Stereochemical Studies. II. Conformational Analysis of ∆⁵-3-Keto Steroids

The conformation of ring A in ∆⁵-3-keto steroids was determined through ultraviolet absorption, circular dichroism, and optical rotatory dispersion data. In most of the ∆⁵-3-keto steroids investigated, a coupling of the two chromophores was observed as indicated by increased n → π^* transitions and the appearance of π → π^* transitions of the β,γ-unsaturated carbonyl group. Introduction of a C-7 keto function to a “coupled” ∆⁵-3-keto system caused additional enhancement of the n →π^* transitions of the isolated C-3 carbonyl group and changed markedly the normal ultraviolet pattern of the α,β-unsaturated carbonyl chromophore. An absence of the interaction characteristics in 4,4niialkyl ∆⁵-3-ketones indicates a quasi-boat conformation for its ring A. Supporting evidence for the latter was found in the nmr data.