penylidene)triphenylphosphorane reacted with diacylethylenes to give 2-ethoxycyclopentadienes as a 1:1 mixture of the 1,3- and 1,4-dienes, which were converted upon mild acid treatment into cyclopentenones in a single form. Use of acylmethylenemalonate as an unsymmetrical substrate provides an excellent route to substituted cyclopentenones. An application of the annulation to synthesis of (±)-methyl