Expedient synthesis of 3-hydroxypyrroles via Bu3SnH-triggered ionic 5-exo-trig-cyclization of 5-chloro-3-azamuconoate derivatives

Abstract

An unprecedented application of tributyltin hydride (TBTH) as a reagent for the reductive ionic 1,5-cyclization of 3-azamuconoates (3-azahexa-2,4-dienedioates) is described. A novel 1,5-exo-trig-cyclization of 5-chloro-3-azamuconoates was used as a base for the development of an effective two-step method for the preparation of 4-hydroxy-1H-pyrrole-3-carboxylic and 3-hydroxy-1H-pyrrole-2,4-dicarboxylic acids derivatives from readily available diazo esters and 2-chloro-2H-azirines or 4-chloroisoxazoles. The experimental results, as well as the DFT calculations of the pyrroles formation, are in good agreement with the ionic mechanism involving the initial hydride attack at the CN bond of the azamuconoate followed by tin-promoted 1,5-cyclization.