Chlorination of 2 H-1, 2, 3-benzothiadiazine 1, 1-dioxide (1) in wet methylene chloride gives o-formylbenzenesulfonyl chloride (4), but in dry methylene chloride at 0° yields the chlorosultine 3-chloro-3 H-2, 1-benzoxathiole 1-oxide (2); the latter is readily hydrolyzed to the" pseudo-acid" 3 H-2, 1-benzoxathiol-3-ol 1-oxide (3) which on further chlorination forms 4. In 1, 2-dichloroethane at-20° chlorination of 1 gave the" coupled product" 14, whereas ...