Asymmetric Induction with Substrates

In 1994, the scope of this p-hydroxy sulfoximine ligand was extended to the borane reduction of ketimine derivatives by these workers. The corresponding chiral amines were formed with enantioselectivities of up to 72% ee, as shown in Scheme 10.57. It was found that the A -substituent of the ketimine had a major influence on the asymmetric induction, with a ketoxime thioether (SPh) being the most successful substrate. 

Intermolecular Stereoselective Aryl Coupling 2.1.1. Internal Asymmetric Induction with Chiral Substrates 2.1.1.1. Dimerization of Chiral Aryl Compounds... 

Photochemical asymmetric synthesis with native and modified CDx s is certainly one of the most potentially successful examples of supramolecular photochirogen-esis, which is applicable in principle to most photoreactive substrates of appropriate size and shape. Despite this wide applicability, the enantiodifferentiating ability of CDx and hence the product ee obtained are not sufficiently high in many cases. Furthermore, the elucidation of the chiral discrimination mechanism and the rationalization of the product chirality and ee are generally more difficult in photochemical asymmetric induction. For more simple and well-defined host-guest interactions (in the ground as well as excited states), several approaches to photochemical asymmetric induction with newly designed synthetic chiral hosts have been reported. 

In this chapter, we intend to restrict the expression asymmetric catalysis to the specific case of an enantioselective reaction controlled by a chiral catalyst. We will not consider the diastereoselective reactions on a chiral substrate involving a chiral catalyst (double asymmetric induction with matched and mismatched pairs). 

The rearrangement of the 15-membered silyl ketene acetals 12 indicates a high relative asymmetric induction with the preferred formation of 16 from the ( )-substrates through the chairlike and 17 from the (Z)-substrates through the boatlike transition state. The rearranged product 14 obtained from the 14-membered silyl ketene acetal 13 immediately undergoes cyclization to give the tetrahydronaphthalene derivative 15018. 

Efficient asymmetric induction with 1,3-chirahty transfer has also been described with substrates possessing a stereogenic center in the position alpha to the sulfur atom and an allyhc moiety with a controlled configuration. Despite interesting synthetic features, reported examples are scarce and limited to the zwitterionic BeUus variant of the Claisen rearrangement [98,100]. 

In this area, the complex Cp RuCl(cod) has shown to be an excellent catalyst for the [2+2] cycloaddition of various internal alkynes with a variety of substituted norbomenes and norbomadienes [69-83]. A number of aspects of this reaction have been studied, notably the reactivity, the chemo- and regioselectivity of tmsymmet-rical substrates [69-73] and the asymmetric induction with chiral auxiliaries on the alkyne partner [74—77] [Eq. (31)]. 

Meyers has demonstrated that chiral oxazolines derived from valine or rert-leucine are also effective auxiliaries for asymmetric additions to naphthalene. These chiral oxazolines (39 and 40) are more readily available than the methoxymethyl substituted compounds (3) described above but provide comparable yields and stereoselectivities in the tandem alkylation reactions. For example, addition of -butyllithium to naphthyl oxazoline 39 followed by treatment of the resulting anion with iodomethane afforded 41 in 99% yield as a 99 1 mixture of diastereomers. The identical transformation of valine derived substrate 40 led to a 97% yield of 42 with 94% de. As described above, sequential treatment of the oxazoline products 41 and 42 with MeOTf, NaBKi and aqueous oxalic acid afforded aldehydes 43 in > 98% ee and 90% ee, respectively. These experiments demonstrate that a chelating (methoxymethyl) group is not necessary for reactions to proceed with high asymmetric induction. 

Most asymmetric induction processes with Chital auxiliaries involve a stereo-differentiating reaction that affords one diastereomet as the primary product To obtain the desired enantiomer, the Chiral auxiliary must be removed Highly dia-stereoselective reactions between otganocoppet reagents and allylic substrates with... 

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