R and S descriptors

Using R and S descriptors write al for a molecule with three chirality centers... 

There are many procedures to determine the chirality sense of a stereomodcl, the best known being the R and S descriptors of the Cahn-Ingold Prelog (CIP) system. 

Give the Cahn-Ingold-Prelog R and S descriptors for the diol(s) formed from cis-2-pentene and fran5 -2-pentene on treatment with the osmium tetraoxide/tert-butyl hydroperoxide reagent. 

Chirality may exist in many molecules that do not possess a chiral center. Such compounds may possess a chiral plane or a chiral axis, and are said to be dissymetric with respect to either that plane or that axis. Certain optically active allenes, biaryls, alkylidenecyclohexanes, and spiranes provide examples of axially dissymmetric molecules (chiral axis). fra/i5-Cycloalkenes exemplify planar dissymmetry in molecules. The configurations of these classes may be specified by the Cahn-Ingold-Prelog convention using the usual R and S descriptors. Special subrules, which we will not describe here, are applied to this purpose. The interested reader is referred to Refs. 8 (see p. 65) and 9 for details. Scheme 2.1 presents some molecules that are optically active because of planar or axial dissymmetry, and for which the absolute configurations have been determined. 

The diirality of a molecule is described by specifying its configuration. The system that is used is the Cahn Ingold-Prel6g convention, which uses the descriptors R and S. The Fischer convention, employing the descriptors D and L, is historically important and is still used with certain types of molecules. 

The concept of d and l as configurational descriptors is well established, particularly in amino acids and sugars frankly, however, we could live without them and save ourselves a lot of confusion. Since they are so widely used, we need to find out what they mean, but in most cases the information conveyed is less valuable than sticking with R and S. 

Sequence rule 5. i.e., R> S, covers pseudoasymmetric" units, requiring descriptors r and s. Note that the descriptors R and 5 refer to isolated ligands, i.e., the r,s descriptors are not... 

A further shortcoming of the most commonly used topological indices is the inability to take into account stereo-specific properties of molecules, such as atomic chiralities, enantiomers (R-and S-isomers), and - and I J-diastereomers (Z- and E-isomers). Topological descriptors proposed to fill the gap to account for chirality and ZE-isomerism can be found in Schultz et al. (1995), de Julian-Ortiz et al. (1998), and Golbraikh et al. (2001) as well as in Lekishvili (1997 2001), and Golbraikh et al., (2002), respectively. It is probably a question of availability and time until the power of these descriptors is demonstrated and they begin to be used routinely in QSPR analysis. 

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. 

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