I would like to differentiate between two diastereotopic hydrogens using the like/unlike nomenclature. My substance consists of a racemic mixture of a molecule with 3 stereocenters (diastereomerically pure). Which of these 3 stereogenic centers is used to determine the like/unlike hydrogen? Thank you!
I would like suggest to use the pro-R / pro-S nomenclature. That will be clear and unambiguous.
Dear Barbara,
thanks for your reply and for your suggestion. As I understand it, the pro-R/pro-S nomenclature refers to the absolute configuration. So, pro-R hydrogen and pro-S hydrogen don't change when mirroring the molecule, thus looking at the two enantiomers. Therefore, this nomenclature is not applicable to my case. I could just as well use the R*R*/R*S* nomenclature, but still the question would remain: Which of the stereocenters is the one to refer to?
Dear Jan,
Have a look to the example given in Stereochemistry of Organic Compounds (Eliel and Wilen) p. 122. That should solve your problem.
Barbara
https://www1.ethz.ch/loc/people/emerit/Seebach/233_ACI_1982.pdf
Here is a useful article regarding the general nomenclature.
If you are determining like or unlike diastereotopic protons then you should pair them with an adjacent stereocentre.
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