Studies on Intramolecular Alkylation of an α-Sulfinyl Vinylic Carbanion: a Novel Route to Chiral 1-Cycloalkenyl Sulfoxides

Intramolecular alkylation of various β-(ω-haloalkyl) substituted vinylic sulfoxides was investigated. Upon treatment with LDA in THF at −78°C, α-sulfinyl carbanion generated from vinylic sulfoxides cyclized at the α-sulfinyl position to give 1-cycloalkenyl sulfoxides with a five- to seven-membered ring. Although iodide or bromide is normally a good leaving group, chloride affords better results than the corresponding iodide and bromide when the reaction takes place at the benzylic position. The cyclization proceeded even with the secondary iodide in moderate yield. Not only the (E)-isomer but also the (Z)-isomer cyclized via rapid inversion of the olefin geometry. No loss of optical purity was observed during isomerization. Various 1-cycloalkenyl sulfoxides including a fused ring and a polyoxygenated rings were synthesized in good to moderate yields.