Stereogenic plane

An NOE experiment of cyclic ether 40 with irradiation at the methyl group on C-3 showed 3% enhancement in the signal of the vinyl proton at C-8. This result along with the molecular modeling suggests that the C(3)-C(4) and C(7)-C(8) olefinic moieties of 40 form stereogenic planes in the most stable conformation, and proves its planar chiral nature <2005JA12182>. 

Stereogenic plane The descriptors may be modified to Rp, Sp when applied to a stereogenic plane [85], although it is usually more convenient to use the P, M system to specify the configuration of stereogenic axes and planes [86]. 

Stereogenic plane A planar structural fragment that, because of restricted rotation or structural requirements, cannot lie in a symmetry plane. If the stereogenic plane is reflection variant, the element may be called a chirality plane. For example with a monosubstituted paracyclophane, the stereogenic plane includes the plane of the benzene ring. For a 1,2-disubstituted ferrocene, the disub-stituted cyclopentadiene lies in a chirality plane. The absolute configuration may be specified by either R, S or P, M. See also stereogenic element. 

Chirality is the result of arrangement of molecules about a stereogenic plane. 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

A molecule that contains just one chiral carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric or stereogenic carbon atom) is always chiral, and hence optically active. As seen in Figure 4.1, such a molecule cannot have a plane of symmetry, whatever the identity of W, X, Y, and Z, as long as they are all different. However, the presence of a chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since some molecules with two or more chiral carbon atoms are superimposable on their mirror images, and hence inactive. Examples of such compounds will be discussed subsequently. 

The substituted carbons are stereogenic carbons. This means that there are not only two isomers. In the most general case, where W, X, Y, and Z are all different, there are four isomers since neither the cis nor the trans isomer is superimposable on its mirror image. This is true regardless of ring size or which carbons are involved, except that in rings of even-numbered size when W, X, Y, and Z are at opposite comers, no chirality is present, (e.g., 68). In this case, the substituted carbons are not chiral carbons. Note also that a plane of symmetry exists in such compounds. When W = Y and X=Z, the cis isomer is always superimposable on its mirror image, and hence is a meso compound, while the trans isomer consists of a dl pair, except in... 

Fig. 1. Introduction of one or more than one stereogenic elements (center, axis, plane or helix) leads to different types of chiral dendrimers...

Both cis- and trans-1,4-di m ethy 1 cycl ohexanes have a symmetry plane => have no stereogenic centers => Neither cis nor trans form is chiral => neither is optically active. 

Analogous but even more complex is the analysis of syndiotactic vinyl polymers. In the first model, 7, individual macromolecules are achiral through the presence of the c and m planes the sequence of tertiary atoms (stereogenic but not chirotopic) will be. . . r, j, r, j, r, s. . . . 

Moving two-dimensional enantiomorphs out of the plane into three-dimensional space allows them to become congruent, i.e., identical. However, some two-dimensional aspects remain. When the two faces of a particular figure are examined, one perceives that the faces, and the corresponding half-spaces10, of the three-dimensional space, are enantiomorphic, and their chirality sense can be specified by Re/Si descriptors. Furthermore, for figures with more than one stereogenic center the Ikjul and ZjE descriptions are preserved in three-dimensional space. 

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 ... 

For molecules with two stereogenic centers, the traditional descriptors are erythro and threo to which, henceforth, a c (short for carbohydrate) is added for a reason that will become apparent (Table 10). They are unambiguously defined for simple sugars. The backbone is given by the carbon chain the reference conformation is the conformation with all carbon atoms eclipsed. This statement is identical with the requirement of a Fischer projection. If two identical substituents X are located on the same side/opposite sides of the plane traced by the backbone, then the c-erythrojc-threo configuration is assigned. 

Ferrocenes of type 11 (as well as cyrhetrenes such as 14) are characterized as having two elements of chirality a stereogenic center at the oxazoline ring and a plane of chirality due to the two ortho substituents on the ferrocene core. 

In most steroids the B-C and C-D rings are fused, usually in a trans manner. The lower side of the steroid is denoted a, the upper side of the steroid is denoted (3, usually drawn as projected helow the plane of the paper, which is shown as broken lines, and above the plane of the paper, which is drawn as solid lines. Thus, substituents attached to the steroid are also characterized as a and (3. Cholesterol has eight chiral centres, therefore 256 stereoisomers are theoretically possible, but only one exists in nature Stereogenic centres in steroid side chains are denoted preferentially with the R and S nomenclature. 

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