Photolysis of N-(α, β-unsaturated carbonyl)thioamides gave thietane-fused β-lactams in good yields, whereas some of the thioimides formed thiones viaβ-hydrogen abstraction of the thiocarbonyl group. Substituents at the α-position to the carbonyl carbon lead to a preference for [2 + 2 ]cyclisation over β-hydrogen abstraction. From a sensitisation experiment this reaction was shown to proceed via an nπ* triplet excited state.
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Sakamoto et al. (1991) studied this question.