Retro-Hantzsch, mechanism

Rearrangement of 2-hydrazino-4-methylthiazole (291) to 2-amino-5-methyl-l,3,4-thiadiazine (296) in acidic medium has been reported (514. 531). We suggest that this reaction occurs by way of the retro-Hantzsch mechanism (Scheme 175). In this mechanism, a succession of reversible steps leads to the open-chain product (294), which can either revert to the starting product (291) or react further to give the thiadiazine (296). The whole reaction is expected to be reversible through this mechanism. The reaction would proceed from thiadiazine to 2-hydrazinothiazole if the 4-nitrogen atom and the 5-carbon atom of the thiadiazine were both substituted with bulkyl alkyl groups (343, 537). 

Retro-Hantzsch, mechanism of, 84, 85, 102 Rhodanine, alkylation of, 419 ambident reactivity of. 419 reaction with aryldiazonium salts, 419 reaction with halogenothiazoles, 79 Rice cultures, 135, 136, 137 Ring-chain tautomerism, 113 Ring opening, of A-2-thiazoline-5-one, 433 Ring transformation, 5 amino-A-4-thiazo-line-2-thione to 4-mercapto-imidazoline-2-thione, 399 5,5-diphenyl-2,4-thiazolidinedithione, to 4,5-diphenyl-A-4-thiazoline-2-thione, 37 3... 

The amino group is expected to have decreased reactivity in thiazolium salts. 2-Amino-4,5-trimethylene thiazole (224) heated in diluted HQ at 80°C, however, gives the product 227a (463). The probable mechanism is shown in Scheme 139. This mechanism suggests a retro-Hantzsch ... 

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