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.