Constitution and stereochmistry of fagopyrins, the photodynamic pigments of buckwheat - Fagopyrum aesculentum - flowers

CONSTITUTION AND STEREOCHEMISTRY OF FAGOPYRINS, THE PHOTODYNAMIC PIGMENTS OF FAGOPYRUM AESCULENTUM FLOWERS PINTO, Andrea; LATTUADA, Benedetta; BARBIROLI, Alberto; VISCONTI, Giacomo L.; RAGG, Enzio M. Università degli Studi di Milano, Italy Benedetta.lattuada@yahoo.it Introduction: Fagopyrin is a phenantroperylenequinone1 present in the flowers of Fagopyrum aesculentum (buckwheat) endowed with photodynamic activity. It has been reported that fagopyrin extracts actually contain a complex mixture of closely related molecules, differing only on the nature of the perylenequinone substituents2. We wish to report our detailed study about the chemical composition of fagopyrin extracts. Material & Methods: Fagopyrins were extracted from buckwheat flowers with an acetone/ acetic acid/ water mixture. Removal of rutin and other flavonoids was achieved by ethanol/HCl precipitation, column chromatography (silica gel in pyridine/formic acid/ethyl formate and LH20 in methanol/TFA1%). Solid Phase Extraction was used as a final purification step. Structural determination was achieved by HPLC-MS and 1D- and 2D-NMR of single HPLC fractions. Stereochemistry was elucidated by Circular Dichroism supported by Density Functional Theory calculations. Results: Nine isomers have been structurally characterized and their stereochemistry fully elucidated.The presence of these two heterocyclic ring substituents, together with the inherent axial chirality of the aromatic system3 provides three stereogenic centres that introduce complex stereochemical relationships among isomers, thus giving account of the high level of molecular multiplicity found in the extract. Conclusion: The combined use of 1H-NMR and CD spectroscopy was found to be particularly suited to fully characterize all stereochemical aspects of the extracted fagopyrins. 200 Words maximum for abstract References 1. Brockmann, H., Weber, E., & Pampus, G. (1952). Protofagopyrin und Fagopyrin, die photodynamisch wirksamen Farbstoffe des Buchweizens (Fagopyrum esculentum). Justus Liebigs Annalen der Chemie, 575(1), 53-83. 2. Benković, E. T., Žigon, D., Friedrich, M., Plavec, J., & Kreft, S. (2014). Isolation, analysis and structures of phototoxic fagopyrins from buckwheat. Food chemistry, 143, 432-439. 3. Falk, H. (1999). From the photosensitizer hypericin to the photoreceptor stentorin—the chemistry of phenanthroperylene quinones. Angewandte Chemie International Edition, 38(21), 3116-3136.