Competing dimerisation of some N,O-heterocyclics. Thermodynamic versus kinetic control of isomeric products

The behaviour of substituted 1,3-oxazolidin-2-ones (1) and 1,3-oxazolin-2-ones (2) on treatment with acid is reported. Electrophylic addition of iminium cations to 1,3-oxazolin-2-onic monomers occurs, yielding the kinetically preferred dimers (3). The elimination pathway to 3 is in direct competition with the formation of the cyclodimerisation products resulting from a thermodynamically controlled process. The stereochemistry of the cyclodimers is discussed on the basis of X-ray analysis. Qualitative energetic considerations on the overall reaction and NMR characteristics are discussed using deuterium labelling experiments.