Carbohydrate estersDerivatives of carbohydrates with one or more hydroxyl groups substituted with an acid moiety. According to the degree and mode of substitution, they can be partial, full or mixed esters.

The most important carbohydrate esters are:

- with carboxylic acids: • acetates (O-acetyl derivatives) –OCOMe (-OAc)• benzoates (O-benzoyl derivatives) –OCOPh (-OBz)

- with sulfonic acids: • methanesulfonates (mesylates, O-methanesulfonyl derivatives, O-mesyl derivatives) -OSO2Me (-OMs)• toluene-4-sulfonates (tosylates, O-toluene-4-sulfonyl derivatives, O-tosyl derivatives) -OSO2C6H4Me-4 (-OTs)

- with mineral acids: • phosphates -OPO3H2 • sulfates -OSO3H • nitrates -ONO2

• borates • (non anomerc) halogenides

Carbohydrate esters

• They are prepared by reaction of a sacharide with chlorides (benzoates, sulfonates, carbonates, phosphates, sulfates) or anhydrides of acids (acetates, sulfates), usually in basic medium (most often in pyridine), eventually directly with an acid (nitrates, borates).

Preparation of acetates

OOH

HOHO

OH

OH

OOAc

AcOAcO

AcO OAc

OOAc

AcOAcO

OAcOAc

OOAc

AcOAcO

OAc

OAc

Ac2OPy

Ac2O

ZnCl2

Ac2O, NaOAc

70 %

80 %

100 ºC, 1 h

100 ºC, 1 h

Reactions of acetates

• Zemplén deacetylation – O-deacetylation of acetates of sacharides by trans-esterification in anhydrous methanol in the presence of a catalytic amount of sodium methoxide.

O

O

MeO

OH MeO

OMe

+ MeO +MeO

_MeOH _

Migration of esters

• Some partial esters of sacharides (e. g., acetates, benzoates, phosphates) undergo both basic and acid catalyzed migration of the ester group. In general, the direction of this intramolecular arrangement is from a secondary towards a free primary hydroxyl group. Thus, e. g., 1,2,3,4-tetra-O-acetyl--D-glucopyranose in 0,001 N sodium hydroxide is transformed to 1,2,3,6-tetra acetate.

O

CH2OH

AcO

OAc

OAc

OAc O

CH2OAc

HO

OAc

OAc

OAc0,001 M NaOH

81 % from α-acetate,

51 % from β-acetate

In practical synthesis, such acyl migration is observed quite often. For example, when treated with iodomethane in presence of silver oxide, 1,3,4,6-tetra-O-acetyl-α- or -β-D-glucopyranose yields metyl 2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside.

O

CH2OAc

AcO

OAc

OH

OAcMeI

Ag2O

O

CH2OAc

AcO

OAc

OAc

OMe

Migration of esters

Sulfonates

C O S R

O

O

X

OH

X

X(a) (a)

(b)

(b)OSO2R+

_

+ XSO2R

(a)

(b)

+ H+

The very great importance of sulfonate esters in carbohydrate chemistry stems from excellent ‘leaving properties’ of sulfonyloxy group in nucleophilic displacement reactions, i.e. from the propensity of these derivatives to react by alkyl-oxygen fission (route a) rather than by sulfur-oxygen fission (route b). Most other esters react by process equivalent to route (b) (e.g. see acetates (carboxylates) in the Zemplen deacetylation), and cosequently do not offer a means of carrying out chemical operations at the carbon atoms of sugar chains.

C O C R

OCX

C OH

X

X(a) (a)

(b)

(b)OCOR+

_

+ XCOR

(a)

(b)

+ H+Carboxylates

Displacement of sulfonates

OOTs

TsO

BzO OMe

BzO

OBzO

TsO

OBz

OBz

OBz

OBzO

TsO

OBz

BzO OBz

OBzO

BzO OMe

BzO

OBz

OBzO

BzO

OBz

OBz

OBz

NaOBz, DMF

120 oC, 5 h

52 %

Bu4N OBz

140 oC, 24 h

Bu4N OBz

140 oC, 24 h

49 %

+ _

+ _

None 3-benzoate

Reaktivitu pri substitúcii sulfonátov významne ovplyvňuje stereochemické usporiadanie ostatných substituentov na sacharidovom skelete. Podobné prejavy závislosti reaktivity na stereochemickom usporiadaní sa pravidelne pozorujú aj pri iných derivátoch sacharidov. Preto pri posudzovaní reaktivity akejkoľvek funkčnej skupiny v sacharidoch a ich derivátoch ako relatívne zložitých molekulách je nevyhnutné chápať molekulu sacharidu vždy ako celok a nestačí sa obmedziť na príslušnú funkčnú skupinu.

Artificial fats

OOH

HOHO

OH O

O

OH

OH

HO

OH

H2C

HC

H2C

OH

OH

OH

OOR

RORO

OR O

O

OR

OR

RO

OR

H2C

HC

H2C

OR

OR

OR

sucrose glycerine

olestra fats and oils

R = stearoyl, palmitoyl, oleyl

Substitution of at least six hydroxy groups of sucrose by acyloxy moieties of higher fatty acids R gave rise to a non-metabolizable substitute of natural fats. Olestra was approved by the Food and Drug Administration for use as a food additive in 1996, and was commercialized by Procter & Gamble Co. The presence of sucrose esters with fatty acids in foods is declared by symbol E 473.

„Edible“ detergents

• Esters of fatty acids and sucrose are used as non toxic, biologically degradable detergents (degree of substitution 1-3).

• Many non-ionogenic surface-active substances based on sorbitol (D-glucitol) are used in food industry as emulsifiers of water-in-oil type and as antifoaming agents. They are obtained by esterification of sorbitol with fatty acids.

Artificial sweeteners

• Cukralose is a chlorinated sucrose derivative, with a D-galactopyranosyl unit instead of the D-glucopyranosyl unit, and containing three chlorine atoms instead of hydroxy groups. This derivative is 600-times sweeter than sucrose.

• Cukralose is not metabolized. It is fairly soluble and 60-times more stable in acidic media than sucrose. Used in some countries as

noncaloric sweetener, most often under commercial name Splenda. In EU it is known as E955.

two conformations of sucrose with relevant intramolecular hydrogen bonds

sucralose

OOH

HOHO O O

O

OH

OH

HO

OH

H

O

OOH

HOHO O

H

O

HO

HO

O

OH

H

OOH

Cl

HO O O

O

OH

Cl

HO

Cl

H

Artificial sweeteners

H3N CH

C NH

CH

CH2CH2

CO2

O

CO2CH3

O NS

O

OO

KNH

SO3

+

-

Na

++- -

H3N CH

C NH

CH

CH3CH2

CO2

O

C NH

OS

H3C CH3

H3C CH3

+

-

SN Na

C

O

OO

+-

aspartám (dipeptid L-asparagínu a L-fenylalanínu)

alitám (dipeptid L-asparagínu a D-alanínu)

sacharín acesulfám K cyklamát (Na+ alebo Ca2+)

Artificial sweeteners

Relative sweetness* of major nutričných sladidiel-------------------------------------------------------------------------------

Sladidlo Relative sweetness-------------------------------------------------------------------------------Alitam 200 000-290 000Cukralose 55 000-75 000Sacharin 30 000Aspartam 18 000-20 000Acesulfam K 15 000-20 000Cyklamate 3 000-------------------------------------------------------------------------------*Sucrose = 100

http://en.wikipedia.org/wiki/Steviol_glycoside

The steviol glycosides are responsible for the sweet taste of the leaves of the stevia plant (Stevia rebaudiana Bertoni). These compounds range in sweetness from 40 to 300 times sweeter than sucrose.[1] They are heat-stable, pH-stable, and do not ferment.[2] They also do not induce a glycemic response when ingested, making them attractive as natural sweeteners to diabetics and others on carbohydrate-controlled diets.

O

OH

OH

OHO

OH

O

O

OH

OH

OOH

OH

OH

OH

O

OH

CH3

H3C

O

CH2

Stevioside

Steviol

D-glucose

D-glucose

D-glucose

The diterpene known as Steviol is the aglycone of stevia's sweet glycosides, which are constructed by replacing steviol's carboxyl hydrogen atom (at the bottom of the figure) with glucose to form an ester, and replacing the hydroxyl hydrogen (at the top of the figure in the infobox) with combinations of glucose and rhamnose. The two primary compounds, stevioside and rebaudioside A, use only glucose: Stevioside has two linked glucose molecules at the hydroxyl site, whereas rebaudioside A has three, with the middle glucose of the triplet connected to the central steviol structure.In terms of weight fraction, the four major steviol glycosides found in the stevia plant tissue are:5–10% stevioside (250–300X of sugar)2–4% rebaudioside A — most sweet (350–450X of sugar) and least bitter1–2% rebaudioside C½–1% dulcoside A.Rebaudioside B, D, and E may also be present in minute quantities; however, it is suspected that rebaudioside B is a byproduct of the isolation technique.[2] The two majority compounds stevioside and rebaudioside, primarily responsible for the sweet taste of stevia leaves, were first isolated by two French chemists in 1931.[3]

http://en.wikipedia.org/wiki/Steviol_glycoside

http://delibo.sk/images/product/708.jpg

http://www.jakbydlet.cz/images/L%C3%A9%C4%8Divky/Stevia.jpg

http://members.chello.sk/fytomodelovanie/stevia%20v%20crepniku2.jpg

E Numbers

• E100–E199 (colours)• E200–E299 (preservatives)• E300–E399 (antioxidants, acidity regulators)• E400–E499 (thickeners, stabilizers, emulsifiers)• E500–E599 (acidity regulators, anti-caking agents)• E600–E699 (flavour enhancers)• E700–E799 (antibiotics)• E900–E999 (glazing agents and sweeteners)• E1000–E1599 (additional chemicals)

E numbers are codes for chemicals which can be used as food additives for use within the European Union and Switzerland (the "E" stands for "Europe").