Novel Heterocycles by Bis Heteroannulation of Oxazoles

We report the first examples of intramolecular Diels–Alder addition of heterodienophiles N=N, C=N, C=O, C=S to oxazoles. The required 5-ethoxy- and 5-phenyloxazoles were synthesized bearing a side chain of variable length on C-2 to which the different heterodienophiles are attached. The products of the thermal bis heteroannulation are 3-triazolines, imidazolines, oxazolines, or thiazolines fused to a five- to six-membered ring. Relative reactivities were established and the mechanism is discussed.