SILYLMETHYLENE RADICAL CYCLIZATION - A STEREOSELECTIVE APPROACH TO BRANCHED SUGARS

Ethyl yl-2,3-dideoxy-alpha-D-erythro-hex-2-enopyranoside (2) was conve rted, in three steps and in 73% overall yield, into ethyl -(hydroxymet hyl)-alpha-D-ribo-hex-2-enopyranoside. This transformation involved si lylation of 2 with (bromomethyl)chlorodimethylsilane and application o f the Nishiyama-Stork radical cyclisation, followed by Tamao oxidation of the sila cycle. Ethyl nzyl-2,3-dideoxy-alpha-D-threo-hex-2-enopyra noside and benzyl 2,6-di-O-benzyl-alpha-L-threo-hex-4-enopyranoside we re similarly transformed into, respectively, ethyl -(hydroxy-methyl)-a lpha-D-lyxo-hex-2-enopyranoside (50%), and benzyl deoxy-4-C-(hydroxyme thyl)-beta-D-galactopyranoside (71%).