SITE-SELECTIVE PHOTOCYCLOADDITIONS OF 2-PYRONES WITH ELECTRON-POOR OLEFINS AND THE DERIVATION FROM THE CYCLOADDUCTS

Photosensitized cycloaddition of 4,6-dimethyl-2-pyrone (1) with methac rylonitrile (3b) afforded two types of [2 + 2]cycloadducts, 4b and 6b, across the C5-C6 and C3-C4 double bonds in 1, respectively. Photosens itized reactions of 1 with dimethyl maleate and dimethyl cyclobutene-1 ,2-dicarboxylate gave [2 + 2]cycloadducts 4d, 4e across the C5-C6 doub le bond, in addition to [4 + 2]cycloadduct 9d or bicyclo[4.2.0]octadie ne 10e. The photoreactions of methyl 2-pyrone-5-carboxylate (2) with 3 b and 2-chloroacrylonitrile (3c) gave [4 + 2]cycloadducts 5b, 5c in ad dition to [2 + 2]cycloadducts 11b and 11c across the C5-C6 double bond in 2. The photocycloaddition mechanism was explained from results cal culated by means of PM3-CI method. Namely, the site- and/or regio-sele ctive products, 4, 5, 8, 9 and 10 were thought to come from the same s ite-selective radical intermediates in the case of electron-poor olefi ns. Pyrolysis and/or hydrolysis of the cycloadducts 4e, 5b, 5c gave 5, 6-dihydro-2-pyrone 12 or benzene derivatives.