Hydroxamic acids 3,3 -sigmatropic rearrangements

Previously, Ayyangar and colleagues reported the synthesis of 2-chloroanilide and aniline derivatives by a [3,3]-sigmatropic rearrangement of the adduct obtained by the treatment of hydroxamic acid and hydroxylamine derivatives with thionyl chloride. 

Pratasine. The utilisation of the cyclic hydroxamic acid 230 as a late precursor of pratosine has also been described [62] (Scheme 38). The preparation of 230 was achieved in excellent yield by photocyclisation of the borate ester 231 itself obtained from hydroxamic acid 232. Aqueous hydrolysis of the cyclic borate furnished 230, which underwent a smooth Michael addition to methyl propiolate to form the O-vinyl ester 233. The latter, on thermolysis in wet dimethylsulfoxide provided, via a 3,3-sigmatropic rearrangement, pratosine (204, 17% yield) and methyl pratosine-4-carboxylate (234,35% yield). 

Ibotenic acid undergoes an interesting photoreaction comparable to a Lossen rearrangement of hydroxamic acid (Scheme 12). When irradiated in an aqueous solution with a low-pressure mercury lamp, it is transformed to muscazone with a yield of up to 40%. This occurs most probably via an unstable keto-lactam which then undergoes a [1,3] sigmatropic rearrangement 46). 

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