Oxazoles ring-cleavage reactions

Oxazole, 2,2 -p-phenylenebis(5-phenyl-X-ray diffraction, 6, 180 Oxazole, 5-phenyl-2-p-tolyl-reactions, 6, 215 Oxazole, tetrahydro-ring cleavage, 5, 80 Oxazole, C-tolyl-reactions... 

An example of this type of ring-cleavage in an oxazole is provided by the 4-oxazolyllithium 40, which is cleaved when heated in hexane at 70°C (76JCS(P1)989). The intermediate 41 was intercepted by reaction with benzaldehyde. 

The original photoproduct of these reactions has a hydrated oxazolic ring formed by the interaction of the carbonyl group with the neighboring N-alkylated amino group. This photoproduct was transformed into anthra 2,1 -J oxazoline derivatives, which form following oxidative cleavage. 

Quaternized oxazoles are readily attacked by hydroxide, ammonia, and amines. Cleavage of the oxazole ring is observed by attack of hydroxide, undoubtedly at position 2, and iV-methyl-a-acylamino ketone (169) is formed.319 Treatment of 2-amino-5-phenyl-3-phenacyloxazolium bromide with ammonia gives 4-phenyl-l-phenacyl-2-imidazolone (170) by ringopening and reclosure. A similar reaction with aliphatic amines (RNH2) leads to 1-substituted 2,6-diphenylimidazo[l, 2-a]imidazoles (171).320... 

Tanaka and Kuriyama " prepared cii-4,5-disubstituted-oxazolines by catalytic hydrogenation of 2-amino-4,5-disubstituted oxazoles using PtOa or Pd/C. The authors isolated ring cleavage products from reactions employing a high catalyst load. 

Reactions of isoxazoles are less prolific due to facile isoxazole ring cleavage. Isoxazoles can be treated with V-bromosuccinimide to brominate the C4-position of the isoxazole. In contrast to oxazoles, when an isoxazole engages in a [4 + 2] cycloaddition, the isoxazole acts as a dienophile, not a diene. ... 

A 1,5-bond cleavage reaction which involves a 6,5-elimination rather than nitrogen participation has been reported. - The penicillin imino-chloride (130) rearranges to oxazole (132) on treatment with triethylamine a 6,5-elimination followed by jS-lactam ring-opening and oxazole formation has been proposed as the likely pathway. ... 

Few solid-phase syntheses of oxazoles have been reported (Table 15.17). The most general strategy is the dehydration of a-(acylamino) ketones (Entry 2, Table 15.17) or 2-(acylamino)phenols (Entry 1, Table 15.17). Oxazolidin-2-ones have been prepared by intramolecular nucleophilic cleavage of carbamates from insoluble supports (Entries 5 and 6, Table 15.17). Resin-bound 2-aminoethanols, which are accessible by nucleophilic ring-opening of oxiranes with amines, undergo cyclocondensation with aldehydes to yield oxazolidines [220,221]. These compounds are unstable towards acids, and can be released from the support only under neutral or basic reaction conditions. 

Reaction of 2-methyloxazole with sulfur and amines at 160° for 35 hours in an autoclave results in ring rupture. Thus, with aniline, acetanilide and acetamide are the products. It was suggested that the cleavage of the oxazole occurs at the 3-4 and 1-5 bonds [Eq. (17)].378... 

Sun et al. reported a diversity-oriented tactic to access the benzo[d]oxazol-5-yl-lH-benzo[d]imidazole 64 on IL support (Scheme 24). The authors coupled 4-hydroxy-3-nitrobenzoic acid onto IL-immobilized o-phenylenediamine 58 that underwent ring closure to furnish benzimidazole derivatives 60. Further hydrogenation of the nitro group to an amine followed by the reaction with 1,1-thiocarbonyldiimidazols resulted into IL tagged-benzoxazol 63. In order to generate skeletal diversity, S-alkylation with alkyl and aryl bromides was carried out. Cleavage of 64 from IL support was done by treatment with sodium methoxide in methanol imder microwave irradiation [127]. 

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