2.5- diphenyl-4- oxazoles

In contrast to other acids, anhydrous hydrogen fluoride does not cause hydroly SIS and decarboxylation of the malonic acid residues in these reactions [5]. It is a good reagent for the cyclization of a-benzamidoacetophenones to 2,5 diphenyl-oxazoles [6] (equation 7) The same reaction with concentrated sulfuric acid gives cyclic product with only a 12% yield [6]... 

The scintillator 2,5-diphenyl-oxazole (PPO) in DMSO maybe used instead of salicylate ... 

A solution of 5.1 grams 2-chloro-4,5-diphenyl-oxazole, 6.3 grams diethanolamine and 50 ml absolute ethanol was refluxed for 4 hours. The solvent was stripped at 1 mm and the oily residue was added at 60°C to 100 ml 50% ethanol by cooling the hydro-alcoholic solution, 4.5 grams of 2-bis((3-hydroxyethyl)amino-4,5-diphenyl-oxazole was obtained (yield, 69.5%). The product crystallized from ethyl ether + petroleum ether, with a MP of 96 to 98°C. 

The tetrahydro-derivatives of the oxazole and isoxazole system are unstable. As a consequence, only acyclic products have been reported from the reductions with complex metal hydrides. 2,5-Diphenyl-oxazole (119) gave 2-benzylamino-l-phenylethanol (120),141 and 3,5-diphenyl-2-isoxazoline (121) was converted to 3-amino-l,3-diphenylpropanol (122)142 on reduction with lithium aluminum hydride. 3-Phenylbenzisoxazole was resistant to reduction with lithium aluminum hydride and sodium borohydride,143 but benz-oxazole (123), benzoxazol-2-one (124), and benzoxazol-2-thione (125) have been reported 141 to yield 2-methylaminophenol (126) on reduction with lithium aluminum hydride. 

The primary scintillator converts excitation energy into light. Two commonly used primary scintillators are PPO (2,5-diphenyl-oxazole) and butyl-PBD (2-(4 -tert-butyl-phenyl)-5-(4 -biphenyl)-l,3,4-oxadiazole). The pulse voltage produced by butyl-PBD is about 20% greater than that produced by PPO, but its usefulness is reduced because of its limited solubility. Suitable concentrations of PPO and butyl-PBD are about 5 g/1 and 7 g/1 respectively. With alkaline samples butyl-PBD produces a brownish colour in the scintillation cocktail. In highly quenched samples, a higher concentration may be required for optimum efficiency. 

Brown and Ghosh240 have used ultraviolet spectroscopy to study the nature of the bonding in diphenyloxazoles. The spectrum of 2,5-diphenyl-oxazole shows two main peaks, of which the high-intensity one at 314 nm (log t 4.34) is considered to arise from an extended conjugation as in structure 151, cf. p-terphenyl, 2,5-diphenylfuran, and 2,5-diphenyl-l,3,4-oxadiazole.241 The absence of such a high-intensity peak above 276 nm in the spectrum of 2,4-diphenyloxazole (152) suggests that canonical structures such as 153 contribute little to the oxazole resonance, and hence to the conclusion that an oxazole is more like a conjugated diene than a fully aromatic system.240... 

The formation of the fluorenyl cation (156) in the spectra of diphenyl-oxazoles has been noted previously283 and a mechanism has been proposed. It has been suggested286 that the formation of 156 from 2,5-diphenyloxazole... 

Metalated thiazoles are only stable at low temperatures. The formation of an isonitrile in the procedure below clearly shows this poor stability (compare Ref. [189]). On the other hand the methylthio derivative of benzothiazole is formed in good yield upon addition of a methythiolation reagent to the solution resulting from the interaction between benzothiazole and sodamide in liquid ammonia. This situation may be analogous to the ambident behaviour of 2-lithio-4,5-diphenyl-oxazole [193]. 

The synthesis of antimycin A3 (676), a potent antifungal agent, makes use of lactaldehyde 658 to establish the stereochemistry of the three contiguous asymmetric centers in the eastern half of the dilactone skeleton via intermediate 678. Strategically, the 4,5-diphenyl-oxazole heterocycle is used as a template for protection of the latent activated carboxylate group, which is unmasked by photooxygenation [120,200]. The chiral western half of the dilactone framework is derived from the differentially protected L-threonine derivative 677. 

Liebscher and co-workers reported a general method of preparing optically active 2-(a-aminoalkyl)oxazoles 1167 via an asymmetric a-alkylation of 2-(ami-nomethyl)-4,5-diphenyloxazole 1164 (Scheme 1.301). Their work complements the known methodology for asymmetric a-alkylation of other aminomethyl hetero-cycles. Thus Lewis acid-catalyzed condensation of 1164 with commercially available (IS, IS, 5S)-2-hydroxypinanone generated (IS, 2S, 5S)-3-(4,5-diphenyl-oxazol-2-ylmethylimino)-2,6,6-trimethylbicyclo[3.1.1]heptan-2-ol 1165 in 65% yield. 

Phenyloxazole (38) is regioselectively phenylated at C-5 to afford 2,5-diphenyl-oxazole with bromobenzene, and arylation of benz[ ]oxazole (39) with 26 gives the C-2 arylation product 40 as expected [3]. 

Analytical grade p-xylene is mixed with 2.5 diphenyl-oxazole (PPO) and p-bis-(o-methylstyrl)-benzene to a final concentration of 21 g/1 and 4.5 g/1, respectively. Various volumes of dehydrated alcohol (0.5% water) were added to the xylene scintillator. In all cases, 25 ml of the xylene-alcohol mixture was counted. All measurements were made in plastic vials (American Searle Corporation, Chicago, IL) in a Packard Tri-Carb Counter Model 3380 or a Beckman LS-200 Liquid Scintillation Counter. 

A mixture of desyl chloride and acetonitrile liquefied by brief warming treated gradually with SnGl4 with water-cooling and occasional shaking, allowed to stand overnight, and heated 3 hrs. on a water bath -> 2-methyl-4,5-diphenyl-oxazole. Y 75%. F. e. and limitations s. M. Lora-Tamayo, R. Madronero, and H. Leipprand, B. 97, 2230 (1964). 

Anhydro [a Dhenyl-hippars uie-pbei l ester], Ueieht 5 Phencay 2.4 diphenyl-oxazol 14, 470 s. a. 27,120. x-Diphenyl >X [4-o -phenyl] oxaxol, Oxybenzilam 27 II100. 3.4.5-Triphenyl-oxazolon (2) 271295,... 

Benzoxazoies.—Benzoxazoles (559) are obtained by the action of the thio-phosphoryl compound PhPS(OMe)2 on the nitrones (558). The naphthaquinones (560 R = H, Me, Ph, or NH2) cyclise to the naphtho[2,3-condensed oxazole (562) on treatment with ethyl azidoformate. Acenaph-tho[l,2-d]oxazoles (564 R = Me, Et, or Ph) are formed when 2-diazoacenaph-thenone (563) is irradiated in the presence of cyanides RCN. Phenanthro-oxazoles result when 2-aryl-4,5-diphenyl-oxazoles are irradiated in the presence of oxygen or iodine the p-nitrophenyl derivative is obtained by the action of p-nitrobenzyl chloride on the monoimine or the mono-oxime of phenan-thraquinone, ... 

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