An improved coupling procedure for the Barton-Zard pyrrole synthesis

An improved final step in the Barton-Zard pyrrole synthesis uses inexpensiv e potassium carbonate as base in the coupling-cyclization reaction of vic-n itro-acetates with isocyanides. In this modification the isolated yields of synthetically useful 2-carboalkoxypyrroles (1a,b and 3) and 2-(p-toluenesu lfonyl)pyrroles (2a,b) consistently rise to the 78-89% range. Conversion of 2a to 5-(p-toluenesulfonyl)-2-pyrrolinone 4 is conveniently and directly a chieved by reaction with 30% hydrogen peroxide in acetic acid, thus circumv enting the commonly used two step procedure involving bromination followed by solvolysis.