Conformations of the 10‐membered Ring in 5, 10‐Secosteroids. II. (E)‐3α‐Acetoxy‐5, 10‐seco‐1 (10)‐cholesten‐5‐one and (E)‐5, 10‐seco‐1 (10)‐cholestene‐3, 5‐dione

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Abstract
(E)‐3α‐Acetoxy‐5, 10‐seco‐1(10)‐cholesten‐5‐one (3) was synthesized by fragmentation of 3α‐acetoxy‐5α‐cholestan‐5‐ol (1) using the photochemical version [3] of the lead tetraacetate reaction [4], and transformed into the corresponding 3‐oxo‐compound (5). Two conformations (A and B) were deduced for the 10‐membered ring of 3 by analysis of the 1H‐ and 13C‐NMR. spectra in toluene. The major conformation (A) corresponds to that found in the solid state by X‐ray analysis. According to its NMR. spectra in toluene, the medium‐sized ring of the diketone 5 exists also predominantly in two conformations, the major one being analogous to A (the solid‐state conformation of the 3β‐acetoxy isomer (9) [1]) and the minor one to A (see above). The stereochemistry of the acidcatalyzed and thermal cyclisations of 3 as well as of the corresponding 5‐oxime is discussed in terms of conformational factors.