Stereoselective Synthesis of α,β‐ trans 1‐Spiro‐β1‐lactones by Diels‐Alder Cycloaddition of 1,3‐Dienes to α1‐Methylene‐β1‐lactone and Their Decarboxylation by Pyrolysis to ( E )1‐Alkylidenecycloalkenes, a Convenient Olefination Method

A number of spiro‐β1‐lactones was prepared in good to excellent yields through stereoselective [4 + 2] cycloaddition of β1‐isopropyl‐α1‐methylene‐β1‐lactone ( 1 ) with acyclic, cyclic, heterocyclic and aromatic 1,3‐dienes by sealed‐tube reaction at moderate temperatures (50 – 130°C). Flash pyrolysis of the resulting spiro‐β1‐lactones 2 in the gas phase at 400°C afforded by decarboxylation exclusively the corresponding ( E )1‐isopropylidenealkenes in high yields with retention of the initial geometry and without double bond isomerization. This olefination method constitutes an excellent stereoselective synthesis of ( E )1‐alkylidenecycloalkenes.