A new synthesis of chloroheterocycles via metal-halogen exchange between trichloroacetyl derivatives and heteroaromatic lithium and Grignard reagents

The reaction between 2-lithio derivatives of aromatic azaheterocycles and trichlorocetyl derivatives rapidly produces the corresponding 2-chloro derivatives in high yields through a metal-halogen exchange mechanism. The use of ethyl trichloroacetate can give better results with respect to those obtained with trichloroacetyl chloride, which probably involves dichloroketene formation. The reaction with Grignard reagents is more complex: in fact, 2-benzothiazolylmagnesium chloride with ethyl trichloroacetate or trichloroacetyl chloride gives 2-chlorobenzothiazole together with considerable amounts of ethyl 1,3-benzothiazole-2-carboxylate or 2-benzothiazolyl dichloromethyl ketone, respectively. (C) 1999 Elsevier Science S.A. All rights reserved.