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Enantioselective synthesis of imidazolidin-5-ones through a phosphoric acid catalyzed reaction between azlactones and N-substituted β-carbolines is reported. The reaction takes place via an initial formal [2+2] cycloaddition to generate an a-amino-β-lactam, which subsequently undergoes an acid-catalyzed asymmetric penicillin–penillonic acid (PPA) rearrangement with high diastereo- and enantioselectivity. To the best of our knowledge, this represents the first [2+2] cyclization of azlactones with imines and the first asymmetric PPA rearrangement, which are linked together by the phosphoric acid catalyst. [ABSTRACT FROM AUTHOR]