1,2-Benzisothiazoles. Part II. Reactions of 3-chloro-1,2-benzisothiazole with carbanions

Abstract

Pentane-2,4-dione reacts with 3-chloro-1,2-benzisothiazole in the presence of sodium ethoxide to form 2-acetyl-3-aminobenzo[b]thiophen. Under similar conditions, ethyl acetoacetate and diethyl malonate each give mainly ethyl 3-aminobenzo[b]thiophen-2-carboxylate. Under slightly modified conditions diethyl malonate gives, in addition, some of the expected diethyl (1,2-benzisothiazol-3-yl)malonate, whereas ethyl cyanoacetate forms only ethyl (1,2-benzisothiazol-3-yl)cyanoacetate irrespective of conditions. A mechanism is suggested involving S-substituted o-cyanothiophenols as possible intermediates in those reactions which lead to benzo[b]thiophen derivatives. Support for this mechanism comes from the reaction of o-cyanothiophenol in aqueous sodium hydroxide with ethyl bromoacetate, chloroacetone, phenacyl chloride, chloroacetonitrile, or bromonitromethane to give respectively 2-ethoxycarbonyl-, 2-acetyl-, 2-benzoyl-, 2-cyano-, and 2-nitro-3-aminobenzo[b]thiophen.