18-Substituted steroids, part 14. The high-field 1H and 13C nuclear magnetic resonance spectra of aldosterone; full assignments for the main equilibrating forms in solution

Abstract

Aldosterone exists in solution predominantly as an equilibrating mixture of the 20-oxo-11,18-hemiacetal (2) and the 11,18-epoxy-18,20-hemiacetal (1), resulting in distinct but superimposed n.m.r. spectra. Full assignments of the ¹H and ¹³C spectra have been achieved by carrying out a ¹³C–¹³C connectivity experiment (INADEQUATE) to identify the set of ¹³C signals for each of the two forms, followed by the use of ¹H–¹³C heteronuclear shift-correlated spectroscopy, aided by a phase-sensitive double quantum filtered COSY and 2D J-resolved spectra (¹H), with a few selective nuclear Overhauser difference measurements. This permitted assignments of all the ¹H and ¹³C chemical shifts and almost all of the two- and three-bond interproton coupling constants for each tautomer.