Towards α- or β-d-C-glycosyl compounds by tin-catalyzed addition of glycosyl radicals to acrylonitrile and vinylphosphonate, and flexible reduction of tetra-O-acetyl-α-d-glucopyranosyl bromide with cyanoborohydride Original Research Article

Photo-induced radical addition of acetylated α-d-glucopyranosyl bromide (1) to acrylonitrile or diethyl vinylphosphonate, in the presence of catalytic amounts of tri-n-butyltin chloride and sodium (or tetra-n-butylammonium) cyanoborohydride in excess, allowed efficient preparations of α-configurated nonononitrile and 2-(α-d-glucopyranosyl)-ethylphosphonate (79, 70% yields, respectively). These conditions led to 2-(α-d-manno-, and galactopyranosyl)-ethylphosphonates in 68 and 76% yields. Similarly, radical addition of acetylated 1-bromo-β-d-glucopyranosyl chloride (2) to acrylonitrile or diethyl vinylphosphonate afforded mainly intermediate chlorides which, upon radical reduction with excess tri-n-butyltin hydride, afforded the corresponding β anomers (40 and 38%, respectively) by sequential CC and CH bond formation. Stereocontrol relies on the α-stereoselective quenching of d-glycopyranos-1-yl radicals. We found also that UV light irradiation of 1 with excess NaBH3CN in tert-butanol afforded either 1,3,4,6-tetra-O-acetyl-2-deoxy-α-d-arabino-hexopyranose (65% after crystallization) or, when 10% mol thiophenol was added, 2,3,4,6-tetra-O-acetyl-1,5-anhydro-d-glucitol (79%). These are simple, tin-free, and easily controlled conditions, which compare well with known preparations of these reduced sugars.