Chiral substrates

Steric Approach Chelate Control Achiral Substrate Chiral Substrate... 

Substrate Chiral ligand Reaction conditions [temp, time (h)]a) ee [%] (config.)... 

By starting with enantiomerically enriched or pure j8-allenylcarboxylates, it is possible to carry out several of the transformations mentioned above stereoselectively. With regard to the required substrates, chiral 5-alkynylidene-l,3-dioxan-4-ones of... 

Substrate Chiral ligand Metal- to-ligand molar ratio Reaction conditions Con- versionh Yield Hydroformylation product and isomeric composition1 Chiral reaction product Ref. 

Catalytic system Substrate Chirality of the prevailing enantiomer Enantiomeric ... 

The combination of nitrene and substrate chirality improves the diastereoselectivity of the cycloaddition reaction. For example, due to the use of both chiral aroyl azides and chiral alkenes, the pure exo-product (compound 108) was obtained (Sch. 32) [43]. 

Despite the fact that early experiments suggested low selectivity of nitrile-converting enzymes with respect to the substrate chirality (Faber, 1992), many recent works report the successful enantioselective bioconversion of nitriles catalyzed by nitrilases or nitrile hydratases, even if the stereoselectivity of nitrile hydratases remains often lower that that of coupled amidases. 

This method of resolution of polyolefins has been extensively studied for cyclooctatetraene systems where excellent enantiomeric excesses are normally observed. Lanthanide-induced shifting can be used to determine the diastereoisomeric composition of the urazoles. Alternate means for the resolution of polyenes based on kinetic resolution using (+)-tetra-2-pinany Iborane have been described, but this reagent consumes valuable substrate. Chiral platinum complexes can also be used but at prohibitive cost on a large scale and with poor regioselec-tivity when several coordination sites are present. 

Enantioselective reactions (achiral substrate, chiral reagent or catalyst) present a different situation, as a racemic or resolved auxiliary (in the reagent or the catalyst) must necessarily give rise to two different results. The product will be either completely racemic or may have some enantiomeric excess. This is also expressed by Eq. (1) where when... 

Due to facile introduction to the substrate, chiral amines are frequently used as auxiliaries. Their versatility is based on the opportunity of easy transformation into... 

The general approach in elucidating the stereochemical course of an enzyme-catalyzed reaction involves the following steps (i) synthesis of substrates chirally labeled (with 170, 180, or S) at phosphorus (ii) use of chirally labeled substrates to perform the reaction and isolate the product and (iii) determination of the absolute or relative configuration of the substrate and the product. 

The first applications of nickel-catalyzed [3 + 2] cycloaddition to asymmetric diastereose-lective synthesis of metbylenecyclopentanes employed acrylic ester substrates chirally modified with menthol-4 or camphor-derived41,42 auxiliaries. The adducts were obtained in good yield with diastereomeric ratios up to 99 141-42. After hydrolysis, optically active 3-methylene-l-cy-clopentanecarboxylic acids 4 were obtained. 

For the rearrangement of conformationally constrained [2,3] Wittig substrates, chiral bases can exert a more pronounced influence on the stereochemical outcome of the reaction, as evidenced by the macrocyclic ring contraction of cyclic ether 47 86. 

Entry Substrate Chiral LA (eq) Additive Yield (%) ee (%) Config... 

Enantioselective lithiation can be used to introduce and control new elements of planar or of axial chirality. The principal classes of compounds displaying these stereochemical features are ferrocenes, arenechromium tricarhonyl complexes, biaryls, atropisomeric amides and al-lenes. Methods for the enantioselective lithiation of these compound classes, relying on either substrate (chiral auxihary) or reagent (chiral base) control will be reviewed. 

Binding of Chiral Substrates—Chiral Shift Reagents... 

The transfer hydrogenation of ketones was subsequently attempted, although for these more challenging substrates chiral primary amines such as L-valine terf-butyl ester were required in order to obtain good yields (68-99%) and enantioselectivities (70-98% ee) [168]. The reaction most likely involves an iminium-phosphate hydrogen bond. Another important application of asymmetric counteranion-directed catalysis developed by the List group is the chiral phosphate anion-directed epoxidation of a, 3-unsamrated carbonyls [169]. 

In addition, a chiral 1,2-diamine derived from L-proUne was investigated as a catalyst for the KR of primary alcohols with acyl chlorides by Oriyama et al. [50], providing the highest selectivity factors of up to 16 in the case of glycerol derivatives as substrates. Chiral Ph-BOX-Cu(II) complex 17 has also been successfully... 

Stereoselective hydroformylations can benefit from the chirality in the substrate. Chirality can be part of the original substrate, but transient incorporation of... 

Crotti P, Di Bussolo V, Pineschi M (2011) Copper-catalyzed divergent kinetic resolution of racemic allylic substrates. Chirality 23 703-710... 

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