Synthetic Photochemistry. LXXI. Photocycloaddition of Methyl 2,4-Dioxohexanoate to 3-Benzylozy-2-methylpropene

A useful C20 synthon for higher terpenoid syntheses was prepared by photocycloaddition of 3-benzyloxy-2-methylpropene to methyl 2,4-dioxohexanoate. In this combination of two sterically crowded components, a protic solvent, which normally reduces the enol content of B-diketones, unexpectedly made the reaction smooth. It plays probably a catalytic role on the re-enolization of the diketo-isomer which would be accumulated in the relatively slow photo-reaction, C20 synthons, prepared by low-valent chromium salt mediated coupling, we're fully characterized by NMR spectral analysis as well as X-ray crystallographic analysis of the key isomer.