Microwave-assisted regioselective olefinations of cyclic mono- and di-ketones with a stabilized phosphorus ylide

A number of cyclic mono- and di-ketones underwent regioselective olefination with (carbethoxycthylidene)triphenylphospharane under controlled microwave heating. The Wittig reaction of 4-substituted cyclohexanones or 1,2- and 1,4-cyclohexanediones with the ylide at 190 degrees C for 20 min in MeCN afforded the exocyclic olefins in > 94:6 isomer ratios. On the other hand, the same reactions carried out at 230 degrees C for 20 min in the presence of 20 mol % DBU furnished the endocyclic olefins in > 83:17 isomer ratios. The base-mediated isomerization of the exocyclic olefins into the endocyclic isomers was primarily driven by thermodynamic stability of the products and the effect of ring structures on deconjugation was examined. (c) 2006 Elsevier Ltd. All rights reserved.