Conformation of the A ring in 1-oxo and 3-oxotriterpenoids. Vicinal interproton coupling constants

The coupling constants of the protons on the A ring were determined by full analysis of the spin systems in the 1H NMR spectra of 4,4-dimethyl-5α-cholestan-3-one (XIX) and 1-oxo and 3-oxo triterpenoids I, XIV-XVI, XX and their 2-methyl derivatives V, VI, XVII, XVIII, XXIII and XXIV. from the values of vicinal coupling constants it was inferred that in 2α-methyl-1-oxo and 3-oxo derivatives V, XVII and XXIII the A ring assumes a chair conformation, while in 2β-methyl derivatives VI, XVIII and XXIV a boat conformation. The chair form greatly predominates in 4,4-dimethyl-5α-cholestan-3-one (XIX) and in 1-oxotriterpenoid XX, while in triterpenoid 3-oxo derivatives I, XIV-XVI a significant amount of boat conformation is present at equilibrium (up to 40%). The solvent shifts and the lanthanide induced shifts of the protons on C(2) lead to the same conclusion.