THE DETERMINATION OF CONFORMATION BY NUCLEAR MAGNETIC RESONANCE

Author Institution: Cyanamid European Research InstituteIt has been shown by Karplus and by Conroy that the spin-spin coupling constant between vicinal protons should vary as the square of the cosine of the dihedral angle between the two carbon-hydrogen bonds and their relationship has been shown to be valid for a number of rigid molecules of known conformation. Molecules in which a dihydrobornadiene group is fused to another cyclic system provide a suitable class of compounds whose conformation should be determinable from this relationship, since the dihedral angle between the exo-protons and the adjacent tertiary, protons is $40^\circ$ while that between the endo-protons and the tertiary protons is $80^\circ$, so that the corresponding coupling constants should be 3--4 c/sec and 0 c/sec. We have measured the nuclear magnetic resonance spectra of the following four compounds of this type and thereby determined their conformation. The first two compounds, which are Diels-Alder adducts of eyclopentadiene and benzoquinone, have the endo-conformation thus providing a further example of the stereo-specificity of this type of Diels-Alder reaction. The stereo isomer of II, prepared by enolisation of the primary Diels-Alder adduct, has the mixed endo-exo conformation, while compound IV appears to have the exo-conformation. The spin-coupling constants between the gun protons of the methylene groups show that strain has here increased the tetrahedral angle to between $112^\circ-114^\circ$. [FIGURE