NMR Fingerprints of Formyl Phloroglucinol Meroterpenoids and Their Application to the Investigation of Eucalyptus gittinsii subsp. gittinsii

NMR fingerprints provide powerful tools to identify naturalproductsin complex mixtures. Principal component analysis and machine learningusing H-1 and C-13 NMR data, alongside structuralinformation from 180 published formyl phloroglucinols, have generateddiagnostic NMR fingerprints to categorize subclasses within this group.This resulted in the reassignment of 167 NMR chemical shifts ascribedto 44 compounds. Three pyrano-diformyl phloroglucinols, euglobal In-1and psiguadiols E and G, contained H-1 and C-13 NMR data inconsistent with their predicted phloroglucinol subclass.Subsequent reinterpretation of their 2D NMR data combined with DFT C-13 NMR chemical shift and ECD calculations led to their structurerevisions. Direct covariance processing of HMBC data permitted H-1 resonances for individual compounds in mixtures to be associated,and analysis of their H-1/C-13 HMBC correlationsusing the fingerprint tool further classified components into phloroglucinolsubclasses. NMR fingerprinting HMBC data obtained for six eucalyptflower extracts identified three subclasses of pyrano-acyl-formylphloroglucinols from Eucalyptus gittinsii subsp. gittinsii. New, eucalteretial F and (+)-eucalteretial B,and known, (-)-euglobal VII and eucalrobusone C, compounds,each belonging to predicted subclasses, were isolated and characterized. Staphylococcus aureus and Plasmodium falciparum screening revealed eucalrobusone C as the most potent antiplasmodialformyl phloroglucinol to date.

Automated summary

NMR fingerprints are useful for identifying natural products in complex mixtures. By using principal component analysis and machine learning with H-1 and C-13 NMR data, along with structural information, diagnostic NMR fingerprints were generated to categorize subclasses within a group of formyl phloroglucinols. This led to the reassignment of NMR chemical shifts for 44 compounds. The study also involved the reinterpretation of NMR data and the isolation and characterization of compounds within predicted subclasses.