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

The Diels-Alder adduct of phencyclone and N-n-butylmaleimide has been prepared, and NMR studies have been carried out in CDCl3 solution at ambient temperatures by one- and two-dimensional H-1 NMR (300 MHz) and C-13 NMR (75 MHZ) techniques. The resulting spectra appear to be consistent with slow rotation about the hindered C(sp(2))-C(sp(3)) bonds to the bridgehead unsubstituted phenyls, i.e., slow exchange limit (SEL) spectra. Full rigorous H-1 spectral assignments have been made via high-resolution COSY experiments. The number of signals in the C-13 NMR aryl region were also consistent with hindered phenyl rotations; preliminary C-13 assignments are given. Striking evidence for magnetic anisotropic effects due to the phenanthrene moiety, bridging ketone carbonyl, and bridgehead phenyls are discussed, supporting endo stereochemical assignment of the adduct.