carbohydrate esters derivatives of carbohydrates with one or more hydroxyl groups substituted with...
TRANSCRIPT
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").