4,5-disubstituted 2-phenyloxazoles

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 ... 

Cycloadditions can also be achieved with diazomethane giving pyrazoles (equation 8-57),112 and with 4-phenyloxazole to give 3,4-bis(tributylstannyl)- and 3-tri-butylstannyl)-furan (equation 8-58), which can be converted into other symmetrically or unsymmetrically 3,4-disubstituted furans by Stille coupling or via monolithiation.113 114... 

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. ... 

Nilsson and Hacksell isolated 2,5-dimethyl-4-phenyloxazole 245 (Ri = CH3, R2 = CeHs) as the only product from methylation of N-(l-phenyl-2-propynyl)acet-amide 244 (Ri = CH3, R2 = CeHs). The authors found that propargyl amides could be cyclized to 2,5-disubstituted and 2,4,5-trisubstituted oxazoles (Scheme 1.67). Thus a propargyl alcohol 243 was converted to the corresponding amide 244 in... 

Irradiation of a methanolic solution of an oxazole at —78°C in the presence of O2 and rose bengal produced a 2,5-disubstituted 5-hydroperoxy-4-methoxy-4,5-dihydrooxazole 709 (Scheme 1.197). Here again, the oxazole substitution pattern determined the product distribution. For example, 4,5-diphenyl-2-methyloxazole 231a produced only N, iV-dibenzoyl acetamide 706 (Ri = CH3, R2 = R3 = CeHs). On the other hand, 4-methyl-2-phenyloxazole 701a gave 709 (Ri = CgHs, R2 = CH3, R3 = H), which decomposed on warming to room temperature to yield... 

Both 1005 and 1008 (X = H) are readily acylated by an activated ketene, e.g., dichloroketene or f-butylcyanoketene (Scheme 1.271). The authors isolated excellent yields of the a,a-disubstituted 2-acyl-5-phenyloxazoles 1015a/b (X = H). In contrast, the a,a-disubstituted 2-acyl-4-methyloxazoles 1014a/b were isolated in modest to good yield. Nonethless, the yield of 1014a was an improvement over direct acylation of 3 using dichloroketene. ... 

The two a-bonds that are cleaved in the retro Diels-Alder reaction need not necessarily be the same as those formed in the initial forward reaction. Many valuable retro Diels-Alder reactions involve the cleavage of one carbon-carbon and one carbon-heteroatom bond, or two carbon-heteroatom bonds, that were not set up in a forward Diels-Alder reaction. For example, the otherwise relatively inaccessible 3-substituted and 3,4-disubstituted furans can be prepared by way of a tandem Diels-Alder then retro Diels-Alder reaction of 4-phenyloxazole and substituted alkynes, as illustrated in Scheme 3.87. 

The first three structural classes are characterized by a polycondensed aromatic ring, the fourth by an ortho, ort/io -disubstituted phenyl ring, and all four classes by a flexible linker between the 2-phenyloxazole and the 0-atom in the para-position to the propionic acid unit. 

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... 

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