Carbohydrates Wohl degradation

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Two common procedures in carbohydrate chemistry result in adding or removing one carbon atom from the skeleton of an aldose. The Wohl degradation shortens an aldose chain by one carbon, whereas the Kiliani-Fischer synthesis lengthens it by one. Both reactions involve cyanohydrins as intermediates. Recall from Section 21.9 that cyanohydrins are formed from aldehydes by addition of the elements of HCN. Cyanohydrins can also be re converted to carbonyl compounds by treatment with base. 

Further experiments disproved Wohl s theories1-3 4 as to a direct condensation mechanism. By degradation of 2,3,4,5-tetra-O-acetyl-L-arabinononitrile82 (5) in the presence of propionamide or benzamide, l,l-bis(acetamido)-l-deoxy-L-erythritol (6) was obtained in yields comparable to those obtained when the reaction was conducted in the absence of these extraneous amides. Also, an excess of acetamide in the medium did not increase the formation of nitrogenated carbohydrate derivatives. 

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