Functionalized alkylidenecyclopentenes by acid-catalyzed electrocyclic ring closure of (2Z)-(di)vinylallene acetals

Acid-induced electrocyclic ring-closure of (2Z)-4-tert-butyl-3-methyl-2,4,5,7-tetraene acetals 3 afforded a mixture of alkylidenecyclopentene dioxanes Z-4 and E-4. The lack of torquoselective effects on the electrocyclization suggested the transition state structures for the two alternative conrotatory modes to have similar energies. The results of an ab initio study of a model system at the DFT B3LYP/6-31G* level were consistent with this hypothesis. (C) 1997 Elsevier Science Ltd.