NMR-STUDIES OF HINDERED ROTATION - THE DIELS-ALDER ADDUCT OF PHENCYCLONE WITH MALEIC-ANHYDRIDE - RESTRICTED MOTION OF BRIDGEHEAD PHENYLS

H-1 NMR studies at 300 MHz have been performed for the Diels-Alder adduct of phencyclone and maleic anhydride in CDCl3 at ambient temperatures. The 1D spectrum shows four equal (2H) intensity doublets in the aryl region (in addition to other absorptions) which is fully consistent with a slow exchange limit (SEL) spectrum of a system in which the unsubstituted bridgehead phenyls exhibit hindered rotation around the C(sp(2))-C(Sp(3)) bond on the NMR timescale. These protons are assigned to H-1,8 and H-4,5 of the phenanthrene moiety and to H-2' and H-6' of the phenyls based on the two-dimensional (2D) homonuclear chemical shift correlation spectrum (COSY) together with arguments regarding carbonyl and aromatic ring anisotropy. Full proton assignments are given.