Ring-Transformations in the Reaction of 3-(Dimethylamino)-2,2-dimethyl-2H-azirines with 1-Substituted Imidazolidine-2,4,5-triones. Reaction of 1-substituted imidazolidine-2,4,5-triones (= N-substituted parabanic acids; 2) and 3-(dimethylamino)-2,2-dimethyl-2H-azirine (1) in iPrOH or MeCN at room temperature yields 5,6,7,7a-tetrahydro-3H-imidazo[3,4-a]imidazole-5,7-diones 3 (Scheme 1). By 15N-NMR studies, using (3-15N)-2a, it has been shown that only N(1) in (15N)-3a is labelled and, hence, N(4) stems from 1, e.g. the azirine reacts via cleavage of the N(1)=C(3) bond. In MeCN at room temperature, the azacyclols 3 rearrange slowly to give monocyclic 2H,5H-imidazol-2-ones 4 (Scheme 3); the 15N-label in (15N)-4a is in position 1. Both reactions proceed via deep-seated skeletal rearrangements, most probably via ring-expansion/ring-contraction processes.