2-phenyl-4-oxazole triflate

Schaus and Panek also employed oxazole triflates as coupling partners in their palladium-catalyzed synthesis of vinyloxazoles for application to the C(26)-C(31) subunit of phorboxazole. They reported an improved procedure of preparing 2-phenyl-4-oxazole triflate 984 from 2-phenyl-4(57/)oxazolone 983 (Scheme 1.263). With 984 in hand, they developed a one-pot Cp2ZrCl2 catalyzed carboalumination of a terminal alkyne to produce an intermediate vinyl alane (not shown), which was then coupled with 984 to generate a 4-( )-alkenyl-2-phenyloxazole, e.g., 985 or 987, respectively. 

TABLE 1.71. 4-( 0-ALKENYL-2-PHENYLOXAZOLES FROM PALLADIUM-CATALYZED CROSS-COUPLING REACTIONS OF 2-PHENYL-4-OXAZOLE TRIFLATE"... 

Larhed et al. investigated enantioselective Heck reactions with 2,3-dihydrofuran as alkene [86]. In the coupling with phenyl triflate, conditions previously reported by Pfaltz [87] were attempted under microwave irradiation. Interestingly, the catalytic system Pd2(dba)3/(4S)-4-t-butyl-2-[2-(diphenylphosphanyl)phenyl]-4,5-dihydro-l,3-oxazole, identified by the Swiss team, was found suitable for high-temperature microwave-assisted enantioselective Heck reactions (Scheme 76). Using a proton sponge as a base and benzene as a solvent gave the best conversions (Scheme 76). At tempera-... 

In the model studies toward the total synthesis of dimethyl sulfomycinamate, Kelly et al. successfully carried out the Stille couplings of oxazolyl triflate 18 with an array of organostannanes [19, 20]. Thus, 2-aryl-4-oxalone 17 was transformed into the corresponding triflate 18, which was then coupled with 2-trimethylstannylpyridine under the agency of Pd(Ph3P)4 and LiCl to provide adduct 19. The couplings of triflate 18 with phenyl-, vinyl- and phenylethynyl trimethyltin all proceeded in excellent yields. Unfortunately, application to the more delicate system in the natural product failed and the oxazole moiety was installed from acyclic precursors. 

Analogously, Barrett and Kohrt transformed 2-phenyl-4-oxalone into triflate 20, which was then converted to the corresponding stannane 21 using Pd-catalyzed coupling with hexamethyldistannane. Subsequent coupling with 2-iodooxazole 4 elaborated bis-oxazole 22 [2],... 

Fluoride-ion-mediated generation ofbenzyne from silylaryl triflates (41) [20] is mild and excellent in chemical yields andsoithasbeen employed for a variety of electrophilic (42) [21 ], nucleophilic (43) [22], and cycloaddition reactions [23] (Scheme 3.26). N-Phenylations of sulfonamides (44), amines (45) [24], azirines [25], oxazoles [26], pyrroles [27], and imidazoles [28] with benzyne from 41 are also known. 

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