Amine-Catalyzed Cross-Aldol Condensations of Enolizable Aldehydes via Enol Ethers: Experimental Support for the Mannich Addition–Elimination Pathway

Alkyl enol ethers are introduced as efficient pronucleophiles in amino quaternary phosphonium salt (AQPS) catalyzed cross-aldol condensation of two enolizable aldehydes under metal-free and additive-free conditions. The successful coupling of methyl enol ethers with aldehydes provides strong experimental support for the Mannich addition-elimination pathway in amine-catalyzed aldol condensation. This strategy offers a practical solution to the long-standing challenge of chemoselective cross-aldol condensation between two enolizable aldehydes, with the aldehyde-derived enol ethers effectively suppressing competing self-condensation.