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[3 + 2] Annulation of Allylidenetriphenylphosphorane with 1,2-Diacylethylenes and 1,2-Diacylacetylenes:  A One-Step Synthesis of Tri- and Tetrasubstituted Cyclopentadienes and Fulvenes

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journal contribution
posted on 19.09.1997, 00:00 by Yuichiro Himeda, Hiroshi Yamataka, Ikuo Ueda, Minoru Hatanaka
Allylidenetriphenylphosphoranes underwent [3 + 2] annulation with 1,2-diacylethylenes at room temperature to give cyclopentadienes 1117 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.