On Fri, Apr 14, 2017 at 3:28 AM, Wolf Ihlenfeldt <w...@xemistry.com> wrote:
> On Fri, Apr 14, 2017 at 1:37 AM, Robert Hanson <hans...@stolaf.edu> wrote:
> > Can someone shed some light on how this brilliant statement:
> >
> > Rule 3. When two ligands differ only in that one has an atom
> > or atom-group of higher rank in a cis-position and the
> > other in a trans-position to the core of the stereogenic
> > unit, then preference is given to the former.
>
OK -- I see that the 2013 final version is much simpler than I thought, in
that one can simply take care of most of the E/Z business by itself in Rule
3. (I hear you, Wolf. Not pretending I have this totally worked out for
more complex cases. I consider that the realm of Rule 4.)
Q: Aren't there cases where you have to know the R/S to get E/Z, but you
also have to know E/Z to get the R/S? How is that trap evaded?
No, not even that is sufficient. You can have of course stereo double
> bonds determining the fate of an atom stereo center which are
> themselves E/Z only by virtue of having substituents which differ only
> in R and S - which then need to be determined first to get the double
> bond descriptor. You cannot determine atom and bond stereochemistry
> independently in the CIP system. This must be a single, repetitive,
> interwoven process.
>
>
> Please keep us posted on your implementation line count ;-)
>
>
>
Rules 1-3 implemented. Tracking multiple stereocenters, but not carrying
out much in the way of Rule 4. All cases that I have that do not involve
auxiliary R/S determination (such as inositol) appear to be working ---
basically anything but Rule 4. No pseudochirality; no axial or other more
sophisticated forms of stereochemistry. At this point I can say, though,
that the algorithm is certainly suitable for most typical cases and holds
up pretty well with the sorts of cases illustrated in Chapter 9. All
methods are relatively simple, with efficient recursion and structural
analysis only on a "need-to-know" basis.
412 lines
Bob
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