Chirality, examples

For a first non-chiral example see Ikeda, A. Shinkai, S. J. Chem. Soc., Chem. Commun. 1994,... 

For covalently linked heterocalix[4]arenes with two uracil units incorporated via methylen bridges see Kumar, S. Hundal, G. Paul, D. Hundal,M. S. Singh, H. J. Org. Chem. 1999,64,7717-7726. So far all examples described have Cs-symmetry, but chiral examples (Ct, C2) with one or two uracil units should be also possible. 

We are already well accustomed with the historical relationship seen in the development of NHC ligands with special features. They often follow the example of successful phosphane ligands. Chiral NHC are no exception, although in key areas the very difference in shape between NHC and phosphanes prevents the development of NHC ligands as structural phosphane mimics. The axial (atropisomers with binaphthyl backbone [6,7]) and planar (ferrocene derivatives [8,9], [2,2]-paracyclophanes [10,11]) chiral examples, however, are styled on their phosphane predecessors [12-15]. We will first look at the unique options for NHC ligands before we turn to the more familiar phosphane mimics. 

This cluster, which is a chiral example of the large class of alkyne bridged trinuclear clusters, is obtained from the ternary metal carbonyl RuCo2(CO)i by a two-step sequence involving a capping reaction and a metal exchange, with an overall average yield of 42% ... 

A chiral example of phosphazene bases was synthesized by treatment of (5)-2-(dialky-laminomethyl)pyrrolidine derived from 5-oxo-(5)-proline, with phosphorus pentachloride and subsequent addition of gaseous ammonia. The phosphazenes were isolated as HBF4 salts in high yields and fully characterized by H, and P NMR spectroscopy, various ID and 2D NMR experiments and mass spectrometry (El). The molecular structure and the absolute configuration of the HBF4 salts were determined by X-ray analysis [16]. 

In contrast, when an object and its mirror image are superposable, the object is achiral, that is, it lacks chirality. Examples of objects lacking chirality are an undecorated cup, a regular tetrahedron, a cube, and a perfect sphere. 

The Biginelli reaction has also been the focus of an increased interest in the area of asymmetric organocatalysis especially for the development of more efficient procedures and rendering new chiral examples of this process [41]. Great improvements have been reached in this field [42, 43] compared with those obtained in the chiral metal version that has been little explored [37, 38]. 

In 2004, List and co-workers [130] pioneered only one chiral example of a novel iminium catalytic conjugate reduction of a,p-unsamrated aldehyde 47a (Scheme 19a). In 2005, and independently, List s (Scheme 19b) [131] and MacMillan s groups (Scheme 19c) (as reported by the authors in Ref. [21], the complete details of their studies into transfer hydrogenation were first published via oral presentation on... 

Empirical evaluation of the twisting powers of hundreds of chiral molecules of various structural types have revealed regular trends in the magnitude of P with solute structure within specific types by interpolation, it is then possible to derive information on the absolute configuration and structure of new chiral molecules (i.e., that aspect of the structure that is responsible for the solute s chirality). Examples of this include calculations of the twist angle between aryl groups in biphenyls [338] and l,r-binaphthyls [340, 342, 343]. Alternatively, the technique can be used to determine solute enantiomeric purities it requires very small quantities of solute and is particularly useful for compounds with extremely low optical rotations. 

All the chiral examples contain an atom that is connected to four different substituent groups. Such a nucleus is called an asynunetric atom (e.g., asymmetric carbon) or a stereocenter. Centers of this type are sometimes denoted by an asterisk. Molecules with one stereocenter are always chiral. (We shall see in Section 5-6 that structures incorporating more than one such center need not be chiral.)... 

Finally, some molecules having a planar portion with one face distinguished from the other and lacking a plane of symmetry are chiral. Examples include paracyclophane (3.9) and rra s-cyclooctene (3.10) [5]. 

It is known that reaction of metal phenolates with aldehydes or ketones proceeds in regioselective manner to afford ortho-substituted hydroxyalkylation products. Bigi and coworkers reported the chiral examples of this reaction using chiral aluminum Lewis acid prepared in situ from a chiral alcohol and 1 equivalent diethylaluminum chloride. Thus, after treating phenol with the chiral Lewis acid, the resulting aluminum phenolate is reacted with chloral in toluene as shown in Scheme 6.15... 

Chirality plays a major role in the development of drugs. A chiral molecule is defined as nonsuperimposable on its mirror image. Two chiral molecules commonly called enantiomers are often compared to the right and left hands. The same type and number of atoms are in the two enantiomers but the spatial arrangement of atoms is different Usually one enantiomer is preferred over the other. In the pharmaceutical industry chiral molecules constitute a large portion of pharmaceutical sales. More than half the drugs approved worldwide are chiral. Examples include Lipitor and Zocor , as well as ibuprofen sold under the common brand names Motrin and Advil . 

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