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Gold carbene reactivity patterns were accessed by ynamide insertion into a C(sp3)-H bond. A substantial increase in molecular complexity occurred through the cascade polycyclization of N-allyl ynamides to form fused nitrogenheterocycle scaffolds. Exquisite selectivity was observed despite several competing pathways in an efficient gold-catalyzed synthesis of densely functionalized C(sp3)-rich polycycles and a copper-catalyzed synthesis of fused pyridine derivatives. The respective gold–keteniminium and ketenimine activation pathways have been explored through a structure–reactivity study, and isotopic labeling identified turnover-limiting C-H bond-cleavage in both processes. [ABSTRACT FROM AUTHOR]