Catalytic γ‐Position‐Selective Reactions of (2‐Oxopropyl)phosphonates with Aldehydes to Afford γ,δ‐Unsaturated β‐Ketophosphonates

Catalytic formal aldol condensation reactions between β‐ketophosphonates and arylaldehydes, in which the C−C double bond formation occurs at the γ‐position of the β‐ketophosphonate, are reported. The reactions were performed in the presence of a catalyst system composed of p‐anisidine and trifluoroacetic acid under mild conditions. Mechanistic investigations suggest that a Mannich reaction followed by elimination results in the formation of the γ,δ‐unsaturated β‐ketophosphonate. Although the β‐ketophosphonate has an active methylene group, the bond formation occurred selectively at the γ‐position of the β‐ketophosphonate.