Electrophilic 1,5-Addition of Acyl Chlorides to Conjugated Azocyclopropanes

Addition reactions of acetyl and benzoyl chlorides to 1-pyrazolines containing a spiro cyclopropane fragment at the adjacent azo group lead selectively and in high yields to the corresponding 1-acyl-3-(chloroethyl)-2-pyrazolines, resulting in opening of the three-membered ring.