Suzuki enantioselective

In a synthesis of (-i-)-asteltoxin, Cha applied the Suzuki-Tsuchihashi rearrangement to silyloxy epoxide 184 for the enantioselective construction of the unusual... 

Shibasaki and coworkers [87] described the first enantioselective combination of this type in their synthesis of halenaquinone (6/1-162) (Scheme 6/1.43). The key step is an intermolecular Suzuki reaction of 6/1-159 and 6/1-160, followed by an enantioselective Heck reaction in the presence of (S)-BINAP to give 6/1-161. The ee-value was good, but the yield was low. 

Pu and co-workers incorporated atropisomeric binaphthols in polymer matrixes constituted of binaphthyl units, the macromolecular chiral ligands obtained being successfully used in numerous enantioselective metal-catalyzed reactions,97-99 such as asymmetric addition of dialkylzinc reagents to aldehydes.99 Recently, they also synthesized a stereoregular polymeric BINAP ligand by a Suzuki coupling of the (R)-BINAP oxide, followed by a reduction with trichlorosilane (Figure 10).100... 

The development of chiral catalysts for use in enantioselective rhodium-catalyzed hydroborations was pioneered by Burgess9, Suzuki,77 and Hayashi.78 The chiral diphosphine ligands employed in their preliminary investigations 23-26 (Figures 2(a) and 2(b)), had previously been successfully applied in other catalytic asymmetric transformations. 

In concurrent and independent work, Suzuki and Enders found that tethered keto-aldehydes undergo highly enantioselective cross-benzoin reactions using tria-zolium based catalysts [50, 51], The scope includes various aromatic aldehydes with alkyl and aryl ketones (Table 4). Additionally, aliphatic substrate 39a is cyclized in excellent enantioselectivity, albeit in 44% yield. 

Table 4 Suzuki and co-workers enantioselective intramolecular cross-benzoin]...

Suzuki and co-workers have relayed this methodology into the synthesis of (+)-sappanone B (Scheme 5) [52], The authors found that catalysts previously introduced by Rovis and co-workers led to inferior results iV-Ph catalyst 41 gave significant elimination while Al-C Fj gave low enantioselectivities. By tuning the electronics of the M-aryl substituent these workers identified 49 as providing the optimal mix of reactivity and enantioselectivity. Commercially available 2-hydroxy-4-methoxybenzaldehyde 47 was transformed into aldehyde 48, which upon treatment with triazolium salt 49 in the presence of base was cycUzed to afford (R)-50 in 92% yield and 95% ee and subsequently transformed into (-t-)-sappanone B. 

Suzuki and co-workers first published on the topic of enantioselective transesterification in 2004 [140, 141]. This process exploits C -symmetric imidazolium salts with various substitutions. When vinyl propionate 281 acts as the acyl donor, ester 282 is isolated in 68% ee at 19% conversion, corresponding to an s value of 6.1 (Eq. 27). 

Similar results were obtained by Suzuki et al. (55) in their investigation of enantioselective addition of Et2Zn to various N-diphenylphosphinylimines with PAMAM dendrimers functionalized with 4 and 8 chiral amino alcohol groups. They observed 92% ee for the monomeric ligand, 43% ee for GO, and 30—39% ee for G1 when using N-diphenylphosphinylbenzaldimine as the substrate. Again, a high local concentration of chiral active sites leads to a decrease in enantioselectivity. 

In a related study, the Shibasaki group examined cyclizadon of naphthyl triflate 10.1 (Scheme 8G.10) [23], Cyclization of 10.1 under standard cationic conditions gave Heck product 10.2 in 78% yield and 87% ee. Evidently, the reaction is fairly tolerant of the nature of the aryl group, because both 10.1 and 9.3 behaved similarly. An interesting variation of this reaction was also demonstrated in which Suzuki coupling and asymmetric Heck cyclization were performed in a one-pot operation. Thus, treatment of ditriflate 10.3 with borane 10.4 under standard Heck conditions provided 10.2 in similar enantioselectivity to the stepwise procedure, albeit in quite low yield. Heck product 10.2 was converted in several steps to the natural products, halenaqui-none (10.5) and halenaquinol (10.6). 

Suzuki et al. [74] and Maruoka and colleagues [75] made further progress in the enantioselective acylation of secondary alcohols 93. Suzuki et al. reported moderate enantiomeric excesses (up to 51% ee) when employing Ci symmetric chiral imidazolium salts 94 as precatalysts and vinyl acetate as acylation agent. How-... 

In a interesting example of organocatalysis, Suzuki et al. studied the enantioselective acylation of secondary alcohols using chiral NHCs [11,12]. The approach was partly based on the work of Nolan and Hedrick who had independently reported NHC-catalyzed transesterifications [13,14]. The enantioselective acylation was subsequently improved by using more sterically hindered acylating agents such as diphenylacetate derivatives (Scheme 4), leading to selectivity factors (s = kn, ) of up to 80 [15,16]. 

Sakakura A, Suzuki K, Nakano K, Ishihara K (2006) Chiral l,l -binaphthyl-2,2 -diammonium salt catalysts for the enantioselective Diels-Alder reaction with a-acyloxyacroleins. Org Lett 8 2229-2232 Sanwal BD, Zink MW (1961) L-Leucine dehydrogenase of Bacillus cereus. Arch Biochem Biophys 94 430-435... 

Takikawa H, Hachisu Y, Bode JW, Suzuki K (2006) Catalytic enantioselective crossed aldehyde-ketone benzoin cyclization. Angew Chem Int Ed Engl 45 3492-3494... 

Masumoto S, Usuda H, Suzuki M, Kanai M, Shibasaki M (2003) Catalytic enantioselective Strecker reaction of ketoimines. J Am Chem Soc 125 5634-5635... 

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