13C NMR study of spirostanes and furostanes in solution and solid state

A series of compounds deriving from 12α- and 12β-hydroxytigogenin (1, 2), pseudohecogenin 6, 23-bromo derivatives of hecogenin and sarsapogenin (3–5) and products (7–9) of their transformation was analysed by 13C NMR in solution and in the solid state. The model structures containing three or four rings have been calculated by DFT B3LYP/6-31G** method. On the basis of the GIAO DFT shielding constants and experimental data the influence of 23-Br and 12-OH group configuration on chemical shifts of carbon atoms in adjacent rings was estimated. Analysis of CPMAS shifts indicated that the acetate carbonyl group at C-3 in 1a is involved in an intermolecular hydrogen bond with 12α-OH; compounds 8 and 9 exhibit polymorphism.