STEREOSELECTIVE SYNTHESIS OF ALPHA,BETA-TRANS-SPIRO-BETA-LACTONES BY DIELS-ALDER CYCLOADDITION OF 1,3-DIENES TO ALPHA-METHYLENE-BETA-LACTONE AND THEIR DECARBOXYLATION BY PYROLYSIS TO (E)-ALKYLIDENECYCLOALKENES, A CONVENIENT OLEFINATION METHOD

A number of spiro-beta-lactones was prepared in good to excellent yiel ds through stereoselective [4 + 2] cycloaddition of beta-isopropyl-alp ha-methylene-beta-lactone (1) with acyclic, cyclic, heterocyclic and a romatic 1,3-dienes by sealed-tube reaction at moderate temperatures (5 0 - 130-degrees-C). Flash pyrolysis of the resulting spiro-beta-lacton es 2 in the gas phase at 400-degrees-C afforded by decarboxylation exc lusively the corresponding (E)-isopropylidenealkenes in high yields wi th retention of the initial geometry and without double bond isomeriza tion. This olefination method constitutes an excellent stereoselective synthesis of (E)-alkylidenecycloalkenes.