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 ¹H NMR (300 MHz) and ¹³C NMR (75 MHz) techniques. The resulting spectra appear to be consistent with slow rotation about the hindered C(sp²)-C(sp³) bonds to the bridgehead unsubstituted phenyls, i.e., slow exchange limit (SEL) spectra. Full rigorous ¹H spectral assignments have been made via high-resolution COSY experiments. The number of signals in the ¹³C NMR aryl region were also consistent with hindered phenyl rotations; preliminary ¹³C 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.