Brønsted acid promoted synthesis of polysubstituted pyrroles from enamines/imines and diazopyruvates: a metal-free cascade approach

A Brønsted acid-promoted, metal-free cascade reaction of easily available enamines/imines with diazopyruvates has been demonstrated. With triflic acid as the promoter, the reaction proceeds smoothly at room temperature through sequential diazo protonation, nucleophilic enamine N-addition/C-addition and dehydrative aromatisation in a highly regioselective manner. Interestingly, the regioselectivity of the reaction is governed by the unusual enamine N-nucleophilic addition. The method provides an operationally trivial approach to the synthesis of multisubstituted pyrroles including tri-, tetra-, and penta-substituted derivatives as well as N-H free pyrroles with diverse functionalities in good yields under extraordinarily simple and mild conditions. The utility of the method is illustrated by the rapid assembly of polysubstituted bispyrroles and an array of diversely structured pyrrole derivatives.