Carbonyl–Allene Metathesis of S-Allenyl-α-oxo- <i>S</i> , <i>S/N</i> , <i>S</i> -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-<i>S</i>,<i>S</i>-/<i>N</i>,<i>S</i>-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.