Photoadducts of 4,6-Dimethyl- and 4-Methoxy-6-methyl-2-pyrones with Chloroethylenes and Their Dehydrochlorination

Article abstract of DOI:10.1246/bcsj.60.621

Sensitized photoreactions of 2-pyrones with chloroethylenes gave two kinds of <2+2>cycloadducts, 2-oxabicyclo<4.2.0>oct-4-en-3-ones (4,8) and across the C₃-C₄ double bond in 2-pyrone, benzene derivatives, and/or cyclobutanecarboxylic acids: Their relative yields greatly depended upon the nature of the reactants.The dehydrochlorination of the cycloadducts 4 and 8 across the C₅-C₆ double bonds in the pyrones with triethylamine in ethanol gave 5-ethenyl-2-pyrones, whose structures were previously incorrectly assigned as oxocinones, via a symmetry-allowed ring opening of 2-oxabicyclo<4.2.0>octa-4,6-dien-3-ones.Some 5-ethenyl-2-pyrones possessing a chlorinated ethenyl group finally affords 5-ethynyl-2-pyrones by an anti-elimination of hydrogen chloride.The X-ray crystallographic analyses of 5-ethenyl- and 5-ethynyl-2-pyrones are also described.