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

(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 1 H‐and 13 C‐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 C 1 , 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 C 1 are responsible for the absence of intramolecular cyclizations in the (Z)‐ketoesters 6 and 7 ( a and c ).