Zur Photochemie von 2, 1‐Benzisoxazolen (Anthranilen) und thermischen und photochemischen Umsetzungen von 2‐Azido‐acylbenzolen in stark saurer Lösung

On the Photochemistry of 2, 1‐Benzisoxazoles (Anthraniles) and on the Thermal and Photochemical Decomposition of 2‐Azido‐acylbenzenes in Strongly Acidic Solution Anthranils 6 (Scheme 3) , when irradiated with a mercury high‐pressure lamp, in 96% sulfuric acid yielded, after work‐up, 2‐amino‐5‐hydroxy‐acylbenzenes 8 and as side products 2‐amino‐3‐hydroxy‐acylbenzenes 9 (cf. Schemes 5–7 and Table 1) . When C(5) of the anthranils 6 carries a methyl group a more complex reaction mixture is found after irradiation in 96% sulfuric acid (cf. Schemes 8 and 9) : 3, 5‐dimethyl‐anthranil ( 6d ) yielded (after irradiation and acetylation) 2‐acetyl‐ amino‐5‐methyl‐acetophenone ( 15 ), 2‐acetylamino‐5‐acetoxymethyl‐acetophenone ( 18d ) and 2‐acetylamino‐5‐acetoxy‐6‐methyl‐acetophenone ( 12c ). The latter product was also formed after irradiation of 3, 4‐dimethylanthranil ( 6c ) in 96% sulfuric acid. 3, 5, 7‐Trimethyl‐anthranil ( 6f ) formed under the same conditions 2‐acetylamino‐3, 5‐dimethyl‐acetophenone ( 15f ) and 2‐acetylamino‐5‐acetoxymethyl‐3‐methyl‐acetophenone ( 18f ). Since qualitatively the same product patterns were observed when the corresponding 2‐azido‐acetophenones 7 were decomposed in 96% sulfuric acid it is concluded that anthranilium ions (cf. 6b ‐H⊕, Scheme 11) on irradiation are transformed by cleavage of the N , O ‐bond into 2‐acyl‐phenylnitrenium ions (cf . 25b ‐H⊕) in the singlet ground state. The nitrenium ions are trapped directly by nucleophiles ( HSO ⊖ 4 in 96% sulfuric acid), thus, yielding the hydroxy‐acetophenones 8 and 9 (Scheme 11) . If C(5) is blocked by a methyl group a [1, 2]‐rearrangement of the methyl group may occur (cf. Scheme 13) or loss of sulfuric acid can lead to quinomethane iminium ions (cf . 32‐H⊕ , Scheme 13) which will react with HSO ⊖ 4 ions to yield, after hydrolysis and acetylation, the 5‐acetoxymethyl substituted acetophenones 18d and 18f . It is assumed that the reduction products (2‐acetylamino‐acetophenones 15 ) are formed from the corresponding nitrenium ions in the triplet ground state.