MAGNETIC RESONANCE IN CHEMISTRY, VOL. 33, 923-953 (1995) REVIEW NMR Spectroscopy of Steroidal Sapogenins and Steroidal Saponins: an Updatet,t Pawan K. Agrawal, * Dharam C. Jain and Ashish K. Pathak Central Institute of Medicinal and Aromatic Plants, Lucknow-226015, India The 13C NMR chemical shifts of 126 steroidal sapogenins published between 1983 and 1993 are listed and critical spectral features and advances made in the NMR characterization of these compounds are discussed as a guide for the identification of the parent skeleton and the determination of substitution patterns. The :\MR spectroscopic methods applicable to deduce the complete structure of the oligosaccharide moiety and its linkage to the sapogenin residue are also presented to elucidate the structure of steroidal saponins. KEY WORDS NMR; 1H NMR; 13C NMR; steroidal sapogenins; steroidal saponins; structure elucidation; review LIST OF CONTENTS 1. 1.1. 2. 2.1. 2.1.1. 2.1.2. 2.1.2.1. , 1.2.2. 1.2.3. .1.2.4. 2.1.3. 2.2. 2.2.t. 2.2.3. 2.2.4. 2.3. 3. 3.1. 3.2. 3.3. 4. Introduction Nomenclature Steroidal Sapogenins NMR strategies for structural elucidation of steroidal sapogenins and saponins Determination of parent skeletal type Determination of substitution pattern General spectral features Applications of 1D- and 2D-pulse experiments Keto group assignment Unsaturation Determination of ring fusion Individual classes of compounds Spirostanes Furospirostanes 16-Hydroxylated cholestanes Tables of 13C NMR shielding data Steroidal Saponins Identification of the oligosaccharide unit Identification of the sugar-aglycone linkage Three-dimensional NM R spectroscopy Conclusion Acknowledgements References 1. INTRODUCTION Steroids of the spirostane and furostane series (steroidal sapogenins) and their glycosides (steroidal saponins), widely distributed in various plant families,1-7 are attracting the attention of research workers not only as * Author to whom correspondence should be addressed. t NMR Spectral Investigations, Part 37, for Part 36, see P. K. Agrawal and A. K. Pathak, :'v/agn. Reson. Chern. 32, 753 (1994). CIMAP Publication No. 94-111. +Dedicated to Professor H.-J. Schneider, Universitiit des Saar- Ides, Saarbrlicken, Germany, on the occasion of his 60th birthday. CCC 0749-1581/95/120923-31 1995 by John Wiley & Sons, Ltd. economically important raw materials convertible into various steroid hormonal drugs,4.8.-1o but recently also as biologically active materials having independent value. 3 ,6,8,1l From 1973 to 1983, there was a dramatic increase of interest in the application of l3C NMR spec- troscopy to the study of these compounds, which prompted us to publish a general review of the use of 13C NMR in structural analysis of steroidal sapogenins and steroidal saponins,12 This paper extends that review from 1983 to the present. Nowadays, 13C NMR spectroscopy has become an integral part of the procedure for establishing the struc- ture of new steroidal sapogenins and saponins, The sim- plicity of proton-decoupled 13C spectra, with a single line for each carbon atom, and the availability of various pulse sequences for distinguishing between carbon atoms according to the number of attached protons have facilitated the interpretation. Moreover, the structural and substituent effects that control 13C chemical shifts are well documented for steroids,13-16 allowing good estimates to be made for individual carbon atoms. The IH NMR spectra, in contrast, are complicated by extensive interproton coupling, as a result of which only a few signals are readily recognizable, such as singlets for angular methyl groups (18-CH 3, 19-CH 3) and doub- lets for secondary methyl groups (21-CH3' 27-CH3) whereas the rest of the methylene and methine reso- nances appear as a series of overlapping multiplets in the so-called 'methylene envelope' which was difficult to resolve with 60, 80 and 100 MHz spectrometers. This is the most probable reason why most of the early publications l7 incorporate mainly only the 1H NMR chemical shifts for the methyl signals and for the more downfield signals (0 > 3 ppm) such as olefinic, oxy- substituted methylene and methine resonances, Signal linewidths and spectral integration identify the kinds of protons and their relative abundance present in various spectral regions. Full assignments of the 1H NMR resonances for a steroid with a low degree of functionalization has been Received 14 September 1994 Accepted (revised) 22 March 1995