The sulfonation of 2,5-dichlorothiophene (I) with ClSO3H -SOCl2 gives 2,5-dichlorothiophene-3-sulfonic acid chloride (II), which by reaction with methylamine in CHCl3 yields the corresponding methylamide (III). The carboxylation of (III) with butyllithium and CO2 in ether affords 5-chloro-3-(N-methylsulfamoyl)thiophene-2-carboxylic acid (IV), which is esterified with PCl5 and methanol to the methyl ester (V). The condensation of (V) with methyl iodoacetate (VI) by means of NaH in DMF gives 5-chloro-3-[N-(methoxycarbonylmethyl)-N-methylsulfamoyl]thiophene-2-carboxylic acid methyl ester (VII), which is cyclized with sodium methoxide in methanol yielding 6-chloro-4-hydroxy-2-methyl-2H-thieno[2,3-e]-1,2-thiazine-3-carboxylic acid methyl ester 1,1-dioxide (VIII). Finally, this compound is treated with 2-aminopyridine (IX) in refluxing xylene.

The sulfonation of 2,5-dichlorothiophene (I) with ClSO3H -SOCl2 gives 2,5-dichlorothiophene-3-sulfonic acid chloride (II), which by reaction with methylamine in CHCl3 yields the corresponding methylamide (III). The carboxylation of (III) with butyllithium and CO2 in ether affords 5-chloro-3-(N-methylsulfamoyl)thiophene-2-carboxylic acid (IV), which is esterified with PCl5 and methanol to the methyl ester (V). The condensation of (V) with methyl iodoacetate (VI) by means of NaH in DMF gives 5-chloro-3-[N-(methoxycarbonylmethyl)-N-methylsulfamoyl]thiophene-2-carboxylic acid methyl ester (VII), which is cyclized with sodium methoxide in methanol yielding 6-chloro-4-hydroxy-2-methyl-2H-thieno[2,3-e]-1,2-thiazine-3-carboxylic acid methyl ester 1,1-dioxide (VIII). Finally, this compound is treated with 2-aminopyridine (IX) in refluxing xylene.