Reaction of ethyl arylhydrazonochloroacetates (1) with 2-aminothiophenol (2) in ethanol in the presence of triethylamine yielded the respective ethyl thiohydrazonate esters (3). Similarly, methyl arylhydrazonochloroacetates (6) gave the corresponding methyl thiohydrazonate esters (7). Treatment of both 3 and 7 with hydrogen chloride in ethanol afforded the respective 1,4-benzothiazine derivatives 4. Identical products (4) were obtained by refluxing 1 or 6 in ethanol in the presence of triethylamine. The structure of 4 was confirmed by their alternate synthesis starting with diethyl chloromalonate in ethanol in the presence of triethylamine which yielded the intermediate 1,4-benzothiazine derivatives 8. The subsequent coupling of 8 with diazotized anilines in ethanol in the presence of potassium hydroxide afforded 4. J. Heterocyclic Chem., 40, 207 (2003).Within the framework of our interest in regioselectivity in cyclization reactions of α-oxohydrazonoyl halides with 1,2-aminothiols [1-4] and azo-hydrazone tautomerism of arylazo derivatives of heterocyclic compounds [5-8], we wish to report the results of our study of the reactions of 2-aminothiophenol (2) with a series of ethyl N-arylhydrazono-2-chloroacetates (1) and elucidation of the tautomeric structures of the respective products. At present, there are two contradicting reports in the literature concerning the structure of the products of such reactions. It has been reported that the reaction of 2 with 1 (X = 4-NO 2 ) afforded 2,3-dioxo-1,4-benzothiazine-2-(4-nitrophenylhydrazone) 4i via elimination of ethanol from the initially formed thiohydrazonate ester 3 [9]. However, it was also claimed [10] that a similar reaction of 2 with 1 (X = 4-Me) yielded a product that was assigned the structure of 2-ethoxycarbonyl-4-(4-methylphenyl)-1,3,4-benzothiadiazine 5, obtained by elimination of ammonia from the intermediate 3 (Scheme 1). The latter assignment seems ambiguous, because it involves nucleophilic aliphatic displacement of an amino group from an aromatic amine residue. This significant difference between the reaction products 4i and 5 prompted us to re-examine the reactions of 2 with a series of hydrazonoyl chlorides (1a-1i) and to study the effect of the ring substituent upon the regioselectivity in the cyclization reaction of thiohydrazonate esters of type 3 (Scheme 2). Also, we have investigated the synthesis of the title compounds 4 by other methods starting from 2 to confirm their structures (Scheme 2). Furthermore, as the title compounds can have more than one tautomeric structure (Chart 1), it was of interest to elucidate their tautomeric structure. For this purpose, the 15 N isotopomer of 4d (4d') was prepared as a typical example of the title compounds, and its 1 H and 15 N nmr spectra were examined. Because only the hydrazone form A can exist with a hydrogen attached to the 15 N, a splitting of the proton resonance as well as of the 15 N resonance would be a conclusive proof of the existence of the hydrazone form. Finally, the electronic absorption ...
