An improved coupling procedure for the Barton-Zard pyrrole synthesis

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