[3+2] Annulation of allylidenetriphenylphosphorane with 1,2-diacylethylenes and 1,2-diacylacetylenes: A one-step synthesis of tri- and tetrasubstituted cyclopentadienes and fulvenes

Allylidenetriphenylphosphoranes underwent [3 + 2] annulation with 1,2-diacylethylenes at room temperature to give cyclopentadienes 11-17 having various substituents except for (Z)-3-acetylacrolein (18) which gave cyclohexadiene 19 derived from [3 + 3] annulation as the sole annulation product. The resulting cyclopentadienes were mixtures of the corresponding double-bond isomers which arose through an equilibrium process from the initially formed 1,3-dienes a under the weakly basic reaction conditions. Similar reactions of the phosphoranes with 1,2-diacylacetylenes afforded substituted fulvenes 24, 25, and 28 directly without the attendant formation of the corresponding benzene derivatives which come from another possible [3 + 3] annulation. The observed high level of the preference for the [3 + 2] annulation is discussed on the basis of the semiempirical PM3 MO calculations. The calculations suggested that the course of the annulation may be kinetically controlled at the oxaphosphetane-forming step of the intramolecular Wittig reaction.