Generation of Chiral Carbanions by Chirality Transfer

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This project was conducted to establish the feasibility of chirality transfer from epoxide to a carbanion. The key feature of the transfer involves our previously-developed epoxysilane rearrangement in which γ-metalated α,β-epoxysilane undergoes anion-induced ring opening followed by Brook rearrangement to give β-siloxy allyl anion. To realize the chirality transfer, we have conducted the following experiments.1.Development of eight-membered carbocycles and heterocycles by Brook rearrangement-mediated [3 + 4] annulation.2.Mechanistic studies of the epoxysilane rearrangement.3.Development of cascade-type ring-forming reactions using the epoxysilane rearrangement.4.Studies on the formation and stability of five-coordinated silicate intermediates.5.Trap of a chiral carbanion generated via the epoxysilane rearrangement by [2,3]-Wittig rearrangement.From these experiments, we obtained the following results.1.We demonstrated the possibility of chirality transfer from epoxide to remote positions in Brook rearrangement-mediated [3+4] annulation2.α,β-Epoxysilanes bearing a γ-tolyl group can function as an acrolein β-anion equivalent.3.Chiral carbanions generated by chiral transfer from epoxide can be trapped by [2,3]-Wittig rearrangement without racemization.4.We have found that chiral nonracemic α-nitrile allylic carbanions can be generated by chirality transfer from epoxysilanes via Brook rearrangement, and they have configurational stability with the aid of chelation by a carbamoyl group to allow trapping before a complete racemization by bimolecular processes.