Reduction stereoselective, glycosyl radicals

Praly, J.-P., Reduction stereoselective d halogenures de glycosyles par le deuteriure de tributyletain. Tetrahedron Lett., 24, 3075,1983 Giese, B. and Dupuis, J., Anomeric effect of radicals. Tetrahedron Lett., 25, 1349, 1984. 

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

Stereoelectronic effects can be invoked for the radical reaction at anomeric centre of carbohydrates. The high stereoselective preparation of a-substituted C-glycosyl phosphonates in a a p ratio of 98 2 was achieved by reductive addition of bromide 2 to a-phosphonoacrylate (Reaction 7.5) [10]. Yields (in parentheses) depend on the sugar configuration D-galacto (80%), D-manno (47 %), D-gluco (30 %) and L-fuco (62 %). 

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