NMR and conformational studies of the cyclobutane ring involved in the bicyclo [4.2. 0] octane system of a tetracyclic diterpene structure

Conformational studies were performed on the bicyclo [4.2.0] octane system of a series of related tetracyclic diterpenes, using molecular mechanics and 1H NMR. Both methods gave compatible results, but did not permit the deduction of any quantitative relationship linking the vicinal coupling constants to the cyclobutane dihedral angles. Nevertheless, the Karplus relationship allowed a qualitative interpretation of the coupling constants in terms of conformation. A set of characteristic 1H NMR coupling constants was obtained which allows the interpretation of the 1H NMR spectrum of any compound presenting a similar system.