Microwave-accelerated synthesis of benzyl 3,5-dimethyl-pyrrole-2-carboxylate

Abstract

Benzyl 3,5-dimethyl-pyrrole-2-carboxylate, a very useful pyrrole in porphyrin and dipyrromethene synthesis, can be synthesized via the Knorr-type reaction, but in low yield. Alternative routes to benzyl 3,5-dimethyl-pyrrole-2-carboxylate have been developed involving the trans-esterification of ethyl 3,5dimethyl-pyrrole-2-carboxylate and the de-acetylation of benzyl 4-acetyl-3,5-dimethyl-2-carboxylate, both precursors being easily obtained using the Knorr reaction. These traditional methods involve treatment of the known products with a strong basic solution or heating for extended periods which often lead to decomposition. The use of microwave energy to promote these two reactions proves to be an extremely efficient way to obtain benzyl 3,5 -dimethyl-pyrrole-2-carboxylate quickly, in high yield, and in excellent purity with no need for recrystallization. Of particular note is the use of catalytic sodium methoxide in benzyl alcohol, rather than stoichiometric amounts of sodium benzoxide, to effect benzylation.