Centred chirality

Ferrocene-based aminophosphines are shown to be effective ligands in the Ru(II)- catalysed asymmetric hydrogenation of ketones. The enantioselectivity is mainly determined by the C-centred chirality of the ligands, but the planar chirality is also important, and (Rc,SFc)- or (Sc,Rfc)- is the matched combination of chiralities.334 Dimethyl oxalate is selectively hydrogenated to methyl glycolate with Ru(acac)3,... 

The second main type of chiral phosphines are those which have carbon centred chirality on the substituents on phosphorus. These types of ligand can again... 

In order to maximize the diastereoselectivity observed for an auxiliary, it would appear reasonable that the stereo controlling functional group is in a position in space as close as possible to the newly forming stereogenic centre. Chiral imide auxiliaries such as Evans N-acyloxazolidinones (1.43) are used for asymmetric alkylation and asymmetric aldol condensation (Scheme 1.10). 

The isotropic part of the first hypcrpolarizability, fi, may be calculated using a number of methods - ranging from a simple single-centre chiral molecular orbital approach to ab / z7/(9 alculations at varying levels of approximation (see Section 4). We have computed fi for a number of chiral molecules in order to establish the typical strength and frequency dispersion of the pseudoscalar [30, 54, 59, 71]. fi is a measure of tlie signal strength in SFG experiments and a convenient measure that facilitates direct comparison with achiral second-order nonlinear optical processes. 

Mezey PG. Tying knots around chiral centres chirality polynomials and conformational... 

In practice, separation of enantiomers by the use of chiral stationary phases is not free from problems. Chiral stationary phases are difficult to prepare reproducibly, are sometimes of lower chromatographic efficiency than expected, and optimization of separation conditions is restricted by the fixed nature of the chiral centres. Chiral mobile phases are free from many of these problems, optimization of the separation is more convenient, and conventional reversed-phase columns may be used. Thus N-(2, 4-dinitrophenyl)-L-alanine-n-dodecyl ester has been used as a non-ionic chiral mobile phase additive for the resolution of 1-azahexahelicenes by reversed-phase chromatography. The resolution obtained was found to be a function of the mobile phase polarity and the concentration of chiral additive used. 

In the preceding examples, the asymmetric catalyst is a Lewis acid and hence the catalytic processes reported so far involve electrophilic activation by a metal-centred chiral Lewis acid. There is another strategy, although less explored, which consists of designing chiral Lewis bases for nucleophilic catalysis. It is well known that Lewis bases such as nitrogen heterocycles and tertiary phosphines and amines catalyse a variety of important chemical processes. For instance 4-(dimethylamino)pyridine (DMAP) catalyses the acylation of alcohols by anhydrides the mechanism by which DMAP accelerates this process provides an instmctive illustration of how nucleophiles can... 

The similarity of both quinic and shikimic acid with the target drug is obvious what is perhaps remarkable is just how many steps it takes to get from one to the other. The majority of these steps are concerned with the introduction of the two amino substituents with inversion of stereochemistry at the coloured stereogenic centres. Chiral pool syntheses often have to take long convoluted routes to correct relatively minor errors of structure and stereochemistry. Here this is simply the price we have to pay for a starting material that has the valuable qualities of enantiomeric purity and the right hydrocarbon skeleton. 

In all 19 optically active amino acids, one of the substituents on the a-carbon is a potentially acidic hydrogen atom. Removal and subsequent reattachment of this proton represents a potential mechanism for enantiomerization of this chiral centre. Chiral integrity is particularly at risk during coupling as the acidity of this proton is greatly enhanced when the carboxy group is activated. In practice, direct enolization does not appears to be an important mechanism for enantiomerization except for amino acids such as phenylglycine, which offer an additional mode of enol stabilization. 

Asymmetric synthesis for quaternary stereo centres Chiral amine synthesis (N-centered chemistry)... 

The first observable transition-metal based Lewis acid that is optically active with metal-centred chirality has been claimed for [CpRe(NO)(PPh3)]PF5. Reactions of this substrate with Lewis bases has also been described. ... 

Conceptually, the chirality of substituted metallocene can be treated by following the rules for compounds with centred chirality (see Reference [5]) by considering... 

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