Crystal Structure and Conformational Analysis of 7,13-Abietadien-18-oic Acid

Abstract

Abstract The crystal and molecular structures of title compound 1 have been determined by single-crystal X-ray diffraction analysis. Crystals are monoclinic, space group P21, with a = 14.058(2), b = 11.911(1), c = 11.753(2) Å, β = 111.73(1)°, Z = 4. The structure identification was refined by full-matrix least-squares calculations to a final R of 0.054 for 2899 independently observed reflections. The crystallographic asymmetric unit contains two independent molecules with similar geometries and planar conformations. In each molecule, cyclohexane ring A has a typical chair form. Cyclohexene rings B and C, containing of two conjugated double bonds, have half-chair conformations. The relative stereochemistry for both molecules is trans fusion for the A/B ring junction [14.9(2) and 14.6(3)°], anti between C9 hydrogen and C10 methyl (steroid numbering), and coplanar for the B/C ring junction [5.7(2) and 3.5(2)°]. The dihedral angles between the mean plane of ring A and the carboxyl group are nearly right angles [74.6(2) and 86.2(2)°]. Thermal motion analysis in rigid-body approximation and potential energy calculations suggest that the isopropyl groups are more freely in these regions. The isopropyl C atoms had large thermal motions. The hydrogen bonds of carboxyl groups are rigid between two independent molecules.