A new procedure for the reduction of α,β-unsaturated pyrrolidinones to 2H-pyrroles and 1H-pyrroles based on initial activation by N-nitrosation

Abstract

A new two step procedure is developed for the half-reduction of lactams to cyclic imines and enamines. N-Nitrosation using dinitrogen tetroxide furnishes N-nitroso lactams, which undergo chemoselective 1,2-reduction to N-nitroso carbinolamines by one equivalent of hydride delivered from lithium triethylborohydride. The nitroso group is cleaved in a novel way using samarium(II) iodide and dehydration then generates the corresponding imine (which may tautomerise to the isomeric enamine). The reduction can be performed in the presence of esters and has proved efficient for the preparation of 2H-pyrroles (pyrrolenines) and 1H-pyrroles relevant to the study of tetrapyrrole biosynthesis.