On 19 April 2017 at 05:31, Robert Hanson <hans...@stolaf.edu> wrote:
> a) The first S on the highest-ranking branch (O...X) sets the reference
> atom for this unit, regardless of how far down the line it is.
>
Nope, because that's not the highest ranked node. Remember it's all about
spheres :-), nodes are ranked hierarchally.
The rule for pairing stereodescriptors is as follows: “A reference
> descriptor for chirality centers, identified as ‘R’ or ‘S’ (not associated
> with any node of the digraph), is chosen in each ligand and is: (a) the one
> associated with the *highest rank node* corresponding to a chiral unit in
> the ligand; (b) the one that occurs the most in the set of equivalent
> highest rank nodes; or (c) sequentially both descriptors (‘R’ and ‘S’), if
> these occur in the same number in the set of equivalent highest ranked
> nodes.”
An easier example by your scheme this would be R. ChemSketch (and centres -
shown) say S:
[image: Inline images 1]
It's been a few years but IIRC this rule is nasty and effectively requires
a power set <https://en.wikipedia.org/wiki/Power_set> in some cases.
John
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