Rearrangements 4-hydroxy-1,8-naphthyridines

The base-catalyzed rearrangement of the cyclic imide (35) affords a 50% yield of the 1,6-naphthyridine (36) and a 20% yield of the isomeric 1,7-naphthyridine (37).61 Treatment of 38, obtained by hydrolysis and decarboxylation of 36, with phosphorus oxychloride is reported to give 5-chloro-8-hydroxy-1,6-naphthyridine (39).88... 

Acetyl-l-benzyl-5-hydroxy-3,4,7,8-tetrahydro-l, 7-naphthyridin-2(l //)-one (10) underwent catalytic dehydrogenation and rearrangement to give 5-acetoxy-3,4-dillydro-l,7-naphthyridin-2(l//)-one (11) (Pd/C, xylene, 135°C, 30 h 45%).1272... 

Ethyl-4-hydroxy-6-iodo-7-methyl[ 1,8]naphthyridine-3-carboxylate has been prepared by thermal rearrangement of diethyl- [(5-iodo-6-methyl-2-pyridinyl)amino]-methylenejmalonate in diphenyl ether as illustrated in Eq. 1 and is described (1). 

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,... 

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