Effect of chirality

Miller, KE. Rich, D.H. Molecular Mechanics Calculations of Cyclosporin A Analogues. Effect of Chirality and Degree of Substitution on the Side-Chain Conformations of (2s, 3r, 4r, 6e)-3-Hydroxy-4-methyl-2-(methylamino)-6-octenoic Acid and Related Derivatives. 

Thus, a novel chiral zirconium complex for asymmetric aza Diels-Alder reactions has been developed by efficient catalyst optimization using both solid-phase and liquid-phase approaches. High yields, high selectivity, and low loading of the catalyst have been achieved, and the effectiveness of chiral catalyst optimization using a combination of solid-phase and liquid-phase methods has been demonstrated. 

NNRTI and a Previous Structurally Related Development Candidate Table 1.8 Effect of chiral modifiers on organo-zinc chemistry. 

Scheme 10 Plausible catalytic mechanism for alkyne-carbonyl coupling as supported by the effect of chiral Bronsted acid catalyst and deuterium-labeling...

The ability of STM to image at the atomic scale is particularly exemplified by the two other chapters in the book. Thornton and Pang discuss the identification of point defects at Ti02 surfaces, a material that has played an important role in model catalyst studies to date. Point defects have been suggested to be responsible for much of the activity at oxide surfaces and the ability to identify these features and track their reactions with such species as oxygen and water represents a major advance in our ability to explore surface reactions. Meanwhile, Baddeley and Richardson concentrate on the effects of chirality at surfaces, and on the important field of surface chirality and its effects on adsorption, in a chapter that touches on one of the fundamental questions in the whole of science - the origins of life itself ... 

This chapter has reported the only extensive and coordinated investigation of the effects of chirality on the properties of monolayer films spread at the air-water interface. Twenty compounds of varied headgroup and chain length have been examined carrying one and two chiral centers. In every case, all of the optical isomers—enantiomers and diastereomers—were made and their properties measured both as pure compounds and as mixed monolayers in order to compare phase changes in the films with mixed melting points of the crystals. 

S. Chang, J. M. Galvin, E. N. Jacobsen, Effect of Chiral Quaternary Ammonium Salts on (salen)Mn-Catalyzed Epoxidation of ds-Olefms. A Highly Enantioselective,... 

Now let us turn to an examination of the A-[Co(en)2(Ala-(S)-AlaOMe)]3+ products. When 0.03 M (S)-AlaOMe was used RP-HPLC gave 50% A-RS and 50% ASS after 8 s, building to 76% A-RS and 24% ASS at the conclusion of the reaction (500 s). With 0.06 M (S)-AlaOMe the result was 32% A-RS and 68% ASS (4 s) building to 58% A-R-S and 42% ASS (24 s) with 0.06 M (R)-AlaOMe the comparison were 34% A-R-S (4 s) building to 55% (24 s), i.e., nearly the same distributions. These results require the A-R ester to react with (S)- or CR)-AlaOMe to give the A-R-S (or A-R-R) product at a considerably faster rate (via k4) than does the A-S ester to give ASS (or A-S-R). Also, either differences between the rate constants for epimerization and aminolysis cancel the effect of chirality of the amine base or the rate constants themselves are little affected by the amine chirality. Subsequent analysis on other systems found the latter explanation to hold. 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

See Section IV.1 for alternative methods of chiral resolution. Partial chemical hydrolysis of proteins and peptides with hot 6 M HC1, followed by enzymatic hydrolysis with pronase, leucine aminopeptidase and peptidyl D-amino acid hydrolase, avoids racemiza-tion of the amino acids281. The problems arising from optical rotation measurements of chiral purity were reviewed. Important considerations are the nonideal dependence of optical rotation on concentration and the effect of chiral impurities282. 

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

The powerful directing effect of bis(isopinocampheyl) allylic boranes has been put to great use in the context of several applications of double diaster-ereoselective allylations in the total synthesis of natural products. As discussed in a previous section, the Brown allylation can be exploited to overcome the stereodirecting effect of chiral a-stereogenic aldehydes, including a-aUcoxy substituted ones. Thus, the simple allylation of aldehyde 154 provides as major product the desired diastereomer needed towards a total synthesis of brasilenyne (Scheme 14). The yield and stereoselectivity is even increased to over 97 3 under the low-temperature, magnesium-free conditions described before. 

Compared with chiroptical methods and nuclear magnetic resonance spectroscopy (NMR), only chiral chromatography by direct and indirect methods is suitable for the accurate determination of enantiomeric impurities of less than 1% and for quantitative stereochemical analyses of small sample amounts (for example, in vivo studies of the metabolic pathway or pharmacokinetic effects of chiral pharmaceuticals.)... 

Finally, the effect of chiral groups at the 4-out position in the conrotatory transition structures (114, 115) was studied at the B3LYP/6-31G level. It was found that both transition structures are almost isoenergetic, thus showing that in electrocyclic conrotatory transition structures the 4-outward disposition is much less efficient as a source of chirality, a result already observed by Pannunzio et al. in their experimental studies [32, 33] (Scheme 29). 

Schafer examined the effect of chiral auxiliaries on the stereochemistry of reduction of a series of a-ketoamides (equation 32)88. Diastereomeric excesses ranged from 42 to 81%. 

Thisayukta J, Niwano H, Takezoe H, Watanabe J (2001) Effect of chiral dopant on a helical Sml phase of banana-shaped N-n-O-PIMB molecules. J Mater Chem 11 2717-2721... 

The effect of chiral symmetry breaking on the physical picture described above is to additionally split the degenerate Andreev levels. A dispersion asymmetry Aa 0 lifts the left-right symmetry of electron transport through the junction and splits the doubly degenerated Andreev levels at

[Pg.222]

The Maruoka group s further efforts toward simplification of the catalyst have led to the design of new, polyamine-based chiral phase-transfer catalysts of type 15, with expectation of the multiplier effect of chiral auxiliaries, as illustrated in Scheme 5.10 [13]. The chiral efficiency of such polyamine-based chiral phase-transfer catalysts (S)-15 was examined by carrying out an asymmetric alkylation of glycine derivative 2 under phase-transfer conditions. Among various commercially available polyamines, spermidine- and spermine-based polyammonium salts were found to show moderate enantioselectivity. In particular, the introduction of a 3,4,5-trifluor-ophenyl group at the 3,3 -positions of chiral binaphthyl moieties showed excellent asymmetric induction. 

S. Goldmann and J. Stoltefuss, 1,4-Dihydro-pyridines effect of chirality and conformation on the calcium-antagonistic and -agonistic effects, Angew. Chem. 1991, 103, 1587-605 Angew. Chem. Int. Ed. En. 1991, 30, 1559-1578. 

Mention of chirality in dendritic architectures can be traced back to patents of Denkewalter et al., which describe the construction of peptide-like dendritic structures from L-lysine units [1]. In spite of the demanding nature of some of these syntheses, numerous chiral dendritic structures have meanwhile been prepared and characterised [2]. This cannot be explained solely by the somewhat academic interest in the effect of chiral monomeric building blocks on the chirality of the overall molecule. The prospect of using chiral dendrimers as model... 

Figure 9.8 Effect of chiral selectors on enantiomeric resolution of FITC-baclofen and FITC-norfenefrine in NCE. (a) CD, HP-a-CD, and DM-0-CD, (b) HP-0-CD and DM-(3-CD [27],...

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