The detection of the precursors of the photorearranged products of 3-hydroxyflavones in selected solvents from UV-visible spectra in situ

2, 9-deoxyflavonoids (= 3-aryl-1-indanones) from Aglaia odorata: Structure elucidation, biosynthetic pathway and lung protective activity

Six previously undescribed 2, 9-deoxyflavonoids (1/2a, 1/2b, 6, and 7), along with six known compounds (3-5, and 8-10), were isolated from the twigs and leaves of Aglaia odorata. Their structures were determined by a combination of spectral analysis, ECD calculation and enzymatic hydrolysis assay. Surprisingly, (±) aglaindanone E (11a, 11b) were unexpectedly formed as the derivatives of compounds 3-6 when exposed to ambient natural light. Furthermore, the plausible biosynthetic pathway of 2, 9-deoxyflavonoids was proposed and chemically mimicked. In biological activity assay, compounds 1/2a, 1/2b, 4, and 6 showed potential protective effects in the 0.75%CSE-induced BEAS-2B cells injury model.

The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η1-flavonolate rather than the η2 -bidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O2. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η1- and η2-coordination modes. Methylation of 3-hydroxythioflavone generates an air stable η1 hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η1-methoxyflavonol does not react with O2 under any condition. We find that any of the studied flavonol derivatives, η1 or η2, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.