24
1 Parent Structures
According to a recent rule revision the numbering of each interior atom is
now related to its lowest numbered peripheral neighbour atom by a superscript denoting the number of bonds between the two atoms. The following example stands among others for the intricacies involved in the application of orientation rules a) through e). Here the decision for the correct
orientation is only possible after lowest locants for the fusion positions
have been evaluated.
1
6arll: ca a 8 a 1
c::c:f
2a
4
4a
4a
\.
)
Y
correct
incorrect
The sequence 2, 3, 6, 8 is lower than and thus senior to 3,4, 6, 8 or 2,4, 7, 8.
Note: not the sum is decisive but the first point of difference.
8
7
6
Benzo [a] anthracene
3
4
7
6
3
4
Dibenzo [e,g] phenanthrene
not: Naphtho [2, l-e] phenanthrene
If additional benzo units are likewise ortho-fused to the above pentacyclic
system, helical structures result which can be specified as hexa-,hepta- etc.
-helicenes. Since the current rules lead to confusing variable peripheral
numberings for individual members of that compound class, a new consistent numbering scheme has been proposed according to which numbering always starts at one end of the helix and proceeds sequentially to
the other.