Acid-catalyzed intramolecular vinylogous Mannich reactions and intramolecular Michael reactions affording pyrrolizinone-fused N-heterocycles from hydroxylactam derivatives bearing enals have been developed. Depending on the substituent on the hydroxylactam, the enal moiety acted either as a nucleophile (i.e., as an enol/enolate) or as an electrophile to react with the N-acyliminium ion or enamide generated from the hydroxylactam moiety, respectively. The reactions were demonstrated in the construction of fused N-heterocycles with 5- to 8-membered rings.