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Photochemical Synthesis of 3,4-Dihydro-2H-1,3-oxazin-4-ones

The first example of a C-O bond formation in the course of the Norrish-Yang reaction is described. Starting with readily accessible α-mesyloxy-β-keto amides 4, a δ-hydrogen transfer to the excited carbonyl group occurs and the diradicals thus formed undergo a very rapid elimination of methane sulfonic acid providing enolate diradicals. These ambidental enolate diradicals undergo a regioselective cyclization to 3,4-dihydro-2H-1,3-oxazin-4-ones 5. In two cases a cleavage reaction is observed, giving cyclic imines. The mechanism is investigated by means of DFT- and ab initio methods.