Carbonyl–Allene Metathesis of S‑Allenyl-α-oxo‑S,S/N,S‑ketene Acetals: A Thermally Driven Intramolecular Tandem [2 + 2] Cycloaddition–Retro-Cyclization

Carbonyl–allene metathesis (CAM) represents an
emerging
class of bond-reorganizing transformations with significant synthetic
potential. Here, we report a thermally controlled, catalyst-free intramolecular
CAM reaction demonstrated via a tandem [2 + 2] cycloaddition and retrocyclization
of S-allenyl-α-oxo-S,S-/N,S-ketene acetals. Control experiments
and kinetic and DFT studies indicate the reaction involved an asynchronous
concerted cycloaddition between the carbonyl and allene moieties,
forming an oxetane intermediate, which subsequently undergoes retro-cyclization
to afford 2,4-disubstituted thiophene derivatives.