Oxazoles, 2-aryl-5-bromo

Cleavage of 171 with TEA was accompanied by desilylation to afford 2-aryl-5-(4-hydroxyphenyl)oxazoles 172. Bromodesilylation of 171 yielded the corresponding resin-bound 4-bromo-2,5-diaryloxazole 173, which was used in a Suzuki coupling to prepare 5-(4-hydroxyphenyl)-4-(4-methylphenyl)-2-phenyloxazole 174 (Ar = C6H5). Selected examples are shown in the Table 1.11. 

Salgado-Zamora and co-workers"" prepared a series of imidazo[5,l-h]oxazoles as potential radiosensitizers or antiprotozoal agents (Scheme 1.59). Alkylation of 4-bromo-2-methyl-5-nitroimidazole 208 with a series of phenacyl halides gave a 1 3 mixture of the regioisomeric imidazoles 209 and 210. These were separated, and 210 was cyclized to the desired 2-aryl-5-methyl-7-nitroimidazo[5,l-h]oxazoles 211. This reaction tolerates a wide variety of phenacyl halides. Selected examples are presented in Table 1.17. 

To functionalize the C5 position, Williams and Fu developed a 2-phenylsulfonyl substituted oxazole. The C5 position of this oxazole can be cleanly deprotonated with LDA and trapped with either NIS or NBS to form the 5-iodo- or 5-bromo-2-phenylsulfonyloxazole in good yield. The same report details that the 2-phenylsulfonyl group can subsequently be displaced with alkyl, alkenyl, or aryl lithium reagents to form 2,5-disubstituted oxazoles efficiently. A triflate at the C5 position can be prepared from the corresponding oxazolone however, the oxazolone decomposes at room temperature, and Kelly reported that attempted Stille coupling with C5 triflates failed due to decomposition of the triflate. ... 

The effect of the halogen on the aryl halide was also investigated using bromo- and chlorobenzene as the electrophile. Using cesium carbonate as the base and tri-(cr(-butylphosphine as the ligand, the arylation of ethyl 4-oxazole carboxylate with bromobenzene proceeded mainly at the C5 position, affording product B (eq 21). However, when chlorobenzene was used, the C2 aryloxazole A was obtained as the major product. Similar results were obtained... 

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