Electrophilic Halogenation of Allenoates and 3-Alkynoates: Synthesis of 1,4-Dicarbonyl (E)-3-Haloalkenes and Mechanistic Investigations

Allenoates traditionally provide halobutenolides upon reaction with halonium ions via electrophile-assisted halolactonization. Herein, an unusual electrophilic halogenation of di- and trisubstituted allenoates and 3-alkynoates is demonstrated with N-halosuccinimides under DABCO promotion. The protocol affords densely functionalized 1,4-dicarbonyl 3-haloalkenes in good yields with excellent (E)-stereoselectivity (up to 83% yield, >20:1 dr). The allenoates are presumed to form γ-haloallenoate intermediates, which further react with halonium ions and provide the desired scaffolds. The role of the nucleophilic base in the unusual transformation is demystified through control experiments and computational studies. The deliverables are identified as good synthons for various synthetic group transformations and valuable targets for biologically active pyridazine scaffolds.