Axial element

Klein F, Mahr P, Galova M et al 1999 A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis. Cell 98 91-103... 

A new class of phosphines (30) containing only an axial element of chirality (atropisomerism) has been made (253, 254). An in situ 1 1 rhodium/2,2-bis(diphenylphosphinomethyl)-1,1 -binaphthyl system (30a) hydrogenated a-acetamidocinnamic acid to a 54% ee (S) using 50 atm H2, the solvent not being recorded (253). The corresponding diphenyl-phosphinite system (30b) in toluene-acetone was particularly effective (76% ee) for hydrogenation (95 atm) of a-acetamidocinnamic and a-acet-amidoacrylic esters (254). 

During the prophase of the first meiotic division (meiosis I) two homologous pairs of partially "condensed" chromosomes must find each other and pair with appropriate orientation. A protein in the telomeres of the chromosomes seems to be involved.269 277 The key structure in meiotic crossing-over is the ribbonlike synaptonemal complex formed by the pairs of homologous chroma fids.271/278 2791 This complex, in which a proteinaceous core or axial element separates the greatly extended chromatid pairs (Fig. 26-13), is fully formed in the pachytene stage of meiosis. Formation of the synaptonemal complex is preceded by development of a few double-stranded breaks in... 

Biaryls merit special interest due to their axial element of chirality and are among the most widely used ligands in enantioselective synthesis and catalysis. Their coordination by a tricarbonyl chromium fragment following benzannulation provides an additional stereogenic element in terms of a chiral plane to the molecule [68]. Biaryl quinones are similarly relevant to natural product synthesis and enantioselective catalysis. 

The vibrational excursions of a molecule may cause it to have sharply changing electrical properties from state to state. This, of course, is essential for mechanisms of absorption and emission of radiation. How sharp these changes may be is illustrated for HF in Figure 3. The curves show the axial elements of a. A, and P in the vicinity of the equilibrium bond length as a function of the H-F distance. The types of changes that may be found in a polyatomic molecule are illustrated by Figures 4 and 5. They show contours of the dipole polarizability and hyperpolarizability elements over the two stretching coordinates of HCN. Both and P yy have zero contours... 

BINAP is a new type of fully aryl-substituted dlphosphine with only an axial element of chirality. Optically pure BINAP was first synthesized by the optical resolution of (l)-BINAP using optically active d1-iJ-chlorob1s[(S)-N,N-dimethyl-a-phenylethylam1ne-2C,N]d1pallad1um. The phosphine Is also obtained by stereospecific transformation of optically active 2,2 -d1bromo-l,l -binaphthyl The procedure outlined here, however, is the best preparative-... 

However, soluble metal chiral complex catalysts are better, even though ketone hydrogenation under these conditions is more difficult to run than olefin hydrogenation. Notable achievements have been made using Rh and Ru metals. Phosphine ligands are often used, which contain either an asymmetric alkyl group have the phosphorous atom as the asymmetric center, or have an axial element of chirality (atropisomerism) . Other chiral ligands commonly used are derived from amino acids, from L-hydroxyproline and from ferrocene. 

Figure 4.2-12 represents the current evaluation of axial xenon oscillation stability to the hypothetical xenon redistribution. The relative power in axial element 4 (in top half) and element 10 (at bottom of core) were used to illustrate the power stability to the h)rpothetical xenon redistribution. The power factors in these two element locations prior to the transient were 1.42 for element 4 and 0.67 for element 10. As noted from the results given in Figure 4,2-12, the power perturbation is damped and the relative power in these two locations returns to values that are close to their original values after about 75 hours. 

Fig. 4.1 The array considered here consists of a finite number of infinitely long columns (stick arrays) with axial elements. Voltages are induced in all elements by an incident plane wave propagating in the direction S = kSx+ Sy + Is. We seek the bistatic scattered pattern.

Bidentate Phosphorus Ligands Supported on Atropisomeric Biaryl Backbones. Most ligands of this group contain binaphthyl or biphenyl moieties-providing an axial element of chirality. The flagship of this class of compounds is the atropisomeric diphosphane BINAP (36), whose complexes (primarily Ru(II) complexes (37)) play a pivotal role in asymmetric scale-up hydrogenations and isomerizations (Fig. 5). 

By steric and electronic tuning of the parent diphosphane (DTB-BINAP (38), Tol-BINAP (39), Cy-BINAP (40), etc), the resulted new ligands show similarly superior enantioface-discriminating abilities. Similarly, the biphenyl moiety served as a basic skeleton for the synthesis of the sets of BINAP analogues possessing axial element of chirality (BICHEP (41), BIPHEMP (42), MeO-BICHEP (43), BIMOP (44), and MOC-BIMOP (45)). 

The molybdenum complex of 1,1 -biphenyl-2,2 -diol-based ligand (BIPHEN) possessing an axial element of chirality proved to be an efficient catalyst in various modes of enantioselective olefin metathesis (71). (The catalytic precursor is known as the Schrock-Hoveyda catalyst.)... 

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