Synthesis of Pyrroles through a 4π-Electrocyclic Ring-Closure Reaction of 1-Azapentadienyl Cations

Abstract

1-Azapenta-1,4-diene-3-ols <b>4a</b>–<b>m</b> are easily accessible from 1-azapenta-1,4-dien-3-ones <b>3a</b>–<b>i</b> and organolithium compounds. Treatment of the compounds <b>4a</b>–<b>m</b> with strong acid (triflic acid) generates 1-azapentadienyl cations in situ upon protonation at the hydroxyl oxygen atom and subsequent water elimination. The intermediate cations undergo facile 4π-electrocyclization under ambient condition to give diversely substituted pyrroles <b>6a</b>–<b>m</b> in moderate to good yield. The product pyrrole <b>6k</b> could be characterized by X-ray diffraction. Quantum chemical calculations were performed to elucidate the mechanism of this reaction with respect to starting compounds, transition states, and products. They support the proposed mechanism of a 4π-conrotatory Möbius-type electrocyclic ring-closure reaction