Under the ‘best anhydrous’ conditions, we were able to achieve, the bicyclic oxazolidinones derived from proline and pivalaldehyde (or cyclohexanone) equilibrate with added carbonyl compounds in (D6)DMSO and in (D6)benzene. With (18O)cyclohexanone, no incorporation of the label into the 1,3-oxazolidinone ring was observed (in-situ NMR investigations; Figs. 1, 3, and 4). Since an iminium-carboxylate zwitterion might be involved in this process, we also studied the reaction of N-isopropylidene-pyrrolidinium perchlorate with cyclohexanone in anhydrous CDCl3 (Fig. 5). We speculated that an interconversion between iminium and carbonyl species might occur in the absence of H2O or other impurities, i.e., formally a metathesis through 1,3-oxazetidinium ions (Schemes 2 and 3). A theoretical investigation with various DFT methods, ranging all the way to CCSD(T)/aug-cc-pVTZ//MP2/def2-QZVPP, shows (Figs. 8–11) that oxazetidinium ions are stable species (more or less equi-energetic with the reactants iminium ion+carbonyl system), but that the transition states leading to these cations are too high in energy for a reaction taking place in the gas phase at room temperature. Further investigations are proposed to study the iminiumcarbonyl interconversion mechanism.