Reaktionen von Allylidenphosphoranen mit Heterokumulenen, II. Neue Synthesen von 1,2,4‐Pentatrienen und 2‐Penten‐1,5‐dionen

Reactions of Allylidenephosphoranes with Heterocumulenes, II. New Syntheses of 1,2,4‐Pentatrienes and 2‐Pentene‐1,5‐diones Cinnamylidenetriphenylphosphorane ( 2a ) undergoes Wittig reaction with ketenes to afford monomeric and dimeric 1,2,4‐pentatrienes 4 and 5 , resp. 4b reacts further with an excess of ketene via a [2 + 2]‐cycloaddition, whereby the 3‐styryl‐1‐naphthyl diphenylacetate 12 is formed. In contrast, with the α‐phenylsubstituted phosphorane 2b , instead of the Wittig reaction addition of ketene takes place at both the α‐ and the γ‐positions. The 1,3‐diacyl‐3‐triphenylphosphonio‐1‐propen‐3‐enolate 6 readily hydrolyzes to the 2‐pentene‐1,5‐dione 9 .