Zur Photochemie von 1, 2‐Benzisoxazolen in stark saurer Lösung

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Abstract
On the Photochemistry of 1, 2‐Benzisoxazoles in Strongly Acidic SolutionThe 1, 2‐benzisoxazoles 1a, 1b and 1d when dissolved in 96% sulfuric acid and irradiated through a quartz filter with a mercury high‐presure lamp yield, after work‐up, mixtures of 2, 5‐ and 2, 3‐dihydroxy‐acylbenzenes (2 and 3, respectively; cf. Schemes 1 and 3 and Table 1). Irradiation of 3, 5‐dimethyl‐1, 2‐benzisoxazole (1c) in 96% sulfuric acid leads to the formation of 2, 3‐dihydroxy‐5‐methyl‐acetophenone (3c) in only 6% yield (cf. Table 1). It is assumed that the 1, 2‐benzisoxazolium ions react in the excited singlet state by heterolytic cleavage of the N, O‐bond to yield the corresponding aryl oxenium ions 7 in the singlet ground state (see Scheme 5). Reaction of 7 with HSO4⊖ ions, present in 96% sulfuric acid, yields, after hydrolysis, the dihydroxy compounds 2 and 3. Photolysis of 3‐methyl‐1, 2‐benzisoxazole (1b) in diluted sulfuric acid (0,5 to 9 M) in methanol or water leads only to the formation of 2‐amino‐phenol (6; see Scheme 3), presumable via photo‐isomerization of 1b to 2‐methylbenzoxazole (5b) which then is hydrolyzed to give 6.