[Paulie]

Hello, I'm trying to understand how I can elucidate the stereochemistry of substituents on a compound from the coupling constants. The example I'm working on is shown in the picture below.



I have been given both 13C and 1H spectra and been asked to determine the relative stereochemistry of the substituents shown with squiggly lines. However I can't find a description anywhere of a step by step method to do this. Does anyone want to take a shot at explaining it to me? The spectra are posted below if anyone is interested.

[Babcock_Hall]

Forum rules require you to show an attempt before we can help you.  However, I will give you a direction.  Did your instructor discuss the Karplus equation?

[Paulie]

Somewhat, but I don't think I understand how to use it. The way I understand it is that the coupling value is largest for dihedral angle of 180° and 0°, and smallest for 90°, but when looking at a coupling constant on a spectrum how do I know if this is a large or small value?

If the coupling constant value between two hydrogen is small, does that mean that they are trans to each other?

[Babcock_Hall]

Let us hypothesize a cyclohexane derivative with two hydrogens that are on adjacent carbon atoms, that are trans to each other, and that don't couple to anything else.  In one ideal chair conformation, they will be 180° and in the other they will be 60°.  Therefore, their observed coupling will depend on the equilibrium constant between the two chair forms.  In some cases (such as glucose) one chair form strongly predominates.

I would approach this problem by assigning the signals, then finding the actual values of the coupling constants in Hz.  Others here may be able to help at that point better than I can.  My understanding is that there are empirical parameters in the Karplus equation that are different for different chemical structures, but it might be overkill to use them in this instance.