Synthesis, conformational analysis, and biological activity of C-thioribonucleosides related to tiazofurin

The syntheses of furanthiofurin [5 beta-D-(4'-thioribofuranosyl)furan-3-car boxamide, 1] and thiophenthiofurin [5 beta-D-(4'-thioribofuranosyl)thiophen e-3-carboxamide, 2], two C-thioribonucleoside analogues of tiazofurin, are described. Direct trifluoroacetic acid-catalyzed C-glycosylation of ethyl f uran-3-carboxylate with 1-O-acetyl-2,3,5-tri-O-benzyl-4-thio-D-ribofuranose gave 2- and 5-glycosylated regioisomers, as a mixture of alpha and beta an omers. Ethyl 5-(2,3,5-tri- O-benzyl)-beta-D -(4'-thioribofuranosyl)furan-3- carboxylate (6 beta) was debenzylated and then converted into the correspon ding amide (furanthiofurin) by reaction with ammonium hydroxide. A similar C-glycosylation of ethyl thiophene-3-carboxylate with 1,2,3,5-tetra-O-acety l-4-thio-D-ribofuranose catalyzed by stannic chloride afforded an anomeric mixture of 2- and Ei-glycosylated regioisomers. Deacetylation of ethyl 5-(2 ,3,5-tri-O-acetyl)-beta-D-(4'-thioribofuranosyl)thiophene-3-carboxylate (13 beta) with methanolic ammonia and treatment of the ethyl ester with ammoni um hydroxide gave thiophenthiofurin. The glycosylation site and anomeric co nfiguration were established by H-1 NMR spectroscopy. Thiophenthiofurin was found to be cytotoxic in vitro toward human myelogenous leukemia K562, alb eit 39-fold less than thiophenfurin, while furanthiofurin proved to be inac tive. K562 cells incubated with thiophenthiofurin resulted in inhibition of inosine 5'-monophosphate dehydrogenase (IMPDH) and an increase in IMP pool s with a concurrent decrease in GTP levels. From computational studies it w as deduced that, among the C-nucleoside analogues of tiazofurin, activity r equires an electrophilic sulfur adjacent to the C-glycosidic band and an en ergetically favorable conformer around chi = 0 degrees. Among these, the mo re constrained (least flexible) compounds (tiazofurin and thiophenfurin) ar e more active than the less constrained thiophenthiofurin. Those compounds which contain a nucleophilic oxygen in place of the thiazole or thiophene ( oxazofurin, furanfurin, and furanthiofurin) show the least activity.