Baylis-Hillman acetates undergo a rapid S(N)2' allylic substitution with ethyl (triphenyl phosphoranylidene) acetate under microwave irradiation to afford ethyl 5-aryl or alkyl-(E)-pent-4-enoates in high yields with high (E)-stereoselectivity. The reaction rates and yields are significantly improved by employing microwave irradiation. A comparative study under thermal conditions and microwave irradiation is also described. (c) 2007 Published by Elsevier B.V.