NMR studies of hindered rotation. The Diels-Alder adduct of phencyclone with N-n-propylmalemimide: Restricted motion of bridgehead phenyls.

The Diels-Alder adduct of phencyclone and N-n-propylmaleimide has been studied in CDCl3 solution at ambient temperatures by one- and two-dimensional H-1 NMR (300 MHz) and C-13 NMR (75 MHz) techniques. Clear evidence is presented from slow exchange limit (SEL) spectra for hindered rotation of the bridgehead phenyls in the adduct. Full H-1 spectral assignments have been made via selective homonuclear decoupling and high resolution COSY experiments. The number of signals in the aryl region of C-13 NMR spectra also indicated slow rotation about the C(sp(2))-C(sp(3)) bond to the unsubstituted bridgehead phenyls. Striking evidence of magnetic anisotropic effects, seen from H-1 NMR, permits stereochemical assignment of the adduct as endo.