Asymmetric reactions Sakurai allylation reaction

Related reactions Roush asymmetric allylation, Sakurai allylation ... 

The use of C2-symmetric 1,2- and 1,3-diols as chiral auxiliaries is a reliable method for asymmetric allylation of acetals [382]. Acyclic acetals derived from homochiral 1-phenylethanol undergo the Hosomi-Sakurai allylation with high diastereoselectivity [383]. Tietze et al. have, on the other hand, reported that the TMSOTf-catalyzed successive acetalization-allylation reaction of aliphatic aldehydes with homochiral silyl ethers 123 and allyltrimethylsilane gives the corresponding homoallyl ethers with complete diastereocontrol these ethers can be readily converted into enantiomerically pure homoallyl alcohols without epimerization (Scheme 10.135) [384]. This method is applicable to asymmetric allylation of methyl ketones [385]. 

The Hosomi-Sakurai-Prins reaction of the easily available enal 142 and allylsilane 143 was performed in the presence of Lewis acids to give the 4-methylene-tetrahydropyran with poor cis-trans selectivity ( 2 1), however, the pro tic acid provided only cis-144, which was converted into the aldehyde 145. The Takai iodoalkenylation, followed by desilylation and Sharpless asymmetric epoxidation, provided 146 with a 4 1 ratio of /Z, which was separable via the treatment of TBAE After protection of the primary alcohol, the alkenyllithium derived from the iodoalkene, reacted with aldehyde 147 to form 148, which was converted into epoxy-carboxylic add 149 in five steps. The key macrocyclization was performed by the treatment of 149 with Ti(Oi-Pr)4 [73] under high diluted conditions (2 mM) at 75 °C to provide the macrolactone 150 in moderate yield with 30% of the starting material recovery. After desilylation, the chemoselective oxidation of the allyl alcohol with 4-acetylamino-2,2,6,6-tetramethylpiperidine- 1-oxoammonium tetraflu-oroborate, followed by oxidative cleavage of the C20-C21 diol, produced (-)-dactyloUde (Scheme 30). 

Enantioselective Mukaiyama-aldol and Sakurai-Hosomi allylation reactions catalyzed by chiral Lewis acid are currently of great interest because of their utility for the introduction of asymmetric centers and functional groups. 

The development of asymmetric catalytic Sakurai reactions offers major challenges. As mentioned earlier, the Bp3-catalyzed allylation stops at the boron alkoxide 86 and FSiMe (87). The silyl ether 88 is not formed, nor is BF3 regenerated. Likewise, the use of TiCU yields the titanium alkoxide 89 and ClSiMe3 (90). The silyl ether 88 is not produced and TiCLt is not released. For the reaction to be catalytic, however, the alkoxide must react with the halotrimethylsilane (as in 91 with 92) to regenerate the catalyst MXn. Furthermore, the acyclic nature of the transition state makes any form of asymmetric control difficult to achieve. 

The aldehyde 218 possessing 2,6-frans-tetrahydropyran, was synthesized as shown in Scheme 48. /3-Keto ester 220 was reduced by Noyori hydrogenation [97] to give 6-hydroxy ester 221 in 94% ee, which was converted into iodide 222. Asymmetric alkylation using Myers chiral auxiliary [98] with 222, followed by acid treatment, furnished 5-lactone 223 with high stereoselectivity. Reductive acetylation, axial allylation by the Hosomi-Sakurai reaction, and ozonolysis completed the synthesis of 218. 

Asymmetric allylation is a valuable method for constructing chiral functionalized structures, and therefore many chiral allylmetal reagents directed toward a high level of asymmetric induction have been designed and synthesized. Although some of them have exhibited good to excellent enantio- and diastereoselectivities in reactions with achiral aldehydes, Yamamoto et al. developed the first method for a catalytic process in 1991 [43a]. CAB 3a has a powerful activity in the Sakurai-Hosomi allyla-... 

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