Conformations of the ten-membered ring in 5,10-secosteroids I. X-ray and NMR. Analysis of (E)-3 β-hydroxy-5,10-seco-1(10)-cholesten-5-one esters

X-ray structure determination of the p-bromobenzoate 2b of one of the (E/Z)-isomeric 3β-hydroxy-5,10-seco-1(10)-cholesten-5-ones confirmed the (E)-configuration proposed previously and showed the cyclodecenone ring to adopt an extended crown conformation of type A ₁ (Fig. 9). Analysis of the ¹H- and ¹³C-NMR. spectra of acetate 2a revealed that in solution the ten-membered ring of this steroid exists in at least two distinct forms, the predominant (about 85%) corresponding to the solid-state conformation of 2b (= A ₁), and the minor most likely to B ₂ (Fig. 9). From NMR. data the energy difference between the two species and the relevant activation energy were estimated. A number of conformational force field calculations using a simplified partial structural model was performed; but the computed energy differences between the various possible conformations do not reproduce the effective situation, neither in solution nor in the crystal lattice, indicating that additional effects such as the transannular interaction of the double bond with the carbonyl group may strongly influence the thermodynamic stability of the system. The conformations deduced were used to rationalize the stereochemical course of different chemical reactions of 2a .