CONFORMATIONAL-ANALYSIS OF DIASTEREOISOMERIC TETRAPHENYL-SUBSTITUTED PYRROLIDINES

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.