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

1-Azapenta-1,4-diene-3-ols 4a–m are easily accessible from 1-azapenta-1,4-dien-3-ones 3a–i and organolithium compounds. Treatment
of
the compounds 4a–m 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 6a–m in moderate to good yield. The product pyrrole 6k 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.