Comparative cotton effects of steroidal 2,4-dienes and related trienes: origin of the folded conformation in levopimaric acid

Abstract

The decreased intensity of the positive Cotton effects of a 9α-methyl- and a 19-nor-steroidal 2,4-diene compared to those of other steroidal 2,4-dienes suggests that the peculiar B/C folded conformation in levopimaric acid (negative Cotton effect) is caused by relief of a 1,3-diaxial repulsion between the C-10 angular methyl group and the 11β-hydrogen, probably enhanced by C-4 methyl buttressing, which the resulting eclipsing of the 9α- and 11α-hydrogens is not sufficient to override.