Synthesis of the C-glycoside of methyl α-D-altropyranosyl-(1→4)-α-D-glucopyranoside

The C-glycoside of methyl alpha-D-altropyranosyl-(1-->4)-alpha-D-glucopyranoside 2 was prepared in a convergent fashion, from readily available precursors, 4-O-tert-butyldiphenylsilyi-1,2-O-isopropylidene-D-erythro-S-phenyl monothiohemiacetal 13 (five steps from D-ribose) and the known acid, methyl 2,3,6-tri-O-benzyl-4-C-(carboxymethyl)-4-deoxy-alpha-D-glucopyranoside 17 (seven steps from methyl alpha-D-glucopyranoside). The key reactions in the synthesis are the oxocarbenium ion cyclization of thioacetal-enol ether 19 to a Cl substituted glycal 20, and the stereoselective hydroboration of 20 to the alpha-C-altroside 21.