Axis of chirality

The asymmetric synthesis of allenes by stereoselective manipulations of enantio-merically pure or enriched substrates relies on the availability of such optically active substrates. In contrast, a direct synthesis of allenes by the reaction of prochiral substrates in the presence of an external asymmetric catalyst is an almost ideal process [102]. Most of the catalytic asymmetric syntheses in organic chemistry involve the creation of chiral tetrahedral carbon centers [103], whereas the asymmetric synthesis of allenes requires the construction of an axis of chirality. 

The meaning of the defining phrases for the axis and the plane of chirality were now clarified by stating (4) that the axis of chirality is derived by de-... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Axis of chirality,... 

Whereas a center or an axis of chirality can by clearly defined 2> 7) there is still some ambiguity in respect to the specification of a plane of chirality. It therefore seems somewhat difficult to define the scope of planar chiral compounds which at a first glance include mostly rather rigid aromatic compounds of special interest from synthetic, structural, spectroscopic and especially chiroptical points of view. 

Transfer of chirality in aldol reactions has been attempted using / -allenyl ester enolates. These ambident nucleophiles have an axis of chirality, and such compounds have been less utilized in stereoselective reactions. They are prepared by transmetallation of the... 

An interesting class of chiral enolates are allenyl enolates. These ambident nucleophiles bear an axis of chirality. Krause and coworkers have found that an axis to center chirality transfer takes place in the aldol reaction of chiral magnesium allenyl enolate with pivalic aldehyde . The aldol reaction proceeds with good diastereofacial selectivity if... 

Molecules display an axis of chirality when two structural conditions are met, namely (a) that they have four groupings occupying the vertices of an elongated tetrahedron, and (b) that these groupings meet the condition a = b (diagrams XXIX). If these two conditions are fulfilled, the XY axis becomes an axis of... 

DMAP itself is achiral but a chiral version would make the achiral reactive intermediate 6 chiral. Since the alcohol 7 reacts with this species, there is the possibility that one of the alcohol enantiomers will react more quickly than the other and there will be a kinetic resolution. All that needs is for a chiral version of DMAP to be developed. Because DMAP has two planes of symmetry, this takes some doing. One way that was developed by Spivey et al. was to use an axis of chirality.6 One plane of symmetry is removed because a naphthyl ring is attached on one side and not the other while the chiral axis differentiates the back and front faces of the pyridine ring 8. For the esterification of 9 the s factor was 27 which means good levels of ee in both the starting material and the product should be possible. Indeed 97% ee of the starting material can be achieved at around 56% conversion. 

Another chiral DMAP 11 has been developed by Fu.8 This contains neither a chiral centre nor an axis of chirality and is probably best described as having planar chirality. The s factor was improved from 14 to a very useful 43 simply by changing the solvent and reaction temperature The references contain useful sample experimental procedures.8,9 Several alcohols including 12-15 have been resolved with this but in looking at the ees, remember to note the conversion necessary to achieve them. Clearly the acetylene 15 was more difficult to resolve.9... 

Axis of chirality See chirality element, stereogenic axis, stereogenic element. 

Each of these compounds presumably possesses an axis of chirality but the configurations of these were not reported. 

Molecules without chirality centers can be chiral. Biphenyls that are substituted can exhibit an axis of chirality. When A B, and X Y, the two conformations are nonsuperimposable mirror images of each other that is, they are enantiomers. The bond connecting the two rings lies along a chirality axis. 

Chiral allenes are another example of molecules that are chiral, but that do not contain a chirality center. Like the chiral biaryl derivatives that were described in Section 7.9, chiral allenes contain an axis of chirality. The axis of chirality in 2,3-pentadiene is a line passing through the three carbons of the allene unit (carbons 2, 3, and 4). 

So far it was elucidated that the polyacetylene chains propagate along the director (an averted direction for the LC molecules within a domain) of the chiral nematic LC. As the helical axis of polyacetylene is parallel to the polyacetylene chain and the director of the chiral nematic LC is perpendicular to the helical axis of chiral nematic LC, the helical axis of polyacetylene is perpendicular to that of chiral nematic LC. Considering these aspects, one can describe a plausible mechanism for interfadal acetylene polymerization in the chiral nematic LC, as shown in Figure 3.7. In the case of a right-handed chiral nematic LC, for instance, the polyacetylene chain would propagate with a... 

Chirality about an axis is exemplified by allenes, such as the 2,3-penta-diene enantiomer 23. In 23 the methyl and hydrogen substituents on C2 lie in a plane (in the page) that is perpendicular to the plane containing the methyl and hydrogen substituents on C4. Here the axis of chirality is coincident with the C2—C3—C4 bond axis. The structure has a C2 symmetry element, so it is dissymmetric, not asymmetric. The C2 rotation axis is perpendicular to the axis of chirality, as illustrated in Figure 2.13. It must be emphasized that not all structures that are chiral about an axis have a C2 rotation axis. For example, the 2,3-hexadiene enantiomer 24 also has an axis of chirality coincident with the C2-C3-C4 bond axis, but it does not have a C2 rotation axis. The adamantane 25 and appropriately substituted spiro compounds, such as 26, are also chiral about an axis. ... 

For illustrations and examples of both an axis of chirality and a plane of chirality, see Prelog, V. (with Cahn, R. S.) Chem. Brit. 1968, 4,382. See also Prelog, V. Science 1976, 193,17. 

The relative rates of faster reacting slower reacting amine enantiomers was shown to be 27 1, by a quantitative measure of each rate on pure enantiomers of 2-methylpiperidine. Interestingly, resolution with diastereomeric quinazoline 32b gave as the benzoylamide product the enantiomer of the above experiment, suggesting that the axis of chirality controls the enantiomer selectivity. For the use of similar quinazolines in enantiodivergent reactions, see Scheme 6.28. 

Another type of chiral quinazoline has been investigated by Atkinson in the resolution of racemic 2-methylpiperidine. Here, the chiral substituent is not present in the 2-quinazoline position but in the N-acyl groups [49]. Crystal structures of these compounds reveal their exo/endo conformation, and NMR experiments indicate that no conformational interconversion exo/endo-endo/exo takes place at room temperature, indicating a N-N axis of chirality. DAQ 33... 

This binaphthol compound, by virtue of restricted rotation about the single bond joining the two naphthalene groups, is a chiral compound. That is, the molecule cannot readily exist in a planar form because of the steric interference of the bulky —OH substituents, and, in fact, the two enantiomers have been separated. Such enantiomeric compounds contain no stereocenter, but rather have an axis of chirality, or a chiral axis, which in this case contains the bond joining the two napthalene rings (see Fig. lO.SW) ... 

A second area concerns the use of the word chiral. This has a specific definition and can only be properly applied to three-dimensional objects. Thus it is incorrect to refer to a chiral centre/axis or a chiral synthesis . The former can be a chirality centre/axis or a centre/axis of chirality although for most purposes we prefer to replace these terms by stereogenic centre/axis (which is, however, not equivalent). [51 As a collective term for enantio- and diastereoselective... 

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