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

Abstract

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 NaBH 3CN 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.