Two-Carbon Ring Expansion of Cyclobutanols to Cyclohexenones Enabled by Indole Radical Cation Intermediate: Development and Application to a Total Synthesis of Uleine

A single-electron transfer (SET)
oxidation of indole or benzo­[b]­thiophene to a radical
cation reverses the intrinsic polarity
of these π-excessive bicyclic heteroarenes. Here we report an
oxidative two-carbon homologation of cyclobutanols to cyclohexenones
under a visible-light photoredox catalysis. 1-(Indol-2-yl)­cyclobutan-1-ols
are converted to 2,3,4,9-tetrahydro-1H-carbazol-1-ones,
important structural motifs found in alkaloids and pharmaceuticals,
with a broad substrate scope. A mechanistic study suggests that the
reaction is initiated by an SET from an indole to an excited acridinium
salt to generate the radical cation, which is followed by two consecutive
1,2-alkyl migrations and a rearomatization. Benzo­[b]­thiophene-substituted cyclobutanols are similarly converted to 2,3-dihydrodibenzo­[b,d]­thiophen-4­(1H)-ones. A total synthesis
of (±)-uleine featuring this ring-expansion process is documented.