Heterocycles from oxazole rearrangements oxazoles reactions

Oxazoles readily participate in cycloaddition reactions as dienophiles and as dienes in Diels-Alder reactions, and suitably substituted oxazoles participate in sigmatropic rearrangements (e.g., aza-Claisen rearrangements). In particular, the Diels-Alder reaction of oxazoles is one of the most widely explored and synthetically useful reactions, and as such, it has been used extensively both in natural product syntheses and to convert oxazoles to other heterocyclic ring systems. For example, a partial list of heterocyclic systems readily accessible from oxazoles via Diels-Alder reactions or other cycloadditions include pyridines hydroxy-pyridines isoindoles pyridazines tetrahydronaphthyridines benzo[h]-l,6-naphthyridines benzopyrano[3,4-b]pyridines 2-substituted, 2,4-disubstituted,... 

There have been reports on the reaction of oxazoles with heterodienophiles, including N=N, C=N, C=0, and types, to give new heterocycles <89X3535, 88JOO 663>. In most cases, the products may be envisioned to arise from ring opening and rearrangement of Diels-Alder adducts followed by reclosure, but so far no proof of this mechanism has appeared. A reaction of 5-ethoxy-... 

---