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 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 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 ).