NEIGHBORING GROUP EFFECTS IN INTRAMOLECULAR ALDOL CONDENSATIONS - RATE ENHANCEMENT BY GAMMA-SUBSTITUENT

The O-acetyl dieneol Ib, obtained from the Grignard addition of 2-(2-b romoethyl)-1,3-dioxolane to 2,4-hexadienal via 1a, and followed by an acetylation, was treated with maleic anhydride to give the crystalline Diels-Alder adduct 2b with good stereoselectivity. The acetal of 2b w as selectively hydrolyzed to the acetoxy aldehyde 3b. This was treated with potassium hydride in tetrahydrofuran. A stereoselective intramol ecular Aldol reaction resulted in the formation of the tricyclic lacto ne 4b, However, attempts to achieve an Aldol condensation under simila r reaction conditions with the analogous aldehyde 3c, lacking the acet oxy group, failed, giving back the starting material 3c, On the other hand, the methoxy aldehyde Sd was cyclized to the tricyclic compound 4 d.