Conformational preference of ring A in 3-oxo-Δ5,10-steroids: X-ray crystal structure analysis of 17α-ethynyl-17β-hydroxyestr-5(10)-en-3-one (norethynodrel)

Abstract

The crystal structure and solid-state conformations of the title compound (5) have been established by X-ray analysis. Crystals are orthorhombic, space group P2₁2₁2₁, with a= 21.593(13), b= 22.075(13), c= 7.015(8)Å, Z= 8. The structure was solved by direct methods and refined by least-squares calculations to R 0.046 over 2 572 statistically significant reflections from diffractometer measurements. The A rings in the two crystallographically independent molecules approximate to envelope conformations in which C(2) is displaced to the α-side in one and to the β-side in the other. Stabilization of the higher energy C(2)β-side envelope form is a consequence of the intramolecular hydrogen bonding arrangement in the crystal. The B ring shapes also differ slightly owing to conformational transmission effects.