[3-DIACYLETHYLENES AND 1,2-DIACYLACETYLENES - A ONE-STEP SYNTHESIS OFTRISUBSTITUTED AND TETRASUBSTITUTED CYCLOPENTADIENES AND FULVENES(2]ANNULATION OF ALLYLIDENETRIPHENYLPHOSPHORANE WITH 1,2)

Allylidenetriphenylphosphoranes underwent [3 + 2] annulation with 1,2- diacylethylenes at room temperature to give cyclopentadienes 11-17 hav ing various substituents except for (Z)-3-acetylacrolein (18) which ga ve cyclohexadiene 19 derived from [3 + 3] annulation as the sole annul ation product. The resulting cyclopentadienes were mixtures of the cor responding double-bond isomers which arose through an equilibrium proc ess from the initially formed 1,3-dienes a under the weakly basic reac tion conditions. Similar reactions of the phosphoranes with 1,2-diacyl acetylenes afforded substituted fulvenes 24, 25, and 28 directly witho ut the attendant formation of the corresponding benzene derivatives wh ich come from another possible [3 + 3] annulation. The observed high l evel of the preference for the [3 + 2] annulation is discussed on the basis of the semiempirical PM3 MO calculations. The calculations sugge sted that the course of the annulation may be kinetically controlled a t the oxaphosphetane-forming step of the intramolecular Wittig reactio n.