6.7 Specifications of Stereochemistry
193
6.7.2.2
Molecules with Helical Stereogenic Units
Stereogenic units of quite different types are responsible for the existence
of enantiomers in the case of axially and planary chira! molecules whose
helicities - POus} or M(inus) - can most easily be derived with the help of
appropriate four-point figures inscribed in the respective structural formulae.
1'00 (P)
2
1 \ ....... 3
.-rV
(M)
4"" 2
The appertaining stereo descriptors depend on whether the syn-periplanar
arrangement of connecting lines 1 - 2 and 3 - 4 (corresponding to a superposition of planes 1,2,3 and 2, 3, 4) requires a right (P) or left (M) turn
around axis 2 - 3.
6.7.2.2.1
Screw-like Molecules (One Chirality Axis)
Screw-like structures are found above all for molecules such as helicenes,
1,3-disubstituted allenes, 2,2'-disubstituted biphenyls, etc., displaying C2
symmetry. With the aid of the 4-point figures indicated in the formulae the
absolute helicities (P or M) can straightforwardly be ascertained. The currently still customary descriptors as (or Sa) and aR (or Ra) regrettably correspond only inversly to the much more generally applicable helicity symboIs P and M.
(P)-Hexahelicene
H
(M}-6,6' -Dinitrobiphenyl-2,2' -dicarboxylic acid
CI
~C"'CI ()
D· hI
d·
,~C 3
4
P -1,3- lC oropropa lene
, 2C
A
H
193
6.7.2.2
Molecules with Helical Stereogenic Units
Stereogenic units of quite different types are responsible for the existence
of enantiomers in the case of axially and planary chira! molecules whose
helicities - POus} or M(inus) - can most easily be derived with the help of
appropriate four-point figures inscribed in the respective structural formulae.
1'00 (P)
2
1 \ ....... 3
.-rV
(M)
4"" 2
The appertaining stereo descriptors depend on whether the syn-periplanar
arrangement of connecting lines 1 - 2 and 3 - 4 (corresponding to a superposition of planes 1,2,3 and 2, 3, 4) requires a right (P) or left (M) turn
around axis 2 - 3.
6.7.2.2.1
Screw-like Molecules (One Chirality Axis)
Screw-like structures are found above all for molecules such as helicenes,
1,3-disubstituted allenes, 2,2'-disubstituted biphenyls, etc., displaying C2
symmetry. With the aid of the 4-point figures indicated in the formulae the
absolute helicities (P or M) can straightforwardly be ascertained. The currently still customary descriptors as (or Sa) and aR (or Ra) regrettably correspond only inversly to the much more generally applicable helicity symboIs P and M.
(P)-Hexahelicene
H
(M}-6,6' -Dinitrobiphenyl-2,2' -dicarboxylic acid
CI
~C"'CI ()
D· hI
d·
,~C 3
4
P -1,3- lC oropropa lene
, 2C
A
H