6.7 Specifications of Stereochemistry
189
H. C02H
~ -< t-S-Nitrocyclohexane-r-l,c-3-dicarboxylic acid
H * * C 02H
02N
H
CX~~
U"" CI
1,t-2-Dichlorocyclopentane-r-l-carboxylic acid
Et
CI
1·"' 0' · Me r -I-Chloro-3-ethyl-l-methyl-t -3-iodocyclohexane
All the above examples contain stereo genic centers and can therefore also
be characterized according to the rules for the specification of absolute
configuration as described in the following paragraphs.
6.7.2
Specification of Absolute and Relative Configuration
6.7.2.1
Compounds with Stereogenic (Traditionally Asymmetric) Carbon
(and Analogous) Centers
Chiral molecules with stereo genic centers of known absolute configuration
are differentiated by the stereo descriptors Rand S assigned according to the
sequence rules stated below. For tetracoordinated centers these descriptors
are obtained by placing the lowest ranked ligand (atom or substituent
group) away from the viewer so that the remaining three ligands exhibit a
clockwise (R) or anticlockwise (S) arrangement, depending on their ranking 1. Such steric arrangements can be characterized by the collective term
tripodal stereo genic unit.
1 For amino acids, carbohydrates, and related compound types the traditional descriptors D and L are preferred.