2.5- Diphenyloxazole, formation from

The reaction of oxazoles with alkynes is entirely different, leading to furans. The adducts (157) eliminate a cyanide in a retro-Diels-Alder process (equation 15). A typical example is the formation of the ester (164) from 5-ethoxy-4-methyloxazole and dimethyl acety-lenedicarboxylate (equation 16) equation (17) illustrates the production of two regioisomers in this reaction (79MI41802) a more elaborate case is the twofold addition of benzyne to 4-methyl-2,5-diphenyloxazole to give the bridged dihydroanthracene shown in equation (18) (80TL3627). 

The reactivity of 2-alkyloxazoles in base-catalyzed additions and condensations is due to the intermediacy of delocalized anions analogous to (142). 2-Methyl-4,5-diphenyloxazole forms the adduct (278) by the action of benzophenone in liquid ammonia containing lithium amide (equation 89), and 2-methylbenzoxazole condenses with diethyl oxalate in the presence of potassium ethoxide to yield the keto ester (279). Anions with more extended conjugation are involved in the condensation of 2-(propen-l-yl)benzoxazole with diethyl oxalate (equation 90) and in the formation of the stilbene derivative (280) from 5-phenyl-2-p-tolyloxazole and benzylideneaniline (78AHC(23)l7l). 

Balaban and Frangopol116 observed that the addition of thionyl chloride precludes the formation of oxazolid-4-one and that the yield of 2,5-diphenyloxazole (PPO) is a function of the amount of added thionyl chloride. Some patents20 21 have described the preparation of hitherto unknown 2,5-diaryloxazoles with amino and dialkylamino substituents at the para position of the benzene ring, for use as photoconductive materials for electrophotography. Recently the reaction has been applied for the one-step synthesis of halfordinol (48), an oxazole alkaloid, from p-hydroxymandelonitrile and nicotinaldehyde.117... 

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... 

Oxazoles.—Formation. ci)-Diazoacetophenone reacts with benzonitrile in the presence of trifluoromethanesulphonic acid to give 2,5-diphenyloxazole (514). Bi-, ter-, and quater-oxazoles have been synthesized from a-metallated isocyanides thus lithiobenzyl isocyanide (PhCHLiNC) and dimethyl oxazole-3,4-dicarboxylate afford compound (515). Stable methylene-oxazolines (516 R = Me or Ph) are obtained by heating the iminesHN=C(Ph)OCHRC=CH. The fluorinated oxazoline (518) results from the action of benzonitrile on the oxiran (517), and also from the reaction of bis(trifluoromethyl)carbene with the benzoylimine (CF3)2C=NCOPh/ Treatment of 2-acylamino-l-aryl-alkan-l-ols (519) under the conditions of the Pictet-Gams reaction, i.e. with phosphorus pentoxide in boiling toluene or xylene, results in cis- and trans-2-oxazolines (520), rather than isoquinolines, as claimed previously. The spiro-oxazolines (521 R = Me or Ph) are formed when the lithiated di-isocyanide CN—CH2CH2CHLi—NC is treated with acetone or benzophenone, respec-tively. ... 

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