4,4-Dimethyl Effect. (6). The Ring A Conformation of 4,4-Dimethyl-3-keto Steroids and Triterpenoid-3-ketones: Predicted and Observed Geometries and Their Chiroptical Properties

The ring A conformations of 4,4-dimethyl-3-keto steroids can be classified into various geometries on the basis of their torsion angle sign sequence (TASS) of the ring; however, only three were predicted to be possible from theoretical considerations; i.e., C, FB³ and T₁ (T₂also occurs in the case of 5-enes) (for details, see the text). They were actually observed by X-ray analyses. The ring A conformation is so flexible that it can readily changes from chair to T₁ when a remote structural change such as introduction of an 8β-methyl occurs. Corresponding to this conformational change, the circular dichroism (CD) spectrum of the compound changes its sign from negative to positive. Ring A can also equilibrate in solution with various contributions of chair and T₁conformations depending on their relative energy difference. This equilibrium is greatly affected by remote changes in the structure and also by the solvent, and is sensitively reflected in the CD spectrum which, more or less, shows a double-humped character. The typical double-humped CD spectra of some compounds can thus be well explained. Those compounds should have FC to FB³conformations at ring A.