Semipinacol Rearrangement Induced by Cleavage of Dibromocyclopropane

A novel semipinacol rearrangement using dibromocyclopropanes as sources of carbocations was developed. Heating dibromocyclopropanes bearing a 1-hydroxyalkyl group with silver perchlorate and 2,6-lutidine induced cleavage of the cyclopropane ring to form allyl cations, which underwent 1,2-shift of a substituent at the alpha-position of the hydroxy group to give beta,gamma-unsaturated carbonyl compounds having a quaternary carbon at the alpha-position. Because the products have convertible functional groups such as bromo, vinyl, and formyl groups, the method is expected to be applicable to the synthesis of various molecules.