Tetracyclic Triterpenes. XVII. The Inverted Conformation of Ring A and B in Stereoids: 3-Substituted-4-oxo-14alfa-methyl-5beta-cholest-8-enes

Full assignment of 13C NMR shifts for a series of _8-5_- and 5_-steroids is presented. These data along with some characteristic features of 1H NMR and IR spectra are in accordance with inverted conformation of Aand B rings in 4-oxo-5_-steroids 5 and 6 in solution. Configuration at C-3 is important for inversion of Aand B ring conformation of _-methyl ketones 6 and 7, while the intramolecular hydrogen bonding appears to be responsible for the inverted, slightly distorted chair conformation of ring A in _-hydroxy ketone 5.