Pyridine-induced shifts in the 1H nuclear magnetic resonance spectra of 20-hydroxypregnane derivatives

Abstract

Solutions of compounds of the pregnane-3,20-diol, -3,16α,20-triol, and -3,17α,20-triol series in [²H]chloroform and in [²H₅]pyridine show ¹H n.m.r. spectral differences which can be used to assign configurations at C-20. The C-18 protons exhibit a chemical shift difference [Δ=δ(C₅D₅N)–δ(CDCI₃)] which is greater for the pregnan-20β-ols than for the -20α-ols; the reverse is generally true of the C-21 proton signal, except when a 16α-hydroxy-group is present. The main features of the pyridine-induced shifts are rationalised in terms of the preferred conformations of 20α- and 20β-hydroxypregnanes, and of the OH ⋯ py hydrogen-bonded complexes.