Stereochemistry of carbanion reactions at sp³-carbon

The stereochemistry (inversion vs. retention) of reactions at sp³-carbanion centers is one of the pressing problems in organic chemistry. This review describes our recent studies on the stereochemistry of carbanion reactions using enantiomerically defined α-(alkoxy) organolithiums generated from the enantio-enriched stannanes via Sn/Li exchange: (1) A new practical method for asymmetric synthesis of α-hydroxystannanes is described. (2) The [2, 3]-Wittig rearrangement was shown to proceed with complete inversion of configuration at the Li-bearing migrating terminus. (3) The carbolithiative cyclization is proved to proceed with complete retention of configuration at the Li-bearing carbon. (4) The [1, 2]-Wittig rearrangement is shown to proceed predominantly with inversion of configuration at the Li-bearing terminus and retention at the migrating center, wherein a significant level of mutual enantiomer recognition is observed in the radical recombination process.