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The I3C 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 NMR 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.

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Quercetin, a naturally occurring dietary flavonoid, is well known to ameliorate chronic diseases and aging processes in humans, and its antiviral properties have been investigated in numerous studies. In silico and in vitro studies demonstrated that quercetin can interfere with various stages of the coronavirus entry and replication cycle such as PLpro, 3CLpro, and NTPase/helicase. Due to its pleiotropic activities and lack of systemic toxicity, quercetin and its derivatives may represent target compounds to be tested in future clinical trials to enrich the drug arsenal against coronavirus infections. There is evidence that quercetin in combination with, for example, vitamins C and D, may exert a synergistic antiviral action that may provide either an alternative or additional therapeutic/preventive option due to overlapping antiviral and immunomodulatory properties. This review summarizes the antiviral significance of quercetin and proposes a possible strategy for the effective utilization of natural polyphenols in our daily diet for the prevention of viral infection.