic planning we have tried to minimize the above problems. The use of protecting groups reduces the selectivity problems, and introduces a hydrophobic character to the hydroxy groups, making the head group precursors soluble in common organic solvents. However, the need for purification of the completed surfactants must be brought to a minimum, and optimal protecting groups are therefore those that can be removed in the last step with no other workup needed but filtration and evaporation.

A number of structural features are desired to provide the surfactants with the optimal properties for pharmaceutical solubilization. During this work, special attention was paid to the development of synthetic sequences offering the possibility to elongate the hydrophilic head groups of the surfactants. Three structurally different polyhydroxy surfactant classes were designed to meet with this requirement (Figure 5). Surfactant A was designed to contain two head groups connected to a single tail group. By using consecutive asymmetric dihydroxylation reactions as key steps, several stereoisomers of A would be accessible, as further discussed in Section 2.2. The single head group of surfactant B could be prepared using a metathesis reaction as a key step. 12-hydroxy stearic acid has previously been shown to provide surfactants with favorable properties, and it was therefore selected as the hydrophobic group in surfactants B and C. The synthetic efforts towards compound B are discussed in Section 2.3. The bulky head group of surfactants C–F was designed to suppress crystallization and enhance the aqueous solubility of the surfactants. Von Corswant et al. have observed a change in haemolytic activity upon acylation of the 12-hydroxy group of the fatty acid tail group.22 Three acyl groups (acetyl, hexanoyl, and myristoyl) in C–F were chosen to study this effect further. The preparations of surfactants C–F are described in Section 2.4.

HOOH

OHN

O

N

O

R OH

OHOH

A R=n-C8H17 or n-C12H25

nn

OOH

OHOH

nn

O

OH

5

9

B

N

O

OHOH

MeO

OH

O

HO OH

OMe

OH

O

OR

5

9

C R=HD R=acetylE R=hexanoylF R=myristoyl

Figure 5. Three target surfactant classes.

12

2.2 Stereodivergent Approach Towards Polyhydroxy Surfactants

(Paper I)

2.2.1 Introduction and Synthetic Outline It has previously been shown that a surfactant with a larger head group has a lower haemolytic activity compared to a surfactant with a smaller one.21 The generic structure A was thus suggested as target (Scheme 3). The size of the hydrophilic part was increased by connecting two hydroxylated chains to the hydrophobic part, and a synthetic sequence offering the possibility to increase the number of hydroxymethylene carbons in the two head groups was envisaged. Relatively few of the works published within the field of synthetic carbohydrate synthesis provide a pathway to both enantiomers of a given hexose through a divergent synthetic approach.31-33,50 However, the recent advances in catalytic asymmetric dihydroxylation51 and in particular the sequential dihydroxylation of polyenes with high enantio- and diastereoselectivity52-54 made us favor the retrosynthetic pathway presented in Scheme 3. So far alkenes comprising up to three conjugated double bonds have been dihydroxylated selectively.54 We expected that polyol E could be accessed in high stereo- and enantioselectivity by stepwise asymmetric dihydroxylation reactions of alkene F. The hydrophobic tail group G could be made from commercially available dimethyl malonate and a suitable alkyl halide.

N

O

N

O

R1OP

OPHO

OP

OPOH

A R1= n-alkyl P= Protecting group

OMe

O

MeO

O

R1

HONHMe

OP

OP

HOOEt

O

OMe

O

MeO

O

R1-Br

E G

F

n

n

n

n

Dimethyl malonate

Scheme 3. Retrosynthetic pathway for surfactant A, via dihydroxylation of a diene.

(50) Takeuchi, M.; Taniguchi, T.; Ogasawara, K. Synthesis 1999, 341-354. (51) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev. 1994, 94, 2483-

2547. (52) Guzman-Perez, A.; Corey, E. J. Tetrahedron Lett. 1997, 38, 5941-5944. (53) Becker, H.; Soler, M. A.; Sharpless, K. B. Tetrahedron 1995, 51, 1345-1376. (54) Park, C. Y.; Kim, M.; Sharpless, K. B. Tetrahedron Lett. 1991, 32, 1003-1006.

13

2.2.2 Method Evaluation Using an Alkene Starting with the simplest possible system, olefin 7 was asymmetrically dihydroxylated,55 which gave 8. Disappointingly, 8 was obtained in only 60% ee, as determined by 1H NMR experiments using Eu(hfc)3 as a chiral shift reagent.56 Nevertheless, it was still decided to explore the chemistry using this substrate, and diol 8 was therefore tosylated, which gave 9 that was transformed into the corresponding amino alcohol 10a (Scheme 4). Amino alcohol 10b, a primary amine with the opposite absolute configuration, was prepared from the enantiomerically pure epoxide 11, via alcohol 12 and tosylate 13.

PMBOa

PMBOOH

OR

7 8 R=H9 R=Ts

b

cPMBO

OHNHMe

10a

dBnO

OHOR

11 12 R=H13 R=Ts

b

eBnO

OHNH2

10bO

OH

Scheme 4. (a) AD-mix α, K2OsO4·2H2O, tBuOH:H2O (1:1), 0 °C, 48 h (60%, 68%, ee). (b) Bu2SnO, TsOH, Et3N, CH2Cl2, rt, 4 h (9: 88%, 13: 91%). (c) aq NH2Me:THF (1:2), 75 °C, 3 h (97%). (d) NaH, BnOH, THF, 0 °C, 12 h (65%). (e) aq NH4OH:THF (2:1), 80 °C, 24 h (92%).

Amino alcohols 10a and 10b were used as model substrates for the evaluation of the coupling conditions between the hydrophilic and the hydrophobic entities (Table 1). Six carboxylic derivatives (14a–f) were used as hydrophobic partners. The amide formation proved to be more problematic than expected. The initial attempt to heat the carboxylic ester 14a together with amine 10a to 140 °C, as previously described for similar systems,57 only resulted in recovered starting material (entry 1). Neither was the attempt to use selected catalysts successful (entry 2). As methyl esters might require harsh conditions to react, it was decided to investigate more reactive carboxylic acid derivatives. Amine 10a was thus treated with malonyl dichloride (14c), but still no product was isolated (entry 3).

(55) Sharpless, K. B.; Amberg, W.; Bennani, Y. L.; Crispino, G. A.; Hartung, J.; Jeong, K.-S.; Kwong, H.-L.; Morikawa, K.; Wang, Z.-M.; Xu, D.; Zhang, X.-L. J. Org. Chem. 1992, 57, 2768-2771.

(56) The absolute configuration was determined in accordance with the mnemonic device proposed by Sharpless and coworkers; J. Am. Chem. Soc. 1994, 116, 1278-1291.

(57) Pitt, A. R.; Newington, I. M. PCT Int. Appl. Wo 9215554, 1992. (58) Other reagents tried were: DCC, Lawessons reagent, and 2-Chloro-1-

methylpyridinium iodide. (59) Klausner, Y. S.; Bodansky, M. Synthesis 1972, 453-463. (60) Kunishima, M.; Kawachi, C.; Hioki, K.; Terao, K.; Tani, S. Tetrahedron 2001, 57, 1551-

1558. (61) Pedersen, U.; Thorsen, M.; El-Khrisy, E.-E. A. M.; Clausen, K.; Lawesson, S.-O.

Tetrahedron 1982, 38, 3267-3269.

14

Selected58,59-61 coupling reagents were used in combination with malonic acid (14d) and 10a, however the most promising reagent, DMT-MM, only provided 50% yield of the desired product (entry 4). At this point the secondary amine 10a, was exchanged for the primary amine 10b, with the intention to increase the reactivity. Indeed, when 10b was heated together with malonic ester 14b some product was obtained, although the reaction was slow and the yield never exceeded 50% (entry 5). The use of DMT-MM and malonic acid 14d did not increase the yield (entry 6). When 2,2-dimethyl malonic acid (14e) was used in combination with amine 10b and DMT-MM, the starting material was converted into one main compound, however much product was lost at purification (entry 7). With these results in mind, it was decided to utilize 2,2-dimethyl malonyl dichloride together with amine 10b. Indeed, this protocol readily provided the desired product as determined by 1H NMR (entry 8).

Table 1. Amide coupling using 10a–b and 14a–f as model substrates.

HN R4R3

10

14a R1=H, R2=n-C4H9, X=OMe14b R1=R2=H, X=OMe14c R1=R2=H, X=Cl14d R1=R2=H, X=OH14e R1=R2=Me, X=OH14f R1=R2=Me, X=Cl

PMBOOH

BnOOH

10a R3=

10b R3=

X

O

X

O

R1 R2N

O

N

O

R1 R2R3

R4

R3

R4

; R4=Me

; R4=H

Entry Amine Carboxylic derivative Yield

1 10a 14a no reactiona 2 10a 14a no reactionb 3 10a 14c no reactionc 4 10a 14d

15

2.2.2.1 Conclusion These studies showed that the generic surfactant A (Scheme 3) was not easily synthesized. To obtain synthetically applicable yields, a primary amine had to be used rather than a secondary one. Furthermore, two alkyl chains had to be incorporated in the α-position of the malonate ester. The forthcoming synthetic work was therefore adapted to these requirements.

2.2.3 Preparation of Head Groups from a Diene An electron-rich double bond can be selectively dihydroxylated over an electron-deficient bond.53 Starting from diene 15 the pathway outlined in Scheme 5 was suggested to reach four different stereoisomers, 19, ent-19, 20, and 21, by employing sequential asymmetric dihydroxylations (Compounds 19 and 21 being enantiomers if P1 = P2.).

P1OOEt

O

P1OOEt

OH

OH

OP1O

OEt

OH

OH

O

P1OOP2

OH

OH

P1OOP2

OH

OH

OH

OH

OH

OH

P1ONH2

OH

OH

OH

OH

H2NOP2

OH

OH

OH

OH

P1ONH2

OH

OH

OH

OH

P1ONH2

OH

OH OH

OH

P1OOP2

OH

OH OH

OH

15

ent-1616

17 18 ent-17

ent-19212019

AD mix αAD mix β

AD mix β AD mix β AD mix α

Scheme 5. Synthetic strategy for the preparation of four stereochemically different head groups.

16

Compound 2266 was converted into compound 23 as PMBz has been shown to contribute to the diastereoselection in the dihydroxylation step (Scheme 6). Further, it is less prone to undergo migration to adjacent hydroxyl moieties compared to the similar benzoyl ester.67

By treating compound 23 with AD-mix β, all-syn alcohol 24 was accessed with good diastereoselectivity (syn:anti 33:1, mismatched case).68 When ent-23 was subjected to AD-mix β, Anti-alcohol 25 was obtained with an even higher diastereoselectivity (syn:anti 1:56, matched case) that could be further improved by flash chromatography (syn:anti 1:97).69 The all-syn enantiomer ent-24 could in analogy be made from ent-23 by treatment with AD-mix α.

b AD-mix βPMBO

OPMBz

O

O OH

OH

PMBOOPMBz

O

O OH

OH

25 anti

24 all-syn

R1OOR2

O

O

22 R1=PMB R2=H23 R1=PMB R2=PMBz

a

ent-23 b AD-mix β

Scheme 6. Second consecutive dihydroxylation. Reaction conditions: (a) PMBzCl, Et3N, DMAP, CH2Cl2, 24 h (53%). (b) AD-mix β, K2OsO4·H2O, tBuOH:H2O 1:1, 12 h, (82–91%).

PMBz ester 25 was hydrolyzed in NaOH/MeOH/H2O at 70 °C to give 26 (Scheme 7). Attempted transformation of alcohol 26 into the corresponding monotosylate 27, using the previously developed method (Scheme 4), was troublesome. Having a triol rather than a diol lowered the selectivity for the primary hydroxy group considerably. Variation of temperature, base and solvent did not increase the yield, and therefore the synthetic sequence outlined in Scheme 8 was utilized instead.

(66) This was prepared as described in: Somfai, P. Marchand, P, Torsell, S., Lindström U. M. Tetrahedron 2003, 59, 1293-1299, using AD-mix β in place of AD-mix α.

(67) Corey, E. J.; Guzman-Perez, A.; Noe, M. C. J. Am. Chem. Soc. 1995, 117, 10805-10816.

(68) As determined by HPLC (69) When treated with K2OsO4·H2O in the absence of chiral ligands a syn:anti relationship

of 1:2.7 was obtained.

17

R1OOR2

O

O OH

OH

25 R1=PMB R2=PMBz26 R1=PMB R2=H

a

bPMBO

OPMBz

O

O OH

OH

27

Scheme 7. Hydrolysis and tosylation to 27. Reaction conditions: (a) NaOH, MeOH, H2O, 60 °C, 48 h, (95%). (b) TsCl, Bu2SnO, Et3N, CH2Cl2 (

18

2.2.4 Alkylation of Dimethyl Malonate The solubilizing capacity of a surfactant generally increases with an increasing size of the hydrophobic part. However, if the head group is not hydrophilic enough to balance the tail group, the aqueous solubility might not be sufficient. As concluded above, dimethyl malonate must be dialkylated in the α-position to undergo the amide coupling smoothly. It was therefore decided that two different hydrophobic tail groups should be prepared, one featuring two n-C8H17 chains and one incorporating two n-C12H25 alkyl chains. Hydrolysis would then provide the corresponding malonic acids, which could be further transformed to the corresponding acid chlorides prior to the amide coupling.

OMeMeO

O O

a OMeMeO

O O

R1 R1b OHHO

O O

R1 R1

37a R1=C8H1737b R1=C12H25

38a R1=C8H1738b R1=C12H25

Dimethyl malonate

Scheme 10. Preparation of the tail groups 38a and 38b. (a) KHMDS, R1Br, ∆, 20 h (37a: 51%, 37b: 23%). (b) KOH, H2O, EtOH, ∆, 4 days (38a: 94%, 38b: 73%).

The alkylation of dimethyl malonate was not straightforward (Scheme 10). The reaction was sluggish and the isolated yields of 37a and 37b were low. Using two equiv. of NaH together with two equiv. of alkyl bromide in refluxing DMF:THF (1:3) produced a mixture of mono- and dialkylated products, together with a large amount of polymerized material that was difficult to separate from the product. Changing the base to KHMDS allowed for the addition of 1 equiv. of base at a time, which was an improvement, but separation of undesired compounds70 was still difficult. Nevertheless, the desired alkylation products 37a and b were obtained in 51 and 23% yield, respectively.

Malonic esters are usually refluxed in alcoholic solvents with an excess of KOH or NaOH to obtain the corresponding acids.71,72 In this study, the reaction time had to be extended considerably to obtain complete conversion. The amphiphilic character of 38a and 38b made extractive workup difficult, but gratifyingly, the reaction mixture could be neutralized by the addition of a strongly acidic ion exchange resin, allowing an easier workup.

(70) Unreacted dimethyl malonate, alkyl bromide, monoalkylated product and polymerization products.

(71) Sutton, A. C.; Nantz, M. H.; Hitchcock, S. R. Organic Preparations and Procedures Int. 1992, 24, 39-43.

(72) Denmark, S. E.; Nakajima, N.; Nicaise, O. J.-C.; Faucher, A.-M.; Edwards, J. P. J. Org. Chem. 1995, 60, 4884-4892.

19

2.2.5 Completion of the Surfactants The transformations to the final surfactants are depicted in Scheme 11. The carboxylic acids 38a and 38b were transformed into the acid chlorides 39a and 39b with oxalyl chloride and DMF.

a

38a R1=n-C8H1738b R1=n-C12H25

bOH

O

HO

O

39a R1=n-C8H1739b R1=n-C12H25

R1 R1

c

R2O

HN

O

O O

O

HN

O

R1 R1

OR2

O

OO

OO

40a R1=n-C8H17 R2=PMB40b R1=n-C12H25 R2=PMB

d

41a R1=n-C8H17 R2=H41b R1=n-C12H25 R2=H

HO

HN

OH

OH OH

OH

HN

O

R1 R1

OHOH

OHOH

OHO

42a R1=n-C8H1742b R1=n-C12H25

Cl

O

Cl

O

R1 R1

Scheme 11. Completion of the synthetic sequence. Reaction conditions: (a) (COCl)2, DMF, CH2Cl2, 1 h. (b) amine 31, Et3N, DMAP, CH2Cl2, 12 h (56-63% over two steps). (c) H2, Pd(C), MeOH, 30 min, (99%). (d) Dowex-50W, methanol, ∆, 48 h (99%).

The fully protected surfactant precursors 40a and 40b were formed almost instantly when the head group precursor 31 was added to a solution of 39a or 39b together with Et3N and DMAP in CH2Cl2. The highly hydrophobic character of the products 40a and 40b, especially the C12-comprising compound 40b, made purification on normal and reversed phase silica gel difficult. Repeated purification with flash chromatography was needed to obtain a pure product and the yields thereby dropped. As it was still possible to obtain the desired amides in 56–63% yield, the slight loss was accepted as it was considered to be more difficult to purify the compounds at a later stage.

It was of great importance to use a deprotection protocol that did not require any other purification than filtration or evaporation in the last steps of the synthetic sequence. Two protocols were considered for the cleavage of the PMB ether in compounds 40a and 40b; oxidation with DDQ or hydrogenolysis with H2 and Pd/C. Both reactions worked equally well, but as the oxidative procedure required flash chromatography, hydrogenolysis was a superior method. To remove the acetonides, a strongly acidic cation exchange resin was added to 41a and 41b dissolved in methanol. At heating, the desired products 42a and 42b were obtained in high purity as white powders after filtration and evaporation.

20

The stereoisomers ent-42a and b, 43a and b, and ent-43a and b could be prepared from 32, 24 and ent-24, respectively (Scheme 12) in analogy with the synthesis reported above.

24 HOHN

OH

OH OH

OH

HN

O

R1 R1

OHOH

OHOH

OHO

43a R1=n-C8H1743b R1=n-C12H25

ent-24 HOHN

OH

OH OH

OH

HN

O

R1 R1

OHOH

OHOH

OHO

ent-43a R1=n-C8H17ent-43b R1=n-C12H25

32 HOHN

OH

OH OH

OH

HN

O

R1 R1

OHOH

OHOH

OHO

ent-42a R1=n-C8H17ent-42b R1=n-C12H25

Scheme 12. Representation of surfactants ent-42a and b, 43a and b, and ent-43a and b.

2.2.6 Conclusions With a set of surfactants prepared by consecutive dihydroxylations of a diene, the next step would be to develop a protocol for the chemo- and enantioselective dihydroxylation of a triene. However, the aqueous solubility of the surfactant types 42 and 43 were too low for the compounds to have an application as solubilizers. It is known that glucamine surfactants might suffer from low water-solubility,49 and it was therefore decided not to continue this investigation with the triene, but instead focus on a new set of surfactants, designed and prepared with the intention to increase the water-solubility.

Nevertheless, we demonstrated a synthesis of four isomerically different surfactant head groups that were coupled with two different hydrophobic tail groups. The phydicochemical properties of these surfactants are further discussed in Sections 3.2 and 3.3.

21

2.3 Synthesis of a Surfactant Head Group by a Metathesis Approach

(Paper II)

2.3.1 Introduction and Synthetic Outline Surfactant H in Figure 6 offers several opportunities for the introduction of diversity. Diastereomeric surfactants can be obtained by the choice of starting material (a), and the polyhydroxy chain can be elongated (b). The internal double bond can be oxidized or reduced (c), and the linkage between the hydrophilic and the hydrophobic parts can be varied depending on heteroatom (d). To further modify the hydrophilic/lipophilic balance, the fatty acid hydroxyl group can be alkylated or acylated (e).

XOP1

OP3OP2

nn

O

OH

5

9

(c) Oxidize or reduce

(a) Control of stereochemistry

(d) Choice of heteroatom

(e) Alkylate or acylate H

(b) Elongate via a Swern/Wittig/Metathesis protocol

Figure 6. Target surfactant.

The retrosynthetic pathway for the preparation of surfactant I (n = 3) is presented in Scheme 13. The ester linkage between the hydrophilic and the hydrophobic groups in surfactant I could be prepared from (R)-12-hydroxy stearic acid and alcohol J, which could be accessed via a metathesis reaction between fragments K and L. Hexenes K and L could be obtained by a reductive fragmentation of a selected pyranose derivative M, taking advantage of the stereochemistry already present in the carbohydrate.

22

OOP1

OP2OP1

33

O

OH

5

9

I

(R)-12-hydroxy stearic acid

P1OOP1

OP2OP1

J

P3OOP1

OP2OP1

33

K L

O OH

OHOH

HO

I

3 3

M

OH

O

OH

5

9

Scheme 13. Retrosynthetic pathway for the synthesis of surfactant I.

The synthetic outline in Scheme 14 was proposed to afford an efficient way of elongating the head group. Treatment of olefin N with OsO4 would give the hydroxylated compound O. Protecting group manipulation; oxidation and olefination would present P, which could be used as a metathesis partner to give the cross-coupled product Q. This sequence would increase the number of hydroxy groups with 2n+2 for each cycle.

P1OOP

OP2OP

nn

P1OOP

OP2OP

nn

OH

OH

P1OOP

OP

nn

OH

OHP1O

OP

OP

nn

OH

OH

OP2OP

OP

nn

OH

OH

N O

PQ

AD

Swern/Wittig

Metathesis

Scheme 14. Surfactant head group from elongation.

23

2.3.2 Synthesis of the Polyhydroxylated Head Groups Glucose and galactose were selected for the preparation of building blocks K and L. The 3,4-di-O-isopropylidene protected galactoside 4473 (Scheme 15) was transformed into iodide 45 following a literature procedure. 74,75 The subsequent fragmentation of iodide 45 by standard conditions76,77 produced a mixture of byproducts, among which the 2-deoxygenated aldehyde 46a was identified. 2-Deoxygenation under similar conditions has been reported,74,78 and could be suppressed by acetylation of the hydroxy group in iodide 45. Although alcohols normally are the solvents of choice for zinc-promoted reductive eliminations,76,77 the use of ethanol or methanol was not successful in the fragmentation of 45. Neither THF was a suitable solvent, but the reactivity was somewhat increased when water was added to the mixture.78 The reaction was still slow, and the aldehyde formed in the reaction probably started to decompose before complete conversion was reached. Yet, the addition of 4 equiv. of NH4Cl to the reaction mixture turned out to increase the reactivity, and the starting material was instantly converted to the aldehyde 46b.79 Aldehyde 46b was subsequently reduced with LiAlH4 to the desired olefin 47a that was protected to give 47b.

OO

OMeOH

O R

a44 R=OH45 R=I

b

46b

cOO

O

AcO

46a

OO

O

ROOR1

OR1

OR

d47a R=H, R1=C(Me)247b R=R1=C(Me)2

Scheme 15. Preparation of terminal olefins based on D-galactose. Reaction conditions: (a) PPh3, I2, imidazole, toluene, 5 h (48%). (b) i) Ac2O, pyridine, DMAP, CH2Cl2, 12 h (100%); ii) Zn, NH4Cl, THF:H2O (10:1), 10 min. (c) LAH, Et2O, 30 min (55% over two steps). (d) 2-methoxypropene, TsOH, DMF, 10 min (87%).

(73) Rashid, A.; Mackie, W.; Lamba, D. Can. J. Chem. 1990, 68, 1122-1127. (74) Desire, J.; Prandi, J. Eur. J. Org. Chem. 2000, 3075-3084. (75) Garegg, P. J.; Samuelsson, B. J. Chem. Soc., Perkin Trans. 1 1980, 2866-2869. (76) Schmitt, C. G.; Boord, C. E. J. Am. Chem. Soc. 1931, 53, 2427-2428. (77) Bernet, B.; Vasella, A. Helv. Chim. Acta 1979, 62, 1990-2016. (78) Skaanderup, P. R.; Hyldtoft, L.; Madsen, R. Monatshefte für Chemie 2002, 133, 467-

472. (79) Kleban, M.; Kautz, U.; Greul, J.; Hilgers, P.; Kugler, R.; Dong, H.-Q.; Jäger, V.

Synthesis 2000, 1027-1033.

24

2.3.2.1 Olefin Metathesis Cross metathesis (CM) has lately emerged as a powerful reaction tool for the intermolecular coupling of two olefinic compounds (R and S) as described schematically in Scheme 16.80-82 The reaction is catalyzed by a Ru-carbene complex (e.g. 48–50), and proceeds under the evolution of ethylene gas. The reaction typically yields a mixture between cross metathesis and self metathesis products T–V, and depending on the electronic and steric properties of R1 and R2, the relative distribution between these three possible reaction products can be controlled, as well as the E:Z ratios of the formed products.81

R1 R2

Ru cat.R1

R2R1

R1R2

R2

RuPCy3Ph

ClCl

NMesMesN

49

RuN Ph

NMesMesN

50

ClN

Cl

BrBr

RuPCy3

PCy3PhClCl

48

R ST U V

Scheme 16. Schematic metathesis reaction between olefins R and S. Three ruthenium catalysts: Grubb’s 1st generation catalyst (48), 2nd generation catalyst (49), and phosphine free catalyst (50).

The cross metathesis product T is not always easily obtainable if the steric and electronic properties of the two metathesis partners R and S are similar.81 We therefore initiated our study with the intention of obtaining the self-metathesis product 51a (Scheme 17) by treating olefin 47a with either of the two selected Ru-catalysts 49 or 50, 50 being reported to be more active (Scheme 17).83 In this study however, no significant differences between the two catalysts were observed. Surprisingly, olefin 47a was completely unreactive under the applied conditions. Previous experiences made us believe that this was due to sterical crowding in the cis-substituted five-membered ring,84 a conclusion that was further supported by the finding that olefin 47b was equally unreactive at room temperature and up to 40 °C.

(80) Connon, S. J.; Blechert, S. Angew. Chem. Int. Ed. Engl. 2003, 42, 1900-1923. (81) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003,

125, 11360-11370. (82) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413-4450. (83) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew. Chem. Int. Ed. Engl.

2002, 41, 4035-4037. (84) Restorp, P.; Somfai, P. Chem. Commun. 2004, 18, 2086-2087.

25

From the literature it was not obvious whether allylic alcohols or ethers facilitate the metathesis reaction, or rather have an adverse effect.80,85-88 It was therefore decided to continue the work with a brief investigation of the effect of O-protecting groups on the outcome of the metathesis reaction.

O

O

ORRO

O

O

OROR

47a R=H47b R=CMe2

51a R=H51b R=CMe2

aRO

O

O

OR

Scheme 17. Self metathesis reaction. (a) 49 or 50 (10 mol%), CH2Cl2 or toluene, rt → 70 °C (0%).

Five additional metathesis substrates were prepared as described in Scheme 18. Olefin 47a served as starting material for 47c, and olefins 47d–g were prepared from protected glucoside 5289, which was iodinated and reductively fragmented as described for 44.

OPMBOBnO

OMeBnO

R

R2OOR1

OBn

OBn

c

b52 R=OH53 R=I

OOPMB

OBn

OBn

46c

d

e47d R1=PMB, R2=H47e R1=R2=H47f R1=PMB, R2=TBS47g R1=H, R2=TBS

e

f

47aa

47c

BnOOH

OH

BnO

Scheme 18. Synthesis of metathesis partners 47c–g. (a) i) NaH, Bu4NI, BnBr, THF, 12 h (61%); ii) HCl, THF, 1 h (85%). (b) PPh3, I2, imidazole, toluene, 5 h (94%). (c) Zn, NH4Cl, THF:H2O (10:1), 10 min. (d) LAH, Et2O, 30 min (71% over two steps). (e) DDQ, CH2Cl2:H2O (25:1), 1 h (47e: 36%, 47g: 81%). (f) TBSCl, imidazole, DMF, 12 h (69%).

(85) BouzBouz, S.; Simmons, R.; Cossy, J. Org. Lett. 2004, 6, 3465-3467. (86) Lautens, M.; Maddess, M. L. Org. Lett. 2004, 6, 1883-1886. (87) Hadwiger, P.; Stütz, A. E. Synlett 1999, 1787-1789. (88) Maishal, T. K.; Sinha-Mahapatra, D. K.; Paranjape, K.; Sarkar, A. Tetrahedron Lett.

2002, 43, 2263-2267. (89) Johansson, R.; Samuelsson, B. J. Chem. Soc., Perkin Trans. 1 1984, 2371-2374.

26

The results from the metathesis study with substrates 47a–g are summarized in Table 2. Olefin 47c, which possessed a more flexible structure compared to 47a and 47b, was subjected to the same reaction conditions. Indeed, the reactivity was higher, but only byproducts were isolated (entry 5). Continuing the investigation with the glucose-derived olefins 47d–g, 47d was recovered quantitatively after reflux in CH2Cl2 (entry 6), and recovered together with decomposition products upon heating at 60 °C in toluene (entry 7). The deprotected diol 47e was even less suitable for the metathesis reaction, and gave a mixture of unidentified products even at room temperature (entry 8). It was therefore speculated that the free hydroxy groups at C(1)–C(3) were not tolerated in the reaction, but a sterically demanding protecting group at the allylic hydroxy group suppressed the reactivity of the adjacent double bond. To prove this theory, substrates 47f and 47g were subjected to the same reaction conditions, and indeed 47f was recovered quantitatively even after stirring for several days, while 47g was almost instantly converted to the desired dimer 54g in good yield and as a single isomer (entries 9–11).

Table 2. Olefin metathesisa

OR

OR1

OR2R3O

OR

OR1

OR2R3O

OR

OR1

OR2

OR3

47 54

a

Entry Olefin Solvent /

Temp. (°C) Cat. Prod/

Yield (%) 1 47a (R=R1=CMe2, R2=R3=H; S,S) CH2Cl2 / 40 49 — / 0b

2 47a (R=R1=CMe2, R2=R3=H; S,S) Toluene / 70 49/50 — / 0b 3 47b (R=R1= R2=R3=CMe2; S,S) CH2Cl2 / 40 49/50 — / 0b 4 47b (R=R1= R2=R3=CMe2; S,S) Toluene / 70 49/50 — / 0c 5 47c (R=R1=H, R2=R3=Bn; S,S) Toluene / 70 49 — / 0c 6 47d (R=PMB, R1= R2=Bn, R3=H; R,R) CH2Cl2 / rt 49 — / 0b 7 47d (R=PMB, R1= R2=Bn, R3=H; R,R) Toluene / 60 49 — / 0d 8 47e (R=R3=H, R1= R2=Bn; R,R) CH2Cl2 / rt 49 — / 0c 9 47f (R=PMB, R1= R2=Bn, R3=TBS; R,R) CH2Cl2 / rt 49 — / 0b 10 47f (R=PMB, R1= R2=Bn, R3=TBS; R,R) Toluene / 60 49 — / 0d 11 47g (R=H, R1= R2=Bn, R3=TBS; R,R) CH2Cl2 / 40 49 54g / 67e

aReaction conditions: (a) Catalysts 49 or 5083 (10-20 mol%) in CH2Cl2 at rt → 40 °C or in toluene at 60 → 70 °C. No significant differences in reactivity were seen between the two catalysts. bStarting material was recovered. cStarting material was consumed. dStarting material was recovered together with decomposition products. eE:Z >99:1, determined from coupling constants of the vinylic protons in compound 61.

27

2.3.3 Completion of the Surfactants The next step in the synthetic sequence towards the desired surfactants was to realize the ester bond between the polyhydroxy compound and 12-hydroxy stearic acid. Cleavage of the silyl protecting groups in 54g gave 55 (Scheme 19). Despite the C2 symmetry of this molecule, selective esterification at only one out of four possible sites was believed to be a problem. Simple symmetrical diols have been monobenzylated by the use of Ag2O and benzyl bromide, but applying these conditions to alcohol 55 did not provide the expected compound 56.90,91 As an alternative, 54g was treated with a deficit of TBAF to give alcohol 57, but unexpectedly 57 was obtained in only 80% purity, probably as a result of silyl group migration. As it was not possible to purify 57, an alternative approach was initiated.

OH

OBn

OBnOTBS

OH

OBn

OBnTBSO

54g

OH

OBn

OBnOH

OH

OBn

OBnTBSO

57

OH

OBn

OBnOH

OH

OBn

OBnBnO

56

OH

OBn

OBnOH

OH

OBn

OBnHO

55

a

b c

Scheme 19. Attempts to desymmetrize/monofunctionalize olefin 54g. (a) HOAc:THF:H2O (3:1:1), rt, 24 h, (88%). (b) TBAF·3H2O (0.25 mol%), THF, 0 °C, 1 h (>99%). (c) Ag2O, BnBr, CH2Cl2 (0%).

It was decided to investigate if the esterification could be accomplished directly using a five-fold excess of alcohol 55 to (R)-12-hydroxy stearic acid92 (Scheme 20). Under the Yamaguchi esterification protocol,93 ester 58 was indeed obtained, however together with a significant amount of byproducts which were impossible to separate from the desired product.

OH

OBn

OBnOH

OH

OBn

OBnHO

55

aO

O

OH

5

9OH

OBn

OBn

OHOH

OBn

OBn

58 Scheme 20. Attempt to esterify alcohol 55. (a) (R)-2,4,6-trichlorobenzoyl-12-hydroxy stearate, Et3N, DMAP, 12 h.

(90) Bouzide, A.; Sauvé, G. Org. Lett. 2002, 4, 2329-2332. (91) Löfstedt, J.; Pettersson-Fasth, H.; Bäckvall, J.-E. Tetrahedron 2000, 56, 2225-2230. (92) The enantiomeric purity of 12-hydroxy stearic acid was determined to be >95% in

analogy with the method described in: Sonnet, P. E., Hayes, D.; J. Am. Oil Chem. Soc. 1995, 72, 1069-1071

(93) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989-1993.

28

To circumvent these problems, the secondary hydroxyl groups in alcohol 54g were benzylated to provide 59, followed by cleavage of the silyl groups that gave alcohol 60 (Scheme 21). Gratifyingly, treatment of alcohol 60 under the above described conditions provided the desired surfactant precursor 61 in excellent yield with no detected formation of bis-ester. The unreacted alcohol could be recovered and reused after the reaction. Hydrogenolytic cleavage of the benzyl groups was accomplished with H2 and Pd/C and provided the expected surfactant 62 in good yield.

OR1OBn

OBn

OBn

R1OOBn

OBn

OBn

O

O

OH

5

9OBn

OBn

OBn

OHOBn

OBn

OBn

59 R=TBS

60 R=H

61

c

a54g

b

d

O

O

OH

5

9OH

OH

OH

OHOH

OH

OH

62

Scheme 21. Coupling between head group and tail group. Reaction conditions: (a) KHMDS, BnBr, THF, –78 °C → rt, 12 h (71%). (b) HOAc:H2O:THF (1:3:3), 50 °C, 24 h (79%). (c) (R)-2,4,6-trichlorobenzoyl-12-hydroxy stearate, Et3N, DMAP, 12 h (73%). (d) H2, Pd/C, MeOH, 24 h (61%).

Somewhat surprisingly, surfactant 62 turned out to have very low water solubility. To compensate for this, the three derivatives depicted in Figure 7 were suggested. Compound 63 was expected to have increased water solubility due to the shorter hydrophobic tail group. Compounds 64 and 65, being 4,7-bis-O-acetylated and 4,7-bis-O-methylated respectively, were believed form weaker hydrogen bonds, therefore being less crystalline and having a higher aqueous solubility.

29

O

O

OH

5

9OR

OH

OH

OHOR

OH

OH

64 R=Ac65 R=Me

O

O

9OH

OH

OH

OHOH

OH

OH

63

Figure 7. Alternative surfactants designed to have higher aqueous solubility.

Surfactant 63, having a C11 hydrophobic tail group, was synthesized from alcohol 53 and undecenoic acid following the same procedure as developed for 62 (Scheme 22). Unfortunately, the water solubility of surfactant 63 was still unsatisfactory, which suggested that the solubility issue was more complicated than just changing the hydrophilic/lipophilic balance.

O

O

9OH

OH

OH

OHOH

OH

OH

60a, b

63

Scheme 22. Synthesis of surfactant 63 comprising a C11 hydrophobic tail group. (a) 2,4,6-trichlorobenzoyl-10-undecenoate, Et3N, DMAP, 12 h (83%). b) H2, Pd/C, EtOAc, 24 h (92%).

Surfactants 64 and 65 were prepared from 54g following the synthetic pathway outlined in Scheme 23. Acetylation and methylation gave 66 and 67, respectively, which were deprotected as previously described to provide alcohols 68 and 69. Esterification of alcohols 68 and 69 with (R)-12-hydroxy stearic acid yielded the surfactant precursors 70 and 71, respectively.

30

OR1OR2

OBn

OBn

R1OOR2

OBn

OBn

c

O

O

OH

5

9OR1

OBn

OBn

OHOR1

OBn

OBn

66 R1=TBS, R2=Ac67 R1=TBS, R2=Me

68 R1=H, R2=Ac69 R1=H, R2=Me

70 R1=Ac71 R1=Me

d

a or b

54g

eO

O

OH

5

9OR1

OH

OH

OHOR1

OH

OH

64 R1=Ac65 R1=Me

Scheme 23. Synthesis of surfactants 64 and 65. (a) Ac2O, Et3N, DMAP, CH2Cl2, 64: 99%. (b) KHMDS, MeI, THF, –78 °C → rt, 12 h (65: 99%). (c) HOAc:H2O:THF (1:3:3), 50 °C, 24 h (68: 73%, 69: 99%). (d) 12-hydroxy stearic acid, 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, 12 h (70: 84%, 71: 82%). (e) H2, Pd/C, MeOH.

Hydrogenolysis of surfactant precursors 70 and 71 was for unknown reasons cumbersome. Compound 64 was obtained together with a mixture of isomers, probably as a result of acyl group migration. The deprotection of compound 71 gave also mixture of products, with 65 as the main component. It was however impossible to purify 65 enough for a confident structural assignment and for surface chemical evaluation.

2.3.4 Conclusions A polyhydroxylated surfactant head group, featuring seven hydroxy groups, was prepared from glucose via a metathesis strategy. This head group was derivatized and coupled to (R)-12-hydroxy stearic acid and undecenoic acid that gave four different surfactants. Unfortunately, none of the surfactants were enough water soluble for surface chemical characterization, the reason for which will be discussed in Chapter 3. To circumvent further solubility problems, it was decided to not proceed with the intended chain elongation presented in Scheme 14, as this approach may not contribute to increased solubility of the surfactant.

31

2.4 Synthesis of Surfactants from Glucose and 12-Hydroxy stearic acid

(Paper III)

2.4.1 Strategy Having encountered severe solubility problems of surfactant types 42 and 43, and 62–63, we were still interested in finding a novel surfactant that could be useful as a drug solubilizer. One way of increasing the water solubility of a surfactant could be to increase the sterical hindrance around the ester or amide bond connecting the hydrophilic and hydrophobic parts. Surfactant W, which could be made from (R)-12-hydroxy stearic acid and the amine X, was therefore suggested as a target (Scheme 24).

O

OR

O

OHOH

(R)-12-hydroxy stearic acid X

NHO O

PO

POOMeOMe

OP

OPOP OP

N

O

OHOH

MeO

OH

O

HO OH

OMe

OHW

Scheme 24. Retrosynthetic outline for surfactant W.

32

2.4.2 Synthesis of the Amine Head Group The first attempt to synthesize a sugar derivative with the generic structure X is presented in Scheme 25. Protected galactoside 7273 was transformed into the corresponding mesylate 73, which was subsequently treated with aqueous NH4OH to provide amine 74. Mesylate 73 and amine 74 were heated together with the intention of obtaining the cross-coupled product 75, but this met with no success. Mesylate 73 was also heated with 0.5 equiv. of p-methoxy benzyl amine with the purpose of obtaining 76, but only starting materials were recovered even after several days.

O

OMeOH

O

O

RO

72 R=H73 R=Ms

a

b O

OMeOH

O

O

H2N

74 R=H

O

OMeHO

OO

O

OMeOH

OO

NH

75

c

76

O

OMeHO

OO

O

OMeOH

OO

NPMB

d

Scheme 25. Synthesis of a polyhydroxylated head group from galactose. SN2 approach. (a) MsCl, pyridine, CH2Cl2, 0 °C, 1.5 h (61%). (b) NH4OH, THF, 120 °C, 12 h (59%). (c) 73, MeCN, ∆ (0%). (d) p-Methoxybenzyl amine, MeCN, ∆ (0%).

Assuming that this low reactivity was due to sterical hindrance, a second approach was set up, starting from the protected glucoside 7794 as described in Scheme 26. The 6-OH group in 77 was via mesylate 7895 transformed into amine 79. To circumvent the unsuccessful SN2 reaction a reductive amination strategy was suggested. Alcohol 77 was oxidized to yield the corresponding aldehyde 80, which was immediately treated with amine 79 and NaBH3CN, which gave the expected amine 81.

(94) Ishikawa, T.; Shimizu, Y.; Kudoh, T.; Saito, S. Org. Lett. 2003, 5, 3879-3882. (95) Kobertz, W. R.; Bertozzi, C. R. J. Org. Chem. 1996, 61, 1894-1897.

33

77

a

78 R=OMs79 R=NH2

80

c

b

81

d

NHO O

BnO

BnOOMeOMe

OBn

OBnOBn OBn

OHO

BnO

BnOOMe

OBn

RO

BnO

BnOOMe

OBn

OO

BnO

BnOOMe

OBn

Scheme 26. Synthesis of the amine head group 81. (a) MsCl, pyridine, 0 °C → rt, CH2Cl2, 12 h (98%). (b) NH4OH, THF, 100 °C, 48 h (86%). (c) (COCl)2, DMSO, Et3N, –78 °C, 1 h (86%). (d) Ti(OiPr)4, NaBH3CN, MeOH, 2 days (88%).

2.4.3 Completion of the Surfactants To accomplish the amide coupling between amine 81 and (R)-12-hydroxy stearic acid, the carboxylic acid was treated with oxalyl chloride with the intention of obtaining acid chloride 82 (Scheme 27). This failed however, probably as a result of internal esterification of the 12-hydroxy group.

OH

5

9

O

OHa

OH

5

9

O

Cl

82(R)-12-hydroxy stearic acid Scheme 27. Attempt to transform (R)-12-hydroxy stearic acid into the corresponding acid chloride. (a) (COCl)2, DMF, CH2Cl2, rt (0%).

34

It was instead decided to employ a peptide-coupling reagent. As tedious purification steps must be avoided in surfactant synthesis, EDC was chosen as a coupling reagent, as the urea formed in the reaction can easily be removed in an aqueous extraction step. Indeed, following this strategy, the surfactant precursor 83 was readily accessed in 64% yield (Scheme 28). Previous studies have revealed that the hydrophobic part influences the haemolytic activity of the surfactant, therefore the 12-hydroxy group was functionalized as an acetyl-, hexanoyl-, or myristoyl ester respectively,22 which yielded esters 84–86. Debenzylation under standard conditions was slow, but surfactant 87 was still obtained in nearly quantitative yield from 83. Addition of a catalytic amount of HOAc to the remaining reaction mixtures decreased the reaction time and provided surfactants 88–90 in excellent yields from their respective precursors 84–86. To our delight, having a branched sugar head group, surfactants 87–89 were readily water-soluble. Surfactant 90, having a rather hydrophobic tail group was not soluble, this was however anticipated.

81a

83 R1=H84 R1=Ac

85 R1=Hexanoyl86 R1=Myristoyl

c or d

87 R1=H88 R1=Ac89 R1=Hexanoyl90 R1=Myristoyl

b

N

O

OBnOBn

MeO

OBn

O

BnO OBn

OMe

OBn

OR1

5

9N

O

OHOH

MeO

OH

O

HO OH

OMe

OH

OR1

5

9

O O

Scheme 28. Completion of surfactants 87–90. (a) (R)-12-hydroxy stearic acid, EDC, Et3N, DMAP, CH2Cl2, 24 h (62%). (b) RCl, pyridine, CH2Cl2, 2-5 h (84: 94%; 85: 94%; 86: 93%) (c) H2, Pd/C, MeOH, 48 h (98%). (d) H2, Pd/C, HOAc, MeOH, 19 h (100%).

2.4.4 Conclusions Our attempt to access a series of surfactants featuring a branched head group was indeed successful. Four surfactants (87–90) were accessed in only seven steps from 77. This strategy would also allow for the combination of different hexoses for the head group, although it was not included in this study. Surfactants 87–90 were evaluated with respect to their CMCs and haemolytic activities, and these results are presented and discussed in Chapter 3.5.

35

3

Surface Chemical Characterization 3.1 Introduction

As addressed in the first chapter, we were interested in measuring the haemolytic activity of the prepared surfactants. Moreover, we wanted to study the properties of surfactant stereoisomers. Unfortunately, the water solubilities of surfactant types 42 and 43 and 62–63 were very low, therefore the use of these surfactants in pharmaceutical applications was limited. Instead, surfactants 42b and ent-42b, being enantiomers, and 43b, being a diastereomer to 42b and ent-42b, were compared at the air-water interface using a Langmuir film balance, Brewster angle microscopy and Grazing incidence X-ray diffraction. The results for these observations are presented and discussed in Section 3.3. Surfactants 62 and 63 were not obtained in sufficient amounts and/or purity, and were therefore not further characterized with respect to their physicochemical properties. Surfactants 87–89 were water soluble, and were characterized with respect to their CMCs and haemolytic activities (Sections 3.4–3.5). Surfactant 90 was not water soluble, and was therefore excluded from the CMC and haemolysis measurements.

3.2 Solubility

External variables such as temperature, pressure, additives, and electrolytes may affect surfactant solubility. Furthermore, the surfactant structure also influences the solubility in mainly two ways. The hydrophilic and the hydrophobic parts must be properly balanced, which means that a surfactant with a large hydrophobic part needs a large hydrophilic part to maintain enough monomer solubility.96 For polyhydroxylated surfactants however, strong intermolecular hydrogen bonds might facilitate tightly packed monolayers and thus reduce the water solubility. Such effects have been observed for sugar-based surfactants, and have in particular been shown to be important for surfactants containing strong hydrogen bond donors such as unstubstituted or N-monosubstituted amides. The

(96) Rosen, M. J. Surfactants and interfacial phenomena; 2 ed.; Wiley, New York, 1989.

36

C12-gluconamide 91 is for example insoluble in water while the C12-N-methyl gluconamide 92 has a CMC of 0.148 mM (Figure 8).97,98

HOOH

OH

OH

OH HN

O11 HO

OH

OH

OH

OHN

O11

91 92 Figure 8. C12-gluconamide (91) and C12-N-methyl-gluconamide (92).

Neither the surfactants of types 42 and 43, nor surfactants 62–63 had high enough monomer solubility to form micelles, not even at heating. The originally proposed surfactant A, would probably have a higher aqueous solubility than the actually synthesized surfactant types 42 and 43. The more hydrophobic tail groups of the types 42 and 43 compared to compound A, as well as the N-monosubstituted amide connecting the hydrophilic and hydrophobic parts of types 42 and 43, might have contributed to the low water solubility of the these compounds.

Surfactants 64 and 65 were suggested to be more water soluble than 62 and 63. The methyl and acetyl groups in 64 and 65 were supposed to reduce the intermolecular hydrogen bonding and thereby reduce the crystallization energy, with increased water solubility as a result. The aqueous solubility was somewhat disappointingly equally low for 64 and 65 as for 62 and 63, which either implied that the hydrophobic part was still to large to be balanced by the hydrophilic part, or that the two methyl groups in 64 and the two acetyl groups in 65 did not disturb the molecular packing efficiently. The fact that 64 and 65 were obtained as impure mixtures also needed consideration, however impurities may either increase or reduce the water solubility of a surfactant.

Surfactants 87–90 were designed to minimize the intermolecular hydrogen bonding, comprising N,N-disubstituted amide bonds and rigid head groups to disfavor crystallization. This was very successful, as surfactants 87–89 were soluble in water, while 90 was insoluble in water due to the large hydrophobic part.

Surfactant 89 showed an interesting temperature–solubility dependence. The physicochemical properties of polyhydroxy-based surfactants are generally regarded as being independent to temperature,99 but a cloud point was reached upon heating 89 to a temperature at approximately 60 °C. This behavior was not seen with any of the other synthesized surfactants.

(97) Syper, L.; Wilk, K. A.; Sokolowski, A.; Burczyk, B. Prog. Colloid Polym. Sci. 1998, 110, 199-203.

(98) Zhu, Y.-P.; Rosen, M. J.; Vinson, P. K.; Morrall, S. W. J. Surfactants Deterg. 1999, 2, 357-362.

(99) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160-170.

37

3.3 Stereochemistry

(Papers I and IV)

3.3.1 Background The features of many chiral amphiphiles have been studied at the air-water interface, and the literature on the topic was recently reviewed.100 Fuhrhop and Vollhardt have in particular contributed to the study of sugar-based amide surfactants, and both have observed differences between enantiomerically pure vs. racemic amphiphiles, as well as differences between diastereomeric surfactants.101-104 Hato and coworkers have investigated monolayers consisting of oligosaccharide amphiphiles, and have observed differences in monolayer compressibility depending on the number of glucoside residues in the surfactant head group, as well as on the character of the glycosidic linkage.105,106

Possible applications of chiral surfactants would be to mimic biological membranes, and the study of chiral surfactants provides an opportunity to study molecular recognition under controlled circumstances.107 By studying surface pressure isotherms of a monolayer at an air-water interface using a Langmuir-Blodgett trough, valuable information regarding intermolecular forces can be obtained. Such isotherms are preferable complemented with Brewster Angle Microscopy (BAM), and Grazing incidence X-ray diffraction (GIXD).

3.3.2 Introduction to Langmuir Monolayers 3.3.2.1 Surface Pressure The technique to measure surface pressure (π)–area (A) isotherms, which was first described by Irvin Langmuir, is despite its relative simplicity a powerful method to study intermolecular interactions.108 Figure 9 shows a simple model of a Langmuir film balance. This consists of a trough made of PTFE, a barrier made of a hydrophilic material to be wetted by the subphase, and the Wilhelmy plate made of platinum. The Wilhelmy plate is the link between the monolayer at the surface and the microbalance that registers the shift in surface pressure.

(100) Nandi, N.; Vollhardt, D. Chem. Rev. 2003, 103, 4033-4075. (101) Fuhrhop, J.-H.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 1768-1776. (102) Fuhrhop, J.-H.; Schneider, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc.

1987, 109, 3387-3390. (103) Vollhardt, D.; Emrich, G.; Gutberlet, T.; Fuhrhop, J.-H. Langmuir 1996, 12, 5659-

5663. (104) Vollhardt, D.; Gutberlet, T.; Emrich, G.; Fuhrhop, J.-H. Langmuir 1995, 11, 2661-

2668. (105) Hato, M.; Minamikawa, H. Langmuir 1996, 12, 1658-1665. (106) Tamada, K.; Minamikawa, H.; Hato, M.; Miyano, K. Langmuir 1996, 12, 1666-1674. (107) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131-138. (108) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848-1906.

38

Movement of the barrier

Balance

Wilhelmy plate

Trough

Barrier

Top view

Side viewWater subphase

Figure 9. Schematic representation of a Langmuir film balance

The surface pressure (π) is defined as the difference in surface tension between the pure subphase (γ0) and the surface tension in the presence of a monolayer (γ) (equation 2).

(2)

The surfactant to be investigated is dissolved in a spreading solvent to a concentration of approximately 1 mM. A small volume (50-200 µL) is carefully spread onto the aqueous subphase, and after evaporation of the spreading solvent the barrier is slowly moved to compress the monolayer, and the increase in surface pressure is registered continuously. At compression the monolayer might pass a few typical phases. At large molecular areas the monolayer passes through the gaseous state. At somewhat smaller molecular areas cooperative intermolecular forces might become more pronounced, and the monolayer takes the form of a liquid film. Two types of liquid films can usually be recognized. The more compressible liquid expanded (LE) phase that after a gradual transition enters the liquid condensed (LC) phase. At even smaller molecular areas the monolayer takes the form of a solid film that commonly has compressibility comparable to that for bulk matter. At a certain surface pressure the monolayer collapses, forming a multilayer instead.109 Visualization methods have recently

π = γ0 − γ

(109) Adamson, A. W. Physical Chemistry of Surfaces; 5 ed.; Wiley: New York, 1990.

39

emerged, registering the reflection patterns from monolayers spread at the air-water interphone Brewster Angle Microscopy (BAM) and Grazing Incidence X-Ray Diffraction (GIXD) are such visualization techniques.

3.3.3 Strategy and Experimental Methods Surfactants 42b and ent-42b were chosen for the study to represent one pair of enantiomers, which upon mixture (1:1) provided a racemate (rac-42b). Isomer 43b was chosen, as it is a diastereomer to 42b and ent-42b. To be successful, it is important to have substrates with very low solubility in the subphase, otherwise the material might dissolve during the experiment. In this regard, the C8-comprising surfactants 42a and 43a and their corresponding enantiomers would probably have been too soluble, and were therefore excluded.

The purity of the compounds was a central issue because of the high sensitivity of the method. To ensure a high purity the surfactants were recrystallized from mixtures of MeOH and H2O prior to use.

Mixtures of MeOH/CHCl3 (5–10 % MeOH) were used as spreading solvents, and 5 minutes were allowed to elapse to ensure that all spreading solvent had evaporated before the monolayers were compressed. The isotherms were recorded at ambient temperature with a compression speed of 0.5 Å2 molecule–1 min–1.

3.3.4 Results 3.3.4.1 π–A Isotherms There was reason to believe that upon spreading, surfactants 42b and ent-42b were drawn into the subphase by the methanol used in the spreading solution (10% MeOH in CHCl3). Surfactant 43b was dissolved in 5% MeOH/CHCl3 which possibly made this problem less obvious. As a result it was decided to set the area/molecule to 40 Å2 at π=30 mN/m for 42b, ent-42b and rac-42b. For this reason the isotherms obtained from the experiments could only be compared qualitatively.

The π–A isotherms recorded for the enantiomerically pure surfactants 42b, ent-42b and 43b, as well as for the racemic mixture (rac-42b) are presented in Figure 10. The two scalemic surfactants 42b and ent-42b gave similar isotherms within experimental limits, which were also expected.

40

0

5

10

15

20

25

30

35

30,00 40,00 50,00 60,00 70,00 80,00 90,00 100,00 110,00

Area/molecule Å2/molecule

43b

rac-42b

42b

ent-42b

Figure 10. Results of π–A isotherms for 42b, ent-42b, rac-42b and 43b at a compression rate of 0.5 Å2 molecule–1 min–1.

A pronounced difference in compressibility was observed between monolayers composed of 43b vs. 42b, ent-42b and their racemate (rac-42b, Figure 10). While monolayers of compounds 42b and ent-42b featured a distinct liquid expanded–li