Photo-Cycloaddition Reactions of 2-Pyrones

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Abstract
Sensitized photoreactions of two 2-pyrones with five unsaturated compounds were investigated. The reactions of 2-pyrones with electron-deficient ethylenes gave [4+2]cycloadducts (3) and [2+2]cycloadducts (4) across the C5–C6 double bonds in the 2-pyrones. On the other hand, reactions with an electron-donating ethylene, ethyl vinyl ether (2i), gave other kinds of [2+2]cycloadducts (5) and a [4+2]cycloadduct (6), site- and regio-selectively. These two kinds of reactions were inferred to start at the C-6 and C-4 positions of the 2-pyrones to give 3 or 4, and 5 or 6, respectively. The regioselectivity of 6 was opposite to that for a thermal [4+2]cycloadduct (7). The substituents on 2-pyrones also influenced the product-distribution and the triplet energy-level.