Plane chirality

In most common chiral molecules, chirality arises from chiral tetravalent atoms. A conformation-independent chirality code (CICC) was developed that encodes the molecular chirality originating from a chiral tetravalent atom [42], For more generality, a conformation-dependent chirality code (CDCC) is used [43]. CDCC ti cats a molecule as a rigid set of points (atoms) linked by bonds, and it accounts for chirality generated by chirality centers, chirality axes, or chirality planes. 

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

Enantioselective enzymatic desymmetrization is the transformation of a substrate that results in the loss of a symmetry element that precludes chirality (plane of... 

Planar Chirality. Planar chirality arises from the desymmetrization of a symmetric plane in such a way that chirality depends on a distinction between the two sides of the plane and on the pattern of the three determining groups. In the definition of this chiral system, the first step is the selection of a chiral plane the second step is to identify a preferred side of the plane. The chiral plane is the plane that contains the highest number of atoms in the molecule. 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Chirality plane,... 

In a tour de force in 1956, Cahn, Ingold and Prelog introduced the terms chirality axis (descriptors aR/aS) and chirality plane (descriptors pRjpS) in order to deal with compounds such as allenes, biaryls and cyclophanes. Rules for assigning the chirality sense were devised ad hoc. In 1966 the helieity concept was introduced and it was recognized that its use allows the corresponding models to be treated in an alternative way. The specific proposals, as illustrated in Table 1, however, were only published in 19821. 

Fortunately, the original assignment rule for the chirality plane is identical to the helieity assignment defined above with descriptors aRjaS and PjM, respectively, corresponding. However, the rule for the chirality axis was based on an elongated tetrahedron as the stereogenic unit and the descriptors aR and P or aS and M are not equivalent ... 

Recently new definitions for the chiral plane were put forward 6,21,27) and appropriate procedures for the application of the descriptors (R)p and (S)p proposed 21,27). They will not be discussed in this article since we shall follow the revised and refined proposal as presented in Ref. 7) 1,2... 

Chirality (handedness, from Greek cheir = hand) is the term used for objects, including molecules, which are not superposable with their mirror images. Molecules which display chirality, such as (S)-(+)-lactic acid (/, Fig. 1) are called chiral. Chirality is often associated with a chiral center (formerly called an asymmetric atom ), such as the starred carbon atom in lactic acid (Fig. 1) but there are other elements that give rise to chirality the chiral axis as in allenes (see below) or the chiral plane, as in certain substituted paracyclophanes.1,2)... 

The compound possesses a chirality plane. Before establishing the configuration, it is necessary to determine the pilot atom. This is the atom of highest priority according to the CIP rules outside the chirality plane that is directly bonded to an atom in the plane. This is indicated in the formula below with an arrow. A view from this atom onto the chirality plane shows that the atoms on the way to the cyano group will be reached by a counter-clockwise curve. The configuration is therefore Sp. 

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 benzannulation reaction further allows the concomitant generation of an axial and chiral plane in a single reaction step (Scheme 28) [68f]. The diastereomeric ratio of the benzannulation products depends on the protocol used for phenol protection. Thus, in situ protection gives the kinetic ratio of 74a 74b — 11 89, whereas a two-step benzannulation/ protection sequence results in thermodynamic control to give a ratio of 74a 74b >99 1. These results can be explained in terms of a possible or arrested rotation around the biaryl axis in the benzannulation product before protection to give either 74a or 74b. 

V/101, and V/103, have been prepared in 80, 50, and 74 % yield, respectively, by [2.3] sigmatropic rearrangement of the ylides, see Scheme V/16. The ring expansion of a mixture of trans- and cis-l-ethyl-2-vinylthiolanium hexafluoro-phosphates (V/105) gave a mixture of three sulfides, (Z)-2-methyl-thiacyclooct-4-ene (V/106) and the two diastereoisomerically related ( )-(SR,RS)- and ( )-(RR,SS)-2-methylthiacyclooct-4-enes (V/107) and (V/108)3 Scheme V/17. The existence of the two (E) isomers is evidence for the structure of the molecule, holding two elements of chirality, a chiral center, and a plane. The dia-stereoisomers are stabile because of restricted conformational inversion around the chiral plane [45] [60]. 

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