The target thiadiazine dioxide has been obtained by reaction of 7-methoxy--3-(methylsulfanyl)-4H-1,2,4-benzothiadiazine-1,1-dioxide (I) or 3-(1-imidazolyl)-7-methoxy-4H-1,2,4-benzothiadiazine-1,1-dioxide (II) with isopropylamine (III) at 120-150 C in a pressure vessel.

Reaction of 4-fluoroaniline (I) with chlorosulfonyl isocyanate (II) in the presence of aluminum trichloride gives rise to the benzothiadiazine dioxide (III). After conversion of the 3-oxo compound (III) into the corresponding thioxo derivative (IV) by means of phosphorus pentasulfide in pyridine, methylation with iodomethane and NaHCO3 affords the 3-methylsulfanyl benzothiadiazine (V) (1). Finally, the methylsulfanyl group in (V) is displaced by cyclopentylamine (VI) to furnish the target compound (1,2).