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

Applicability of C-13 and H-1 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 C-13 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 C-13 NMR chemical shifts for such stereochernical determinations. In contrast, H-1 NMR chemical shifts (delta(a), delta(b)) for geminal protons of glycosyloxy methylene (H-2-26) exhibit pronounced dependence and the difference (Delta(ab) = delta(a) - delta(b)) among their chemical shifts [Delta(ab),= < 0.48 for 25R; Delta(ab) = >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. ((c) 2005 Elsevier Inc. All rights reserved.