Heterocycles from oxazole rearrangements

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,... 

The thermal rearrangement of O-allyl trichloroacetimidates 1 affords the corresponding 7V-allyl amides 31 2, which have been cyclized with the aim of obtaining amino diols or amino alcohols different to those from the cyclization of the trichloroacetimidates. Cyclization of these latter substrates 1 leads to 4,5-dihydro-4-(l-iodoalkyl)oxazoles 2, while treatment of unsaturated trichloroacetamides 3 with /V-iodosuccinimide3 in chloroform affords the corresponding 4,5-dihydro-5-(l-iodoalkyl)oxazoles 4 in high yield. These heterocyclic products are protected forms of the corresponding amino alcohols. 

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-... 

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