Coupling of triflates

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. 

Triflates are used in this reaction [107]. 5-Phenyltropone 241 is prepared by the coupling of triflate 240 with PhZnCl [108], Instead of rather expensive triflates, triflate equivalents such as phenyl mesylates [100], fluoroalkanesulfonates [109] and the fluorosulfonate 242 [110] are used for the coupling. 

It was shown that palladacycles 1, prepared from palladium(II) acetate and the tris(a-tolyl)- or trimesitylphosphane, are excellent catalysts for the Heck coupling of triflates and halides (including certain aryl chlorides). In this case, oxidation states -l-U and +IV appear to be involved in the catalytic cycle [16]. 

In a similar vein, in a bold application of Stille chemistry, Danishefsky completed the synthesis of eleutherobin by the remarkable coupling of triflate 125 with (trimethylstannylmethyl)arabinose donor... 

The Pd-catalyzed intermolecular C—O bond formation has also been achieved, and is also most effective with bulky, electi on-rich phosphines [91b, 95]. In addition, Buchwald has shown that these types of ligands facilitate the Pd-catalyzed diaryl ether formation [96]. When 2-(di-/ 7-butylphosphino)biphenyl (95) was used as the ligand, the coupling of triflate 93 and phenol 94 afforded diaryl ether 96 in the presence of PdtOAcfj and K,PO, . The methodology also worked for electron-poor, neutral, and electron-rich aryl halides. 

Palladium-catalyzed cross-coupling of terminal alkynes with 4-trifloyloxazole was studied forthe construction of the C(26)—C(31) subunit of phorboxazole A [44].The StiUe coupling of triflate oxazole (54) with vinyl stannane (55) afforded the key intermediate for phorboxazole A [8, 9]. 

A pyridazine-containing bioisostere of (-)-ferruginine was obtained via the coupling of triflate 159 with 4-(tributylstannyl)pyridazine (156) [71]. Several reaction conditions were tried to optimize the yield of this reaction. The best results were obtained using 10 mol% Pd(PhCN)2Cl2, 20 mol% Ph. As. and 20 mol% Cul in combination with three equivalents of LiCI in V-mcthyl pyr roll done. Removal of the carbamate group of 160 finally afforded the bioisostere of (-)-ferruginine (161). 

Chiral oxazolines Stille carbonylative coupling Pd-catalyzed carbonylative coupling of triflates of ketones and phenols with chiral amino alcohols provides )3-hydroxy amides, which cyclize to chiral oxazolines when treated with thionyl chloride. 

The step following was a Suzuki coupling of triflate 765 and boronic acid 769 in the presence of tetrakis(triphenylphosphine)palladium, potassium phosphate, and potassium bromide to obtain the coupling product 770 504). Alkaline hydrolysis of... 

Catalytic asymmetric intramolecular Mizoroki-Heck reactions have been employed to synthesize several polyaromatic polyketide natural products. In one such endeavour, the catalytic asymmetric cyclization of alkenyl naphthyl triflate 84 was a central step in total syntheses of halenaquinone (88) and halenaquinol (89) recorded in the Shibasaki group laboratory (Scheme 16.19) [52]. Naphthyl triflate 84, which arose from Suzuki coupling of triflate 81 with alkylborane 82, was cyclized using the conditions that had become nearly... 

The nucleophihcity of nickel is well suited to activate phenol-derived electrophiles. This is particularly interesting, as phenols are widely available at low cost. Efficient conditions have been developed for the couplings of triflates, mesylates, tosylates, carboxylates, cabamates, suUamates, ethers, and even unprotected phenolates [46, 47]. Recently, it has been evidenced that catalysts such as 61 can promote the... 

Yamamoto has reported the synthesis of (4-boronylphenyl)alanine (BPA), used dinically for treatment of malignant melanoma and brain tumors in neutron capture therapy by Pd-catalyzed coupling of triflate 47 with the diboron derivative 48 [22]. The boronic ester 49 could be easily cleaved by hydrogenolysis to give I-BPA 50 in 74% yield, whereas the corresponding pinacol ester yielded mixture... 

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