Hélio Albuquerque

Chalcones as Versatile Synthons for the Synthesis of 5- and 6-membered Nitrogen Heterocycles

Chalcones belong to the flavonoid family which constitutes one of the major classes of naturally occurring oxygen heterocyclic compounds. The , -unsaturated carbonyl system of chalcones possesses two electrophilic reactive centers allowing them to participate in addition reactions via attack to the carbonyl group (1,2-addition) or involving the -carbon (1,4-conjugate addition), leading to the synthesis of promising bioactive heterocyclic compounds. The purpose of this review is to present a systematic survey of the most recent literature that uses chalcones in the synthesis of biologically active 5and 6-membered nitrogen heterocycles such as pyrroles, indoles, isoxazoles, imidazoles, pyrazoles, indazoles, triazoles, tetrazoles, pyridines and pyrimidines. Efficiency, easy-to-handle and cheap reagents, alternative heating conditions and greener protocols will be highlighted. In this review we will cover the literature since the beginning of the 21 century in more than 400 publications.

Novel chromone and xanthone derivatives: Synthesis and ROS/RNS scavenging activities

Chromones and xanthones are oxygen-containing heterocyclic compounds acknowledged by their antioxidant properties. In an effort to develop novel agents with improved activity, a series of compounds belonging to these chemical classes were prepared. Their syntheses involve the condensation of appropriate 2-methyl-4H-chromen-4-ones, obtained via Baker-Venkataraman rearrangement, with (E)-3-(3,4-dimethoxyphenyl)acrylaldehyde to provide the corresponding 2-[(1E,3E)-4-(3,4-dimethoxyphenyl)buta-1,3-dien-1-yl]-4H-chromen-4-ones. Subsequent electrocyclization and oxidation of these compounds led to the synthesis of 1-aryl-9H-xanthen-9-ones. After cleavage of the protecting groups, hydroxylated chromones and xanthones were assessed as scavenging agents against both reactive oxygen species (ROS) [superoxide radical (O2(•-)), hydrogen peroxide (H2O2), hypochlorous acid (HOCl), singlet oxygen ((1)O2), and peroxyl radical (ROO(•))] and reactive nitrogen species (RNS) [nitric oxide ((•)NO) and peroxynitrite anion (ONOO(-))]. Generally, all the tested new hydroxylated chromones and xanthones exhibited scavenger effects dependent on the concentration, with IC50 values found in the micromolar range. Some of them were shown to have improved scavenging activity when compared with previously reported analogues, allowing the inference of preliminary conclusions on the structure-activity relationship.

First intramolecular Diels–Alder reactions using chromone derivatives: synthesis of chromeno[3,4-b]xanthones and 2-(benzo[c]chromenyl)chromones

A series of novel (E)-2-(2-propargyloxystyryl)chromones and (E,E)-2-[4-(2-propargyloxyphenyl)buta-1,3-dien-1-yl]chromones were designed and synthesized via aldol condensation of 2-methylchromones with 2-propargyloxy(benzaldehyde and cinnamaldehyde), respectively. Both chromone derivatives were used as substrates in microwave-assisted intramolecular Diels–Alder reactions, affording chromeno[3,4-b]xanthones and 2-(benzo[c]chromenyl)chromones. This is the first report involving chromone derivatives in intramolecular Diels–Alder reactions for the synthesis of new oxygen heterocycles, namely xanthone- and flavone-type compounds.