Abstract Several disubstituted pyrazolo [3, 4-d] pyrimidine, pyrazolo [1, 5-a] pyrimidine and thiazolo [4, 5-d] pyrimidine ribonucleosides have been prepared as congeners of uridine and cytidine. Glycosylation of the trimethylsilyl (TMS) derivative of pyrazolo [3, 4-d]