( http://www.ch.ic.ac.uk/ectoc/papers/jones/ ):
We understand that you observed epimerization of your 1,3-dipolar
cycloadducts during purification but could not corroborate your
experimental findings with modeling studies.
Applying higher levels of theory in your calculations may change
this observation though we understand that such calculations may
be prohibitive due to high CPU requirements.
But have you considered performing the calculations using a Lewis-acid
complex with your cycloadducts? BF3 could provide you an inexpensive
mode; and this model may, perhaps, represent your experimental conditions
for purifications and characterizations better.
see Guner et al. J. Org. Chem., 1990, Vol. 55, page 28, for a similar