Carbohydrate esters
Derivatives 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
OH
Ac 2O
O
HO
HO
OH
ZnCl2
AcO
AcO
100 ºC, 1 h
OH
OAc
O
AcO OAc
70 %
Ac 2O
Py
Ac 2O, NaOAc
100 ºC, 1 h
AcO
AcO
OAc
O
OAc
AcO
AcO
OAc
OAc
O
OAc
OAc
80 %
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
Me
MeOH
O
_
OH + MeO +
O
Me
OMe
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.
CH2OH
O OAc
OAc
AcO
CH2OAc
0,001 M NaOH
OAc
O OAc
HO
OAc
OAc
Migration of esters
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.
CH2OAc
OAc
CH2OAc
O
MeI
OAc
OAc
Ag2O
AcO
OH
O OMe
AcO
OAc
81 % from α-acetate,
51 % from β-acetate
Sulfonates
(a)
(a)
O X (b)
X
(a)
_
OSO2R
+
C O S R
X
(b)O
(b)
+ H+
OH
+
XSO2R
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.
(a)
X
(a)
O X (b)
X C
(a)
+
_
OCOR
C O C R
(b)
Carboxylates
(b)
+ H+
C OH
+
XCOR
Displacement of sulfonates
TsO
OTs
O
NaOBz, DMF
BzO
OBz
O
BzO
BzO
120 oC, 5 h
BzO OMe
52 %
BzO
TsO
_
+
Bu4N OBz
OBz
O
OBz
OBz
O
BzO
140
oC,
BzO OMe
OBz
24 h
OBz
BzO
OBz
49 %
BzO
TsO
OBz
O
_
+
Bu4N OBz
140
BzO OBz
oC,
None 3-benzoate
24 h
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
OH
OH
O
O
OH
HO
O
OH
HO
HO
OH
sucrose
OR
OH
HC
OH
H2C
OH
glycerine
OR
O
O
OR
RO
RO
H2C
RO
O
OR
OR
olestra
H2C
OR
HC
OR
H2C
OR
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-inoil 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 Dgalactopyranosyl 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.
OH
OH
O
HO
HO
O
O
H
O
OH
O
OH
HO
HO
HO
OH
OH
HO
OH
Cl
Cl
O
O
HO
O O
H
O
O
HO
O
H
HO
O
Cl
OH
H
sucralose
two conformations of sucrose with
relevant intramolecular hydrogen bonds
Artificial sweeteners
+ H O
H
H3N C C N C CO2CH3
H
CH2
CH2
H3C
O
+ H
H O
H3N C C N C C N
H
H
CH3
CH2
H3C
CO2-
CO 2
aspartám
(dipeptid L-asparagínu a L-fenylalanínu)
O
C
+
N Na
S
O
sacharín
O
S
CH3
alitám
(dipeptid L-asparagínu a D-alanínu)
O
O
-
CH3
O
S
N
-
K
+
-
N SO3
H
+
Na
O
acesulfám K
cyklamát (Na+ alebo Ca2+)
Artificial sweeteners
Relative sweetness* of major nutričných sladidiel
------------------------------------------------------------------------------Sladidlo
Relative sweetness
------------------------------------------------------------------------------Alitam
200 000-290 000
Cukralose
55 000-75 000
Sacharin
30 000
Aspartam
18 000-20 000
Acesulfam K
15 000-20 000
Cyklamate
3 000
------------------------------------------------------------------------------*Sucrose = 100
http://en.wikipedia.org/wiki/Steviol_glycoside
Stevioside
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, pHstable, 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.
OH
HO
D-glucose
O
HO
D-glucose
OH
OH
O
D-glucose
O
H3C
HO
HO
OH
CH3
O
OH
CH2
Steviol
O
HO
O
O
OH
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 bitter
1–2% rebaudioside C
½–1% 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
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").
•
•
•
•
•
•
•
•
•
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)