A convenient protocol for the synthesis of α-substituted-β-oxo azaheterocycles through the oxoammonium salt-promoted cascade reaction of saturated cyclic amines with different nucleophiles based on β-oxo cyclic iminium ion intermediates has been developed. Interestingly, when active methylene compounds containing the acetyl or cyano group were used as nucleophiles, piperidocyclopent-2-enones or 7-hydroxypyrrolo[3,2-b]piperidine-2-ones could be respectively obtained via a further intramolecular nucleophilic addition of the in situ generated α-alkyl-β-oxo azaheterocycles. In addition, the reaction of morpholines with nucleophiles under similar conditions furnished α-substituted-β-TEMPO decorated amines via β-TEMPO-cyclic iminium ion intermediates. Experimental studies and DFT calculations unveiled the origin of the chemoselectivity in the formation of different intermediates.