An alternative synthesis of 2-(N-arylhydrazono)-1-benzothiophen-3-ones

Abstract

In the presence of triethylamine, 2-mercaptobenzoic acid readily adds onto acylhydrazonoyl chlorides (1a–c) (precursors of the reactive nitrile imine 1,3-dipolar species) to afford good yields of the corresponding 2-[(2-oxo-1-arylhydrazonopropan-1-yl)mercapto]benzoic acids (2a–c). The latter acyclic adducts, in THF in the presence of 1,1′-carbonyldiimidazole, undergo intramolecular cyclization involving the activated carboxy and the enol functionality to deliver the respective 2-(N-arylhydrazono)-3-oxobenzothiophenes (3a–c). In the solid state, the latter compounds adopt the (Z)-geometry around the CN double bond as evidenced by single crystal X-ray structure determination for 3b.