Steroidal analogues of unnatural configuration. Part XI. Crystal and molecular structure of 3α,17α-dihydroxy-4,4,14α-trimethyl-19-nor-10α-pregn-5-ene-11,20-dione and a circular dichroism study of the conformation of the acetyl side-chain of the 17-hydroxy-isomers in solution

Abstract

The crystal and molecular structure of the title compound (3) has been determined from three-dimensional, single-crystal X-ray analysis by direct methods, and refined by least squares methods to R 0.037. Crystals are monoclinic, space group P2₁, with a= 12.80(1), b= 7.52(1), c= 10.81(1)Å, β= 97.18(4)°, Z= 2. Ring C is considerably distorted and the solvent-dependence of the 11-carbonyl chromophore c.d. spectra can be explained by conformational mobility of ring C.

The conformation of the 17β-acetyl side-chain in methanol is close to that found in the solid state but in non-polar solvents, c.d. studies showed that a second conformer, stabilized by intramolecular hydrogen bonding, exists. In the 17α-acetyl isomer only one conformation without intramolecular hydrogen bonding is observed in c.d. studies in both polar and non-polar solvents.