Indolizines .3. The synthesis of 2-vinylindolizines and their reaction with dienophiles

The Wittig olefination of 2-acetylindolizine 2 by methylenetriphenylphosphorane gives 2-isopropenylindolizine 3 with a 99% yield; its reaction with dimethyl acetylenedicarboxylate, maleic anhydride, benzoquinone, dimethyl maleate, dimethyl fumarate, N-ethylmaleimide and 4-methyl-1,2,4-triazoline-3,5-dione gives mainly Michael adducts to the available 3 position of the indolizine, and only low amounts of [2 + 4] cycle-addition products. Reaction of 2 with methoxymethylenetriphenylphosphorane gives 54% of 2-(alpha-methyl-beta-methoxy)vinylindolizine 4; this very electron-rich diene undergoes extensive decomposition with most dienophiles; only dimethyl maleate and N-ethylmaleimide gave identifiable products, Michael adducts and cyclo-adducts, and with low yields.