Dear Dang,

It might be that after recrystalization you still have the two conformations: the first one is the crystalline conformer and the second conformation is the one remains in the solvent and did not undergo crystallization.

Therefore, 70% of the total appears as crystals (conformation a) and 30% is in solution (conformation B).

Hoping this will be helpful,

Rafik

We had a similar case some time ago. Quite interesting. Please see: Polish Journal of Chemistry 77(11):1599-1606 · October 2003.

Regrettably, I cannot provide you with a copy at the moment. If you get it, would you send it to me, please? Thank you, Dr. Dang Quoc Hung.

Can you give a bit more detail about the structure? (1,3- or 1,4-dioxane? Substitution pattern?)   It's difficult to think about the problem without knowing what we're talking about.

In general, the inversion barrier should not be high enough to allow non-equilibrium populations of conformers to exist at room temperature.  Are you sure you're seeing conformers, and not geometrical isomers?

In principle, there is no reason for a change in conformational equilibrium after recrystallisation. The compound is the same (different conformations of the same configuration). Recrystallisation removes impurities, does not change the strucure of the compound. Unless you are dealing with diastereomers (which have different configurations), there is no reason for a change in the conformational equilibrium before or after recrystallisation.

...or, in some cases, if there was a deep energy well between the two conformers, heat (e.g. delivered during the crystallization operation) or different polarity of the medium and interactions with solvent molecules, factors like that just might allow for equilibration/ conformational transition. Also, the product may form a solvate in one conformation and not in the other, which ads to the pool of possibilities... I would think such phenomena have been observed more often than not...

     I'm so sorry. I have too much work and had forgotten that i had put a question in here

    This my structure : (2R,3R,5R,6R)-5,6-Dimethoxy-5,6-dimethyl-1,4-dioxane-2,3-dicarboxylate de méthyle. First, i have mixture ( ~ 7 :3 ) two conformation. After recrystallisation, I have obtained crystals pur and one solution ( ~6 : 4, two conformation).

     My question is too complex (as Abel J S C Vieira said). So, we have decided to ignore it. Because it is only an intermediate step...... :(

    To Jacek Martynow  : This my email : [email protected] . I want read your paper. Thank you ! . I'm just master student (M2). 

Thanks Rafik Karaman, Jacek Martynow, Abel J S C Vieira and James Demers !

If I'm drawing it right, there should be two equatorial -CO2Me groups, and no 1-3 diaxial interactions at all.  It should be 100% in that conformation.

I believe you have two isomers, most likely the 2R,3S and 2R,3R dastereomers.  Aldrich sells the material; it might be worth getting a sample for comparison if the stereochemstry matters in subsequent steps.