Structure Revision of Hassananes with Use of Quantum Mechanical ¹³C NMR Chemical Shifts and UV−Vis Absorption Spectra

Geometry optimization and gauge including atomic orbitals (GIAO) (13)C NMR chemical shifts in chloroform solvent calculated at the level of MPW1PW91/6-31G(d,p) were applied to przewalskins A (3) and B (4), which are novel diterpenoids with a 6/6/7 carbon ring skeleton. The good linear correlations between the calculated and experimental (13)C NMR chemical shifts indicated the reliability of the computational method. This method was employed to the structural revision of natural product hassananes (1), which was reassigned to 2 with a similar skeleton as przewalskins A (3) and B (4). Furthermore, the UV-vis absorption spectra in gas phase and solvents were also predicted in order to further support our structure revision of hassananes.