- Reaction Conditions for the Regiodivergent Direct Arylations at C2- or C5-Positions of Oxazoles using Phosphine-Free Palladium Catalysts
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Two sets of reaction conditions for the regiodivergent C2- or C5- direct arylations of oxazole are reported. In both cases, phosphine-free catalysts and inexpensive bases were employed allowing the access to the arylated oxazoles in moderate to high yields. Using Pd(OAc)2/KOAc as catalyst and base, regioselective C5-arylations were observed; whereas, using Pd(acac)2/Cs2CO3 system, the arylation occurred at the C2-position of oxazole. The higher reactivity of C5-H bond of oxazole as compared to the C2-H bond in the presence of Pd(OAc)2/KOAc system is consistent with a concerted metalation deprotonation mechanism; whereas the C2-arylation likely occurs via a simple base deprotonation of the oxazole C2-position. Then, from these C2- or C5-arylated oxazoles, a second palladium-catalyzed direct C?H bond arylation affords 2,5-diaryloxazoles with two different aryl groups. We also applied these sequential arylations to the straightforward synthesis of 2-arylphenanthro[9,10-d]oxazoles via three C?H bond functionalization steps. The Ru-catalyzed C?H arylation of the aryl unit of 2-aryloxazoles is also described. (Figure presented.).
- Shi, Xinzhe,Soulé, Jean-Fran?ois,Doucet, Henri
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p. 4748 - 4760
(2019/09/12)
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- Synthesis and Antitrypanosomal Activity of Some Bis(4-guanylphenyl) Five- and Six-Membered Ring Heterocycles
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2,5-Bis(4-guanylphenyl)-1,3-oxazole, 2,5-bis(4-guanylphenyl)-1,3,4-oxadiazole and -1,3,4-thiadiazole, and 3,6-bis(4-guanylphenyl)pyridazine and several of their ''cyclic guanyl'' analogues have been synthesized. 2,5-Bis(4-guanylphenyl)-1,3-oxazole and -1,
- Das, Bijan P.,Wallace, Rebecca A.,Boykin, David Withers
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p. 578 - 581
(2007/10/02)
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