Selective Syntheses of Δα,β and Δβ,γ Butenolides from Allylic Cyclopropenecarboxylates via Tandem Ring Expansion/[3,3]-Sigmatropic Rearrangements

Allylic cyclopropenecarboxylates undergo ring expansion reactions to give 2-allyloxyfuran intermediates, which subsequently rearrange to Δβ,γ butenolides via a Claisen rearrangement or to the corresponding Δα,β butenolides via further Cope rearrangement. Also described are methods for chirality transfer in the rearrangement of nonracemic allylic esters.