A highly stereoselective transformation of carboxylic esters to trisubstituted olefins via cationic cyclopropyl–allyl rearrangement of sulfonates of cis-1,2-disubstituted cyclopropanols

Methanesulfonates of readily available cis-1,2-disubstituted cyclopropanols on reaction with magnesium bromide in diethyl ether undergo cyclopropyl–allyl rearrangement to afford allyl bromides, with an (E)-trisubstituted double bond, as main products. The amount of the corresponding (Z)-isomers did not exceed 5% when the reaction was performed at 0 °C, and the major concomitant products were the regioisomeric secondary bromides. The latter are sufficiently less reactive towards sulfur, phosphorus and hydrogen nucleophiles than the dominant (E)-disubstituted allyl bromides and these reactions are suitable for the stereoselective preparation of trisubstituted olefins.