Relative and Absolute Structures of Diospongin A, B and C

While screening 60 extracts for their stimulatory activity on proliferation of osteoblast-like cell line and on inhibition of osteoclastic formation, the water extract of Dioscorea spongiosa displayed the strongest stimulation on osteoblastic proliferation and strong inhibition on osteoclastic formation. This water extract was separated using bioassay-guiding fractionation and three new diarylheptanoids were isolated and purified. The structures of three new diarylheptanoids were elucidated by analysis of NMR, IR spectra and high resolution FAB-MS. The relative stereochemistry of diospongin A and B was determined by ROESY spectra and coupling constants in ¹H-NMR spectra and their absolute structures were identified by advanced Mosher method. By analyzing the NMR data, diospongin C was found to be an acyclic diarylheptanoid with four hydroxyl groups at C-1, C-3, C-5 and C-7; i.e., 1,7-diphenylheptan-1,3,5,7-tetraol. So there was some difficulty in the decision of its relative and absolute configuration. The relative configuration of diospongin C also can be determined by analysis coupling constants of two protons of C-2, C-4 and C-6 in Newman projections of one corresponding acetonide derivative and optimizing dihedral angles [1]. Its absolute stereochemistry was identified by the CD spectrum of its dibenzoat product [2]. All the three compounds were examined the inhibitory activity on osteoclast formation and bone resorption induced by PTH in bone organ culture system. Except for diospongin A, diospongin B and C showed potent inhibition even at a concentration of 20 µM, which demonstrates that the stereochemistry was important to structure-activity relationship of these diarylheptanoids.

Structures of diospongin A, B and C.

Acknowledgement: Thanks to Professor Qishi Sun and Yingjie Chen of Shenyang Pharmaceutical University in China for purchasing and identifying the crude drug. Toyama Medical and Pharmaceutical University is also appreciated for the structural measurement of compounds. References: [1] Rychnovsky SD, Richardson TI et al (1997). J Org Chem, 62: 2925–2934. [2] Wiesler WT, Nakanishi K. (1989). J Am Chem Soc, 111: 9205–9213.