Stereoselectivity in the anionic oxy-Cope rearrangement of acyclic vinyl sulfides

Abstract

Novel anionic oxy-Cope (AOC) rearrangements of substituted 5-alkylthio-1,5-dien-3-ols were studied. Aqueous quench gave new access to compounds with a nucleophilic vinyl sulfide and an electrophilic aldehyde in a 1,5 relationship. 3,4-Anti and 3,4-syn 1E,5Z-5-hexylthio-4-methyl-1-phenylhepta-1,5-dien-3-ols were synthesised separately by stereoselective aldol reaction, thioesterification and alkylidenation of the resulting thioesters to give Z-vinyl sulfides with ⩾90% stereoselectivity. AOC rearrangement of the 3,4-syn substrate gave predominantly 3,4-syn Z-vinyl sulfide while the 3,4-anti substrate gave mostly 3,4-syn E-vinyl sulfide, via chair-like transition states with the oxyanion pseudo-equatorial. Stereochemistry was assigned by NOE taking advantage of the conformational stability of the products.