Synthesis of 4-Pyrrolin-3-ones via the 5-<i>endo</i>-<i>trig</i> Cyclization of Conjugated 2-Azaallyl Radicals

4-Selanylcarbonyl-substituted 2-azabuta-1,3-dienes, synthesized from Se-aryl 2<i>H</i>-azirine-2-carboselenoates and α-diazo esters, generate ketene-tethered 2-azaallyl radicals under UV light (365 nm) irradiation. These conjugated radical species, in the presence of catechol as a HAT donor, undergo practically barrierless 5-<i>endo</i>-<i>trig</i> cyclization to give 4-pyrrolin-3-ones in high yields. This synthetically valuable reaction, proceeding via an "unfavorable" anti-Baldwin cyclization, is the first example of an intramolecular transformation of azaallyl radicals.