2-aryl-substituted-4-phenyloxazoles

The 2-aryl-substituted 4-phenyloxazole 144 has been used in the synthesis of the pharmacologically active tetrahydrofuran lignan natural products (Fig. 3.43). Refluxing a mixture of 144 with 2-butyn-l,4-diol diacetate in the presence of sodium carbonate and hydroquinone for 22 h provided an 89% yield of the 2,3,4-trisub-stituted furan 145. Elaboration of 145 by hydrolysis of the diacetate and selective oxidation of the 4-carbinol then afforded the lignan 146 as a mixture of diaster-eomers after three additional steps. ... 

A -Acylamino aromatic ketones 790 can be prepared by arylation of saturated oxazolones in the presence of Lewis acids. Cyclodehydration of 790 leads to 2,5-diaryloxazoles 791. For example, saturated 5(4//)-oxazolones 789 from 7/-benzoyl-alanine or A-benzoylvaline undergo Friedel-Crafts arylation to afford substituted A-benzoylphenacylamines 790. In the presence of POCI3, 790 cyclizes to produce 5-aryl-2-phenyloxazoles 791 (Scheme 7.241). ... 

Aryl-substituted oxazoles are reducible [275] at rather negative potentials [—1.0 to —2.1 V (SCE)], and the electrode reaction consumes in most cases six electrons 2-(l-naphthyl)-5-phenyloxazole, however, consumes two electrons. 

TABLE 1.14. MICROWAVE ASSISTED SYNTHESIS OF 2-PHENYL-4-SUBSTITUTED OXAZOLES AND 4,5-DISUBSTITUTED 2-PHENYLOXAZOLES FROM ARYL KETONES, BENZONITRILE, AND MERCURY(II)TOSYLATE ... 

The Diels-Alder reaction of oxazoles with alkynes has become a preferred method for the synthesis of substituted furans with diverse applications. A large number of early examples of this reaction have been tabulated. Activated dienophiles such as acetylenic ketones and esters can be used, although unactivated alkyl, aryl, and silyl alkynes have been used as well. In particular, the reaction of 4-phenyloxazole with substituted acetylenes is frequently used for preparing 3,4-disubstimted furans. Cycloadducts derived from 4-phenyloxazole typically decompose under milder conditions than 4-alkyloxazoles, allowing the synthesis of a wider range of functionalized furans. Several examples are shown below. 

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