Rearrangement of sulphonium ylides of the 1H-1,4-thiazine type

Abstract

1H-Phenothiazinones 1 bearing benzyl and 3-methylbut-2-enyl substituents on sulphur have been generated. These ylides are not isolable but undergo rearrangement to 2H-phenothiazinones. Migration of the methylbutenyl substituent takes place with allylic inversion at low temperature, consistent with a [3,2] sigmatropic shift mechanism. Benzyl group migration to nitrogen competes with the 1,2-migration. Migration of a 1-(3-methylbut-2-enyl) substituent also takes place in the ylide derived from methyl 3-methyl-1,4-thiazine-2-carboxylate 16: this leads to the isomeric 2H-benzothiazines 17 and 18, the [3,2] sigmatropic shift product 17 predominating at low temperature. Attempts have been made to generate the first monocyclic 1H-1,4-thiazines and evidence is presented for the formation of two such ylides, the 1-ethyl-1H-thiazines 24a and 24b, as isolable but labile solids. An analogous preparation of 1-(3-methylbut-2-enyl)-3,5-diphenyl-1H-1,4-thiazine 25 leads to the isolation of the product of [3,2] rearrangement, the 2H-thiazine 26.