CONFIGURATIONAL ISOMERS
119
N
Me
OH
pseudotropine
R
S
r
N
Me
OH
tropine
R
S
s H
H
N
Me
CO 2 Me
OH
(−)-methylecgonine
S
H
S
R
R
Now, you may have noticed that the hydroxyl group in methylecgonine is oriented differently from that in
tropine. In methylecgonine it is easy to define the position of the hydroxyl, since this is a chiral centre and we
can use the R/S nomenclature. An alternative stereoisomer of tropine exists, and this is called pseudotropine.
How can we define the configuration for the hydroxyl when the plane of symmetry of the molecule goes through
this centre and means this centre is not chiral but can exist in two different arrangements?
This is a situation allowed for in the IUPAC nomenclature rules, because if we are faced with two groups which
are the same but have opposite chiralities, then the group with R chirality has a higher priority than the group
with S chirality. Applying this rule, tropine would have the S configuration and pseudotropine the R configuration
at this centre. Because of the plane of symmetry, these atoms are not strictly chiral, and this is taken into account
by using lower-case letters; tropine is s and pseudotropine is r.
119
N
Me
OH
pseudotropine
R
S
r
N
Me
OH
tropine
R
S
s H
H
N
Me
CO 2 Me
OH
(−)-methylecgonine
S
H
S
R
R
Now, you may have noticed that the hydroxyl group in methylecgonine is oriented differently from that in
tropine. In methylecgonine it is easy to define the position of the hydroxyl, since this is a chiral centre and we
can use the R/S nomenclature. An alternative stereoisomer of tropine exists, and this is called pseudotropine.
How can we define the configuration for the hydroxyl when the plane of symmetry of the molecule goes through
this centre and means this centre is not chiral but can exist in two different arrangements?
This is a situation allowed for in the IUPAC nomenclature rules, because if we are faced with two groups which
are the same but have opposite chiralities, then the group with R chirality has a higher priority than the group
with S chirality. Applying this rule, tropine would have the S configuration and pseudotropine the R configuration
at this centre. Because of the plane of symmetry, these atoms are not strictly chiral, and this is taken into account
by using lower-case letters; tropine is s and pseudotropine is r.