A novel and convenient synthesis of β-dicarbonylated tetrahydropiperidines from the cascade reactions of piperidine derivatives with methyl ketones is presented. Mechanistically, the formation of the title compounds involves a hitherto unreported oxidative cross-coupling between different C(sp)-H bonds through the in situ generation of a cyclic enamine and an α-keto radical as the key intermediates followed by their radical addition and further oxidation of the addition adduct. To our knowledge, this is the first example in which β-dicarbonyl-substituted tetrahydropiperidines were prepared using readily available saturated cyclic amines as substrates and inexpensive and ubiquitous methyl ketones as dicarbonylation reagents. In addition, the product thus obtained could be easily transformed into structurally and pharmaceutically interesting polycyclic compounds in good efficiency.