Regioselective synthesis of N-substituted 4-methylene-2-oxazolidinones and 4-oxazolin-2-ones. Study of reactivity in thermal Michael conjugate additions

N-Substituted 4-methylene-2-isoxazolidinones 9a-9e have been prepared from the tandem condensation of isocyanates 3 with α-ketol 7. In a more polar solvent (DMF) the same reaction led to stereoisomeric alcohols 11 and 12, which could be transformed to the thermodynamically more stable isomers 4-oxazolin-2-ones 10 in good yield. Thermal conjugate additions of both heterocycles 9a and 10a to enone 13 provided the C-5 adduct 15. When the reaction was carried out with the captodative olefin 17, the unexpected aniline 18 was isolated. The regiochemistry of the Michael addition was rationalized in terms of FMO theory by ab initio calculations. (C) 2000 Elsevier Science Ltd.