TPR Organic Chem Passage

[Kminht89]

Hi everyone,

I would really appreciate it if someone can help me with this advance passage-based question (see attached file for passage):

Compounds C & D are best described as:
Diastereomers. Compound C has configuration R,R, while compound D has configuration S,R. They are different only at 1 center and thus diastereomeres.

So I understand what diastereomers are...no need for an explanation there...but I don't understand HOW the absolute configurations were determined/inferred based on the passage. I tried mapping out the separation process but not having any luck.

Thanks y'all!

P.S Are we allowed to take photos of passages and post them on SDN for help? Saw a few forums with pictures but not sure if it's allowed...please let me know if it isn't and I'll know for next time. Thanks!

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[NextStepTutor_1]

Hi @Kminht89,

Interestingly, you can get the right answer here without knowing the absolute configurations of the molecules, because it's describing a technique known as diastereomeric recrystallization, which is one of the most common forms of chiral resolution (or ways of separating enantiomers). I'd suggest consulting chemistry textbooks (or even Wikipedia is not half bad) for more details, but the idea is that when they add the R stereoisomer of Compound B (which I'll call B_R), they're forming two salts with the R and S stereoisomers of Compound A, which are floating around in solution (I'll call these A_R and A_S): salt 1 = {A_R, B_R} and salt 2= {A_S, B_R}. These two salts are described as Compounds C and D in the passage. You don't need to know which one is which in order to recognize that they are diastereomers, because that's inherent to the logic of the technique.

As a side note, does the passage tell you that the R form of Compound A is the medically active one? We can work out that Compound C contained the medically active diastereomer, but unless I'm completely missing something here, I don't see any reason based on what the passage says why the active analgesic must be R. This piqued my curiosity so I actually consulted a textbook on optical resolution via diastereomeric salt formation and it doesn't seem like there's any straightforward way to correlate the melting points of diastereomeric salts with the absolute orientation of their chiral centers (and I've certainly never seen anything along those lines on the MCAT).

Anyway, hope this is helpful!

 

[NextStepTutor_1]

Oh, P.S., also, an even quicker hint that Compounds C and D are diastereomers is the simple fact that they have markedly different MPs, indicating that their physical properties are different, which you expect for diastereomers. Depending on the answer choices, that may be the simplest and quickest way of solving the problem.

 

[aldol16]

It could just as easily be S,S and R,S. There's no easy way to tell from the passage and this kind of question really wouldn't show up on the MCAT. The MCAT is more interested in applying broad principles in biological/medical settings and not so much on the process chemistry behind drug development.

 

[Kminht89]

Hi @NextStepTutor_1 - thanks for the input! To answer your question, no, the passage doesn't mention which absolute configuration is the active form, just that one of the stereoisomers is active. But from your second explanation, I think the difference in melting points would be the best way to solve this. Really appreciate the input!