Fructose dithioacetal, preparation

Preparation of fructose dithioacetals involves the keto peracetate 6.3 which is the normal peracetylation product. However, the carbonyl group of sialic acid is transformed into diethyl dithioacetal 6.4 in excellent yield at room temperature. The a-carboxyl and lactonization in this medium no doubt help in the conversion. 

Like D-glucose and D-fructose, however, D-xylose can be utilized chemic ly or microbially—to generate a variety of interesting five-ca n c emica s o er than furfural (vide supra) or xylitol, a noncaloric sweetener, both being duectly produced from xylan hydrolysates, that is, without the actual isolation of the sugar. Other readily accessible intermediate products of high preparative utiUty (Scheme 2.14) are the open-chain fixed dithioacetal, the D-xylal, and D-hydroxy-xylal esters, or pyrazol or imidazol A -heterocycles with a hydrophilic trihydroxypropyl side chain. 

Bredereck and coworkers also studied the 1,6-ditrityl ether, and definite proof of its structure was afforded by an outstanding series of experiments. The amorphous material (m.p. 94r-96°) was treated with the ordinary acetylation mixture, and yielded 3,4,5-tri-O-acetyl-l,6-di-O-trityl-fce(o-D-fructose. Preparation of the oxime followed by further acetylation gave the crystalline acetate of the oxime [ m.p. 207-208° Ca ]D - -118° (in chloroform)]. < The oxime of l,6-di-t>-trityl-D-fructose affords the same product on acetylation. The keto form was proved by formation of the crystalline diethyl dithioacetal from the acetyl derivative. 

The sugar dithioacetals are perhaps the best intermediates except that the reaction conditions may be too severe for some sugars, such as the disaccharides, which may be hydrolyzed (21). The ketoses are too acid-sensitive for direct mercaptalation D-fructose diethyl dithioacetal has been prepared by Wolfrom and Thompson only by an indirect method (i). The reduction of the acid chloride has not been extensively used but should be limited only by the availability of the acetylated acids (p. 309) 22). 

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