A-ring conformational stability and progesterone-receptor binding affinity of 4-en-3-one steroids

The molecular structure and strain energy of 4-en-3-one steroids in two different A-ring conformations are calculated by means of a molecular mechanics technique. The computations for the isolated molecules provide the following order of increasing stability of the inverted A-ring conformers: 10-methyl, 19-nor, 9-ene compound. This tendency is in agreement with X-ray structure data for single crystals. The normal 1 alpha, 2 beta-half chair conformation of 10-methyl steroids is found to be stabilized by bond angles, mainly at C10, and non-bonded interactions from the 10-methyl group. Pitzer strains favour the inverted 1 beta, 2 alpha-half chair conformation in the case of 4,9-diene-3-one compounds. Binding affinities to the progesterone receptor decrease in the series: 19-nor, 9-ene, 10-methyl compound. In view of this ordering, the calculated relative stabilities of A-ring conformers are in conflict with a conformation-controlled receptor binding. Variations of receptor bond strengths are supposed to be more strongly influenced by a steric hindrance of the 10-methyl group and/or steroid-backbone flexibility.