Resveratrol is a well-known natural polyphenol with a plethora of pharmacological activities. As a potent antioxidant, resveratrol is highly oxidizable, and readily reacts with reactive oxygen species (ROS). Such a reaction not only leads to a decrease in ROS levels in a biological environ-ment but may also generate a wide range of metabolites with altered bioactivities. Inspired by this notion, in the current study, our aim was to take a diversity-oriented chemical approach to study the chemical space of oxidized resveratrol metabolites. Chemical oxidation of resveratrol and a bioactivity-guided isolation strategy using xanthine oxidase (XO) and radical scavenging activities led to the isolation of a diverse group of compounds, including a chlorine-substituted compound (2), two iodine-substituted compounds (3 and 4), two viniferins (5 and 6), an eth-oxy-substituted compound (7) two ethoxy-substituted dimers (8 and 9). Compounds 4, 7, 8 and 9 are reported here for the first time. All compounds without ethoxy-substitution exerted stronger XO inhibition than their parent compound, resveratrol. By enzyme kinetic and in silico docking studies compounds 2, 3 and 4 were identified as potent competitive inhibitors of the enzyme while the viniferins acted as mixed-type inhibitors. Further, compounds 2 and 9 had better DPPH scavenging activity and oxygen radical absorbing capacity than resveratrol. Our results suggest that the antioxidant activity of resveratrol is modulated by the effect of a cascade of chemically stable oxidized metabolites, several of which have significantly altered target specificity as compared to their parent compound.

Many redox diseases, including diabetes, are associated with an imbalance of calcium homeostasis, regulated mainly by a Ca2+-ATPase pump (SERCA). Dysfunction of this enzyme may be prevented by natural polyphenolic compounds, representing a possible supporting treatment. Compounds that increase SERCA activity/expression may be useful for the treatment of diabetic complications. Stimulation of SERCA1 activity was analyzed experimentally and by molecular modeling. SERCA1 activity under methylglyoxal- and palmitate-induced oxidative stress was evaluated. The viability of INS-1E cells and insulin secretion were determined. [6]-Gingerol, resveratrol, and ellagic acid increased SERCA1 activity and exerted a protective effect under oxidative stress in the noncellular system. We found for the first time that the binding of polyphenols ([6]-gingerol, resveratrol, ellagic acid) to Glu439 in the SERCA1 P-domain may be critical for the stimulation of its activity. Moreover, this binding may also be important in the protective effects against oxidative stress. Direct stimulation of SERCA1 activity by ellagic acid was observed for the first time. In INS-1E cells, these compounds increased insulin secretion.

Fluorine represents a privileged building block in pharmaceutical chemistry. Diethylaminosulfur-trifluoride (DAST) is a reagent commonly used for replacement of alcoholic hydroxyl groups with fluorine and is also known to catalyze water elimination and cyclic Beckmann-rearrangement type reactions. In this work we aimed to use DAST for diversity-oriented semisynthetic transformation of natural products bearing multiple hydroxyl groups to prepare new bioactive compounds. Four ecdysteroids, including a new constituent of Cyanotis arachnoidea, were selected as starting materials for DAST-catalyzed transformations. The newly prepared compounds represented combinations of various structural changes DAST was known to catalyze, and a unique cyclopropane ring closure that was found for the first time. Several compounds demonstrated in vitro antitumor properties. A new 17-N-acetylecdysteroid (13) exerted potent antiproliferative activity and no cytotoxicity on drug susceptible and multi-drug resistant mouse T-cell lymphoma cells. Further, compound 13 acted in significant synergism with doxorubicin without detectable direct ABCB1 inhibition. Our results demonstrate that DAST is a versatile tool for diversity-oriented synthesis to expand chemical space towards new bioactive compounds.

Protoflavones, a rare group of natural flavonoids with a non-aromatic B-ring, are best known of their antitumor properties. The protoflavone B-ring is a versatile moiety that may be explored for other pharmacological purposes, but common cytotoxicity of these compounds is a limitation to such efforts. Protoapigenone was previously found to be active against the lytic cycle of Epstein-Barr virus (EBV). Further, the 5-hydroxyflavone moiety is a known pharmacophore against HIV-integrase. The aim of this work was to prepare a series of less cytotoxic protoflavone analogs, and to study their antiviral activity against HIV and EBV. Twenty-seven compounds including 18 new derivatives were prepared from apigenin through oxidative de-aromatization and subsequent continuous-flow hydrogenation, deuteration, and/or 4′-oxime formation. One compound was active against HIV at the micromolar range, and 3 compounds showed significant activity against the EBV lytic cycle at the medium-low nanomolar range. Among these, protoapigenone 1′-O-isopropyl ether (6) was identified as a promising lead due to its 73-times selectivity of its antiviral over its cytotoxic effect, which exceeds that of protoapigenone by 2.4-times. Our results open new opportunities to design new, potent and safe anti-EBV agents based on the natural protoflavone moiety.