A facile access to isocoumarins has been established via rhodium(III)-catalyzed C-H bond activation and intramolecular C-C cascade annulation of enaminones and cyclic 1,3-dicarbonyl compounds. The synthetic protocol features a wide range of substrates with high functional group tolerance, mild reaction conditions, and the selective cleavage of the enaminone C-C bond. Notably, the cyclic 1,3-dicarbonyl compounds can in situ -generate iodonium ylide as a carbene precursor to prepare polycyclic scaffolds by reacting with PhI(OAc) 2 . The application of this method to prepare useful synthetic precursors and bioactive skeletons is also exemplified.