Dihydro thiazinones

A number of oxidative ring contractions of dihydrothiazines are known. For example, when a cyclohexane solution of the compound 117 was exposed to the air at room temperature, a 1 1 mixture of the derivatives 186 and 187 was produced, probably by way of the hydroperoxide 185. As already indicated (Section V,C,l,a), the product of the reaction of the dihydro-thiazinone 97 with hydrogen peroxide and acetic acid was the thiazoline 136 other examples of this oxidative ring contraction have been de-... 

Takamizawa et al. developed a general ring-expansion reaction of heterocycles that, applied to thiazolium salts, yields 1,4-thiazines (496, 497) thiamine (220) reacts with dialkyl acylphosphonates (221) to give the tricyclic 1,4-thiazine (222) (498), which is easily hydrolyzed to dihydro-1,4-thiazinone (223) (499) (Scheme 106). In the case of thiazolium slats containing no functional groups (224), 1,4-thiazine derivatives (226) were directly obtained in fairly good yields (Scheme 107). 

Pyridooxazines (92) react with NaOMe to give pyridine-3-alkylcarboxamide derivatives (85PJS297). Reduction of pyrido[2,3-6]thiazinone with NaBH4 gives the 2,3-dihydro derivative <89T4153>. 

A similar approach with pyridazine and 3-methylpyridazine, but using 1-trichloroethoxycarbonyl derivatives, allows the nucleophilic addition of thiazolones and 1,4-thiazinones (Scheme 16). The initial 1,4-dihydro adducts are oxidised and deacylated by treatment with sulfur in boiling DMF <87CPB2243, 90CPB45>. 

Dihydro-1,3-thiazines, dihydro-1,3-thiazinones, 2 i-l,3-benzothiazines, 4H-7,5-benzothiazines, and related compounds... 

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