Chiral conformations

Cahn et al. (4) thought it beneficial to have two methods available for specifying the sense of chirality of the biaryls, as examples of either axial chirality or conformational chirality. As we have merged both into a single class—the line of stereoisomerism—any justification for two modes of description has vanished. Except for the modifications envisaged earlier, the procedures of Section 4 of the Sequence Rule (4) seem well suited to describe the isomerism of the line. Thus the terms MIP would become the general descriptors of the chiral line of torsion. 

Crystal Chemistry of an Atropisomer Conformation, Chirality, Aromaticity AND Intermolecular Interactions ofDiphenylguanidine... 

Ab initio methods also appear to be useful for predicting the M- to P-conformational transition barrier for reactive species, such as enolate 8. It is known that the presence of an (iS7-C3-substituent will favor the /17-con former in which the C-3 substituent adopts a pseudoequatorial arrangement. Consequently, deprotonation of C-3 at low temperature of certain benzodiazepines can result in single, conformationally chiral, nonracemic enolates locked in the /(/-configuration. The inversion barrier for enolate 8 at 195 K is calculated by DFT methods to be 17.5 kcal mol-1, which compares with 12.4 kcal mol-1 for the derivative where the N-l group is methyl instead of isopropyl <2003JA11482>. These results were used to explain the enantioselective C-3-alkylation method discussed in... 

Given a pair of stereoisomers, classify them as configurational or conformational, chiral or achiral, and enantiomers or diastereomers. 

A homodimer of a tetra-urea calix[4]arene consisting of identical phenolic units A is composed of two enantiomers with C4-symmetry, which results in overall S8-symmetry. Consequently, a heterodimer with a second calixarene consisting of four units B must be chiral, but this chirality is due only to the directionality of the hydrogen-bonded belt or (in other words) to the orientation of the carbonyl groups [42,43]. Rotation around the (four) aryl-NH bonds leads to the opposite enantiomer (conformational chirality). 

The free energy difference between the two rotamers 126a and 126b was estimated to be 0.8 kcal/mol in favor of 126a by temperature-dependent CD curves (40). Free energy differences of derivatives 103,131, and 132 were estimated to be 1.0, 0.7, and 0.2 kcal/mol, respectively. This shows that the important factor in determining the conformational chirality of the biphenyl moiety is the presence of the 2,6-trans disubstituted piperidine ring in the molecule. 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

Figure 1 shows typical chiral organic host compounds. Deoxycholic acid 1 [21] and cyclodextrin 2 [22] are well-known natural compounds. Tri-0-thymo-tide 3 is a conformationally chiral host compound [23]. The host compounds 1-3 were utilized in the early historical stages, but the enantioselectivities were not... 

In this chapter, we mainly discuss and deal with photoresponsive, chiral biorelated and synthetic polymers bearing a configurational and/or conformational chirality in the polymer main chains. The photocontrol of the chiral recognition ability of chiral polymers and chirality induction on achiral polymer films by circular polarized light (CPL) are also briefly reviewed. 

The low-temperature H NMR spectra for calix[4]- and calix[5]arenes reveal that all four or five phenolic units are identical.225 This is compatible with a cone conformation with C4v and C5v symmetry, respectively.226,227 For calix[8]arenes the same pattern is interpreted by a / -symmetrical pleated loop conformation, with a regular up-and-down of the methylene bridges. Calix[7]arenes, however, show seven singlets for the OH protons and seven (overlapping pairs of doublets) for the methylene protons. This total lack of any symmetry element (C,) makes this conformation chiral.228... 

In some cases, conformational chirality of the guest included into cyclodextrin cavity can be clearly deduced from the ICD spectra Lighter, D. A. Gawronski, J. K. Gawronski, K. J. Am. Chem. Soc. 1985, 707,2456-2461. 

The accessibility component, represents the recognition between the specific CYP-protein and the ligand when the ligand is positioned in the CYP-protein and exposes the atom i to the heme. It depends on the ligand 3D structure, conformation, chirality, and on the 3D structure and sidechain flexibility of the CYP-enzyme. Thus the score is proportional to the exposure of the ligand atom i to the heme group of a specific CYP-enzyme. 

In certain cases, seemingly simple enolates can have a chiral memory effect . For example, treatment of a-imino lactam (5)-88 with f-BuOK in CD3OD for 6-13 days at 25°C gave the corresponding enantiomerically deuteriated a-imino lactam l-d-(S)-89 in quantitative yield with 98% D incorporation and ee 97% (equation 15) , via a conformationally chiral enolate. This methodology has been extended towards enan-tioselective alkylation of enolates. Excellent levels of enantioselectivity (ee 88%) were achieved for a-imino lactam (S)-SS using KHMDS as Brpnsted base and benzyl iodide as the electrophile . Interestingly, to prevent unwanted racemization of the intermediate enolate, the reaction time for deprotonation was lowered to 10 seconds, and to ensure rapid alkylation, 20 equivalents of Bnl were used . 

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