Photoinduced molecular transformations. Part 145. Regioselective [3 + 2] photoadditions of 2-hydroxyphenanthrene-1,4-dione with electron-rich alkenes and phenylacetylene: new one-step synthesis of 9,10-dihydrophenanthro[2,3-b]furan-7,11-diones and 2-phenylphenanthro[2,3-b]furan-7,11-dione

Abstract

The irradiation of 2-hydroxyphenanthrene-1, 4-dione and various electron-rich alkenes such as ethyl vinyl ether, isobutene, 1-methoxycyclohexene and 3,4-dihydro-2H-pyran in acetone with Pyrexfiltered light gave 9,10-dihydrophenanthro[2,3-b]furan-7,11-diones arising from regioselective [3 + 2] photoaddition in 25–50% yields. A [3 + 2] photoadduct, 9-phenylphenanthro[2,3-b]furan-7,11-dione, can be similarly formed in low yield when 2-hydroxyphenanthrene-1,4-dione and phenylacetylene in acetone are irradiated with Pyrex-filtered light. In contrast, the irradiation of 2-hydroxyphenanthrene-1,4-dione and alkenes such as methyl acrylate, acrylonitrile, cyclohexene, isopropenyl acetate, propenyl acetate, vinyl acetate, 1-phenylprop-1-yne or trimethylsilylacetylene in acetone gave only a dimer in 42–52% yields, no [3 + 2] photoadducts being formed.