Divergent Synthesis of Pyrrolizine- and Indolizine-Fused Heterocycles via Pd(II)-Catalyzed Cascade Transformation of Functionalized Alkynes

A palladium(II)-catalyzed synthesis of pyrrolizine- and indolizine-fused heterocycles was established involving a cascade functionalization of internal alkynes in good yields (up to 84%). A catalyst- and solvent-dependent intramolecular carboxypalladation of the multifunctional alkynes followed by trapping the in situ generated σ-vinylpalladium species delivered isobenzofuran-fused pyrrolizines and isochromeno[3,4-<i>f</i>]indolizin-5-ones. Further, benzo[<i>d</i>][1,3]oxazin-2-one-fused pyrrolizines were obtained by replacing the carboxylic ester with a <i>N</i>-Boc-protected amine as the nucleophilic partner. This cascade process features regioselectivity, high skeletal diversity, broad substrate scope, and high step and atom economy.