Synthesis of the 3-methyl ether and 4-deoxy derivatives of 4-cyanophenyl 1,5-dithio-β-d-xylopyranoside (Beciparcil)

Treatment of the 2,3-isopropylidene acetal of the title dithioxyloside with 2,4,5-triiodoimidazole–PPh3 caused replacement of the 4-hydroxyl group by iodine to afford 82% of the 4-axial iodide 6, converted by base into 4-cyanophenyl 2,3-O-isopropylidene-1,5-dithio-β-d-glycero-pent-3-enopyranoside (8). Acid treatment of 8 gave 87% of the deacetonated glycos-3-ulose, borohydride reduction of which afforded 63% of 4-cyanophenyl 4-deoxy-1,5-dithio-α-l-threo-pentopyranoside (3), together with 27% of the 3-axial epimer. The 3-methyl ether of the title dithioxyloside was satisfactorily prepared via 2,4-protection as the cyclic phenylboronate, methylation, and deprotection; alternative strategy via the 2,4-bis(triisopropylsilyl) ether was complicated because of silyl-group migration under methylation conditions.