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9780521704632c05 CUFX213A/Peck 9780521618168 December 27, 2007 13:38
5 Isomerism——R 4R 2R 1 — C — R 3——R 4R 1 R 3R 22-Dimensional representation3-Dimensional structureThese structures cannot be superimposed——R 4R 3 R 1R 2
C CFigure 5.2.Chiral centres.plane of the double bond or ring, or on opposite sides. If the groups are on the
same side the conformation is called cis- and if on opposite sides trans-. The bis-
benzylisoquinolinium muscle relaxants, such as mivacuium, have two identical
heterocyclic groups linked through an ester-containing carbon chain. Each of the
heterocyclic groups contains a planar ring with groups that may either be arranged
in the cis- or trans-conformation. So each of these compounds needs two prefixes
describing their geometrical conformation, one for each heterocyclic group, hence
cis-cis- in cis-atracurium. Mivacurium contains three such geometric isomers, trans-
trans- (58%), cis-trans- (36%) and cis-cis (6%).Optical stereoisomers
Optical stereoisomers may have one or more chiral centres. A chiral centre is a car-
bon atom or a quaternary nitrogen surrounded by four different chemical groups.
The bonds are so arranged that in three dimensions they point to the vertices of a
tetrahedron; thus, there are two mirror image conformations that could occur but
which cannot be superimposed.A single chiral centre
The absolute spatial arrangement of the four groups around a single chiral centre
(Figure5.2), either carbon or quaternary nitrogen, is now used to distinguish iso-
mers as it unambiguously defines these spatial relationships. In the past the two
isomers were distinguished by the direction in which they rotated plane polarized
light (dextro−and laevo, d−and l−,+and−), but now their absolute configura-
tions are designated R or S. This is determined by the absolute arrangement of the
atoms of the four substituent groups directly attached to the chiral atom. First, the
atom of lowest atomic number is identified and the observer imagines this group