Benzothiazole ring, construction

Ortholithiation of thioanilides 81 has been used to construct benzothiazole rings 83 via the benzyne 82 (Scheme 39)". ... 

While studying the chemistry of 4,5-dichloro-l,2,3-dithiazolium chloride (Appel s salt) and its derivatives, Besson reported the synthesis of various benzothiazoles from N-arylimino-l,2,3-dithiazoles, which could be synthesised from commercially available aromatic amines28. In this work, the authors explored a variety of strategies to construct the benzothiazole ring and demonstrated that in all cases the focused microwave methodologies were more productive and under well-defined conditions provided convenient methods for scale-up (Scheme 3.17)28. Comparisons were also made between reactions performed under solvent-free conditions and in the presence of solvent29. It is noteworthy that there is no general rule and some reactions performed in the presence of solvent may sometimes be more convenient than the same dry media reaction. 

The vinylbenzothiazoles can also function as very efficient Michael acceptors (78TL13). The a-lithio derivative of acetone dimethylhydrazone undergoes clean conjugate addition to the vinylbenzothiazole derived from cyclohexanone to provide, upon quenching the a-lithiobenzothiazole anion with methyl iodide and hydrolyzing the dimethylhydrazone group, the ketone (579) (93% overall, 86 14 cis trans). The pure cis keto benzothiazole was converted to the cis keto aldehyde (580), and then cyclized with p-toluenesulfonic acid to the A -3-octalone (581 Scheme 127). Many other examples of the use of this chemistry for the construction of fused and spiro ring systems have been developed. 

It has been found possible to construct the 1,4-benzothiazine ring by direct interaction of bis-(2-aminobenzene) disulfide (96) with carbonyl compounds.139,140 The reaction is most efficient when conducted under a nitrogen atmosphere with a 1 1 ratio of reactants otherwise, the principle products are benzothiazoles. While reduction of the benzothiazine 97 with sodium borohydride gives a stable dihydro derivative, the unsaturated benzothiazines themselves were prone to autoxidation, giving rise to benzothiazoles and benzothiazine sulfoxides.141... 

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