Condensed 1,3-benzothiazinones. 2. Synthesis of 2-substituted 1,2,4-triazolo[5,1-b][1,3]benzothiazin-9-ones

Heating 3-amino-2-imino-3,4-dihydro-2H-1,3-benzothiazin-4-one (1) with formic acid neat or with trimethyl orthoformate ( 2a ) in xylene gave 1,2,4-triazolo5,1-b]1,3]benzothiazin-9-one ( 3a ) in 49 and 81% yields, respectively. An analogous reaction of 1 with triethyl orthoacetate ( 2b ) and orthopropionate ( 2c ), trifluoro-acetic and trichloroacetic anhydride ( 2d,e ), trichloroacetonitrile, dichloroacetyl, chloroacetyl, 2-chloro-propionyl, 3-chloropropionyl, ethoxyoxalyl chloride ( 2f-j ) and benzoyl chloride ( 2k ) under suitable conditions afforded a series of 2-substituted 1,2,4-triazolo5,1-b]1,3]benzothiazin-9-ones (3b-k) in 59-98% yields. Facile procedures for the preparation of 2-hydroxy and 2-mercapto derivative 31,m were developed by treating 1 with N,N′-carbonyl- and thiocarbonyldiimidazole (21,m) . The structures of all products were assigned on the bases of spectral and elemental analyses. Compound 3m was found to exist predominantly in the 2-thioxo form.