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‐3 β ‐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 1 H‐ and 13 C‐NMR spectra showed that the unsaturated nine‐membered ring of ( E )‐isomer 3 in C 6 D 6 and (D 6 )acetone solution exists in a sole conformation of type  B 1 , which is similar to its solid‐state conformation. The ( Z )‐isomer 2 in C 6 D 6 , CDCl 3 , and (D 6 )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 CDCl 3 solution.