A New Synthesis of Polysubstituted Cyclopentadienes using Allylidenephosphorane

A convenient method for the synthesis of substituted cyclopentadienes has been developed using allylidenetriphenylphosphoranes. (3-Ethoxycarbonyl-2-substituted-2-propenylidene)triphenylphosphoranes 1 reacted with ƒ¿-halo carbonyl compounds under 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. The annulation is believed to proceed through a sequence involving a stepwise alkylation at the ƒÁ position of 1 and an intramolecular Wittig reaction. The resulting ethoxy- and ethylthiocyclopentadienes can be converted easily to the corresponding cylopentenones.