DFT-GIAO study of aryltetralin lignan lactones: conformational analyses and chemical shifts calculations.

The conformational properties of polygamain and morelensin, two aryltetralin lignan lactones, have been investigated in both the gas-phase and chloroform solution using DFT calculations at the B3LYP/6-311G(d,p) level. Results indicate that the conformation of polygamain is very rigid. Thus, the conformational flexibility of its five-membered rings is considerably restricted as reflects the pseudorotational parameters of the corresponding envelope conformations. On the other hand, morelensin shows a notable conformational flexibility, which is mainly due to its two methoxy groups. Accordingly, 16 minimum energy conformations with relative energies smaller than 2.4 kcal/mol were detected. Furthermore, chemical shifts for 13C nuclei have been calculated using the GIAO method, results being compared with experimental data. A good agreement was found for both polygamain and morelensin.