Silylmethylene radical cyclization. A stereoselective approach to branched sugars

Ethyl 6-O-benzyl-2,3-dideoxy-α-d-erythro-hex-2-enopyranoside (2) was converted, in three steps and in 73% overall yield, into ethyl 6-O-benzyl-2,3-dideoxy-3-C-(hydroxymethyl)-α-d-ribo-hex-2-enopyranoside. This transformation involved silylation of 2 with (bromomethyl)chlorodimethylsilane and application of the Nishiyama-Stork radical cyclisation, followed by Tamao oxidation of the sila cycle. Ethyl 6-O-benzyl-2,3-dideoxy-α-d-threo-hex-2-enopyranoside and benzyl 2,6-di-O-benzyl-α-l-threo-hex-4-enopyranoside were similarly transformed into, respectively, ethyl 6-O-benzyl-2,3-dideoxy-3-C-(hydroxymethyl)-α-d-lyxo-hex-2-enopyranoside (50%), and benzyl 2,6-di-O-benzyl-4-deoxy-4-C-(hydroxymethyl)-β-d-galactopyranoside (71%).