Stereo descriptors

The consecuhve levels of 3D informahon are illustrated in Fig. 7.2. The pure cormechvity informahon is usually referred to as 2D. If stereo information is available, it can be referred to as 2.5D since the stereo descriptors add some 3D informahon. From this, a single 3D structure is obtained from the program GORINA [5] and, subsequently, a mulh-conformer ensemble from the program OMEGA [6] (bottom left). In this chapter, we will refer only to the step from 2D (2.5D) to 3D. 

Which stereo descriptors are used to describe them ... 

Explain in a few short sentences what is meant by the term relative configuration. Which stereo descriptors can be used to describe the relative configuration ... 

Insert at the appropriate place a stereo descriptor endo, exo, syn or anti into... 

Identify the stereogenic units in the molecules represented by the following formulae, assign appropriate stereo descriptors and indicate what type of isomerism is possible in these compounds. 

Diamminedichloridoplatinum(II) can exist as the two isomers shown below. Isomer A is the cytostatic agent cisplatin and usually described by the stereo-descriptor cis, whilst its cy to statically inactive diastereomer B is assigned the stereodescriptor trans. 

The stereo descriptor Z indicates that the configuration of both chirality centres in a molecule with only two chirality centres is the same. However, this does not give any information about the absolute configuration. Thus Z- 1,2-dichlorocyclobutane can mean a mixture of the enantiomers (R,R) and (S,S)-1,2-dichlorocyclobutane but can also be used for just one of the pure enantiomers. The priority order of the atoms attached to the chirality centres is Cl > C(C1,C,H) > C(C,H,H) > H. 

To denote a stereo descriptor for a chirality axis a subscript a is used (not italicised). Thus in this example the compound is (.Ra)-l-bromobuta-1,2-diene. Although it has been common practice to omit the a in the stereo descriptor, for clarity this is not recommended and should not be done. 

The stereo descriptor u denotes that two chirality centres are present the configuration of which is unlike. One of these chirality centres is due to the unshared electron pair on the substituted pyramidal sulfur atom. The unshared electron pair is given the lowest priority in determining the configuration. 

The structural formula of this new cytostatic (picoplatin) is shown below. In determining the stereo descriptor, the situation arises here that the highest ranking ligand occurs twice. In such cases the priority number of the ligand coordinated trans to it with the lowest priority (i.e. with the highest priority number) is used as the configuration index (principle of trans maximum difference). 

In the crossed aldol reaction between acetaldehyde and propiophenone, two chirality centres are created and consequently, four stereoisomers will be produced. Compounds A and B are enantiomers of each other and can be described with the stereo descriptor u. Similarly, C and D are enantiomers and are /-configured. Since both starting materials are achiral, without the use of a chiral base or chiral auxiliary, racemates will be produced. Likewise the choice of base, the addition of a Lewis acid and the reaction conditions used to form the enolate can control which diastereomer is preferentially formed. If the Z enolate is formed, the u product is the preferred product, whilst the E enolate yields predominately the / product. 

When stereospecific search options are used, only compounds that include stereochemical information are regarded. The stereo property of atoms is defined by specifying wedged and dashed bonds. The resulting stereo descriptor is calculated internally. If no stereo center has been specified, the stereo configuration is usually presumed as undefined. Stereo bonds can be defined in several ways ... 

Isomers selective Checks only if the database molecule has stereo information at the same positions as the query (independent of the stereo descriptors). 

For more information on this and other topics cited above, as well as for descriptions of the use of geometrical and stereo descriptors, polyhedral... 

Keywords FuUerenes (higher fullerenes). Chirality, Stereo descriptors. Reactivity, Regioselectivity. 

CD/ChemSketch is a freeware for drawing chemical structures including organics, organometallics, polymers, and Markush stmctures [31]. It has options for structure cleaning, viewing and naming, inch conversion, stereo descriptors etc. For freeware, no technical support is provided and the functionalities are less compared to the commercial version which has structure search capabilities (Fig. 1.5). 

The last rule defines codes for atom, weight, volume, or unspecified percentage. The term string indicates that some word, name, or descriptor is required and in several places a more complex syntax has been abbreviated by enumerating required notions in normal text. These rules for the term substance-data define the substance identification information, what now remains is the description of the composition of the substance. The specification of the stereochemistry ha,s been shortened, suffice it to say that one can define a complex system of stereo descriptors which take care of every kind of stereochemistry such that a given isomer is unambi-geously characterized. Following are the refinements for the terms component data and component ... 

This finishes the specification of the atoms which constitute a compound. Again, the definition was simplified, and the specification of both atom symbol and atom attribute was not further elaborated. For atom symbol, the terms should be self-explaining. The atom attributes allow a very precise specification of the properties of an atom. The attributes atom stereo descriptor, coordination, and atom parity flag are part of the stereochemistry description. The different counts and the valency are just positive integers, the cartesian coordinates are provided for a graphical display of a structure with a suitable program. 

CAS represents stereochemistry in polymers by text descriptor terms when the necessary information is reported such terms include isotactic, syndiotactic, threo-diisotactic, erythro-diisotactic, and disyndiotactic. The term atactic (for a random configuration) is not employed by CAS in indexing specific polymers. In addition to the special terms above, polymer stereochemistry is defined, when appropriate, by the stereo descriptors E, Z, R, S, R, and S. ... 

Figure 13 represents a chemical structure record as It Is Input to the system. The L-number In the upper left Is the registration number. This Is followed by the molecular formula, then any stereo descriptors, reference to source, and finally the chemical structure. This Is a typical single record. The file contains approximately 135,000 such records. The year of registration Is added to the record automatically at the time of Its Input to the system. 

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