Abstract α-Bromoacrylates, α-bromoacrylonitrile and diethyl alkylidenemalonates all react with bromoform and base under phase transfer conditions to give dibromocycloprapanes. When γ-bromine is present in a crotonate or an alkylidenemalonate, the intermediate derived by Michael addition of tribromomethylanion is trapped by cyclisation at that position to produce a (tribromomethyl) cyclopropane.
