Total Synthesis of Pyrrolnitrin. VIII. A New Method of Pyrrole Ringclosure using Aminoacetal

Abstract

A new method of pyrrole ringclosure using an aminoacetal (V) was investigated. Condensation of 2-nitro-3-chlorophenylpyruvic acid (VIII) and 2-nitro-3-chlorophenylacetone (XVI) with V afforded 3-(2-nitro-3-chlorophenyl) pyrrole (XII) and 1-acetyl-2-methyl-3-(2-nitro-3-chlorophenyl) pyrrole (XX) respectively in an AcOH-AcONa·3H₂O mixture. Cyclization of an enamine (XIV), prepared from ethyl 2-nitro-3-chlorophenylpyruvate (XIII) and V, under anhydrous condition afforded ethyl 3-(2-nitro-3-chlorophenyl)-2-pyrrolecarboxylate (XV). Hantzsch reaction was applied to VIII, which gave lastly XII. The reaction mechanism of these reactions was proposed.