X-ray elucidation of 17α-pregna-2,4-dien-20-yne-(2,3-d) isoxazole-17β-ol

Synthetic pregnen steroid 17α-pregna-2,4-dien-20-yne-(2,3-d) isoxazole-17β-ol crystallizes in the orthorhombic space group P2₁2₁2₁, with a = 6.601(4), b = 10.530(4), c = 26.177(9) Å, and Z = 4. The structure was solved from Enraf Nonius CAD4 diffractometer data by direct methods and refined to R = 0.0399 for 2041 observed reflections (I > 2σ(I)). The molecule contains five rings of which rings A, B, and C are cyclohexene; ring D is cyclopentene; and ring E is heterocyclic cyclopentene. Ring A acquires the conformation of a half-chair; ring B tends toward a half-chair conformation with a bias toward a chair conformation that contains a β hydrogen at C8 and an α hydrogen at C9. Ring C acquires a near chair conformation with an a hydrogen at C14. Ring D is a 13β 14α-half chair, and ring E is a O3α-envelope. Both the ring junctions A/B and A/E are quasi-trans, whereas the ring junctions B/C and C/D are both trans. The molecule as a whole is slightly convex toward the β-side, with an angle of 16.7(2)° between the C10-C19 and C13-C18 vectors. Structural association is due to hydrogen bonding and stacking interactions.