2-(3,4-dihydroxyphenyl)-3,5,7-4H-1-benzopyran-4-one, commonly known as quercetin is, as one of the most important representatives of flavonoids, known for its antioxidant, antiviral, anti-inflammatory and anticarcinogenic properties. Due to its low bioavailability, intensive research efforts have been focused on possible modifications of the molecule with the aim of medical and pharmaceutical application. The answer could lie in its halogenated derivatives that exhibit high biological activity. We have successfully prepared and isolated three halogenated derivatives of quercetin: 8-bromoquercetin, 6,8-dibromoquercetin and 6-chloroquercetin.
Bromination was performed in two different ways, with N-bromosuccinimide as reagent and with oxidative system HBr-H2O2. The synthesis of mono- and dibromo derivates of quercetin was successful with both methods. The advantages of both methods are their simple procedure and mild reaction conditions, as they do not require direct use of elemental bromine. Above that, the benefit of using the oxidative method is in its better environmental parameters, as the reactive species is regenerated during the reaction and water is the only by-product. Using the oxidative system HBr-H2O2, we have also successfully identified and isolated one of the by-products present in halogenation reactions, resulting from oxidation of quercetin, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. Furthermore, chlorination was performed with N-chlorosuccinimide and only monochlorinated derivative of quercetin was formed.
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