DFT, solution, and crystal conformation of eremophilanolides

The minimum energy conformation of five eremophilanolides (1-5) from the tubercles of Psacalium paucicapitatum was calculated using density functional theory (DFT) at the B3LYP/6-31G* level. Comparison of the experimental H-1-H-1 coupling constant values of 1-5 with those generated employing a generalized Karplus-type relationship using dihedral angles extracted from the DFT calculation and from the crystal structures for I and 3-5 shows good agreement. The A ring of 1-5 adopts an almost perfect chair conformation with the Me-14 group in an axial position and the Me-15 in an equatorial position. (c) 2006 Elsevier B.V. All rights reserved.