Geometrical chirality defined

In his Nobel Lecture, Vladimir Prelog defined geometrical chirality by conjoining the two definitions17 ... 

Then, the 3D coordinates of A are used for atom t, those of B forj, those ofC for h, and those of U for I. The first three atoms (in the order established by the ranking) define a plane if they are ordered clockwise and the fourth atom is behind the plane, the chirality signal, obtains a value of -rl for the opposite geometric arrangement, obtains a value of-1. 

Intermolecular, enantioselective Heck reactions require a cyclic olefin as substrate, since syn carbopal-ladation of a cyclic olefin results in a geometrically defined a-alkyl-palladium compound. By necessity, the subsequent syn dehydropalladation must take place away from the newly formed chiral centre, thereby affording a chiral product. 

Roald Hoffmann, personal correspondence, letter of October 16, 1990. Also, R. Hoffmann, "Nearly Circular Reasoning," American Scientist, 76 (1988) 182185. And see Kurt Mislow and Paul Bickart, "An Epistemological Note on Chirality," Israel Journal of Chemistry 15 (197677) 16 "Thus chiral and achiral are used with two different connotations When the terms are applied to a geometric model, they are sharply defined, whereas when used in conjunction with observables, they necessarily entail a certain fuzziness" (6). 

Lord Kelvin lla> recognized that the term asymmetry does not reflect the essential features, and he introduced the concept of chiralty. He defined a geometrical object as chiral, if it is not superimposable onto its mirror image by rigid motions (rotation and translation). Chirality requires the absence of symmetry elements of the second kind (a- and Sn-operations) lu>>. In the gaseous or liquid state an optically active compound has always chiral molecules, but the reverse is not necessarily true. 

The condition of handedness or lack of superimposabil-ity of molecules differing only with respect to the stereochemical arrangement of identical substituents around a tetrahedral center. In 1893, Lord Kelvin succinctly defined chirality I call any geometrical figure, or any group of points, chiral, and say that it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself. ... 

Geometrically, CNTs can be described in terms of a two-dimensional graphene (graphite) sheet. A chiral vector is defined on the hexagonal lattice as... 

Unlike desferrioxamine analogs designed for specific therapeutic purposes described above, chiral DFO analogs that form conformationally unique complexes with iron(lll) were designed to serve as chemical probes of microbial iron(lll) uptake processes. As mentioned above, ferrioxamine B can form a total of five isomers when binding trivalent metal ions, each as a racemic mixture. Muller and Raymond studied three separate, kinetically inert chromium complexes of desferrioxamine B (N-cis,cis, C-cis,cis and trans isomers), which showed the same inhibition of Fe-ferrioxamine B uptake by Streptomyces pilosus. This result may indicate either that (i) ferrioxamine B receptor in this microorganism does not discriminate between geometrical isomers, or that (ii) ferrioxamine B complexes are conformationally poorly defined and are not optimal to serve as probes. 

The term Molecular Clip has been coined for molecules of type 2. That these molecules do indeed possess the geometric features of a clip is apparent from the X-ray structure of the tetramethoxy derivative 3a (Fig. 2) [lla,b]. The o-xylylene moieties of this molecule define a tapering cavity, the walls of which are at an angle of 39.5 with the centers of the benzene rings 6.67 A apart. The carbonyl groups of the glycoluril moiety, which are hydrogen-bond acceptor sites, are separated by 5.52 A. It was also possible to obtain a crystal structure of the chiral dibromo-derivative 4 of clip 3 (Fig. 3). This compound was separated into its enantiomers by HPLC on a chiral stationary phase [12]. 

Asymmetric aldol additions of geometrically defined trichlorosilyl enolates of ketones to aliphatic and aromatic aldehydes have been carried out uncatalysed, and with a chiral phosphoramide as Lewis base promoter.54 Significant differences in rates and diastereoselectivities are interpreted in terms of the changeover from a boat-like transition state, with pentacoordinate siliconate, to a chair-like transition state with hexacoordination. 

Boron enolates bearing menthol-derived chiral ligands have been found to exhibit excellent diastereo- and enantio-control on reaction with aldehydes34 and imines.35 Highly diastereo- and enantio-selective aldol additions of geometrically defined trichlorosilyl ketone enolates (31) and (32) have been achieved by promoting the reactions with chiral Lewis bases, of which (,S., S )-(33) proved to be the most effective.36 Moderate enantiomeric excesses have been achieved by using chiral ammo alcohols as catalysts for the Baylis-Hillman condensation of aldehydes with methyl vinyl ketone the unexpected pressure effect on the reaction has been rationalized.37... 

We have defined stereoisomers as isomers whose atoms are bonded together in the same order but differ in how the atoms are directed in space. We have also considered enan- Diastereomers tiomers (mirror-image isomers) in detail. All other stereoisomers are classified as diastereomers, which are defined as stereoisomers that are not mirror images. Most diastereomers are either geometric isomers or compounds containing two or more chirality centers. 

Liu et al. have reported on the total synthesis of 182 and 239 to verify their structural relationship and to confirm the relative and absolute configurations.The overall strategic plan involves (1) chemical conversion of epoxy alcohol 182 to lactone 239 via an intramolecular lactonization, (2) construction of the 14-membered cembrane ring with geometrically defined double bonds and substiments, (3) implementation of C-1 stereogenic center via a readily available chiral pool (i.e., 5-limonene), and (4) introduction of chiral epoxide functionality via a Sharpless... 

The first catalytic, diastereoselective and enantioselective cross-aldol reactions of aldehydes have also been documented. Geometrically defined trichlorosilyl enolate derivatives of aldehydes undergo diastereoselective addition to a wide range of aldehyde acceptors with good enantioselectivity. The use of chiral Lewis base (138) was critical for achieving useful enantioselectivity. ... 

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