[Woodwardt84]

Hi Guys

I have kind of a easy question about diastereomeric ratio.
When i have a reaction converting two vicinal carbons into two stereogenic centers (no meso compounds) then we can have 4 different compounds. SS, RR, SR and RS.
SS-RR and SR-RS are enantiomers. Every compound haves 2 diastereomers, so for example SS has SR and RS as diastereomers. Then what is the stereomeric ratio?
I see in articles that for a reaction where 4 different stereoisomers can be formed the diastereomeric ratios is presented as for example 98:2...I mean for enantiomeric ratio this makes sense because there are only one enantiomer to the other but for diastereomers there is 3 in total, SS, RS and SR. Then i should be presented as 96:2:2 for example???
Or is the enantiomeric couple counted as one diastereomer? Like RR and SS is formed 98% and SR and RS in 2%??
Please help me.

Thanks!!!

[synthon]

Just an educated guess, but it seems like the diastereomeric ratio would assume that the enantiomeric ratio is 1:1.  So if the ratio is 98:2, SS/RR to SR/RS, there's really 4 compounds @ 48:48:1:1 (SS:RR:SR:RS).

If that's not the case, then the situation is more complicated and would likely not be displayed in that fashion, needing more description than a simple ratio.

[Dan]

Just an educated guess, but it seems like the diastereomeric ratio would assume that the enantiomeric ratio is 1:1.  So if the ratio is 98:2, SS/RR to SR/RS, there's really 4 compounds @ 48:48:1:1 (SS:RR:SR:RS).

^this

It means the ratio of (SS+RR):(RS+SR) is 98:2

If no enantiomeric ratio (or enantiomeic excess) is given, then the product is either racemic or enantiopure  (perhaps if enantiopure starting materials were used).

[fledarmus]

Since you can't generate a chiral compound without using a chiral center or other chiral compound, any reaction which generates a chiral center has to generate equal amounts of the two enantiomers. If there is no chiral center and no chiral auxiliary, there is no need to distinguish anything other than diasteromeric ratio - the two enantiomeric pairs of each diastereomer will always be present in equal amounts.

There was a lot of buzz a couple of decades ago about using polarized light or super-strong magnets to try to generate chiral molecules from achiral precursors, but I don't think any of it ever went anywhere.