Conformations of the Nine‐Membered Ring in the B‐Nor‐5,10‐secosteroids. X‐Ray and NMR Analysis

The conformations of (Z)‐ and (E)‐5‐oxo‐B‐nor‐5,10‐secocholest‐1(10)‐en‐3β‐yl acetates ( 2 and 3 , resp.) were examined by a combination of X‐ray crystallographic analysis and NMR spectroscopy, with emphasis on the geometry of the cyclononenone moiety. The ¹H‐ and ¹³C‐NMR spectra showed that the unsaturated nine‐membered ring of (E)‐isomer 3 in C₆D₆ and (D₆)acetone solution exists in a sole conformation of type B ₁ , which is similar to its solid‐state conformation. The (Z)‐isomer 2 in C₆D₆, CDCl₃, and (D₆)acetone solution, however, exists in two conformational forms of different families, with different orientation of the carbonyl group, the predominant form (85%) corresponding to the conformation of type A ₁ and the minor (15%) to the conformation A ₂ present also in the crystalline state. In this solid‐state conformations of the nine‐membered ring of both compounds, the 19‐Me and 5‐oxo groups are ‘β’‐oriented. The NMR analysis suggests that the nine‐membered ring of 4 has a conformation of type C ₁ in CDCl₃ solution.