23 Total Synthesis of Flavocommelin, a Component of Commelinin, Supramolecular Metal Complex Pigment, from Blue Day Flower and its Contribution for Color Development
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Commelinin is a stoichiometric self-assembled supramolecular pigment from the blue petrals of dayflower, Commelina communis, composed with six molecules of malonylawobanin, six moleclues of flavocommelin and two atoms of magnesium ion. A fine molecular recognition occurs on formation of commelinin. The atomic structure of commelinin was determined by X-ray crystallography. We synthesized its various chiral analogues of flavocommelin and clarified the role of flavocommelin on formation of commelinin and its color development. We succeeded in the first total synthesis of flavocommelin by development of novel method in C-glycosylation to flavan skeleton. At first, naringenin was reducted to the flavan, which was C-glucosylated at C-6 using glucosyl fluoride in presence of Lewis acid followed by oxidation with CAN to give the naringenin 6-C-β-D-glucoside. The glucosylation at OH-4' of the naringenin glucoside according to our method and then oxidation with DDQ achieved total synthesis of flavocomelin. And also flavocommelin-analogues bearing D- and/or L-glucose were prepared. Reproduction experiments of the metalloanthocyanin from the anthocyanin, various chiral flavocomelin or its analogues in the presence of Mg^<2+> was performed. For formation of the metal-complex showing the blue color, D-glucose at 4'-O of apigenin was essential while the solution from the 4'-O-L-glucosides only gave purple color, which was very unstable. The 6-C-D-glucoside played a role for the stability of the color. However, surprisingly, apigenin 6-C-D-glucoside formed the supramolecule to show blue color. Now we are attempting to reveal the reason of the chiral structural recognition for the metal-complex formation by the structural analysis in detail. The metal-complex formation on the basis of the molecular interaction causes the beautiful blue color of dayflower.
