Isolation and characterization of a novel tetrahydro-[2,2 ']bipyrrolyl dimer as an impurity from a Knorr reaction

Abstract

A novel dimer, tetraethyl 2,2',3,3'-tetramethyl-1,1',2,2'-tetrahydro-4H,4'H-2,2'-bipyrrolyi-5,5,5',5'-tetracarboxylate, has been isolated as an impurity (0.4% yield) from a Knorr reaction for the synthesis of ethyl 3,5-dimethylpyrrole-2-carboxyl ate from 2,4-pentanedione and diethyl oximinomalonate in a dissolving zinc reduction. The solid-state structure of this novel dimer was determined by X-ray crystallography. Knorr reactions typically rely upon the requisite pyrrole being the only water-insoluble crystalline material present in the reaction mixture, and so work-up and purification procedures for Knorr reactions should be monitored carefully given the water-insolubility of this dimer. Investigations regarding mechanistic implications and reductive dimerization are underway.