Total Synthesis of Pyrrolnitrin. I. Synthesis of 3-Arylpyrrole Derivatives by Knorr's Condensation

Abstract

Alkyl 3-aryl-5-methyl-4-pyrrolecarboxylate (IIIa-j) and alkyl 2, 5-dimethyl-3-aryl-4-pyrrolecarboxylate (IIIk-m), two kinds of key intermediates for synthesizing pyrrolnitrin (I), were obtained from 2-aminoacetophenones (IV) and alkyl acetoacetate by Knorr's pyrrole synthesis. The mechanism of the pyrrole ringclosure of the above compounds was proposed since alkyl 3-substituted benzoylmethylaminocrotonates (VIII) could be isolated as intermediates presumably because of their difficult cyclization to the corresponding pyrroles ; for which the infrared spectra of nitro and carbonyl group of VIII were discussed.