Intermolecular Pauson-Khand reactions of cyclopropene: a general synthesis of cyclopentanones.

[reaction: see text] The Pauson-Khand reaction of cyclopropene with a variety of terminal alkynes has been studied. The best reaction conditions involve NMO activation in CH(2)Cl(2) at -35 degrees C. In this way, 3-substituted-bicyclo[3.1.0]hex-3-en-2-ones have been obtained in good to excellent yields. As a synthetic application, several types of substituted cyclopentenones have been prepared from these cycloadducts by protocols involving conjugate addition and reductive ring opening.