The Synthesis of Some 5-Alkyl (Cycloalkyl)-Substituted 2′ -Deoxy-4′-Thiouridines

The silylated pyrimidine bases IIa-d were condensed with the benzyl 3,5-di-O-benzyl-2-deoxy-1,4-dithio-d-erythro-pentofuranoside III in acetonitrile under activation by N-iodosuccinimide, giving ca 1.5: 1/α: β anomeric mixtures of the blocked nucleosides IVa-d and Va-d. in yields of 55–58%. After the separation on a silica column the pure anomers were deprotected by BCI3 or TiCI4, providing the free nucleosides VIa-d and VIIa,c,d in moderate to good overall yields. The β- or α-anomeric configuration, anti-glycosidic conformation and prevailing C2′endo(S) thiosugar pucker in the synthesized compounds were established by the combined use of the 1H, 13C NMR and X-ray crystallography. Dedicated to Dr. Yoshihisa Mizuno on the occasion of his 75th birthday