NMR assignment of the absolute configuration of C-25 in furostanol steroidal saponins

Steroids
Victoria L ChallinorJames J De Voss

Abstract

The chemical shifts of the geminal proton resonances of H(2)-26 (δa and δb) are a widely used predictor of C-25 stereochemistry in furostanol steroidal saponins, being in general more resolved in 25S than 25R compounds. Unexpectedly, we found that application of this empirical rule in different solvents led to conflicting assignments of stereochemistry. An experimental survey revealed that, while the chemical shifts of H(2)-26 exhibit a dependence on C-25 configuration, it is less pronounced in methanol-d4 than pyridine-d5 solvent, and thus the general rule derived for pyridine-d5 fails when NMR spectra are acquired in methanol-d4. We propose a modified empirical method for the direct assignment of C-25 stereochemistry in furostanol saponins in methanol-d4 (Δ(ab)=0.45-0.48 ppm for 25S; Δ(ab)=0.33-0.35 ppm for 25R), and provide several detailed examples. In addition, the absolute configuration of compound 8, a steroidal saponin isolated in previous work from Ruscus colchicus, is corrected from 25R to 25S stereochemistry.

References

Aug 25, 2004·Journal of Ethnopharmacology·S G SpargJ van Staden
Oct 16, 2007·Phytochemistry·Patricia Y HayesJames J De Voss
Mar 1, 2011·Bioorganic & Medicinal Chemistry Letters·Ki Hyun KimKang Ro Lee
Mar 1, 2011·Phytochemistry·Assunta NapolitanoSonia Piacente
Sep 7, 2011·Phytochemistry·Tamar MuzashviliSonia Piacente
Jan 31, 2012·Steroids·Victoria L ChallinorJames J De Voss

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Citations

Apr 23, 2013·Journal of Asian Natural Products Research·Yu GuoBai-Ping Ma
Feb 5, 2013·Natural Product Reports·Victoria L Challinor, James J De Voss
Mar 19, 2014·Chinese Journal of Natural Medicines·Lu ZhengTing Xiang
Nov 28, 2012·Molecules : a Journal of Synthetic Chemistry and Natural Product Chemistry·Simona De MarinoMaria Iorizzi
Oct 14, 2017·Bioscience, Biotechnology, and Biochemistry·Hyoung Jae LeeMasaharu Mizutani
May 12, 2018·Planta·Swati UpadhyayRakesh Kumar Shukla
May 11, 2017·Journal of Agricultural and Food Chemistry·Yihai WangXiangjiu He

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