Isolation and characterization of a novel tetrahydro‐[2,2′]bipyrrolyl dimer as an impurity from a knorr reaction

A novel dimer, tetraethyl 2,2′,3,3′‐tetramethyl‐1,1′,2,2′‐tetrahydro‐4H,4′H‐2,2′‐bipyrrolyl‐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‐carboxylate 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.