STEREOCHEMISTRY OF CARBANION REACTIONS AT SP(3)-CARBON

The stereochemistry (inversion vs. retention) of reactions at sp3-carb anion centers is one of the pressing problems in organic chemistry. Th is review describes our recent studies on the stereochemistry of carba nion reactions using enantiomerically defined alpha-(alkoxy) organolit hiums generated from the enantio-enriched stannanes via Sn/Li exchange : (1) A new practical method for asymmetric synthesis of alpha-hydroxy stannanes is described. (2) The [2,3]-Wittig rearrangement was shown t o proceed with complete inversion of configuration at the Li-bearing m igrating terminus. (3) The carbolithiative cyclization is proved to pr oceed with complete retention of configuration at the Li-bearing carbo n. (4) The [1,2]-Wittig rearrangement is shown to proceed predominantl y with inversion of configuration at the Li-bearing terminus and reten tion at the migrating center, wherein a significant level of mutual en antiomer recognition is observed in the radical recombination process.