Conformations of the Ten‐membered Ring in 5, 10‐Secosteroids. III: (Z)‐3β‐ and (Z)‐3α‐Hydroxy‐5, 10‐seco‐1 (10)‐cholesten‐5‐one Esters and (Z)‐5, 10‐seco‐1 (10)‐Cholestene‐3, 5‐dione

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Abstract
(Z)‐3β‐Acetoxy‐ and (Z)‐3 α‐acetoxy‐5, 10‐seco‐1 (10)‐cholesten‐5‐one (6a) and (7a) were synthesized by fragmentation of 3β‐acetoxy‐5α‐cholestan‐5‐ol (1) and 3α‐acetoxy‐5β‐cholestan‐5‐ol (2), respectively, using in both cases the hypoiodite reaction (the lead tetraacetate/iodine version). The 3β‐acetate 6a was further transformed, via the 3β‐alcohol 6d to the corresponding (Z)‐3β‐p‐bromobenzoate ester 6b and to (Z)‐5, 10‐seco‐1 (10)‐cholestene‐3, 5‐dione (8) (also obtainable from the 3α‐acetate 7a). The 1H‐and 13C‐NMR. spectra showed that the (Z)‐unsaturated 10‐membered ring in all three compounds (6a, 7a and 8) exists in toluene, in only one conformation of type C1, the same as that of the (Z)‐3β‐p‐bromobenzoate 6b in the solid state found by X‐ray analysis. The unfavourable relative spatial factors (interdistance and mutual orientation) of the active centres in conformations of type C1 are responsible for the absence of intramolecular cyclizations in the (Z)‐ketoesters 6 and 7 (a and c).

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