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Encyclopedia of Physical Science and Technology EN002C-80 May 25, 2001 20:18

394 Carbohydrates

aldono-1,4-lactones to the corresponding acetylated
aldofuranoses.

c. Sodium amalgam. Sodium amalgam in water
and sodium in liquid ammonia are commonly used as re-
ducing agents. The metal alloy has an electron-rich surface
that reacts with protons of a proton-donating solvent to
form hydrogen atoms, which may combine to form molec-
ular hydrogen or react with the substrate to give a radical
ion. The same radical-ion can be directly obtained from
the metal, which, in both cases, is oxidized to a cation,
giving up electrons to the substrate. The hydrogen radi-
cals and the substrate radical-ions, which are bound by
adsorption to the electron-rich surface of the metal, com-
bine to afford the alkoxide ion, which then accepts a proton
from the solution. In the early literature, the reduction of
lactones to aldoses (a key step in ascending the series)
had been carried out with sodium amalgam in acid solu-
tion (pH 3), using such buffers as sodium hydrogenox-
alate, sulfuric acid, and, later, ion-exchange resins. These
buffers are needed because the reduction does not proceed
in alkaline solution (formation of the sodium salt of the
aldonic acid impedes reaction with the nucleophilic re-
ductant). The free acids are not reduced, but esters can be.
Thus, methyl D-arabinonate is converted to D-arabinose by SCHEME 19 Action of reducing agents.
sodium amalgam. The susceptibility of lactones to reduc-
tion and the inertness of the free acids have been exploited
catalyst. Rh[(Ph) 3 P] 3 Cl. Alternatively, trans addition
in the preparation of glycuronic acids by reduction of the
is achieved by reduction with Li in EtNH 2 Satlow
monolactones of aldaric acids.
temperature.
d. Catalytic hydrogenation. In the course of hydro-
genation with palladium or platinum catalysts, surface re- III. OLIGOSACCHARIDES
actions occur between such electron-deficient species as
the double bonds of carbonyl groups (which are attracted Oligosaccharides are polymeric saccharides that have, as
to the surface of the catalyst) and the hydrogen atoms their name denotes, a low degree of polymerization, DP
generated by splitting hydrogen molecules. Catalytic hy- (oligo means “few” in Greek). They are composed of 2 to
drogenation is used to convert keto groups to secondary 10 glycosidically linked monosaccharides, which can be
alcohol groups. Thus, Raney nickel reacts with D-xylo- liberated by depolymerization (e.g., by acid hydrolysis).
5-hexulosonic acid to give both D-gluconic and L-idonic Oligosaccharides having DPs of 2 to 3 are sweet tasting
acid. Usually, the axial alcohol is the preponderant prod- and are included among sugars, whereas higher members
uct, because the reductant approaches the substrate from are devoid of taste and are not referred to as such.
the less hindered side of the carbonyl group, to form an Sucrose is the world’s most widely used sweetener and
equatorial C H bond. Reduction of carbonyl groups can the one produced in largest quantities. Its industrial prepa-
be carried beyond the alcohol stage, to the deoxy stage. ration from sugar cane and from beets is now well es-
For example, methyl β-D-ribo-hexopyranosid-3-ulose is tablished. The process starts with pressing and concen-
converted to the 3-deoxy derivative by hydrogen in the tration of the juice. This is followed by precipitation of
presence of platinum, and 1-deoxyalditols can be obtained sucrose as a calcium complex with lime and its regenera-
from aldoses by the reduction of their dialkyl dithioacetals tion with SO 2 .Refining is achieved by charcoaling and re-
with Raney nickel. crystallization. Recent innovations in the process includes
The stereo specific syn hydrogenation of unsaturated shifting to continuous flow operations, which have con-
sugars (glycals and acetylenes) may be carried out het- siderably increased the efficiency of production. Up until
erogeneously, using Lindlar catalyst (Pd/CaCO 3 ) condi- recently the only competitor of sucrose was glucose syrup
tioned in quinoline, or homogeneously using Wilkinson’s produced by acid hydrolysis, or enzymatic hydrolysis of