Theoretical prediction of the stereochemistry of the OH groups in Turraesterodione

The stereochemistry of the OH groups in 17α,20S-dihydroxypregn-4-ene-3, 16-dione 1 (named Turraesterodione) has been predicted by quantum chemical calculations. Turraesterodione is a natural product found in the stem bark of Turraeanthus africanus, which is found in the equatorial rain forest in Africa. The results of our calculations show that the OH groups at centers C17 and C20 are syn-oriented, in support of a recent X-ray analysis that found the OH groups to be on the same side of the molecule. Additionally, another isomer with the two OH groups anti-oriented with respect to the methyl group at C18, as is the orientation in the X-ray structure, was found to be energetically comparable to the X-ray structure. Equilibrium geometries and thermodynamic stabilities of five isomers, including the one found in the X-ray study, have been calculated at the MP2 and B3LYP levels of theory, using the 6-311G** basis set. Harmonic vibrational frequencies have been calculated at the B3LYP/6-311G** level, and the predicted IR spectrum shows very good agreement with experiment.