Synthetic approaches to biologically active xanthones: an update
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REVIEW
Synthetic approaches to biologically active xanthones: an update Shurutishria Ramakrishnan1 · Sakunthala Paramewaran1 · Nadiah Mad Nasir1 Received: 5 March 2020 / Accepted: 12 August 2020 © Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract Many xanthone structures show promising biological activities. Accordingly, a breadth of synthetic strategies toward xanthone derivatives have been developed. This review surveys these methods published from 2013 to 2019, and in an integrated manner details the biological activities of key xanthone structures. The review highlights the synthesis of xanthones via the classical and the modified Grover, Shah, and Shah reaction; the use of ytterbium, palladium, ruthenium, copper catalysis; the use of chromen-4-ones as building blocks; the use of the Friedel–Crafts reaction; Ullmann-ether coupling; metal-free oxidative coupling; intermolecular and intramolecular couplings; xanthone synthesis via the intermolecular Diels–Alder reaction; a novel decarboxylative aminocatalytic strategy; use of the Michael reaction, and the use of the Knoevenagel–Michael, Michael/Michael, Michael/Henry cycloadditions; and [4+2] cycloaddition. Keywords Synthesis · Xanthone derivatives · Pharmacology
Introduction
Synthesis of simple xanthone
Xanthones (9H-xanthen-9-ones) are yellow-colored and oxygen-containing heterocycles. The dibenzo-γ-pyrone framework of the parent xanthone corresponds to the molecular formula of C13H8O2 (Fig. 1). Many oxygenated heterocycles possess pharmacological activities, and the xanthone class is not an exception (Hepworth 1984). At present, nearly 1000 naturally occurring xanthones are known. Each has different substituents at different positions, ultimately leading to a large variety of pharmacological activities (Sousa and Pinto 2005). The multitude of biological activities found for xanthone derivatives include α-glucosidase inhibition, anticancer activities, anti-Alzheimer activities, and anti-inflammatory activities (Klein-Júnior et al. 2020; Feng et al. 2020). Accordingly, the isolation of xanthone natural products and the synthesis of new xanthones have both received extensive effort. This review highlights these efforts, with emphasis on the new developments since the previous review (published in 2012) in this area (Shagufta and Ahmad 2016).
Classical and modified method
* Nadiah Mad Nasir [email protected] 1
The first successful xanthone synthesis was reported by Michael and by Kostanecki using a mixture of polyphenol and different salicylic acids heated with acetic anhydride as the dehydrating agent (Kostanecki and Gentisin 1891; Michael 1883). In 1955, it was found that the use of zinc chloride/phosphoryl chloride produced xanthones in better yield and with shorter reaction times (Scheme 1) (Grover et al. 1955). Liu et al. (2017) synthesized a series of xanthones via this classical method with microwave heating. Most of these xanthones exhibited effective inhibitory activities against the growth of five cancer cell lines wi
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