THE SYNTHESIS OF SOME 5-ALKYL (CYCLOALKYL)SUBSTITUTED 2'-DEOXY-4'-THIOURIDINES

The silylated pyrimidine bases IIa-d were condensed with the benzyl en zyl-2-deoxy-1,4-ditkio-D-erythro-pentofuranoside III in acetonitrile u nder activation by N-iodosuccinimide, giving ca 1.5 : 1/alpha: beta an omeric mixtures of the blocked nucleosides IVa-d and Va-d. in yields o f 55-88%. After the separation on a silica column the pure anomers wer e deprotected by BCl3 or TiCl4, providing the free nucleosides VIa-d a nd VIIa,c,d in moderate to good overall yields. The beta- or alpha-ano meric configuration, anti-glycosidic conformation and prevailing C2'-e ndo(S) thiosugar pucker in the synthesized compounds were established by the combined use of the H-1, C-13 NMR and X-ray crystallography.