Effects of Linkages on Alkaline Degradation

As stated previously, the alkaline degradation of polysaccharides proceeds by a peeling process in which the reducing end-group is liberated from a chain by elimination of the rest of the chain asa glycoxyanion. Elimination takes place when the chain is in the position beta to a carbonyl group of the reducing end-unit. Eliminations of this type can be exemplified by the action of alkali on 4-ethoxy-2-butanone (IV) to form 3-buten-2-one (V). 

Elimination of alkoxy anions takes place under alkaline conditions not only when there is a carbonyl group in the position beta to the alkoxide 

An alkoxy or glycoxy anion is more easily eliminated by the ionized enediol than is a hydroxyl ion, and the pyranose rings of the released glycoxy anions will open readily because of the tendency of negatively charged oxygen to form a double bond with carbon. 

The released end-group forms an a-dicarbonyl structure which rearranges by an intramolecular, Cannizzaro type of reaction to yield saccharinates. Intramolecular reactions of the Cannizzaro type can occur not only with dialdehydes but also with a-ketoaldehydes and a-diketones ( benzilic acid rearrangement ) they can be exemplified by the base-catalyzed rearrangement of phenylglyoxal (VIII) [or of (2,4,6-trimethylphenyl)-glyoxal ] to the salt of mandelic acid (IX) (or of 2,4,6-trimethylmandelic acid). 

No carbon-bound deuterium is found when the reaction takes place in 

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