Circular dichroism,proton magnetic resonance and conformation of steroid 4-en-3-ones, 4, 9-dien-3-ones and 4, 9, 11-trien-3-ones

Room and low-temperature CD of the title dienones and trienones (3 to 9) clearly show that a conformational equilibrium occurs between two ring A half-chair conformers. The relative stability of the two conformers depends on substitution of ring A. These results may be extended to other series of steroids and in particular to 4-en-3-ones. CD of these enones (14 to 28) is consistent with a conformational equilibrium between the known quasi-cis-quasi-trans conformers. The controversial conformational behaviour of 2 β-substituted 19-nor-4-en-3-ones is explained by a dynamic equilibrium in solution between the two afore-mentioned conformers rather than by single twist or deformed boat conformer.