4.1.2- Benzothiadiazin-3-carboxylate 4,4-dioxides

Aminobenzenesulfonamides are ring closed directly to AH- 1,2,4-benzothiadiazine 1,1-dioxides (500) with a variety of carboxylic acids, employing polyphosphoric acid trimethylsilyl ester as the cyclization agent (83S851, 85JAP(K)6025984, 90JMC172l). 

The cyclization of the 0/7/fo-substituted aniline 487 with acetic acid and sodium nitrite affords 4,l,2-benzothiadiazin-3-carboxylate 4,4-dioxides 488 via diazonium salt intermediates (Scheme 255) <1996JHC347, CHEC-III(9.08.8.1)429>. 

Ethyl 3,4-dihydro-l//-2,l,3-benzothiadiazine-4-carboxylate 2,2-dioxide (294) is prepared in moderate yield (50%) by intramolecular a-sulfamidoalkylation of A -phenylsulfamide using ethyl 2,2-diethoxyethanoate as the source of the one carbon fragment (Scheme 38) <90JOC6098>. 

Previously, ethyl 1/7-4,l,2-benzothiadiazine-3-carboxylate 4,4-dioxide (222) had been obtained unexpectedly by diazotization of the (2-aminophenyl)sulfonylhydrazone (221) (Equation (13))... 

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