Conformational analysis of gossypol and its derivatives by molecular mechanics

Conformations and inversion pathways leading to racemization of all the tautomers of gossypol, gossypolone, anhydrogossypol, and a diethylamine Schiff's base of gossypol were investigated with MM3(2000). All forms have hindered rotation because of clashes between the methyl carbon atom and oxygen-containing moieties ortho to the bond linking the two naphthalene rings. Inversion energies generally agree with available experimental data. Gossypol preferentially inverts in its dihemiacetal tautomeric form through the cis pathway (where similar groups clash). Gossypolone inverts more easily than gossypol, and preferentially through the trans pathway (where dissimilar groups clash) when one of its outer rings has an enol-keto group and the other has an aldehyde group. Anhydrogossypol racemizes through the cis pathway. The bridge bond and the ortho exo-cyclic bonds in all the structures bend from planarity, and the inner naphthalene rings pucker to accommodate the inversion. For gossypol, the transition is achieved through greater bending of the exo-cyclic bonds (up to 12°) and less distortion of the inner benzyl rings (q≤0.34 Å), (up to 12.7°) . For gossypolone the transition occurs with greater distortion of the inner benzyl rings (q≤0.63 Å) and less out-of-plane bending (up to 8.4°). By isolating individual clashes, their contribution to the overall barrier can be analyzed, as shown for the dialdehyde tautomer of gossypol.