NEW CHEMISTRY OF OXAZOLES

Oxazoles are versatile compounds that can undergo reaction with electrophiles (e.g. bromine) leading either to aromatic substitution or to addition products. The latter reactions are shown to proceed via N-bromooxazolium salts and can also be used for introduction of 4-substituents. Intramolecular formation of oxazolium salts also can take place and subsequent reaction with nucleophiles leads via azomethine ylides to interesting products. Diels-Alder reactions of oxazoles with olefins or acetylenes continue to be useful for synthesis of natural products containing pyridine or furan rings respectively. Heterodienophiles (N=N, C=O, C=N, C=S, N=O) are shown to react with electron rich oxazoles either inter or intramolecularly and give rise to triazolines, oxazolines, imidazolines, thiazolines or oxadiazolines. Lithiated oxazoles are useful intermediates either for isocyanides or for alkylated oxazoles.