2.5- diphenyl-4-substituted oxazoles

Clapham and Sutherland developed a general synthesis of 2,5-diphenyl-4-substituted oxazoles 1659 in the context of preparing monomers for scintillating polymers. Stille coupling of 4-bromo-2,5-diphenyloxazole 1658 with a variety of stannanes produced the target oxazoles in modest to excellent yield (Scheme 1.426). Alternatively, the authors reversed the StiUe-couphng approach... 

As in the synthesis of other bipyridines, several routes to 4,4 -bipyridine have been devised where one of the pyridine rings is built up from simpler components. For example, a dimer of acrolein reacts with ammonia and methanol in the presence of boron phosphate catalyst at 350°C to give a mixture of products including 4,4 -bipyridine (3.4% yield), and in a reaction akin to ones referred to with other bipyridines, 4-vinylpyridine reacts with substituted oxazoles in the presence of acid to give substituted 4,4 -bipyridines. ° ° Condensation of isonicotinaldehyde with acetaldehyde and ammonia at high temperatures in the presence of a catalyst also affords some 4,4 -bipyridine, and related processes give similar results,whereas pyran derivatives can be converted to 4,4 -bipyridine (56% conversion), for example, by reaction with ammonia and air at 350°C with a nickel-alumina catalyst. Likewise, 2,6-diphenyl-4-(4-pyridyl)pyrylium salts afford 2,6-... 

Simple triazoles are thermally stable to ca. 300C. However, triazole fV-carboxamides, when heated to 150 C in sulfolane, rearrange with the elimination of nitrogen to give 2-substituted oxazoles. The reaction is general and it is useful for the synthesis of oxazoles with diverse 2-substituents in excellent yields even for bulky substituents (Scheme 1). 4-Ethyl-3,S-diphenyl-47f-l,2,4-triazole is converted into 1-ethyl-3,5-diphenyl-l,2,-4-triazole in the presence of 15-crown-S in octadecane at 330"C <2001JHC955>. 

A simple MO calculation of the longest-wavelength band in the absorption spectra of oxazole and its mono- and diphenyl-substituted derivatives has been performed by Balaban et aZ.242 their results are in agreement with experimental data. 

Oxazole, 2-dimethylamino-4-phenyl-nitration, 6, 190 Oxazole, 2,5-diphenyl-irradiation, 6, 189 nitration, 6, 190 oxidation, 6, 187 reduction, 6, 194 synthesis, 6, 222 Oxazole, 4,5-diphenyl-mercuration, 6, 190 2-substituted... 

Rearrangement of alkoxycarbonyl imidazole acryl azides in diphenyl ether at high temperatures afforded imidazo[l,5-c]pyrimidinone or imidazo[4,5-c]pyridinone derivatives <02TL5879>. Efficient synthesis of imidazopyridodiazepines from peri annulation in imidazo[l,2-a]pyridine has been described <02TL9119>. A convenient synthesis of 3,6-disubstituted-2-aminoimidazo[l, 2-a]pyridines has been published <02TL9051>. Novel 2,3-dihydroimidazo[2,l-h][l,3]oxazoles were prepared from intramolecular nucleophilic i/wo-substitution of 2-alkylsulfonylimidazoles <02S2691>. 4,4 -Bi-l//-imidazol-2-ones were efficiently synthesized from 5-amino-ot-imino-1 //-imidazole-4-acetonitriles and isocyanates <02JOC5546>. 

Reactions of Oxazoles. The reaction of 4-phenyloxazole with di(acetoxy-methyl)acetylene gives the furan (531 R = CH2OAC) by extrusion of benzo-nitrile from the intermediate Diels-Alder adduct (530 R = CH20Ac). Similarly, 5-cyano-4-methyloxazole and hept-l-yne yield mainly the furan (532) and methyl cyanide a minor product, the 4-substituted isomer (533), arises from the alternative mode of addition of the acetylene. The sensitized photooxidation of 2,4-diphenyl-2-oxazolin-5-one (534) affords, inter alia, the coupled product (535). ... 

Irradiation of a methanolic solution of an oxazole at —78°C in the presence of O2 and rose bengal produced a 2,5-disubstituted 5-hydroperoxy-4-methoxy-4,5-dihydrooxazole 709 (Scheme 1.197). Here again, the oxazole substitution pattern determined the product distribution. For example, 4,5-diphenyl-2-methyloxazole 231a produced only N, iV-dibenzoyl acetamide 706 (Ri = CH3, R2 = R3 = CeHs). On the other hand, 4-methyl-2-phenyloxazole 701a gave 709 (Ri = CgHs, R2 = CH3, R3 = H), which decomposed on warming to room temperature to yield... 

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