[3-ANNULATION USING ALLYLIDENE(TRIPHENYL)PHOSPHORANES - A ONE-STEP SYNTHESIS OF CYCLOPENTADIENES(2])

A convenient method for the synthesis of substituted cyclopentadienes has been developed using allylidene(triphenyl)phosphoranes. -substitut ed-2-propenylidene)triphenylphosphoranes 1 reacted with alpha-halogeno ketones under very mild reaction conditions to undergo a [3 + 2]-annul ation leading to the regioselective formation of tri- or tetra-substit uted cyclopentadienes in good to excellent yields. Allylidenephosphora nes 1 also reacted with S-ethyl bromoethanethioate to yield 4-ethylthi ocyclopentadienes, which was readily converted into a cyclopentenone.