D. Bhatnagar, Kusum Gupta, Vandana Gupta
1994
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Journal
Heterocyclic Communications
Abstract
One pot synthesis of title compounds is reported by the condensation and oxidative cyclization of 2-aminobenzenethiols with 3-methoxybenzoylacetone in DMSO. The product have been characterised by analytical, IR, NMR and mass spectral data. Introduction 4H-1,4-Benzothiazines resemble structurally to phenothiazines in having a fold along nitrogen-sulphur axis which is one of the structural specificity to impart biological activities to phenothiazines (1). As such 1,4-benzothiazines are anticipated to possess a wide spectrum of biological activities similar to that of phenothiazines. In continuation of our programme to synthesize novel pharmaceutical heterocycles, we report a single step synthesis of title compounds. Results and Discussion Substituted 2-(3-methoxybenzoyl)-3-methyl-4H-l,4-benzothiazines have been synthesized by the condensation and oxidative cyclization of 3and 5-substituted 2-aminobenzenethiols 1 and 3-methoxybenzoylacetone 3_ in dimethylsulphoxide. The reaction is believed to proceed through the formation of an intermediate enaminoketone Under the experimental conditions, 2-aminobenzenethiols are rapidly oxidised to bis-(2-aminophenyl) disulphides 2 (2-5). Disulphides 2 cyclise to 1,4-benzothiazines by scission of sulphur-sulphur bond (2,6) upon attack by nucleophilic enaminoketone system (Scheme 1). 3-Methoxybenzoylacetone can exist in two enolic forms 3̂ and _3' and there is possibility for the formation of two types of benzothiazines. Only CH,-C=CH-C-C,H.-OCH»(m) 3 j I || ο 4 J — OH 0 CH,-C-CH=C-C-Η . -OCH, 3' 3 II I 6 4 3 0 OH