Hypervalent Iodine Reagents in the Synthesis of Flavonoids and Related Compounds

Hypervalent iodine compounds have proved to be very useful reagents to bring about various oxidative transformations including (i) α-functionalization of carbonyl compounds, (ii) oxidation of phenols, and (iii) oxidative rearrangement of ketones and α,β- unsaturated ketones. These reactions find interesting applications in the development of newer and convenient approaches for the synthesis of flavonoids. This review focuses on the use of most common three hypervalent compounds, namely iodobenzene diacetate, [hydroxy(tosyloxy)iodo]benzene, and [bis-trifluoroacetoxy(iodo)]benzene in the synthesis of cis/trans-3-hydroxyflavanones, 3-hydroxyflavones (flavonols), flavones, isoflavones and related compounds.

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

Mechanistic investigation into phenol oxidation by IBX elucidated by DFT calculations

Density functional theory (DFT) at the SMD/M06-2X/def2-TZVP//SMD/M06-2X/LANL2DZ(d),6-31G(d) level was used to explore the regioselective double oxidation of phenols by a hypervalent iodine(v) reagent (IBX) to give o-quinones.

Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

A high-valent nickel(III) compound performs fast concerted proton–electron transfer on O–H and C–H bonds. Thermodynamic analysis suggests that the oxidizing power of the compound and the formation of a strong ligand O–H bond lead to high reactivity.