Synthesis of Conjugated Bromoenynoates via Electrophilic Bromination of Enynyl Hydrazones

A remote electrophilic bromination of conjugated enynyl hydrazone carboxylates has been developed for the synthesis of densely functionalized bromoenynoates. The protocol enables a substrate-controlled approach for (Z)-bromoenynoates (up to 80% yield and >20:1 Z/E) and (E)-bromodienynoates (up to 96% yield and >20:1 E/Z). The divergence in the product formation is rationalized by invoking an allylic 1,3-strain. Gram-scale reactions and the downstream synthetic transformations of the resulting bromoenynoates are demonstrated toward obtaining useful scaffolds.