Pm. Ivanov et al., CONFORMATIONAL-ANALYSIS OF DIASTEREOISOMERIC TETRAPHENYL-SUBSTITUTED PYRROLIDINES, Journal of molecular structure, 377(1), 1996, pp. 19-26
The relative configurations of a diastereoisomeric pair of tetraphenyl
-substituted pyrrolidines were determined from NOE difference spectra.
Low-temperature H-1 NMR spectra indicate the presence of a conformati
onal mixture in solution. According to computational conformational an
alysis utilizing molecular mechanics (MM2 and MM3), for the isomer wit
h the two pairs of phenyl groups (2,5-cis and 3,4-cis) at different si
des with respect to an average plane of the ring, both envelope and tw
ist conformers are present. For the same isomer, AM1 and PM3 calculati
ons determined a strong preference for the envelope conformer. For the
other isomer (2,4,5-phenyls cis), the AM1 and PM3 results are in favo
ur of twist conformers, while the molecular mechanics models and EHMO
computed the envelope forms as having the lower energy. All computatio
nal methods are consistent in determining conformers from one and the
same regions along the pseudorotational itinerary of the pyrrolidine r
ing as the most probable ones.