Priorities stereodescriptors

The basic idea of specifying the priority of the atoms around a stereocenter in order to obtain a stereodescriptor is also incorporated into the most widespread structure representations, the Molfile and SMILES (see Sections 2.3.3, and 2.4.6). 

Fig. 13.46. Explanation of the anti-selectivity of the Ivanov reaction of Figure 13.45 by means of the Zimmerman-Traxler model. The stereodescriptors Re and Si are defined as follows. Suppose you are looking down on the plane of an alkene, in which an sp2-hybridized C atom is connected to three different substituents. You are on the Re side of the double bond if the Cahn-Ingold-Prelog priorities of these substituents decrease going clockwise, and on the Si side otherwise.

A pseudochirality centre exists on a tetrahedrally coordinated atom when two of the substituents are constitutionally identical but have the opposite chirality sense, i.e. they are enantiomorphic. An R-configured group has priority over an S-configured group. Thus it is possible to determine the configuration of a pseudo chirality centre by application of the sequence rules. To distinguish a pseudo chirality centre from a chirality centre the stereodescriptors r and s are used. 

In the six examples of AlO, the assumed planarity presumes implicit overlap of the p orbitals of X, Z and Y. The application of the rules to systems AlOa-c is also illustrated in Figure D.7. As in the related case of A3, the viewer is stationed on the side opposite W, regardless of the priorities of X, Zand W. One then proceeds in the order X/Z (plane K) > ligands m/n (plane Lj) > ligands p/q (plane Lj). Examples AlOa-1, AlOa-2, AlOb-1, AlOb-2, AlO c-1, AlOc-2 have the facial stereodescriptors shown in Figure D.7. 

According to the stereoisogram approach. Type V cases (e.g.. Fig. 10.2) are recognized to be named by the CIP system by virtue of f 5-diastereomeric relationships on the same line as Types 1 and 111. For example, a pair of f 5-diastereomers 2 and 3 located at the horizontal S-axis of Fig. 10.8 is characterized by a pair of f 5-stereodescriptors in terms of such a common priority sequence asa>b>p>p (tentatively specified for the sake of explanation). 

By virtue of the dichotomy between enantiomers and diastereomers in the conventional approach. Type I cases prefer enantiomeric relationships to rationalize l S -stereodescriptors of the CIP system, as found in Table 10.3. In other words, the conventional approach exclusively takes account of the vertical C-axis of Fig. 10.4 and results in the neglect of the horizontal S-axis, which causes systematic neglect of l 5-diastereomeric relationships. Accordingly, l 5-stereodescriptors are presumed to be given to an enantiomeric pair of promolecules 7 and 7 by applying such as a priority sequence a > b > X > Y. Fortunately, in the Type I cases, the priority sequence (a > b > X > Y) for the... 

These two compounds are not the same they are stereoisomers. But which compound should be called cis and which should be called transi In situations like this, lUPAC rules provide us with a method for assigning different, unambiguous stereodescriptors. Specifically, we look at the two groups on each vinyfic position and choose which of the two groups is assigned the higher priority ... 

When an alkene possesses nonsimilar groups, the stereodescriptors E and Z must be used. 2 indicates priority groups on the same side, while E indicates priority groups on opposite sides. 

E (Sect. 8.4) For alkenes, a stereodescriptor that indicates that the two priority groups are on opposite sides of the tt bond. 

Three-dimensional quantitative structure-activity relationship Geometric stereodescriptor used for compounds having achiral elements resulting from double bonds where the groups of highest priority are on the opposite sides of the vertical reference plane Equivalent... 

CAS uses an extension to the CIP system to denote the configuration of mononuclear coordination compounds. The stereodescriptors are composed of a system indicator (e.g., OC-6 for octahedral), a geometry label, and potentially a chirality symbol. The geometry label is assigned on the basis of CIP priorities and the chirality symbol describes the handedness of the ligand arrangement with respect to some principle axis through the center. The DARC system also uses priorities (not based on CIP) and a method has been proposed to apply them to the stereochemistry of coordination compounds, ... 

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