Synthesis, Structure and Facile Rearrangement of a (Bromomethylene)-cyclopropane–Dichloroketene Adduct

5,5-Dichloro-6-bromospiro[2.3]hexan-4-one 2 is produced by the reaction of (bromomethylene)cyclopropane 1 with dichloroketene and rearranges readily at 0°C into 2-(2-bromoethyl)-4,4-dichlorocyclobut-2-enone 4a; reaction of 2 with basic reagents leads to the formation of 1-vinylcyclopropanecarboxylic acid derivatives.