Assigning stereodiversity of the 27-Me group of furostane-type steroidal saponins via NMR chemical shifts

Applicability of 13C and 1H NMR chemical shifts for the assignment of the 25R/25S configuration of the 27-methyl group in the case of furostane-type steroidal saponins has been investigated. A comparative study of 13C NMR data suggest that chemical shift values for C-20, C-21, C-22, C-23, C-24, C-25, C-26 and C-27 resonances were not much influenced by R/S configuration of the 27-Me group, thus reflecting limited application of 13C NMR chemical shifts for such stereochemical determinations. In contrast, 1H NMR chemical shifts (δa, δb) for geminal protons of glycosyloxy methylene (H2-26) exhibit pronounced dependence and the difference (Δab = δa − δb) among their chemical shifts [Δab = < 0.48 for 25R; Δab = >0.57 for 25S] seems to be of general applicability for ascertaining 25R/25S orientation of the 27-methyl group of furostane-type steroidal saponins.