Straightforward Access to Terminally Disubstituted Electron-Deficient Alkylidene Cyclopent-2-en-4-ones through Olefination with α-Carbonyl and α-Cyano Secondary Alkyl Sulfones

Herein we report on a simplified synthesis of scarcely explored, terminally disubstituted electron-poor alkylidene cyclopent-2-en-4-ones through uncommon olefination. Secondary sulfones, activated by electron-withdrawing groups at the adjacent carbon atom, undergo K₂CO₃-promoted coupling with 4-acyloxy- and 4-tert-butyldimethylsilyloxycyclopent-2-en-1-ones giving directly, or after a separate dehydrosulfinylation step, alkylidene cyclopent-2-en-4-ones. A plausible mechanism for these transformations is proposed. Initially, β-arylsulfonyl esters as well as their acetyl or nitrile analogues are allylated by cyclopentenone derivatives via a tandem Michael addition of α-sulfonyl carbanions followed by proton migration and retro-Michael-type O-nucleofuge elimination. The primary allylation products are formed as two diastereomers whose configuration and conformation were elucidated using single crystal X-ray diffraction and NMR spectroscopy. Regardless of stereochemistry, both sets of diastereomers are subjected to Zaitsev-type retro-Michael vinylogous dehydrosulfinylation under either basic or thermal silica gel promoted conditions resulting in E/Z-alkylidene cyclopent-2-en-4-ones. In these reactions activated sulfones serve as bearing electron-withdrawing group alkylidene anion-radical synthons, whereas 4-oxy-substituted cyclopentenones represent cyclopent-2-en-4-one cation-radical surrogates.