[jschulte]

How is absolute configuration (R or S) assigned to a molecule that has two identical groups attached to the chiral center, e.g., how was the configuration (S) assigned to (S)-olean?

[discodermolide]

How is absolute configuration (R or S) assigned to a molecule that has two identical groups attached to the chiral center, e.g., how was the configuration (S) assigned to (S)-olean?

This is a good case for making a model.

[qw098]

This is a good case for making a model.

Here is the model: http://www.pherobase.com/database/synthesis/synthesis-detail-S-olean.php

[jschulte]

I've seen the model.  That doesn't answer my question.  How do you assign priority to the groups around the chiral center if two are identical?

[qw098]

Now I understand your question. Great question... now that I realize it, I have no answer!

Anyone have an answer?

[fledarmus]

Basically you do it by defining a chiral plane that runs through the center of the molecule. Groups on one (arbitrarily chosen) side of the plane have a higher priority than groups on the other.

Try this reference:

http://books.google.com/books?id=S4o0Hhv7K34C&pg=PA11&lpg=PA11&dq=assigning+stereochemistry+to+helical+compounds&source=bl&ots=MBKapPkWIj&sig=2hLau6eaPh9Pm5s-c7U4m9xjIH4&hl=en&sa=X&ei=cY2YT5_7MI-f6QHC14GFBw&ved=0CCAQ6AEwAA#v=onepage&q&f=false

[qw098]

Great stuff as always Fledarmus!

[Dan]

See also: http://en.wikipedia.org/wiki/Axial_chirality

[jschulte]

Thanks to fledarmus and Dan.  I think I recall seeing the Fisher projection in the Wikipedia reference many years ago when I took organic.  That refreshed my memory.  Thanks again.