T. Shimo et al., SITE-SELECTIVE PHOTOCYCLOADDITIONS OF 2-PYRONES WITH ELECTRON-POOR OLEFINS AND THE DERIVATION FROM THE CYCLOADDUCTS, Journal of heterocyclic chemistry, 30(2), 1993, pp. 419-423
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.