2,5-Diphenyloxazol

In 1896, Emil Fisher found that 2,5-diphenyloxazole hydrochloride was precipitated by passing gaseous hydrogen chloride into an absolute ether solution of benzaldehyde and benzaldehyde cyanohydrin. The oxazole hydrochloride can be converted to the free base by addition of water or by boiling with alcohol. Many different aromatic aldehydes and cyanohydrin combinations have been converted to 2,5-diaryloxazoles 4 by this procedure in 80% yield. ... 

The irradiation of 3,5-diphenylisoxazole in the presence of propylamine gave a mixture of 2,5-diphenyloxazole (20%) and A-propyl-2,5-diphenylimidazole (1%). The same distribution of the products was obtained starting from the azirine 46 (75T785). 

The photoisomerization of 2,5-diphenyloxazole (57) was first reported in 1969. 2,5-Diphenyloxazole, irradiated in ethanol at 80°C, gave three products, where the... 

Fig. 11. Relative energy of the excited states of 2,5-diphenyloxazole and of some reactive intermediates.

Radioactivity Analysis. Samples of urine, feces, and tissues were combusted to COo and analyzed for radioactivity (5). By using this method the recovery of radioactivity from samples spiked with C was 95 dt 5%. To determine the radioactivity expired as CO2, 5-ml aliquots of the solution used to trap the CO2 were added to 15 ml of a scintillation counting solution containing 4 grams 2,5-diphenyloxazole (PPO) and 0.1 grams l,4-bis-2(5-phenyloxazolyl)-benzene (POPOP) per liter of 1 1 toluene 2-methoxyethanol. Samples were counted for radioactivity in a Nuclear Chicago Mark II liquid scintillation counter. Counting eflSciency was corrected by the internal standard technique. 

By the end of the nineteenth century around 600 fluorescent compounds had been identified [3], including fluorescein (A. von Baeyer, 1871), eosine (H. Garo, 1874), and polycyclic aromatic hydrocarbons (C. Liebermann, 1880) [5], Although it is generally accepted that fluorescence markers are relatively new analytical benefits, it is surprising to note that their chemical synthesis is rather old, such as the fluorescein reported by Baeyer, the 2,5-diphenyloxazole by Fisher in 1896, and the fluorene by Berthelot in 1867 [18],... 

A unique Pd(II)-promoted ortfto-esterification of 2,5-diphenyloxazole has been described [26], When 2,5-diphenyloxazole was heated with 2.5 equivalents of Pd(OAc>2 in acetic acid and CCLt, regioselective palladation took place, giving rise to arylpalladium(n) o-complex 31 in almost quantitative yield. The regioselectivity observed reflects the strong coordination ability of the oxazole nitrogen atom to the palladium atom. Complex 31 was then dissolved in MeOH-THF (1 1) and the solution was stirred at 0 °C to produce 2-(5 -phenylthiazol2-yl)-benzoate 32. A... 

The electron transfer system has not been studied in detail in fish, but the metabolism of compounds such as biphenyl (37), benzo(a)pyrene (21) and 2,5-diphenyloxazole (38) by fish liver microsomes has been shown to require oxygen and NADPH generating system. The metabolism of BP (21), 2,5-diphenyloxazole (Ahokas, unpublished observation) and aldrin (27.) by fish liver microsomal enzyme system is inhibited strongly by carbon monoxide. This information and the fact that cytochrome P-1+50, as well as NADPH cytochrome c reductase system are present in fish, suggest strongly that fish have a cytochrome P-1+50 mediated monooxygenase system which is very similar to that described in mammals. 

Ahokas, J.T. Metabolism of 2,5-diphenyloxazole (PP0) by trout liver microsomal mixed function monooxygenase. Res. Commun. Chem. Pathol. Pharmacol. (1976) 13, 1+39-1+1+7. 

The least acidic of the three available position on oxazole is the 4-position, but even this can be lithiated under the appropriate conditions (86CRV845). Thus, while 2,5-diphenyloxazole gives a mixture of products on treatment with n-BuLi, clean metalation at C-4 can be achieved using LDA or KDA (86CRV845), or with 5-BuLi and a catalytic amount of LiTMP (Scheme 86)(91JOC3058). 

The main features of the chemiluminescence mechanism are exemplarily illustrated in Scheme 11 for the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) as base catalyst and the chemiluminescent activators (ACT) anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene and rubrene. In this mechanism, the replacement of the phenolic substituents in TCPO by IMI-H constitutes the slow step, whereas the nucleophilic attack of hydrogen peroxide on the intermediary l,l -oxalyl diimidazole (ODI) is fast. This rate difference is manifested by a two-exponential behavior of the chemiluminescence kinetics. The observed dependence of the chemiexcitation yield on the electrochemical characteristics of the activator has been rationalized in terms of the intermolecular CIEEL mechanism (Scheme 12), in which the free-energy balance for the electron back-transfer (BET) determines whether the singlet-excited activator, the species responsible for the light emission, is formed ... 

A recent approach to the synthesis of 5-bromooxazoles reacted 2-phenyl-4,5-dihydrooxazole with NBS in the presence of a.a-azobisisobu-tyronitrile (AIBN). The 2-aryloxazole product (83) resulted from a bromi-nation-dehydrobromination process. Further bromination then gave 84. With two equivalents of NBS the yields of 83 and 84 were 45 and 10%, respectively, whereas three equivalents raised the yield of dibromo product to 56% (89S873) (Scheme 29). When treated with bromine in neutral conditions, 2,5-diphenyloxazole formed an N-bromo adduct [59LA(626)83]. 

Another study of scintillator fluorescence from 2,5-diphenyloxazole excited by sub-nanosecond pulses of electrons in cyclohexane or toluene has been made by Katsumura et al. [179]. They found that the emission intensity of fluorescence grew in two stages, a faster process occurring during the electron pulse (10 ps) and a slower process following the... 

The emission from the radioisotopes is often insufficient to penetrate the window of a Geiger-Muller counter. Therefore, the compound whose activity is to be measured is often mixed in solution with a scintillator, called a fluor, which transforms / rays into luminescence proportional to the number of /3 particles emitted. The sample is dissolved in a solvent (toluene, xylene or dioxane, the latter being used for water-soluble compounds) that acts as a relay to transfer the energy to the scintillator. The scintillation mixture contains PPO (2,5-diphenyloxazole), which emits in the UV and POPOP, which emits in the visible and is well adapted to detection with photomultiplier tubes (Fig. 17.2). The quantum yield of emission will depend on the energy of the emitted particles. 

The resulting solution consisted of 5 ml. of water and 20 ml. of dioxane containing 7 grams 2,5-diphenyloxazole (PPO), 1.5 grams bis-(o-methylstyryl) benzene (bis-MSB), and 120 grams naphthalene/liter dioxane. The Y-value of this solution was approximately 1 nCi/liter. 

The light, hv2, from F2 is of longer wavelength than hvx from F and is more efficiently detected by a PMT. Two widely used primary and secondary fluors are 2,5-diphenyloxazole (PPO) with an emission maximum of 380 nm and l,4-bis-2-(5-phenyloxazolyl)benzene (POPOP) with an emission maximum of 420 nm. 

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