Les dimethyl-2ξ, 17α,δ9 et δ9,11 nor-19-testosterones. Conformation du cycle a et equilibres en solution

The preparation of 2α- and 2β-isomers of the 2,17-dimethyl-17β-hydroxy estra-4,9 diene-3-ones, 4 and the 2,17-dimethyl-17β-hydroxy estra-4,9,11 triene-3-ones, 21, is described and the configuration at C₂ established by chemical means. CD spectra show that these compounds exist in solution as an equilibrium mixture of two conformers, the proportion depending on the configuration of the methyl group on carbon 2. It appears that ring A of the 2β isomers is in a preferred “inverted half-chair” conformation while the “normal half-chair” conformation is favoured with the methyl group in the 2α-configuration. In alkaline medium the methyl group at C₂ of the dienones 4 and the trienones 21 is partially isomerised and at equilibrium the resulting CD spectra are identical to those of the unsubstituted dienone 1 and trienone 15. An explanation of this phenomenon is given.