Regioselectivity in the Reaction of 2‐Aminobenzothiazoles and 2‐Aminobenzimidazoles with Enaminonitriles and Enaminones: Synthesis of Functionally Substituted Pyrimido[2,1‐b][1,3]benzothiazole and Pyrimido[1,2‐a]benzimidazole Derivatives

Abstract

A variety of new polyfunctionally substituted benzo[d]pyrimido[2,1‐b][1,3]thiazole and benzo[4,5]imidazo[1,2‐a]pyrimidine derivatives have been synthesized. The general synthetic procedure used for this purpose involves the condensation of 2‐aminobenzothiazole and 2‐aminobenzimidazole with a variety of enaminonitriles, enaminones, and acrylaldehyde. The regioselectivity for initial attack of either endocyclic ring nitrogen or exocyclic amino group was studied and rationalized for. It has been concluded that ring nitrogen is the most reactive cite in acidic medium, and cyclic intermediate can also be isolated, attesting to this conclusion upon cyclization. However, in basic or neutral medium, the exocyclic amino group was found to be the most reactive center. All structures of the newly synthesized compounds were elucidated by elemental analysis, spectral data, and alternative synthetic route whenever possible.