Synthesis of 2′-O,4′-C-methyleneuridine and -cytidine. Novel bicyclic nucleosides having a fixed C3, -endo sugar puckering

2′-O,4′-C-Methyleneuridine and -cytidine, novel bicyclic nucleoside analogs having a typical C3′-endo sugar puckering, were synthesized starting from uridine via a several-step sequence.