4'-Thionucleosides via in situ pyranose-furanose rearrangements: A short synthesis of the antiherpes agent 2'-deoxy-5-ethyl-4'-thiouridine via direct coupling of a silylated pyrimidine base with a 4-thiopyranose sugar

Article abstract of DOI:10.1021/jo00121a039

Methyl 2-deoxy-3,4-O-thiocarbonyl-β-D-ribopyranoside was converted into the thione carbonate by reaction with thiophosgene. Bromide ion-catalyzed O-S rearrangement produced methyl 2-deoxy-3-O,4-S-carbonyl-4-thio-β-D- ribopyranoside (3a) and the 3-O,4-S isomer 3b. The carbonates were cleaved with ammonia and the 3-O,4-S pyranoside sugar coupled with bis(trimethylsilyl)-5-ethyluracil using trimethylsilyl triflate to provide the antiherpes agenet 2'-deoxy-5-ethyl-4'-thiouridine 9. The reaction proceeded via in situ pyranoside rearrangement of the sugar and subsequent coupling. The pyranoside sugar could also be converted to the furanoside form with Dowes H⁺ acid resin and coupled in conventional fashion to give the nucleoside. Coupling of methyl 4-O-carbamoyl-2-deoxy-3-thio-β-D- ribopyranoside (4b) with the bis(trimethylsilyl)-5-ethyluracil gave 1-[2-[2- (hydroxymethyl)thiiran-1-yl]-1-methoxyethyl]-5-ethyluracil.