Abstract
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.