Lithiation-stannylation chemistry for nucleoside synthesis

Three types of anionic migration of the stannyl (or silyl) group have been observed during our lithiation studies on nucleosides. 6-Chloropurine ribos ide (1), which is lithiated exclusively at the 8-position with LDA, gave th e 2-stannylated product upon reacting the lithiated species with Bu3SnCl, a s a result of migration from the 8- to the 2-position (migration within the heterocycle). The stannyl group at the 6-position of 1',2'-unsaturated uri dine (2) undergoes instantaneous migration to the 2'-position, upon treatme nt with LDA (migration from the heterocycle to the sugar). The 5'-O-stannyl group can be transferred to the 3'-position, as a result of an anionic mig ration (migration within the sugar), as illustrated upon introduction into the anti-HIV agent stavudine (3). These three types of stannyl migrations h ave opened up access to nucleoside derivatives that have been difficult to synthesize by a known methodology.