Asymmetric catalysis Mukaiyama-type

Chiral sulfoximines liganded to copper(II) give highly enantioselective vinylogous Mukaiyama-type aldol reactions under mild conditions.137 A chiral sulfinyl group has been used to achieve 1,5- and 1,6-asymmetric induction in Mukaiyama aldols, using Yb(OTf)3 catalysis.138... 

The third part of this chapter reviews previously described catalytic asymmetric reactions that can be promoted by chiral lanthanoid complexes. Transformations such as Diels-Alder reactions, Mukaiyama aldol reactions, several types of reductions, Michael addition reactions, hydrosilylations, and hydroaminations proceed under asymmetric catalysis in the presence of chiral lanthanoid complexes. 

Keck also investigated asymmetric catalysis with a BINOL-derived titanium complex [102,103] for the Mukaiyama aldol reaction. The reaction of a-benzyloxyalde-hyde with Danishefsky s dienes as functionalized silyl enol ethers gave aldol products instead of hetero Diels-Alder cycloadducts (Sch. 40) [103], The aldol product can be transformed into hetero Diels-Alder type adducts by acid-catalyzed cyclization. The catalyst was prepared from BINOL and Ti(OPr )4, in 1 1 or 2 1 stoichiometry, and oven-dried MS 4A, in ether under reflux. They reported the catalyst to be of BINOL-Ti(OPr% structure. 

As discussed in Section III J, in general, catalytic asymmetric aldol reactions have been studied using enol silyl ethers, enol methyl ethers, or ketene silyl acetals as a starting material. So far several types of chiral catalysis have been reported.75-85 The chiral lanthanoid complex prepared from Ln(OTf)3 and a chiral sulfonamide ligand was effective in promoting an asymmetric Mukaiyama aldol reaction with a ketene silyl acetal.86 The preparation of the catalyst and a representative reaction are shown in Figure 45. 

During the last decade, use of oxazaborolidines and dioxaborolidines in enantioselective catalysis has gained importance. [1, 2] One of the earliest examples of oxazaborolidines as an enantioselective catalyst in the reduction of ketones/ketoxime ethers to secondary alco-hols/amines was reported by Itsuno et al. [3] in which (5 )-valinol was used as a chiral ligand. Since then, a number of other oxazaborolidines and dioxaborolidines have been investigated as enantioselective catalysts in a number of organic transformations viz a) reduction of ketones to alcohols, b) addition of dialkyl zinc to aldehydes, c) asymmetric allylation of aldehydes, d) Diels-Alder cycloaddition reactions, e) Mukaiyama Michael type of aldol condensations, f) cyclopropana-tion reaction of olefins. 

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