6.7 Specifications of Stereochemistry
191
(5)-(1-Dj )Ethanol
(5)-2,2-Di(37CI,35CI)
chlorobutane
Me
rh-CI (5)-1,5-Dichloro-3-methylpenta -1-cis,4-trans-diene
CI
When several stereogenic (formerly: asymmetric or chiral) centers are
present in a molecule the over-all configuration is characterized by a set of
R,S symbols preceding the compound name.
I
ot' CI
N02
~O 0
CI
Br I
( lR,25)-2-Chloro-l-ethyl-l,2-dihydrol-iodocyclobutabenzene
(1 5,2R,35)-1- Bromo-3-chloro-2-methyl2-nitrobutyl acetate
If only relative configurations should be marked the descriptors R* I S* are
applied for both enantiomers in such a way that the center with the lowest
locant is arbitrarily labelled with R*. Alternatively the unstarred symbols
RI5 can be utilized here too when specified by the prefix rel.
OR*, 35*,55*)- oder rel-OR,3S,S5)-3-Methoxycarbonyl-S-nitrocyclohexane-I-carboxylic acid
For compounds derivable (if only in the widest sense) from carbohydrates
and containing only two stereo genic centers the traditional descriptors
erythro/threo are still frequently used and, as indicators of relative ste-