B3LYP/6-31G* conformational landscape in vacuo of some pterocarpan stereoisomers with biological activity

The relative stability of possible arrangements of the pterocarpan fused ring system and the conformational preferences of the side chains present in two natural compounds, 3,9-dimethoxy-4-prenylpterocarpan (bitucarpin A) and 3,9-dihydroxy-4,8-diprenylpterocarpan (erybraedin C), have been investigated in vacuo on a number of stereoisomers (either natural or not) at the B3LYP/6-31G* level. The results obtained using three classical force elds (Tripos/GH, MMFF94 and GAFF), compared to the quantum mechanical ones for selected torsions (hydroxyl and prenyl group rotations), indicate that MMFF94 produces a satisfactory description of structural features of pterocarpans, and thus it is advisable to resort to it when thorough ab initio or DFT investigations are not affordable.