[3 + 2]-Annulation using allylidene(triphenyl)phosphoranes: a one-step synthesis of cyclopentadienes
Abstract
A convenient method for the synthesis of substituted cyclopentadienes has been developed using allylidene(triphenyl)phosphoranes. (3-Ethoxycarbonyl-2-substituted-2-propenylidene)triphenylphosphoranes 1 reacted with α-halogenoketones under very mild reaction conditions to undergo a [3 + 2]-annulation leading to the regioselective formation of tri- or tetra-substituted cyclopentadienes in good to excellent yields. Allylidenephosphoranes 1 also reacted with S-ethyl bromoethanethioate to yield 4-ethylthiocyclopentadienes, which was readily converted into a cyclopentenone.