Solvent trapping of photochemically generated pyran-4-one-derived oxyallyls: A convenient cyclopentannulation method

Fused bicyclic pyran-4-ones 2a-f are readily available via one-step annulat ion with diketene or diketene equivalents. Irradiation of 2b-e in methanol furnished ring-contracted solvent adducts 4 in moderate yields, whereas irr adiation in aqueous H2SO4 gave the corresponding glycols 8 and 9. The unusu al predominance of cis-fused product 8b appears to result from in situ acid -catalyzed equilibration. Substrate 2f could also be trapped in aqueous aci d, and displayed a modest stereoselectivity that may derive from conformati onal control by the methyl group on the starting material's stereogenic cen ter.