"Chiral centres sequence" vs "isostructurality"

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oiggab_odracir
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Joined: 13 Jul 2011, 18:56
Location: Buenos Aires

"Chiral centres sequence" vs "isostructurality"

Post by oiggab_odracir » 22 Aug 2017, 15:04

Dear Colleagues:
I am far away from Organic Chemistry, and came across a puzzling problem which ended up being trivial when correctly analyzed, with the adequate information.
The point is that I solved two natural products with exactly the same "ring bone" presenting 7 chiral centers. The macro-rings were absolutely superimposable and accordingly, isostructural, but the chiral sequences were (1): 1S,4S,5S,6R,7R,9S,10S, but (2):1S,4S,5S,6R,7R,9R,10S, as correctly assigned by PLATON, following the usual rules on the subject. The puzzling situation for me was that in spite of the groups being absolutely isostructural, they presented different chirality at C9. How could that be?
The answerer (probably obvious for an organic chemist) was that there was an H replaced by an OH in a neighbouring site (C8), with what the "weights" used in evaluating the chirality at C9 in each structure were different, and thus the resulting chiralities inverted. In my ignorance of the chirality assignment rules, it took me quite a while to find out the reason.
The conclusion for me was that the chiral sequence could not be taken as a trustable argument to analyze isostructurality. And the final point is if I got trapped in a trivial problem, currently discussed in undergraduate lectures in Organic Chemistry, or it is not that frequent and is worth mentioning when writing a paper…?
Suggestions are welcome
Ricardo (an ashamed illiterate in Organic Chemistry…and quite a few more disciplines)

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oleg
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Joined: 24 Nov 2008, 21:34

Re: "Chiral centres sequence" vs "isostructurality"

Post by oleg » 22 Aug 2017, 17:01

Hi,

when I was developing RSA (R/S analysis) procedure in Olex2 I discovered that Platon produces different results for two atoms in sucrose - the rules for the rings are confusing but Olex2 gives the 'heavier' part of the ring more weight.

Give it a try for comparison - just open the structure and type 'rsa' in the command line (you may need to type 'text' or scroll the output with PgUp/PgDown).

Cheers,
Oleg.

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