A Three-Step Synthesis of Tetrasubstituted NH-Pyrroles

Buta-1,3-dienes appended with electron-withdrawing groups (EWGs), derived from the [2 + 2] cycloaddition–retroelectro­cyclization (CA–RE) cascade, react with (predominately) nitrogen-based nucleophiles affording tetrasubstituted 2-amino-NH-pyrroles in moderate to excellent yields with complete regioselectivity. Penta-2,4-dien-1-ones also undergo a similar transformation, providing analogous products and greatly enhancing the substitution of the pyrrole available. Oxidation from pyrrole to pyrrolidinone affords highly colored compounds that experience a strong bathochromic shift of the longest-wavelength absorption band in the UV/vis spectrum upon protonation, with return to the original spectra following neutralization.