Chemical and Functional Properties of Food Saccharides

OH
© 2004 by CRC Press LLC
OH
O
OH
OH
a) TMSCl, pyridine
2.32 2.33 2.34
Trityl ethers of sugars are important substrates in synthetic carbohydrate chemistry.
Alkylation of the less sterically crowded hydroxyl group proceeds much more
readily, providing a selective substitution of the primary hydroxyl groups. Pentoses
that normally take the pyranose structure may react as furanoses having a more
reactive primary hydroxyl group.
2.3.4 ACYLATION
H, OH
a)
TMSO
OTMS
O OTMS
OTMS
+
OTMS
OTMS
OH
Under laboratory conditions and in industry, esters (2.37) are available from acids
(2.35) and alcohols (2.36) refluxed in the presence of an acid catalyst. To increase
the yield of ester in this reversible reaction, either ester or water should be continuously
evacuated from the reaction mixture. In the presence of water, the hydrolysis
of esters predominates. Because carbohydrates are unstable in the acidic medium at
elevated temperature, this method does not apply to synthesis of esters of saccharides
(acylated saccharides). The reactions either with acyl chlorides in the presence of
tertiary amine (pyridine, triethylamine) or with acyl anhydrides in the presence of
acid catalyst (zinc chloride, strong mineral acid, acidic ion-exchange resin) are the
most suitable routes of acylation of saccharides.
Reducing sugars in solution form a tautomeric equilibrium. The position of the
acylation depends on the catalyst applied and temperature. The peracylation to penta-
O-acetyl-β-D-glucopyranose is achieved at higher temperature with sodium acetate
as the catalyst.
Acylated aldopyranoses readily undergo acid-catalyzed anomerization. Because
of the anomeric effect, the axial acetyl group is preferred, and therefore α-isomer
of aldohexopyranose peracetate (2.38) is the principal product of the reaction performed
on the tautomeric mixture. However, change in the catalyst applied can result
in the domination of the β-isomer (2.39).
A wide variety of the acyl groups have been introduced to saccharides and their
derivatives. Exhaustively benzoylated derivatives are usually obtained by treating
saccharides with benzyl chloride in pyridine. Saccharide esters of higher alkanoic
(fatty) acids can be prepared by transesterification with other esters in the presence
OH
R 1 -COOH + R 2 OH R 1 COOR 2 + H 2O
2.35 2.36 2.37
OH
OH
O