5-bromo-4-methyl-2-phenyloxazole

Further evidence for involvement of an iV-acyl-l//-azirine intermediate is supported by isolation of both 5-bromo-4-methyl-2-phenyloxazole 113 and 4-bromo-5-methyl-2-phenyloxazole 114 from the photolysis of 2-benzoyl-4-bromo-3-methylisoxazol-5-one 112 (Scheme 1.31). In addition, the presence of a 4-halo substituent in 112 seriously limited the synthetic utility of this reaction. Despite extensive and substituent labeling, the authors concluded that further studies were necessary to elucidate fully the mechanism of this rearrangement. 

The halogenated derivatives of oxazoles, thiazoles80 and benzothiazoles81 were also the subject of palladium catalyzed coupling with terminal acetylenes. 5-Bromo-2-methyl-4-phenyloxazole for example coupled efficiently with phenylacetylene using a conventional catalyst system to give an excellent yield of the desired product (6.53.),82... 

A systematic study of substitution reactions of oxazole itself has not been reported. Bromination of 2-methyl-4-phenyloxazole or 4-methyl-2-phenyloxazole with either bromine or NBS gave in each case the 5-bromo derivative, while 2-methyl-5-phenyloxazole was brominated at C(4). Mercuration of oxazoles with mercury(II) acetate in acetic acid likewise occurs at C(4) or C(5), depending on which position is unsubstituted 4,5-di-phenyloxazole yields the 2-acetoxymercurio derivative. These mercury compounds react with bromine or iodine to afford the corresponding halogenooxazoles in an electrophilic replacement reaction (81JHC885). Vilsmeier-Haack formylation of 5-methyl-2-phenyloxazole with the DMF-phosphoryl chloride complex yields the 4-aldehyde. 

Although electrophilic substitution reactions are possible with oxazoles, they are frequently accompanied by addition reactions, as in furan. The bromination of 4-methyl-2-phenyloxazole with bromine or A/-bromosuccinimide yields 5-bromo-4-methyl-2-phenyloxazole, and that of 2-methyl-5-phenyloxazole gives 4-bromo-2-methyl-5-phenyloxazole. Mercury(II) acetate in acetic acid acetoxymercurates 4-sub-stituted oxazoles in the 5-position, 5-substituted oxazoles in the 4-position, and 4,5-disubstituted oxazoles in the 2-position. The acetoxymercury group can be substituted by electrophiles, e.g. ... 

The reactions of 2,5-disubstituted 3-furyl bromides with butyllithium and OFCP in anhydrous THF at -78° afforded bisfurylethenes 76 [84]. Photochrome 77 [85, 86] (29 6 % yield) and a series of its derivatives 78 (55-65 % yield) with various substituents in position 6 of the benzofuran ring [87] were obtained from 3-bromo-2-methyl-1 -benzofuran. 1,2-Bis(5-n-aIkyl-2-phenyloxazol-4-yl)perfluorocyclopent-enes... 

---