synthesis of prostaglandin and phytoprostanes B1 is described. The key step is an intermolecular Pauson−Khand reaction between a silyl-protected propargyl acetylene and ethylene. This reaction, promoted by NMO in the presence of 4 Å molecular sieves, afforded the 3-tert-butyldimethylsilyloxymethyl-2-substituted-cyclopent-2-en-1-ones (III) in good yield and with complete regioselectivity. Deprotection