Click Chemistry – What’s in a Name? Triazole Synthesis and ...xiuzhengrd.com/ejournals/pdf/synthesis/doi/10.1055/s-2007-966071.… · cedure for the preparation of such heterocycles.2

REVIEW 1589

Click Chemistry – What’s in a Name? Triazole Synthesis and BeyondTriazole Synthesis and BeyondMaría Victoria Gil,*a María José Arévalo,a Óscar Lópezb

a Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, SpainFax +34(924)271149; E-mail: [email protected]

b Departamento de Química Orgánica, Facultad de Química, Universidad de Sevilla, 41012 Seville, SpainReceived 21 February 2007; revised 14 March 2007

SYNTHESIS 2007, No. 11, pp 1589–1620xx.xx.2007Advanced online publication: 11.05.2007DOI: 10.1055/s-2007-966071; Art ID: E17507SS© Georg Thieme Verlag Stuttgart · New York

Abstract: The environmentally amiable route to carbon–heteroat-om bond formation, described by Sharpless as ‘click chemistry’, hasbecome known as a fast, efficient, and reliable approach to the syn-thesis of novel compounds with desired functionalities. Readilyavailable starting materials must be used in this methodology andthey should be essentially inert to most biological and organic con-ditions, including water and molecular oxygen. In this review, wecover reactions included in this label such as cycloadditions, nu-cleophilic ring-opening reactions of strained cycles, and amide syn-thesis, as well as their applications in organic synthesis, molecularbiology, macromolecular chemistry and materials science.

1 Introduction2 Click Synthesis of Epoxides and Aziridines3 Nucleophilic Opening of Strained Rings: Epoxides and

Aziridines4 Huisgen 1,3-Dipolar Cycloadditions4.1 Supramolecular Click Chemistry4.2 Liquid Crystals4.3 Self-Assembled Monolayers4.4 Multicomponent Reactions4.5 Creating Drug Candidates under Click Conditions4.6 Click Chemistry with Peptides and Proteins4.7 DNA as Participant in Click Reactions5 Concluding Remarks

Key words: drug research, supramolecular chemistry, Huisgencycloadditions, triazole, aziridine opening, epoxide opening

1 Introduction

Modern research in organic chemistry and pharmacologi-cal areas require the preparation of structurally complexmolecules through a rational design; furthermore, if thesemolecules are to have any practical applications, the syn-thetic route should be both synthetically and economicallyviable. Although combinatorial chemistry emerged as apowerful tool to provide extensive libraries of com-pounds, applications in the pharmaceutical field usuallydepend strongly on the success of individual reactions.1 Inthat sense, a change in the way of considering syntheticchallenges was needed in order to provide reliable reac-tions; this change took place in 2001 when the term ‘clickchemistry’ was coined by Sharpless2 in an attempt at ob-taining chemical diversity suitable for biological screen-ing, by means of rapid and efficient protocols.

Click chemistry refers to a series of reactions that obeycertain criteria, such as being modular, stereospecific,high-yielding and involving simple experimental proce-dures.2,3 They should also make use of readily availablestarting materials, environmentally friendly conditions(water as solvent, or solvent-free conditions), and shouldavoid chromatographic isolations.2 Usually, reactions in-cluded in such a group have a large thermodynamic driv-ing force and involve the formation of a carbon–heteroatom bond. A few synthetically useful reactions ful-fil such criteria: opening of strained rings such as ep-oxides and aziridines,4 oxygen additions to carbon–carbon double bonds (epoxidations),5 and 1,3-dipolar6 andDiels–Alder7 cycloadditions.

Click chemistry has been especially successful in thepreparation of 1,2,3-triazoles by a 1,3-dipolar cycloaddi-tion between azides and alkynes. The original reaction,known as the Huisgen cyclization, involved a thermaltreatment of both reagents and afforded the correspondingtriazoles with a complete lack of regioselectivity, as a 1:1mixture of the 4- and 5-substituted derivatives.6

Nevertheless, the discovery that this reaction can be effi-ciently catalyzed with copper(I) salts to give exclusivelythe 4-substituted regioisomer resulted in an excellent pro-cedure for the preparation of such heterocycles.2

The interesting properties of the triazole scaffold, not onlyfrom a synthetic point of view, but also in the context ofbiological and pharmacological applications, led to a sig-nificant increase in click chemistry research. Further-more, the triazole ring was shown to be compatible with aplethora of functional groups, as well as exhibiting goodstability under several reaction conditions. Thus, much ef-fort has been devoted to the preparation of triazole-con-taining peptides, oligosaccharides,8,9 and natural productanalogues,10,11 and an enormous development in clickchemistry has been achieved in the last few years. Thepreparation of dendrimers,12,13 polymeric materials,14,15

liquid crystals16 and potential pharmacophores17 has beenachieved through this revolutionary concept. Not surpris-ingly, this modified triazole synthesis has become theparadigm of click chemistry in modern literature and bothterms are now invariably associated, even though Sharp-less’ seminal ideas offer promising perspectives on othersynthetic platforms.

In this review, we cover recent applications of click chem-istry, particularly those involving the opening or forma-tion of strained rings, as well as 1,2,3-triazole formation,

1590 M. V. Gil et al. REVIEW

Synthesis 2007, No. 11, 1589–1620 © Thieme Stuttgart · New York

while highlighting their connection to macromolecularchemistry and drug-candidate searches.

2 Click Synthesis of Epoxides and Aziridines

Epoxides and aziridines are highly valuable intermediatesin organic synthesis,18–22 obtained mainly via oxidativeprocesses from olefins.23–26 The reaction of such interme-diates with nucleophiles in a selective fashion is a typicalexample of a click chemistry reaction.2

a,b-Unsaturated acids and amides also undergo aminohy-droxylation and dihydroxylation processes in a click fash-ion (good yields and extremely easy isolation) to affordthe corresponding vicinal aminoalcohols or diols, respec-tively.2,27

A click example for the preparation of aziridines startingfrom olefins was reported by Siu and Yudin (Scheme 1).5

Thus, a series of aziridines were prepared by an electro-

chemical process, in moderate to good yields, and the re-action could be scaled up to a multigram level. With thisgreen procedure, hazardous oxidants and metal additivesare avoided.5

Scheme 1

Voronkov et al.28 accomplished the practical and scale-able synthesis of isomeric limonene aziridines startingfrom commercially available limonene oxides, com-pounds of interest as key chiral intermediates. A 1:1 mix-

María Victoria Gil wasborn in 1973 in Badajoz,Spain. She studied chemis-try at the University ofExtremadura and completedher degree in 1996. She re-ceived her PhD from thesame university in 2000 un-

der the supervision of Pro-fessors Emilio Román andJosé Antonio Serrano. Shethen pursued studies withProfessor Roberto Ballini atthe University of Camerino(Italy). In 2002, she was ap-pointed as research assistant

in the organic chemistry de-partment at the Universityof Extremadura and in 2006was promoted to assistantprofessor. Her research in-terests include asymmetricsynthesis in the field of nitrocompounds.

María José Arévalo wasborn in 1975 in Badajoz,Spain. She obtained her de-gree in chemistry at the Uni-versity of Extremadura in1998. After that, she joinedthe Department of OrganicChemistry at the same uni-versity, where she was in-volved in the study of the

reactivity of 1,3-thiazolium-4-olate system derived fromcarbohydrates, getting herPhD in 2002 under the su-pervision of Drs. MartínÁvalos and José LuisJiménez. She then obtaineda postdoctoral fellowship tojoin the group of Prof.Albert Padwa at Emory

University for two years,studying the 1,3-dipolar cy-cloaddition of nitrones. Cur-rently, she is interested inisocyanide multicomponentchemistry and she is devel-oping a project in that fieldat the Scientific Park of Bar-celona.

Óscar López was born inSeville (Spain) in 1976 andreceived his Diploma inchemistry from the Univer-sity of Seville in 1999. Hecompleted his PhD in thesame university in 2003,concerning new syntheticapproaches for the prepara-tion of sugar-derived ureas,thioreas and selenoureas. InMarch 2004, he was ap-

pointed as lecturer inEnvironmental OrganicChemistry at the Universityof Huelva, Spain. In June2004, he was appointed aslecturer in Organic Chemis-try at the University ofSeville, in the Faculty ofChemistry, where he is cur-rently an assistant professor.From 2005–2006 he carriedout a postdoctoral stay in the

Department of Chemistry ofAarhus University (Den-mark) in the group of Pro-fessor Mikael Bols; duringthis period, his research wasfocused on the preparationof cyclodextrin-based artifi-cial enzymes and newiminosugars as glycosidaseinhibitors.

Biographical Sketches

R-NH2 +

R3

R4

R1

R2 + 1.80 V

MeCN, Et3NH+ OAc–

R3R4

R1

R2

N

R

R = phthalimidoR1–R4 = alkyl, aryl: cyclohexene, cyclohexenone, cyclooctene, trans-methyl cinnamate, etc.

REVIEW Click Chemistry: Triazole Synthesis and Beyond 1591


ture of commercially available limonene oxides 1 wassubjected to sodium azide mediated stereoselective open-ing of the epoxide ring, affording the corresponding iso-meric a-azido hydroxy derivatives 2 and 3 (Scheme 2).Treatment of the mixture with triphenylphosphine at roomtemperature via a pseudo-Staudinger reaction29 allowed akinetic resolution, as the tertiary azide underwent a muchslower reaction. Acid–base extraction followed by vacu-um distillation afforded pure 4; harsher conditions (re-fluxing 1,4-dioxane) gave access to the diastereoisomericazide 5.28

Scheme 2

A different approach for the preparation of aziridines wasreported by Fioravanti and co-workers,30 using, as startingmaterials, the functionalized enones 6, which were eithercommercially available or readily synthesized(Scheme 3). The aziridination step was carried out bytreatment of the enones with arylsulfonyloxycarbamatesin the presence of an inorganic base such as calcium ox-ide. These reactions proceeded in high yield, with highpurity, and the results were not dependent upon the sub-stituents of the aminating reagent; furthermore, aziridines7 could be used without chromatographic purification.

The prepared aziridines were then transformed into thecorresponding alkenyl aziridines 8 via a Wittig reactionwith high conversion and excellent E-stereoselectivity (upto 99%).30

3 Nucleophilic Opening of Strained Rings: Epoxides and Aziridines

The stereoselective opening of strained rings, such as ep-oxides, aziridines and aziridinium ions, is a valuable syn-thetic tool for the preparation of more complexderivatives. In this context, Wong’s group reported a ver-satile methodology for the rapid discovery of enzymaticinhibitors, where the key step is an amine-mediated open-ing of epoxides in an aqueous medium.31

The epoxide-containing core, 12, was prepared from com-mercially available epoxide 9 as depicted in Scheme 4.The epoxide moiety in the starting material underwentring opening upon treatment with p-thiocresol in basicmedium to afford derivative 10; attack of the thiolate re-agent took place selectively at the less-hindered carbon ofthe epoxide moiety. Next, the Boc protective group wasremoved under acidic conditions and the transient freeamino group was converted into the tetrahydrofuran-3-ylcarbamate 11. Finally, the epoxide moiety was regenerat-ed by use of trimethyloxonium tetrafluoroborate, fol-lowed by treatment with potassium carbonate to give 12.31

Epoxide 12 was subjected to ring opening with a series ofprimary amines to afford a small library of derivatives 13that were screened as enzymatic inhibitors against HIV-1PR and mutant V82A proteases. Compounds bearing ei-ther 2-amino-p-cresol or 4-phenoxyanilino moietiesshowed good inhibitory properties against both wild andmutant proteases.

Bulkier substituents in the amino residue were preparedstarting from commercially available 9 and following amodified synthetic approach (Scheme 5),31 where one ofthe key steps was again an amine-mediated epoxide open-ing. In this case, the first step was the epoxide opening of9 using a primary amine, followed by tosylation of theamino residue of 14 with p-methoxybenzenesulfonylchloride. Removal of the protective group of 15, followedby introduction of the tetrahydrofuranyl moiety as indicat-ed in Scheme 4, afforded derivatives 16 (Scheme 5),which exhibited potent viral protease inhibition in the na-nomolar range (Table 1).

Fringuelli’s group has carried out an exhaustive study ofthe epoxide opening with numerous nucleophiles and re-action conditions, such as pH and Lewis acid catalysts.Thus, aminolysis,32 iodolysis, bromolysis,33,34

azidolysis33,35,36 and thiolysis37–42 of this ring have all beenreported by these authors, with the background of thesereactions fulfilling the click chemistry criteria.2 In partic-ular, the thiolysis of epoxides was accomplished in a wa-ter medium.40,42

O

NaN3

NH4CliPrOH

HON3

+

N3

OH

PPh3THF, r.t.

1 2 3

NH

3

4

PPh31,4-dioxane

reflux

NH

5

+

Scheme 3

R2 R3

R4R1

O

ArSO3NHCO2R5

CaO, CH2Cl2, r.t.

R2R3

R4

R1

O

6

N CO2R5

7

Ph3P=CH-R6R2

R3

R4

R1N CO2R5

R6

8

R1 = -(CH2)n-, MeR2 = -(CH2)n-, H, CO2EtR3 = H, CO2EtR4 = CO2Et, HR6 = H, CN, CO2Et



For example, an easy and environmentally friendly thio-lysis of substituted epoxides 17 by aromatic thiols 18 wascarried out40 in basic medium and in the absence of a met-al catalyst (Scheme 6). Under these conditions, the reac-tion took place in a regioselective fashion on the lesshindered carbon of the epoxide; furthermore, the vicinal

diol, a side-product (roughly 5%) obtained by attack of thehydroxide nucleophile, is water soluble, whereas most tar-geted b-hydroxy aryl sulfides 19 can be recovered by sim-ple filtration in good yields (Table 2).

Scheme 6

Bandini et al. described43 an indium(III) bromide cata-lyzed addition of indoles to chiral aromatic epoxides 20(Scheme 7). The reaction proceeded via a purely SN2pathway and the mild conditions allowed for the isolationof the corresponding 2-aryl-2-(3¢-indolyl)ethan-1-olssuch as 22 without significant racemization. Furthermore,this procedure took place regioselectively and with highenantioselectivity (up to 99%).

Scheme 7

Scheme 4

OHN

O

O

p-thiocresol

KOH, EtOH

1) TFA−CH2Cl2 (1:1)

2) (S)-tetrahydrofuranyl N-oxysuccinimidyl carbonate

OHN

O

OHS

O

1) Me3OBF4

2) K2CO3

OHN

O

O

O

9

11

12

RNH2

H2O−DMSO (1:1), 60 °C

OHN

O

OH

O

13

HN R

OHN

O

OHS

10

Ar

Ar

RNH2 = 1° amine

Table 1 Ki Values of Compounds 16 against HIV Proteases

Compound R HIV-1 PR V82A

16a 3 nM 8 nM

16b 8 nM 13 nM

16c 3 nM 8 nM

Me

OH

O

Scheme 5

OHN

O

O

9

RNH2

iPrOH, 60 °C

OHN

O

OH

14

HN R

OMe

SO2Cl

Py, MeCN

OHN

O

OH

15

N S

RO

O

OMe

1) TFA–CH2Cl2 (1:1)

2) (S)-tetrahydrofuranyl N-oxysuccinimidyl carbonate

OHN

O

OHN S

RO

O

OMeO

16

H2O, 30 °C

pH 9.0O

R1 SH

R2

+OH

R1

S

R2

17 18 19

20

NH

O

+

21

InBr3

CH2Cl2 NH

OH

22



To date, aziridines and aziridinium ions have received lessattention in organic synthesis than their isomeric ep-oxides, although this situation seems be changing. Underthe appropriate reaction conditions, these intermediatescan afford vicinal diamino moieties, a structural motif ofparticular relevance.44

Usually, the opening of aziridines that bear no electron-withdrawing group on the nitrogen atom involves the useof acidic activating agents.44 Recently, Krasnova andYudin reported the preparation of novel chelating agents45

where the key step was the opening of an aziridine ringwith nitrogen nucleophiles such as hydrazine or a tetra-zole derivative (Scheme 8). Racemic aziridine derivative23 (7-azabiciclo[4.1.0]heptane) was prepared from thecorresponding epoxide in a manner similar to that de-scribed in Scheme 2, in a two-step process: opening of theepoxide with sodium azide was followed by ring closureto the aziridine motif by treatment with triphenylphos-phine.

Compound 23 was subjected to opening using two differ-ent nucleophiles. Treatment with hydrazine afforded tran-sient 24 that was used for the next step without furtherpurification;45 reaction with pentan-2,4-dione in refluxing

ethanol afforded pyrazole derivative 25. Resolution of thiscompound was carried out using (S)-mandelic and (S)-camphorsulfonic acids, and enantiomerically pure 25could be condensed with different aldehydes; however,the corresponding ligands showed limited stability.

When the opening of aziridine 23 was carried out with 5-phenyl-2H-tetrazole, derivative 26 was obtained as a race-mic mixture; enantiomerically pure forms of 26 could beisolated upon resolution with (S)-mandelic and (S)-cam-phorsulfonic acids. Condensation of the S,S enantiomerwith 2,6-diformyl pyridine afforded the dimeric ligand 27(Scheme 8), which was complexed with copper salts andshowed high diastereoselectivity in the styrene cyclopro-panation reaction. Reaction of (R,R)-26 with 1,4-dibro-mopropane gave access to tertiary amine 28 in moderateyield.

Sharpless and co-workers have exploited44,46,47 the clickopening of aziridinium ions with different nucleophiles.Unlike aziridine moieties, these useful synthetic interme-diates can be opened at moderate temperatures, and atneutral and even basic pH. For example, pyrazolidin-3-ones were prepared using a spontaneous aziridinium ringopening as the key step.46

Reaction of the oxirane-containing ester 29 with second-ary amines afforded derivatives 30 (major) and 31 (mi-nor), that could be separated by crystallization.46

However, separation of the regioisomers is not necessary,as mesylation of the free hydroxyl group in the crude re-action leads, in both cases, to the aziridinium ion 32. The

Table 2

Epoxide 17 Aryl sulfide 18 Yield (%)

90

90

90

90

95

94

95

O

SH

SHCOOH

SHNH2

SH

O SH

OOPh

SH

NH2

O

n-Bu

SH

NH2

Scheme 8

NH

23

NH2-NH2·H2OH2O, r.t.

NH2

NH-NH224 (racemic)

O O

EtOH, reflux

NH2

NN

25 (racemic)

HNN N

NPh

NH2

N

N N

NPh

26 (racemic)

1) resolution N

N

N N

NPh

(S,S)-27

2)

NOHC CHO

NN

N

N

Ph

NN

Br Br1) resolution2)

N

N

N N

NPh

(R,R)-28



latter undergoes a spontaneous nucleophilic displacementby chloride ion to afford the building block 33(Scheme 9), which is a valuable starting material that hasbeen used in the preparation of pyrazolidin-3-ones,46 b-lactams or benzodiazepinones47 (Figure 1).

Scheme 9

Figure 1

4 Huisgen 1,3-Dipolar Cycloadditions

The 1,3-dipolar cycloaddition that involves azido andalkyne (terminal or internal) derivatives is known as theHuisgen cyclization, and gives access to 1,2,3-triazoles asa mixture of 1,4- and 1,5-regioisomers (Scheme 10).48

The lack of selectivity can be explained by the similarityin activation energies for both processes.49

Such intrinsic features of the Huisgen cycloaddition makeit unsuitable for being considered as a click reaction. Nev-ertheless, the observation that copper(I) salts promote

faster (up to 107 times) and regiospecific couplings be-tween terminal alkynes and azides allowed for the rapiddevelopment of this reaction; these results were reportedindependently by the groups of Sharpless50 and Meldal.3

Moreover, the cycloaddition is usually carried out at roomtemperature in aqueous media, and is compatible withmost common functional groups present in the molecules.

Usually, the source of copper(I) is the reduction of cop-per(II) sulfate in the presence of sodium ascorbate,50 al-though some other conditions have been reported, such ascopper(I) salts,3 copper(I) complexes,51 and copper(I)-stabilized derivatives.52

The accepted mechanism,49,50,53 shown in Scheme 11, in-volves the initial formation of a p complex between a ter-minal alkyne and copper(I), thereby lowering the pKavalue of the alkyne. Under these conditions, the acetylenederivative is acidic enough to be deprotonated in aqueousmedium.

Scheme 11

This copper(I) acetylide evolves to a copper(I) adductupon attack of the organic azide, and then undergoes anintramolecular cyclization to give a copper-containing1,2,3-triazole. Final protonation regenerates the catalystand gives the 4-substituted 1,2,3-triazole derivative.Overall, the postulated mechanism is a stepwise, not aconcerted, pathway.

Recently, Sharpless and co-workers demonstrated54 thatruthenium complexes such as Cp*RuCl(PPh3)2 catalyzedthe Huisgen 1,3-dipolar cycloaddition; interestingly, theobserved regioselectivity was reversed when comparedwith related copper(I) catalysis. A tentative mechanismsuggested by the same authors (Scheme 12)54 involves asix-membered ruthenacycle intermediate, which under-goes reductive elimination, affording a 5-substituted tri-azole derivative. Unlike in the case of the copper(I)-mediated reaction, both terminal and internal alkynes canparticipate, which would mean that the formation ofruthenium acetylides can be discarded as a hypothesis.

Weinreb’s group reported the preparation of 5-substituted1,2,3-triazoles 40 by reaction of b-tosylethylazide (38)with alkynes in the presence of ruthenium catalysts inrefluxing benzene (Scheme 13).55 When the standardcopper(I)-catalyzed protocol was used, it was found thatreaction of b-tosylethylazide with terminal alkynes led tothe expected 4-substituted 1,2,3-triazoles.55

OEt

OO R2NH

EtOH, reflux

OEt

ONR2

29

OH

+

OEt

OOH

NR2

MsClEt3N

OEt

ON

R R

ClOEt

O

NR2

Cl

30

31

3233

R2NH = morpholine, diallylamine

OEt

O

NR2

Cl

33

NHHN

O

NR2

Ph

34

NO

Ph NR2

R1

35

HN NH

Ph ON

O36

NR2 = morpholino, diallylaminoR1 = t-Bu, CH2C=CH, n-Bu

Scheme 10

R1N

N N

R2

R1

N

N N

R2

R1+ R2N3 +

1,4-regioisomer 1,5-regioisomer

80–120 °C

LnCuR1 H

R1 CuLn

R2N

NN

N

R1LnCu

NN

N

R1

R2R2

N N

R2

N N N

LnCu R1

B BH

BH B

LnCu+



Scheme 13

As an attempt at improving the catalysis for 1,3-dipolarcycloaddition reactions between azides and terminalalkynes, active and stable nanometric copper clusters havebeen proved to be efficient ligand-free catalysts.56 Theseclusters are prepared with a narrow cluster-size distribu-tion as a stable solution by reducing copper(I) chloride insolution with tetraoctylammonium formate.

To prove the efficiency of this new catalyst, the kineticsof a model reaction between benzyl azide and prop-2-yn-1-ol catalyzed by copper shavings, copper powder, coppernanoclusters, and copper(II) sulfate/ascorbate were moni-tored. Copper nanocluster catalysis displayed the highestactivity of the four systems tested, affording 100% con-version after 18 hours, and this result was in good agree-ment with the increased surface area of the nanoclusters.The specific surface area of the clusters and the powderwas 168 m2/g and 0.15 m2/g, respectively.56

An extended application of this reaction to the synthesisof electronic and optoelectronic materials is the couplingof azides with ruthenium-containing alkynyl biosensors.57

The corresponding di(ruthenium)-containing click prod-uct is obtained in excellent yields.

With the aim of exploring the feasibility of the clickchemistry with ynamides and azides, coupling between N-benzyl-N-tosylynamide and N-Boc-2-azidoethylaminewas also tested. This approach was broadened to poten-tially bioactive compounds, and carbohydrates and aminoacids containing azido groups were clicked with a seriesof ynamides to yield the corresponding cycloadducts.58

Nevertheless, when the click Huisgen cycloaddition wasattempted between copper(I) acetylides and sulfonyl

azides, N-acylsulfonamides were isolated in aqueous me-dium,59 whereas amidines were obtained in the presenceof secondary amines.60

Another unexpected reaction was reported by Whitingand Fokin to take place between phenylacetylene and p-toluenesulfonyl azide.61 Despite obtaining the corre-sponding 1-sulfonyl-4-phenyl-1,2,3-triazole, the only iso-lated product was a cyclobutene derivative. Furthermore,when the reaction was accomplished in the presence of N-benzylideneaniline, an azetidinimine was obtained, to-gether with a small amount of the 1,4-disubstituted tri-azole.61

A catalyst-free alternative to carry out the covalent modi-fication of biomolecules in living systems consists of astrain-promoted [3+2] cycloaddition. Following thismethodology, Agard et al. reported the synthesis of tri-azole compounds by reaction of azides and cyclooctynederivatives under physiological conditions.62

The click Huisgen 1,3-dipolar cycloaddition has also beenextended to the synthesis of tetrazoles when organicazides and nitriles are used. Upon simple heating of neatp-toluenesulfonyl or acyl cyanides 42 and 43 with oneequivalent of various unhindered azides 41, quantitativeconversion to the corresponding 1-alkyl-tetrazole deriva-tives 44 and 45 was achieved (Scheme 14).63 The reactionwas run neat, there were no side products, yields werequite high and isolation was simple. In all cases, the reac-tion was highly regioselective and only one isomer wasobserved. Unfortunately, bulky azides and aryl azides arenot appropriate for this reaction, and only certain highlyelectron-deficient nitriles are good enough as dipolaro-philes to engage organic azides in an intermolecular fash-ion.63

Scheme 14

4.1 Supramolecular Click Chemistry

4.1.1 Dendrimers

Dendrimeric materials are extremely attractive candidatesfor a variety of surface-active applications, such as multi-valent binding sites for interaction with biological recep-tors and cell surfaces in the construction of targeted drugdelivery systems.64–66 Dendrimers contain three distinct

Scheme 12

Me

Me

Me Me

Me

RuCl

NH

R1

NN

R2

Me

Me

Me Me

Me

Ru Cl

N

R1

NN

R2N

NN

R1

R2

KOt-BuTHF

PhR TsN3

N

N

N

R

Ts

N

N

NH

R

Cp*RuCl(PPh3)2PhH, reflux

–78 °C to 0 °C

37 38

39 40

+

R = Ph, 4-MeC6H4, Bu, Hex, PhOCH2, 4-MeOC6H4

N3 N

NN

N

R1 R2

NR2[2+3]

4142 R2 = Ts43 R2 = COR3

44 R2 = Ts45 R2 = COR3

R1

R1 =

OH

, , ,

, , S

N3



structural parts, namely the core, end-groups, andbranched units connecting core and periphery. These ma-terials have been synthesized by methodologies based onboth convergent and divergent routes.67,68 The convergentapproach, introduced by Fréchet’s group,69 introduces thecore scaffold in the final step. This method, on the onehand, enables the introduction of the desired functional-ities in the structural building blocks,68 and on the otherhand, implies fewer coupling reactions, more exact mac-romolecular architectures, greater monodispersity, andgreater control over the placement of desired functional-ities. In contrast, the divergent approach allows for theformation of the dendrimeric structure by way of iterativeadditions to the core scaffold.

It happens that the Huisgen 1,3-dipolar cycloaddition is asimple and reliable procedure for the efficient synthesis ofchemically differentiated dendrimers.70–73 The methodol-ogy is characterized by 1,4-regiospecific 1,2,3-triazoleformation, water tolerance, and acceptance of a widerange of functionalities.74 There are three strategies for tri-azole dendrimers: coupling reaction between a dendronazide and a dendron alkyne, between a dendron azide andpolyalkynes or between a dendron alkyne and polyazides.

In this context, polyvalent dendrimeric peptides that maybe useful in the preparation of synthetic vaccines havebeen accessed via an efficient Huisgen 1,3-dipolar cy-cloaddition. Thus, Rijkers and co-workers accomplishedthe microwave-promoted coupling of polyvalent alkynesand azides in the presence of copper(II) and sodium ascor-bate (Scheme 15).15 Using these conditions, the tetrava-lent dendron 46 was coupled with the azido-containingamide 47 to afford derivative 48. Dendrimeric cycload-ducts were isolated in moderate to good yields (50–95%).

Sharpless, Hawker, and co-workers75 used the Huisgen1,3-dipolar cycloaddition of dendrons containing azide(49) and alkyne (50) moieties to obtain third-generationdendrimer 51 as a biocompatible building block(Scheme 16). It is noteworthy that, as indicated above,Huisgen cycloaddition reactions are compatible with aplethora of functional groups; for instance, as depicted inScheme 4, the reaction is not hindered by to the presenceof free hydroxyl groups and ester moieties.

The introduction of mannose and coumarin units to theperiphery allowed for the preparation of unsymmetricaldendrimers that proved to be efficient agents for the inhi-bition of hemagglutination.75

Propargyl-functionalized Fréchet-type dendrons 52, thatis, poly(benzyl ether) dendrons, have been applied to theconvergent synthesis of dendrimers using tripodal azidecore 53 or azide-focal-point-functionalized Fréchet-typedendrons 55.76 Reactions were carried out in aqueousN,N-dimethylformamide, under the standard copper-cata-lyzed Huisgen reaction conditions, and the correspondingsymmetrical and unsymmetrical triazole-containing den-drimers were obtained in good yields (Scheme 17).76

Polyamidoamine-based dendrimers, so-called PAMAMs,represent a new class of macromolecular structures, andthey are sometimes referred to as ‘dense star’ polymers.77

A standard synthesis of these polymers is based on a di-vergent approach that involves a Michael addition of me-thyl acrylate and the amine core to afford the ester moiety,which is subjected to saponification and coupling with adiamine; subsequent iteration leads to the title com-pounds.78 These polymers were recently found to solubi-lize water-insoluble drugs and to promote their transportthrough biological membranes.79,80 Furthermore, the pres-ence of a large number of exo-functional groups might al-low their use as molecular biosensors.81 A high-yieldingsynthesis of this interesting family of dendrimers was re-cently achieved by Lee et al.,12,82 in which the azido moi-eties were efficiently connected to the bis(alkyne) coreunit 58 via a copper(I)-catalyzed Huisgen reaction to fur-nish PAMAM dendrimers 59 in 99% yield (Scheme 18).

Introduction of carbohydrate units to dendrimeric struc-tures might lead to a glycoconjugate that could participatein recognition processes with biological receptors.83 How-ever, synthetic routes described so far for the preparationof glycodendrimes84,85 usually involve long syntheses thatafford unprotected coupled sugar scaffolds. In this con-text, Riguera’s group accomplished86 the anchoring of aseries of carbohydrate-derived acetylenes into azido-ter-minated dendrimers through regioselective 1,2,3-triazoleformation (Scheme 19); the reported dendrimers incorpo-rated up to 27 monosaccharidic units (L-fucose, mannose,and lactose), with yields up to 92%. The inherent condi-tions of this reaction (environmentally friendly, regiose-lective, high-yielding) make this dendrimer derivatizationa good candidate for the preparation of glycoconjugates ofpotential biological and pharmacological interest.

Scheme 15

N3

O

HN

O

O

O

O

O

N N

MeO O

O

O

O

O

O

O

N N

MeO O

O

O

O

NN

N

N NNN

NN

NN

NR

R

R

R

R

46

47

48



Scheme 16

O

OO

O

O

O

O

O

OHO

HO

HO

HO

O

O

OO

HO

HOO

HOHO

N3

O

O

O

O

O

O

OO

O

O

O

O

O

OO

O

OO

OO

O

H

O

OO

O

O

O

O

O

OHO

HO

HO

HO

O

O

OO

HO

HOO

HOHO

N

O

O

O

O

O

O

OO

O

O

O

O

O

OO

O

OO

O

O

NN

49 50

51

Cu(Ph3P)3Br (2 mol%/acetylene)i-Pr2NEt, THF, 50 °C

O

O

Scheme 17

O

D1D2D3D4

N3

N3

N3

52 53

N

N

NN

N

NN

O

N

N

O

O

D1

D1

D1

D2

D2

D2

D3

D3

D3

D4

D4

D4 54

N3

D1 D2 D3 D4

55

NN

N

O

D1D2D3D4

D1 D2 D3 D4

56



Fréchet’s group has also carried out the preparation oflinear dendrimer-containing polymers using click chemis-try.13,87 The sizes and shapes of these polymers can bemodulated; the existence of a linear architecture might im-

prove their use in some nanoscale applications, such as ca-talysis or molecular electronics. Furthermore, sometimesthe spherical shape of a classical dendrimer is not appro-priate for some applications.88,89 The approach used by

Scheme 18

N3 N

NH

NH

N

OMeO

N

OMe

O

O

OMe

O

O

OMeO

O

O

O

O

N

NN

NN

NN

HN

HN

N

MeOO

N

MeO

O

O

MeO

O

O

MeOO

NH

NH

O

N

N

OMeO

N

OMe

O

O

OMe

O

OMeO

nn = 0, 1, 2

CuSO4·5H2O, sodium ascorbate

n

n

n = 0, 1, 2

m = n + 1

59m = n + 1

57

58

Scheme 19

O

HOOH

OH

OO

O

OH

HOHO

OHO

OHHO

HO

O

OHHO

OO

OH OH

N3 NNN

O

60 α-L-fucoside 61 α-D-mannoside 62 β-D-lactoside

n Cu(I)-catalyzedHuisgen conditions

60, 61, 62

nO

OH

63 64



this group was to start from a linear polymer with pendantreactive groups (for example, alkyne moieties) and to car-ry out the coupling with an azido derivative.13 A den-dronized polymer was obtained from the first-generationdendron 65 and poly(vinyl acetylene) 66 in quantitativeyield, as depicted in Scheme 20. The same kind of reac-tion was also reported to occur in almost quantitative yield(> 98%) for third-generation azide-containing dendrons.

Scheme 20

4.1.2 Polymers

The control of the molecular structure, and particularlythe chemical functionalities, of polymers is a pivotal taskin modern polymer synthesis, because complex macro-molecules are needed in the rapidly growing fields ofnanotechnology90–92 and nanobiotechnology.93–95 In thiscontext, click chemistry provides a powerful tool for theselective modification of copolymers, and the copper-cat-alyzed [2+3] Huisgen cycloaddition has provided accessto triazole-containing derivatives.96–99

Carroll et al. reported the preparation of flavin-derivedpolymers100 with potential analytical applicability, for ex-ample as potential sensors.101 Nucleophilic displacementof the chlorine atoms in chloromethylstyrene copolymer68 with sodium azide, followed by copper(I)-mediatedHuisgen cycloaddition with alkyne derivative 70 affordedthe flavin-functionalized polymer 71 (Scheme 21). It isremarkable that the triazole scaffold has a strong dipolarmoment and subsequently can establish hydrogen bondswith hydrogen donors; at the same time, they providesome hydrophilicity in the system while being stable un-der biological conditions. These specifically flavin-functionalized polymers displayed reversible redox-modulated recognition with complementary 2,6-diami-dopyridine (DAP)-derived units, the binding associationbetween the polymer and DAP being quantified throughthe use of fluorescence spectroscopy.100

Scheme 21

Polystyrene derivatives analogous to 68 have been con-verted into a triazolylmethyl acrylate (TMA) resin as asolid support for the efficient parallel synthesis of differ-ently substituted tertiary amines.102

The nucleophilic substitution of a Merrifield-type resinwith sodium azide allowed for the formation of azide-sub-stituted polystyrene 72. The subsequent 1,3-dipolar cy-cloaddition with commercially available propargylacrylate was conducted in N,N-dimethylformamide/tetrahydrofuran in the presence of N,N-diisopropyl-ethylamine and a catalytic amount of copper(I) iodide,leading to the TMA resin 73 (Scheme 22). Then, N-benz-yl-N-methylamine was added in N,N-dimethylformamideand the aminopropionate 74 was alkylated with 2,6-dichlorobenzyl bromide. Final cleavage to the TMA resin,induced by basic medium, led to the tertiary amine 75 in a68% yield, with the progression of the cycloaddition be-ing monitored by IR spectroscopic methods.102

Electrooptic polymers have been generated from the clickDiels–Alder reaction between maleimide-containing non-linear optical (NLO) chromophores and polymers thatpossess pendant anthryl-containing diene moieties. Thissynthetic approach is very mild, versatile, quantitative,and free of ionic species and catalysts.103

The bromine chain-ends of a polystyrene prepared usingatom-transfer radical polimerization were successfullytransformed into various functional end-groups (w-hy-droxy, w-carboxyl and w-methylvinyl) by a two-step path-

5 mol% CuSO4·5H2O

10 mol% sodium ascorbateTHF–H2O (1:1)

H

n

N N

N

O O

n

O O

N3

65

66

67

+

Cl N3

NN

N

N

N

N

N

O

O

H

H

H

N

N

N

O

O

N

N

N

N O

H

O

x y

x = 36, y = 4

NaN3, DMSO

x y

x = 36, y = 4

Cu(I), DMSO

x = 36, y = 4

69

71DAP

4

70

68



way: first, nucleophilic substitution of the terminalbromine atom of 77 by an azide function, and then 1,3-di-polar cycloaddition of the terminal azide and functional-ized alkynes to give derivatives 79–81 (Scheme 23). The1,3-dipolar cycloaddition was catalyzed by the combina-tion of copper(I) bromide and 4,4¢-di(5-nonyl)-2,2¢-bipy-ridine (a new catalytic system for this reaction which iswell known in the atom-transfer radical polymerizationfield), and was quantitative after three hours.104

Scheme 23 PMDETA = N,N,N¢,N¢¢,N¢¢-pentamethyldiethylenetri-amine

The derivatization of aliphatic polyesters by click reac-tions under very mild conditions allowed for the graftingof a wide range of functional groups onto poly(e-capro-lactone) without any protection or deprotection reac-tions.105 a-Chloro-e-caprolactam (82) was converted intoazido-containing polyester 84 through two different syn-thetic routes (Scheme 24): by copolymerization with e-caprolactam followed by displacement of the chlorineatom by sodium azide, or by initial displacement of thechlorine atom in the monomer to give 85 and final copo-lymerization of the latter with e-caprolactam to afford 84.This azido-containing polymer was then subjected105 toclick cycloaddition with propargylic acid derivatives inthe presence of 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU) to afford grafted triazoles 86 under mild condi-tions and in short reaction times, with IR spectroscopicmonitoring of reaction progression. Thus, this methodol-ogy provides a robust synthetic pathway for grafting a

plethora of functional groups onto a polymeric structure,either cationic, anionic or neutral, including labile lacticacid. The authors claim that the title polymers might be ofinterest in drug delivery and gene therapy.105

Scheme 24

Methyl triflate initiated polymerization of 2-(pent-4-ynyl)-2-oxazoline or copolymerization of this monomerwith 2-methyl- or 2-ethyl-2-oxazoline afforded the corre-sponding poly(2-oxazolines) 87 and 88 (Scheme 25).106

Subsequent copper-catalyzed Huisgen 1,3-dipolar cy-cloaddition with a series of azides afforded triazole deriv-atives 89–91 in quantitative yield; these compounds areinteresting synthetic targets because they can be consid-ered as prototypes for carrier systems in a new class ofpolymer-based therapeutics.106

Polymers made by a click post-modification of poly(p-phenylene ethynylene)s (PPEs) were compared107 to thosesynthesized through a conventional approach. Whereverthe structural repeat unit of the polymer was the same, theoptical properties of these polymers in solution proved tobe identical; nevertheless, thin-film optical and thermal

Scheme 22

NN

N

O

O N

Me

Bn

*

NN

N

O

O

*

N3

*

Cl

Cl

N

Me

HN

propargyl acrylateCu(I)I, DIPEA

i. 2,6-dichlorobenzyl bromideii. Et3N

72 73 74 75

Me

O

MeO

Br

n

O

MeO

N3

n

O

MeO

NN

N

R

n

Br

O

OMe

NaN3

R

CuBr, PMDETA

77

78 79 R = CH2OH80 R = COOH81 R = CH(Me)=CH2

76

O

OCl

DSOP

O

Cl

O

O

O

O

ON3

O

O

DSOP

O

N3

O

O

O

CH2R

O

N

O

O

O

N

N

R

4n m

5

x

NaN3DMF, 45 °C, 3 d

NaN3DMF, 45 °C, 3 d

4n m

5

x

CuI, DBU,THF, 35 °C

4n m

5

x

R = OCOPh, Et3N+Br–

NMe2, Me2N+PEOBr–

8283

8485

86

O

O

[PEO = poly(ethylene oxide)]



properties differed as a result of varying degrees of poly-merization and perhaps a different distribution of triazoleregioisomers.

Another important tool for the synthesis and use of mac-romolecules in fields ranging from biomedical devices tonanotechnology is the development of selective syntheticapproaches that are orthogonal to the diverse array offunctional groups present in many polymeric systems.Thus, by combining copper(I)-catalyzed 1,3-dipolar cy-cloaddition with other families of chemical transforma-tions, the synthesis of macromolecule 96 and the cascademethodology for the preparation of 102 were recently si-multaneously reported by Hawker and co-workers.108

In the first case, an 8:1:1 terpolymer was prepared fromstyrene, p-(trimethylsilylacetylene)styrene, and 2-(tri-methylsiloxyethyl)methacrylate using living-free-radicalpolymerization, and deprotection of both protectinggroups with tetrabutylammonium fluoride (TBAF) gavedifunctional macromolecule 95 [Mn = 31900 amu;PDI = 1.16 (PDI = polydispersity index)].108 The acety-lene and hydroxyl functionalities present in 95 affordedreactive sites that could couple simultaneously with bothmethyl 4-(azidomethyl)benzoate and the acetonide-pro-tected bis-MPA [MPA = 2,2¢-bis(hydroxymethyl)propri-onic acid] anhydride in the presence of copper(I) and N,N-diisopropylethylamine (Scheme 26). The orthogonallyfunctionalized macromolecule, 96 (Mn = 39900 amu;

Scheme 25

N

O (CH2)3

R1O

N

NBoc

m n stat

87 n = 45, m = 5 88 n = 35, m = 12

+ N3CH2R2

CuSO4·5H2Osodium ascorbate

N

O (CH2)3

R1O

N

NBoc

N N

N

R2

m n stat

O

O89 n = 45, m = 5; R2 = TMS90 n = 45, m = 5; R2 = 91 n = 35, m = 12 ; R2 =

Scheme 26

OO

OSiMe3

ON

ON

OO

OH

H

O

O O

O O

O

O

N3

O

OMe

CuBr(PPh3)2

ON

OO

OO

O O

NN

N

OMeO

Sia)

120 °C

b) TBAF, THF

1

8

1

92

95

DIPEA

1

8

1

96

93

94



PDI = 1.19), was obtained by transforming the terminalacetylene group into a methyl benzoate substituted triaz-ole motif and by esterification of the hydroxyl group(Scheme 26).108

For the synthesis of 102 (Scheme 27),108 to a solution of a9:1 random copolymer of tert-butyl acrylate and N-acryl-oyloxy succinimide (97) in tetrahydrofuran were addedpropargylamine (98) and azo compound 99. In this case,copper(I) acts as a catalyst for the click reaction between98 and 99, while the N,N-diisopropylethylamine catalyzesthe amidation reaction of 97 with either 98 or the interme-diary click adduct 101. This sequence of reactions wasmonitored by GPC/HPLC and 1H NMR spectroscopy,

which showed that all reactions reached completion after16 hours at 50 °C and that the amidation chemistry andclick reactions occurred at approximately the same rate.108

Conjugated polymers have been synthesized through thecopper(I)-catalyzed 1,3-dipolar cycloaddition of azidesand alkynes.109 The novel polymers obtained, linked via1,4-disubstituted 1,2,3-triazole units, have interesting lu-minescent properties. Fluorene-derived 103 reacted withderivatives 104 and 105 in a degassed tetrahydrofuran–acetonitrile solution. The reaction started with the addi-tion of copper(II)/copper(0) and trisbenzyltriazolylamineas a ligand (Scheme 28). The polymerization reactionswere stopped at both ends of the growing chain by the ad-

Scheme 27

H

NH2

N

N3

NN

Cl

NO2

CuBr(PPh3)3

NO

NHOOtBuO

H2N

N

N

Cl

NO2

N

NN

N

O O

OOO

ON

N

OtBu

N

O

NHOO

N N

N

N

N

Cl

NO2

N

OtBu

97 98 99

DIPEA

9 1

100

102

101

9 1

9 1



dition of excess azidobenzene as stopper, and after twohours, 2-ethynyl pyridine. The reactions were sufficientlyfast to proceed at lower temperatures; when the reactionwas accomplished at these low temperatures and under di-lute conditions, polymers with high molecular mass and amore transparent appearance were obtained.109

Copper(I)-catalyzed reactions have also been used suc-cessfully in macromolecular chemistry for the preparationof stable and thermoreversible cross-linked organogelsthrough coupling between alkynes and azide-containingorganogelators in a 10:1 ratio (Scheme 29).110 This meth-od allows for the modification of some of the parent gel’sproperties, while maintaining the overall structure and thethermoreversibility.

Scheme 28

N3

Me Me

N3

n n

N

Me Me

N

NN NN

N

X

n n

N

X

103a n = 5103b n = 11

104 X = N105 X = CH

Cu/Cu(OAc)2

106 X = N, n = 5107 X = N, n = 11108 X = CH, n = 11

y

+

Scheme 29

N

NH

OO

H

N

NH

OO

H

N

NH

OO

H

N

N

H

H

OO

N

N

H

H

OO

N

N

H

H

OO

gel

N3 N3 CuI

N

NH

OO

H

N

NH

OO

H

NH

O

NNN

NN

N

NNN

N NN

N

N

H

H

OO

N

N

H

H

OO

N

N

H

H

OO

cross-linked gel



Poly(vinyl alcohol)-based hydrogels have been preparedusing this biocompatible copper(I)-catalyzed version ofHuisgen [3+2] cycloaddition by two different cross-link-ing strategies.111 Firstly, poly(ethylene glycol) diazide canbe used as a cross-linker for the alkyne-functionalizedpoly(vinyl alcohol). Moreover, a multicomponent reac-tion involving two poly(vinyl alcohol)s plus amine-func-tionalized alkyne and azide substrates can also beconsidered (Scheme 30). Hydrogel properties are expect-ed to be highly dependent upon the structure of the poly-mer components. The level of 100% gelation was notachieved,111 probably because of incomplete 1,3-cycload-dition cross-linking reactions, or perhaps because of someside reactions that could occur as a result of the difficultyin achieving the proper mixing of reagents. The reactionappears to be quite fast, and the gels were formed essen-tially immediately upon addition of the copper catalyst tothe polymer solution.

Scheme 30

Synthetic glycopolymers are currently the subject of in-creasing attention;112,113 the reason is that, while simplemonosaccharides can only establish weak interactionswith protein receptors and elicit only a weak response toin vivo events, these interactions can be enhanced if car-

bohydrates are used in the form of macromolecular glyco-conjugates. In fact, natural carbohydrate–proteininteractions take place through higher-order oligomericstructures, and this observation is known as the ‘clusterglycoside effect’.114

Du’s group reported the convergent synthesis of C3-sym-metric (1→6)-N-phthalimido-b-D-glucose-derived octa-decasaccharide 116 in a 62% yield (Scheme 31); 115 thetitle compound was obtained by the coupling of azide 114,comprised of a hexasaccharide and an aliphatic spacer,with alkyne 115 in the presence of copper(II) sulfate (2–5mol%) and sodium ascorbate (5–10 mol%) in aqueous tet-rahydrofuran at 50–60 °C. The final octadecasaccharide116 showed good antitumor activity in preliminary mousetests.115

Scheme 31

Triazole rings were also made in the direct assembly offunctionalized sugars by reaction of alkynyl and azidoglycosides. The assembly proceeded to completion in re-action times no longer than 45 minutes when organic-soluble copper(I) complexes such as tris(triphenyl-phosphine)copper(I) bromide [(Ph3P)3·CuBr] and triethyl-phosphite–copper(I) iodide complex [(EtO)3P·CuI] wereused under microwave irradiation.51

The functionalization of polyalkyne materials by co-click-ing reactions of appropriate mixtures of mannose- and ga-lactose-based azides has been reported116 as a simple andefficient route to synthetic libraries of materials that differonly in the nature of the sugar moiety. In this procedure,(PPh3)3CuBr and N,N-diisopropylethylamine were usedas the catalytic system. 1H NMR and FT-IR analysis con-firmed that the conversion of the alkyne groups of 117

H2NOHO

ONH

N3

OHO

ON

NH2N3

360–mm

m 360–m

98 109

111 112

O

OHO

NH

NNN

H2N

OHO

ON

N

NN

H2N

360–mm

360–mm

110

113

O N3

O

OO

114

115

CuSO4·5H2O,sodium ascorbate

O

OO

NN

N

NNNNN

N

O

O

O

116n

n

n



into triazoles was achieved in nearly quantitative yield,and the molecular weight distribution of glycopolymers120 was not significantly variable (Scheme 32).116

Scheme 32

A visibly fluorescent tag was also co-clicked into a poly-meric scaffold via fluorescent azide 121 (Scheme 33). 116

The resulting polyvalent ligands 123 and 124 are quiteuseful in protein–carbohydrate binding interaction stud-ies.117 The reported glycopolymers were subjected tobinding studies with model lectins, that is, a series of plantproteins that selectively binds to carbohydrates; in partic-ular, concanavalin A and Ricinus communis agglutininbind to mannose and galactose, respectively.116

6-Azido-6-deoxycellulose (125), obtained by low-tem-perature tosylation of parent cellulose and subsequent nu-cleophilic displacement by sodium azide in N,N-dimethylformamide, was subjected to copper(I)-catalyzedHuisgen cycloaddition with a series of acetylene deriva-tives. Thus, cellulose analogues 126–128, bearing meth-oxycarbonyl, 2-aniline, and 3-thiophene motifs(Figure 2), were prepared in a selective and efficient man-ner.118

Tetrazole-containing polymers are also of interest for theproduction of ion-exchange and chelating resins and su-perabsorbents.119 This novel class of polymers can be ob-tained by means of click chemistry as a post-polymerization modification of polyacrylonitrile deriva-tives.119 The starting polymeric materials can be poly(ho-

moacrylonitrile), random styrene–acrylonitrile (SAN)copolymer, block copolymer of acrylonitrile and styrene,and also silica or polystyrene particles grafted with poly-SAN. These polymeric materials are prepared by atom-transfer radical polymerization (ATRP),120 a technique forcontrolled radical polymerization. This technique yieldshalogen-terminated polymers that can be further used as

O

O

Br

O O

n

OH

HO

OHO

OH

O N3

OO

OHHO

HO OH

N3

117

118

119

O

O

O O

OH

HO

OHO

OH

O

N N

N

Br

O O

OO

OH

HO

OH OH

N N

N

x

Cu(I)

y

α-mannoside

β-galactoside

120

Scheme 33

Br

O O

O

O

n

N

O

O

O

N3

O

121 122

O

O

O O

O

N N

N

Br

O O

N N

N

O

O ONRO

n

Cu(I)

m

123 R = α-mannopyranoside124 R = β-galactopyranoside

Figure 2

OO

OHHOO

N3

OO

OHHOO

NN

N

COOMe

OO

OHHOO

NN

N

NH2

OO

OHHOO

NN

N

S

125

126

127

128



macroinitiators for the synthesis of block copolymers oras precursors of other chain-end-functionalized polymericmaterials.

4.2 Liquid Crystals

The first example of a liquid crystal that incorporated a re-gioisomeric 4-substituted 1,2,3-triazole in its structurewas recently reported.16 Target compounds were designedto present cholesteric and ferroelectric mesophasesthrough the incorporation of units in the mesogenic corethat had the potential to confer such properties. In order toobtain the desired 1,4-disubstituted triazole, azide 129 (or134) was added to a suspension of alkyne 130 (or 135),copper(I) iodide, and triethylamine in refluxing aqueousethanol (Scheme 34). After 48 hours, the correspondingchiral liquid crystal was isolated in moderate to good yield(60–90%).

Thermal properties of the final compounds were investi-gated by using polarizing optical microscopy (POM) anddifferential scanning calorimetry (DSC). 16 The analysisof the mesomorphic behavior showed that by displacingthe 1,2,3-triazole ring to a central core position, the melt-ing point is substantially lowered, thereby favoring thesmectic phases.

4.3 Self-Assembled Monolayers

Self assembled monolayers (SAMs) are of significant in-terest in interfacial reactions and many technological ap-plications such as sensors, catalysis, microarrays, andmolecular electronics.121–123 A wider use of SAMs in tech-

nology requires, as a first important step, the search forversatile organic reactions that can be performed on them,allowing for the introduction of required functionalities tothe surface. For this purpose, click chemistry seems to bea good option.

The 1,3-dipolar cycloaddition of azides and alkynes toform 1,2,3-triazoles has been used to couple monolayer-covered azide-terminated silica substrates with acety-lenes.124 The development of this reaction would permitthe attachment of desired functionalities needed for spe-cific surface properties in these alkylsiloxane monolayers.Azide-terminated monolayers were prepared in situ from11-bromoundecylsiloxane monolayers and were subse-quently reacted with three different acetylene compounds,as depicted in Scheme 35.124

The reactivity of acetylenyl-terminated SAMs on gold to-wards click chemistry has also been investigated.125,126 Toa reaction mixture containing the acetylenyl-terminatedSAM-coated gold substrates in aqueous ethanol were add-ed an azido compound, copper(II) sulfate, and sodiumascorbate, and the mixture was stirred at room tempera-ture for 12 hours (Scheme 36). This procedure proved tobe a mild and efficient way of tailoring surface function-alities.125

In a similar manner, the azido groups existing in mixed as-sembled monolayers on gold electrodes from various w-functionalized alkane thiols and azido alkane thiols, in thepresence of copper(I) catalysts, reacted rapidly and quan-titatively with terminal acetylenes to form 1,2,3-triazolesvia click chemistry (Scheme 37).127 The coupling reactionproceeded in high yield and under very mild conditions.

Scheme 34

N3

O Ph

+ Cu(I)

X

N

NN

129 130 131 X = OBn132 X = OH Pd(OH)2/C

C10H21O

NOCOCl

C10H21O

NOO

O

N

NN

133

C10H21O

N3

O+

134 135

Cu(I)

C10H21O

N

NN

O

136



Scheme 35

O O O

Si Si SiO O

N3 N3

OO

N3

SiO2

R1 C C R2

O O O

Si Si SiO O

N N

OO

N

SiO2

N

N

N

N

N

N

R1

R2

R1

R2

R1

R2

R1 = C6H13, R2 = HR1 = COOMe, R2 = HR1 = R2 = COOEt

Scheme 36

S

O

O

O

O

Au

RN3CuSO4·5H2Osodium ascorbate

H2O–EtOH

S

O

O

O

O

Au

NN

N

R

R = COOH

OO

ONH2

OHO

N

O

MeO

H

Scheme 37

Br(CH2)11SH BrNaN3 N

NN R

O

N NN

R

O

R =

Fe

NO2

O OMe

3 NH



Surfaces containing organic azides are excellent plat-forms for further modification. It is possible to easily con-trol and measure the amount of a surface-bound azide that,once incorporated, reacts quantitatively with acetylene.This action is carried out by monitoring the intense infra-red stretching mode of organic azides. The major limita-tion in these reactions appears to be steric hindrance.

4.4 Multicomponent Reactions

The increasing demand for novel biologically active smallmolecules and the laborious process of lead discoveryhave made the synthetic efficiency of multicomponent re-actions very attractive to organic chemists. 128,129 In thesereactions where more than two starting materials react toform a product which incorporates essentially all of the re-agent atoms, the selectivity of procedures and the compat-ibility between different functional groups represent keyaspects to be considered.130 Click chemistry, and morespecifically, the Huisgen 1,3-dipolar cycloaddition, hasemerged as a simple, fast, and efficient approach to syn-thesize functionalized 1,2,3-triazoles. For obvious rea-sons, the combination of both these techniques could bean important strategy for the generation of novel com-pound libraries.131,132

One of the first contributions to the multicomponent vari-ant of the copper-catalyzed Huisgen cycloaddition was re-ported by van der Eycken.133 In this process, azides weregenerated in situ from the corresponding halides, and un-derwent cyclization with copper(I) acetylides (t-BuOH–H2O, 1:1), furnishing the expected 1,4-disubstituted 1,2,3-triazoles (Scheme 38). Performing both steps under mi-crowave irradiation (100 W and 125 °C) significantly re-duced the reaction time. The copper(I) catalyst wasprepared by the disproportionation of the copper(0) andcopper(II) species. The final products were obtained inroughly 90% yield.

Scheme 38

Barbas and Ramachary accomplished the preparation of aproline/copper(I)-catalyzed spirotrione-triazole 148 in a90% yield.131 This synthesis is based on the stereospecificassembly of simple substrates like phosphorane, alde-hydes, cyclic 1,3-diketones, and azides under organo-cop-per(I) catalysis by means of Wittig/Knoevenagel/Diels–Alder/Huisgen cycloaddition reaction sequences in a one-pot fashion (Scheme 39).

The Huisgen cycloaddition has also been combined withthe Biginelli multicomponent reaction134 for the efficientsynthesis of libraries of compounds such as 149.132 Thiswas carried out by performing rapid and high-yielding mi-crowave-assisted azide–acetylene couplings. For this

purpose, readily available 6-(azidomethyl)dihydropyrimi-dones 150 were prepared as key intermediates using amulticomponent approach, and then reacted with a seriesof terminal acetylenes under copper(I) catalysis. The bestresults were obtained when microwave irradiation wasused, heating at 80–100 °C. This led to an attractive link-age of two important N-heterocyclic pharmacophores(Scheme 40) with four points of diversity that can easilybe accessed by choice of the appropriate buildingblocks.132

4.5 Creating Drug Candidates under Click Conditions

The traditional process for the design and discovery of anew pharmaceutical drug is both time-consuming and ex-pensive, despite the development of combinatorial chem-istry.1 Thus, the revolutionary click chemistry concept isbeing exploited in drug and biomedical research,17,135–137

as click chemistry constitutes a powerful tool for the rapiddiscovery of target enzyme inhibitors that exhibit greatpotency and specificity.138,139 In this context, Lee and co-workers prepared a library of triazole compounds thatwere shown to be potent inhibitors of human a-1,3-fuco-syltransferase VI, a pivotal enzyme which is involved inthe catalysis of the final glycosylation step in the biosyn-thesis and expression of many important saccharides.140

Srinivasan and co-workers accomplished the preparationof a vast series of enzymatic inhibitors in processes relat-ed to obesity and diabetes.141

Furthermore, click adducts have been employed, not onlyfor the development of new drug candidates, but also inglycobiological studies, such as DNA sequencing142 or thecoupling of ligands to vesicles and liposomes.143

The triazole scaffolds are expected to be more than justpassive linkers, as they might be associated with biologi-cal targets through intermolecular hydrogen-bonding anddipole interactions.17

It is reasonable to postulate that the combination of het-erocyclic pyrazinones and triazoles could lead to com-pounds with pharmacological interest, as both structuralmotifs have been reported to exhibit biological activi-ties.144,145 In this context, Kaval et al. considered two dif-ferent synthetic approaches,146 the first of which was 1,3-dipolar cycloadditions with acetylene-containing 2(1H)-pyrazinones 153. In this case, the active catalyst was gen-erated in situ from copper wire and copper(II) sulfate, andthe reaction was carried out with microwave irradiation inaqueous tert-butyl alcohol (Scheme 41).

The other approach was a 1,3-dipolar cycloaddition withazide-containing 2(1H)-pyrazinones, which are in equi-librium with the corresponding tetrazoles via an intramo-lecular cyclization that involves the vicinal pyrimidinicnitrogen (Scheme 42).146 This equilibrium was found tobe shifted to the bicyclic derivative, thus it was assumedthat an increase in temperature should reverse the equilib-rium to the unstable azide. Nevertheless, microwave-

R1 X NaN3 R2 Cu(0), CuSO4

NN

N

R2 H

R1

R1 = aryl, HX = Cl, Br, I

R2 = Ph



mediated reaction attempts resulted either in no reactionor in reagent decomposition.

Surprisingly, when the reaction was carried out at roomtemperature using a tertiary amine as a copper(I) stabiliz-ing agent, the desired substituted triazoles were obtainedin moderate to good yields (Scheme 42).146

Scheme 41

The Huisgen [3+2] cycloaddition has also been employedin the search for inhibitors of acetylcholinesterase(AChE), a pivotal central nervous system neurotransmit-ter.147,148 For this purpose, a special technique, namedtarget-guided synthesis (TGS)149 has been used; this ap-proach is based on the assembly of two building blocks,with weak affinity for different structural features of thetarget enzyme, to produce a bivalent inhibitor.

Thus, in situ click chemistry between alkynes and azideswith chains of differing length was used, without catalystbut instead in the presence of the enzyme as a template.Under these conditions, only those building blocks whichare involved in an interaction with the active site of the en-

Scheme 39

Ar1 = Ar2

Ar1 = Ar2

O

O

O

OAr2

H

R2R1N

O

O

Ar1

Ar2

#

O

O

O

Ar1

Ar2

O

O

O

O

O

N

NN

R3

N

NNR3

Ar1CH2O Ar2CH2O

O

Ar1

O

OAr2

H

142 143

X

O

N

Ar1

R1 R2

Ar1 = Ar2 = 4-[HC CCH2O]C6H4

137 X = H, H138 X = PPh3

R1R2NH

domino aldol–Knoevenagelor

domino Wittig–Knoevenagel

144 145

#

organocatalyticDiels–Alder

– R1R2NH

+ H2O

147

Cu(I)-catalyzedHuisgen cycloaddition

CuSO4, CuR3N3

R1R2NH

139 140 141

146

148

Scheme 40

N

NH

O

OR3

N3

R2

O

R1

N

NH

O

OR3

NN

N

R4

R2

O

R1

R4 H

Me

O

R3R2

ON

NHO

R1

OR3

O

OMe

O

R2

H NH2

HN

R1O

149 150

151 152

R1 = H, MeR2 = Me, aryl, heteroarylR3 = Et, i-Pr, BnR4 = aryl, heteroaryl

N3R3

N

N

R1

O

O

R2

Cl N

N

R1

O

O

R2

Cl

NN

N

R3

153

154

155

R1 = CH2C6H4OMeR2 = HR3 = 4-MeO, 4-NO2, 4-NMe2, 4-Cl, 3,5-(CO2Me)2, 4-CONH2



zyme are close enough to each other to react and producethe potent inhibitor. It is remarkable that only syn-deriva-tives, that is, 1,5-disubstituted triazoles, were formedfrom among all the possible combinations in the presenceof the enzyme; this is in contrast to what happens underthermal Huisgen conditions, wherein, as indicated above,a roughly 1:1 mixture of regioisomers is obtained.48

Some of these compounds exhibit femtomolar inhibitionof AChE, the lowest dissociation constants reported todate for non-covalent inhibition of this enzyme.147,148 Tri-azole derivatives such as 158 (Figure 3) simultaneouslymimic the structure of three AChE inhibitors: propidiumand decamethronium ions, and tacrine.150,151

Figure 3

The in situ click chemistry approach has also been suc-cessfully applied to the formation of HIV-1 protease in-hibitors; for this purpose, alkyne 159 and azide 160 wereincubated in the presence of the protease, HIV-1-PR(Scheme 43). This protease, which plays an important rolein the inhibition of viral replication, itself acts as a tem-plate for the reaction and even increases the rate of forma-tion of the anti 1,4-triazole product 161.152

Other examples of an enzyme-mediated preparation of tri-azole derivatives were reported by the groups ofSharpless153 and Gmeiner154 for the preparation of carbon-

ic anhydrase II inhibitors and dopaminergic agonists, re-spectively.

Another way of fighting against AIDS infection is theblockage of a viral envelope protein to antigens in the hostT-cell surface.155 Gopi et al. carried out the modificationof a proline unit with 4-phenyl-1,4-disubstituted 1,2,3-tri-azole, formed through a click [3+2] cycloaddition reac-tion.156 This led to a peptide (Figure 4) that binds to theviral protein with an affinity two orders of magnitudegreater than that of the parent peptide and, thus, stronglydisrupts the interaction of this glycoprotein with the hostcell surface.156

Scheme 42

N

N

R1

OR2

Cl

N N

N N

N

R1

OR2

Cl N

N N

R3N

N

R1

OR2

Cl N3

156 157

Cu wire,

CuSO4·H2O

R3 H

R1 = Ph, 4-MeOC6H4R2 = H, 4-MeOC6H4, 4-MeO2CC6H4R3 = Ph, CO2Et, CH2NMe2, CH2OH, CH2CH2OH, C(Me)2OH, C(Me)(Et)OH

N NN

HN

N

H2N

N

NH2

158

Scheme 43

OHNH

O

O

MeO

SN

O

O N3

OH

HIV-1-Pr

MeO

SN

O

O N

OH N N O

O

HN

HO

159 160

+

161

Figure 4

N

HN

O

N

HN

O

N

N

N

native peptide with proline peptide with substituted proline



In the search for inhibitors of carboxylesterase I (CE-I), anenzyme involved in some important biological processessuch as drug metabolism, a series of rhodamine–biotin-tagged forms of (–)- and (+)-166 were prepared(Scheme 44). These compounds were obtained by thecopper(I)-catalyzed reaction of 164 with an alkyne-derivatized rhodamine–biotin agent 165. Binding studiesrevealed that association of (–)-166 with CE-I was stableeven to protein denaturation.157

Click chemistry has also been applied toward the synthe-sis of resveratrol (167), a polyphenolic compound foundin some plants, such as in the skin of red grapes.158 Thiscompound exhibits some beneficial properties, includingcardioprotective, neuroprotective, antiviral, and anti-inflammatory properties,159 so derivatives of resveratrolare of interest as potential biologically active compounds.Pagliai et al. accomplished the preparation of a library ofresveratrol derivatives 168–172, wherein the carbon–car-bon double bond of the stilbene moiety was replaced witha triazole ring (Figure 5).160 After the evaluation of the cy-totoxicity of these compounds, the preliminary biologicalanalysis suggested that some of the compounds screenedwere more potent cytotoxic and antiproliferative agentsthan resveratrol itself.

The broad spectrum of biological activities exhibited byvitamin D161 has prompted researchers to carry out the

synthesis of analogues such as the triazole-bearing vita-min D.162 Click chemistry allowed for the cycloaddition ofa vitamin D side-chain terminal acetylene with phenylazide, and separately with a vitamin D side-chain azide, toyield the corresponding 1,2,3-triazole analogues of 1-a-hydroxy-vitamin D3 and a dimeric vitamin D derivative(173), in which the monomeric units are linked through atriazole tether (Figure 6).163

Scheme 45

Taking advantage of the click strategy, several functional-ized resins were prepared and evaluated for the parallelsolid-phase synthesis of a series of aryl carboxamides,which frequently serve as key pharmacophoric elementsin drugs.164 In this context, Gmeiner and co-workers re-ported the preparation of a library of solid-supported N-benzyl 1,2,3-triazole carboxamides.165 These compounds

Scheme 44

OH

OH

O

H

H H

H

HHOOH

HCl

N3O

OH

OH

O

H

H H

H

HHO

HO

N3tag

OH

OH

O

H

H H

H

HHO

HO

N

NNtag

tag = rhodamine–biotin

166

165 9

162 164

163 9 9

Figure 5

OH

OH

HO

resveratrol (167)

R3

R1 N

NN

R2 R4

R5

R6

168, R1 = OH; R2 = H; R3 = OH; R4 = H; R5 = OH; R6 = H

169, R1 = H; R2 = OH; R3 = H; R4 = OH; R5 = H; R6 = OH

170, R1 = OMe; R2 = H; R3 = OMe; R4 = H; R5 = OMe; R6 = H

171, R1 = H; R2 = OMe; R3 = H; R4 = OMe; R5 = H; R6 = OMe

172, R1 = OH; R2 = H; R3 = OH; R4 = H; R5 = OMe; R6 = H

Figure 6

N N

N

HH

HO OH HO OH173

R1NH

NR1

O

R2

N O

R1

N

NNR3

R2 N

NNR3

R2

R1NH

O

reductive amination

acylationclick

cycloaddition

acidic cleavage

H

O

R1 = N Bn R2 = Me R3 =

H or Br



were prepared as depicted in Scheme 45, in four steps: re-ductive amination of a benzaldehyde derivative, N-acyla-tion with alkynoic acids, [3+2] cycloaddition, and a finalacidic cleavage. All target compounds thus obtained werescreened for G-protein coupled-receptor binding, andsome demonstrated excellent receptor recognition.165

Another target for click reactions is daunorubicin, one ofthe most important drugs employed in cancer chemother-apy, whose possible mechanisms of action include,among others, DNA intercalation.166 In order to increasethe drug’s DNA binding affinity, dimeric species derivedfrom daunorubicin derivatives offered the possibility ofpreparing a novel series of anticancer analogues.137 Thus,through click reactions, several dimeric derivatives suchas 174 (Figure 7), bearing triazole tethers with differingchain lengths and varying flexibility, were successfullysynthesized using (EtO)3PCuI as catalyst. Cytotoxicitystudies revealed that shorter linkers presented a strongeractivity against cancer cells, with flexibility being anotherkey aspect.

Bitriazolyl compounds 177, which are of interest becauseof potential use in agrochemical research as well as in ma-terials science, were synthesized through a copper(I)-cat-alyzed Huisgen cycloaddition by using the azidotriazole

175 and several terminal acetylenes as starting materials,followed by aminolysis of the ester moiety (Scheme 46).Tobacco mosaic virus was used as a model system fortesting the antiviral activity of these bitriazolyl products,and some were shown to be even more potent than com-mercial products used in the treatment of this agriculturalpest.167

Scheme 46

Another interesting target for the design of new pharma-cological drugs is the modulation of protein–carbohydrateinteractions; for example, inhibitors of galectins (b-galac-toside-binding lectins), might be potential active agents in

Figure 7

OHHO

OO

OMe

O

OHO

O

OH

N

N N

O O

N NN

HOOHHO

HO O

O

O

MeO

O O

4

174

Scheme 47

PP

N3

P

N3

O O

O

HO OH

O

HO

O

HO

OH

HOO

OO

N3

P

N3

P

NNN

sugarsugar spacer

= photoaffinity label

galectingalectin and other proteins

spacer

photoaffinity probe

hν

sugargalectin spacerclick chemistry

sugargalectin spacer

fluorecent labelfluorecent label

N N

NN3

OMe

O

R C CH NN

N

R

N NH

NOMe

O

NN

N

R

N NH

NNH2

O

CuI

175 176

177

NH3, MeOH

R = alkyl, aryl, carboxyl



cancer treatment.168 Pieters developed a method for thedetection of galectins, even mixed with other proteins(Scheme 47).169 For this purpose, he used a benzophe-none-based photoaffinity label, in which the benzophe-none group was bound to the C-3 position of the galactoseresidue of lactose, and the glucose moiety of lactose wasderivatized with a polyether chain that had a terminal azi-do group. When binding of galectin and the modified di-saccharide took place, irradiation of the system resulted inan interaction of the photoaffinity label with the protein,followed by triazole formation, thus enabling galectin de-tection.169

Another example of protein–carbohydrate interactionstudied with triazole derivatives was reported by van derPeet et al.170 The authors proved that only a-configuredmannose derivatives bearing triazole substituents couldact as substrates for Leishmania b-1,2-mannosyltrans-ferases.

4.6 Click Chemistry with Peptides and Proteins

Methods are needed for profiling the activity of enzymesin vivo in order to understand the role that these proteinsplay in physiological and pathological processes. Activi-ty-based protein profiling (ABPP)171 uses active-site-directed chemical probes to determine the functional stateof enzymes in complex proteomes, distinguishing activeenzymes from their inactive precursors and/or inhibitor-bound form. Scheme 48 illustrates a standard ABPPprobe, consisting of a reactive group and one or more re-porter tags, the size of the latter being the main limitation

for the in vivo application of ABPP probes.172 In contrast,click chemistry ABPP allows for the profiling of livingcells and organisms by treating them with tag-free azide-or alkyne-modified probes, which are then conjugated invitro to the complementary alkyne- or azide-modified tagunder cycloaddition reaction conditions to visualizeprobe-labelled proteins (Scheme 48).

Cycloaddition-based ABPP is a versatile method that al-lows, for example, for the in vivo and in vitro labelling ofenzymes and for their detection from among the wholeproteome; this methodology gives access to in vivo stud-ies to determine whether a certain drug inhibits its tar-get.173 The combination of both approaches, ABPP andclick chemistry, has contributed to the design and synthe-sis of potent and selective fatty acid amide hydrolase in-hibitors.174

A further approach to aid in the study of protein bindingand function is the incorporation of synthetically modifiedamino acids or peptides into the parent structure.175,176 Inthis sense, model studies have also been performed for theC-terminal lipidation of proteins by exploiting the Huis-gen cycloaddition.177

For instance, Marik and Sutcliffe reported the [18F]-label-ling of peptides via click conjugation of w-[18F]-fluoro-alkynes to several peptides that bear azide moieties, tofurnish the corresponding [18F]-labelled target peptides(Scheme 49).178 Derivatives such as 180 are widely usedas in vivo imaging agents of various physiological andpathological processes, using positron emission tomogra-phy.178

Scheme 48

tagRG

RG = reactive group

in vitro

standard ABPP

N3 N3RG

in vivo

N

in vitro,CuI

tagNN

click chemistry ABPP

(or and )RG N3 tag

tag

Scheme 49

18FN3 peptide

O

NN

N

18F

peptide

O

n

+n

178 179 180



An additional example that demonstrates the enormouspotential of click chemistry connected with proteins is thepreparation of small cyclic peptide analogues that are toostrained for ring closure via lactamization. In order to

demonstrate such a cyclization of peptides, the triazoleanalogue 182 of tyrosinase inhibitor 181 (Figure 8) wassynthesized.179

Thus, compound 182 was prepared starting from com-pounds 185 and 186 (Scheme 50), which were deprotect-ed and coupled to afford the linear tetrapeptide analogue184 that was then subjected to click conditions and there-by yielded 182.

Carbohydrate-derived azides have also been used in thestructural modification of peptides (Scheme 51).180 Thus,coupling of per-O-acetylated glycopyranosyl azides 187with bromoalkynamide 188 gave the corresponding gly-copyranosyl triazole derivative 189.

Figure 8

N

NH

N

HN

OO

O O

HO

N

N

HN

O

O O

HO

NN

N

tyrosinase inhibitor, 181

triazole analogue, 182

Scheme 50

N

N

HN

O

O O

HO

NN

N

N

OBn

O

H2NN

NN O

N

COOH

NN3

O

NH

N

O

O

BnO

NN3

O COOt-Bu

BnO

BocHNN

O

Triazole analogue, 182

triazoleformation

a: peptide bondformation

b:

ba

183 184

185 186

TFA

Scheme 51

HN

NNN

OROAcO

NHAc

OAc

O

OROAcO

NHAc

OAc

N3

HN

Br

O

CuI

Br187

188

189

R = Ac, (OAc)3-β-GlcNAc-



The bromine atom in 189 was displaced through nucleo-philic attack by the thiol groups of the cysteine units inpeptide 190 (Scheme 52).180 Subsequent deprotectionsteps yielded the target glycotriazole-functionalized pep-tide 191.

Biomolecules can be subjected to immobilization in theirnative state by the use of some click reactions like Diels–Alder and [3+2] cycloadditions. In this context, Sun andco-workers reported the use of a bifunctional poly(ethyl-ene glycol) linker with an alkyne terminal group(Scheme 53)181 for the immobilization of both proteinsand carbohydrates.

4.7 DNA as Participant in Click Reactions

Click chemistry has successfully been evaluated as aneasy and efficient method for DNA alkylation.182 Theinterest in this chemical modification is because of theimportance of biological methylation in regulatory mech-anisms of gene transcription. Thus, the coupling of the nu-cleoside analogue 192, which bears an alkyne moiety,with an organic azide, accompanied by covalent bond for-mation with a DNA fragment mediated by a methyltrans-ferase enzyme, proceeded in an efficient manner and

effectively demonstrated the possibility of using DNA inclick modifications (Scheme 54).182

Scheme 52

BocHN

HN

NH

HN

NH

HN

NH

HN

NH

TrtS

O

O

O

O

O

O

O

O

Ot-BuO

Ot-BuO

S

SOt-Bu

Ot-Bu

HN

NH

HN

NH

OS

O

O

O

O

O

S

S

Ot-BuO

O

Ot-Bu

H2N

HN

NH

HN

NH

HN

NH

HN

NH

HS

O

O

O

O

O

O

O

O

OHO

OHO

S OH

OH

HN

NH

HN

NH

OSAcm

O

O

O

OH

OHO

O

OH

HN

NN

NORO

HO

NHAc

OH

O

S

HN

NN

NORO

HO

NHAc

OH

O

189

190

191

Scheme 53

O

O

O

O

glass slide

X

N3

carbohydrate, protein

click chemistry



The modification of either a 3¢- or 5¢-terminus or an inter-nal position of an oligonucleotide with a primary alkyl-amine group is a widely used method for the introductionof additional functional groups into DNA.183 In this sense,the oligonucleotide 5¢-amino-GTT TTC CCA GTC ACGACG-3¢ was used to prepare the azido-labeled DNA 194,which by coupling with the alkynyl 6-carboxyfluorescein

195 led to a fluorescent oligonucleotide with high selec-tivity, yield and stability (Scheme 55).184 This kind of de-rivative can be detected with laser-induced fluorescencetechniques.184

In the search for DNA metallation procedures, in order toincrease the conductivity of these modified DNA nano-structures, an efficient and selective method for the depo-

Scheme 54

N

N

N

NO

OHOH

N

NH2

I

H

N

N

N

N

O

NH2

O-DNA3'5'DNA-O

T DNA5'

RN3

CuI

T DNA5'

N

N

N

N

O

HN

O-DNA3'5'DNA-O

N

N

N

NO

OHOH

N

NH2NN

NR

3'DNA

methyltransferase

3'DNA

192 193

Scheme 55

O

HN

GTT TTC CCA GTC ACG ACG-3'N3

O OHO

OOC

N

O

NN

CH2

HN

O OHO

OOC

O

NH CH2 C CH

O OHO

OOC

O

NH CH2

NN

N NH

GTT TTC CCA GTC ACG ACG-3'

O

NH

GTT TTC CCA GTC ACG ACG-3'

O

196

194

+

195

+

197



sition of silver(0) around aldehyde-containing DNA hasbeen achieved.185 Acetylene-labelled nucleotide triphos-phates were inserted by using DNA polymerases and theproduct DNA was employed in a click approach with al-dehydo azides; the latter process can be

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Click Chemistry – What’s in a Name? Triazole Synthesis and ...xiuzhengrd.com/ejournals/pdf/synthesis/doi/10.1055/s-2007-966071.… · cedure for the preparation of such heterocycles.2