Alkaloid synthesis via [3+2] cycloadditions

Abstract

Tin-lithium exchange on (2-azaallyl)stannanes affords nonstabilized 2-azaallyllithiums (2-azaallyl anions) that undergo [π4s+π2s] cycloadditions with alkenes to afford pyrrolidines. The scope of this method has been expanded to include 2-azapentadienyllithiums and heteroatom-substituted 2-azaallyllithiums. Alternatively, the (2-azaallyl)stannanes may also be used to generate nonstabilized azomethine ylides via N-alkylation/destannylation or N-protonation/destannylation, and these ylides were also found to be useful for the synthesis of pyrrolidines by [π4s+π2s] cycloadditions with alkenes. Applications of both methods to the total synthesis of alkaloids such as (+)-coccinine, lepadiformine stereoisomers, lapidilectine B, and indolizidine 239CD have been accomplished.