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

Abstract

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 1 H‐ and 13 C‐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.