Facile dibromoolefination of lactones using bromomethylenetriphenylphosphorane

Dibromoolefination of lactones was achieved in high yields using bromomethylenetriphenylphosphorane in refluxing tetrahydrofuran. This reaction is believed to proceed via bromination of the monobromophosphorane from the corresponding bromomethyltriphenylphosphonium bromide with concomitant formation of methylenetriphenylphosphorane as shown by 31P NMR. Transylidation should occur to afford the reactive dibromomethylenetriphenylphosphorane.