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A base-catalyzed direct oxidation of rhodamine, carborhodamine, and siliconrhodamine pyronines to the corresponding xanthones is studied. This methodology utilizes addition of water to split pyronines into xanthone and reduced xanthene, the latter of which is returned to pyronine by oxidation with iodine. The transformation is general, working on the three most recalcitrant versions of N, N, N', N'-tetramethylpyronines in good to excellent yields.

A biosynthetic gene cluster of the fungal xanthone heterodimer neosartorin (1) was discovered in Aspergillus novofumigatus, and its biosynthesis was investigated by a series of gene-deletion experiments. The results indicate that the two monomeric units of 1 are synthesized by the same set of enzymes, with chrysophanol (5) as a common precursor. Furthermore, the P450 monooxygenase NsrP for the heterodimerization was discovered, which also accepts non-native substrates to afford novel xanthone dimers.

A versatile and highly efficient strategy to construct a xanthone skeleton via a ligand-free intermolecular catalytic coupling of 2-substituted benzaldehydes and a wide range of phenols has been developed. For this purpose, a novel and magnetically recoverable catalyst consisting of copper nanoparticles on nanosized silica coated maghemite is presented. The reaction proceeds smoothly with easy recovery and reuse of the catalyst. The methodology is compatible with various functional groups and provides an attractive protocol for the generation of a small library of xanthones in very good yield.

Substituted tetrahydroxanthones are constructed using a DMAP-promoted tandem nucleophilic addition process. The reaction yields range from 39% to 73%. Disubstituted tetrahydroxanthones are generated as a ∼2.3:1 mixture of diastereomers favoring the formation of the trans-isomer.

NH aldimines, generated in situ from the corresponding aldehydes by reaction with ammonium acetate, serve as nitrogen nucleophiles in reactions with 3-(1-alkynyl)chromones and 3-cyanochromones that generate functionalized azaxanthones. These processes take place under mild conditions that do not require dry solvents. The products of the reactions described represent new chemical entities. We believe that the newly developed cascade process will serve as a potent method for the synthesis of N-heterocycles and in diversity-oriented synthesis.

Efficient syntheses of the highly substituted benzophenone graphisin A and the xanthone sydowinin B are described. Key steps involve aryl anion addition to substituted benzaldehyde derivatives, subsequent methyl ester installation, and dehydrative cyclization. Oxidation of graphisin A led to a spirodienone derived from a highly substituted benzoquinone intermediate.

A concise and straightforward strategy to construct a xanthone skeleton via an intramolecular cross-dehydrogenative coupling (CDC) of 2-aryloxybenzaldehydes has been developed. The reaction proceeded smoothly without any need of preactivation of the aldehyde group. It can tolerate various functional groups and provides an applicable protocol to construct a wide range of xanthone derivatives., (© 2012 American Chemical Society)

A facile entry to 1,4-dioxygenated xanthones having a variety of substitution patterns and substituents was developed that features a novel application of the Moore cyclization using substrates that were readily assembled in a highly convergent fashion by an acetylide stitching process. The practical utility of the methodology was demonstrated by an efficient synthesis of a naturally occurring xanthone and correction of the structure of dulcisxanthone C.

Readily available C-acylated cycloalkanones undergo efficient Pd catalyzed ring closure/cross-coupling providing 7-substituted tetrahydroxanthones in a single operation. One of the synthesized derivatives (depicted) is shown to selectively kill pancreatic cancer (PANC-1) cells under conditions of nutrient deprivation indicating that the tetrahydroxanthone is responsible, in part, for the "antiausterity" effects of the naturally occurring kigamicins.

An efficient tandem process was developed to synthesize diversified benzo[a]xanthones from 2-methyl-3-(1-alkynyl)chromones with electron-deficient chromone-fused dienes. This unusual reaction, involving multiple steps and not requiring the use of transition metal catalysts or an inert atmosphere, results in the formation of three new C-C bonds and one C-O bond.

A panel of new fluorophores with emission wavelengths from blue to yellow regions using the Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction of alkyne-functionalized xanthones and xanthenes with various azides have been synthesized. Screening of the "click" products led to the identification of "hit" fluorophores which showed a fluorescence increase upon triazole formation. These novel "click" fluorophores could potentially be used for bioconjugation and bioimaging purposes.

The reaction of salicylaldehydes with benzyne prepared from o-trimethylsilyphenyl triflate and CsF gave xanthenes and xanthones. When the reaction was carried out under basic conditions, 9-hydroxyxanthenes (xanthols) were obtained in good yields.

A new catalytic Cu(II)/sparteine system has been developed to promote beta,beta-phenolic couplings of styrenyl phenols en route to carpanone and related unnatural congeners in yields exceeding 85%.

Several 4-methoxyxanthones and 3,4-dimethoxyxanthones were synthesized in good yield via inverse-electron-demand Diels-Alder (IEDDA) driven domino reactions between a series of electron-deficient chromone-fused dienes with 1-(2,2-dimethoxyvinyl)pyrrolidine or tetramethoxyethene, respectively.

[reaction: see text] The reaction of silylaryl triflates, CsF, and salicylates affords a general and efficient way to prepare biologically interesting xanthones and thioxanthones. This chemistry presumably proceeds by a tandem intermolecular nucleophilic coupling and subsequent intramolecular electrophilic cyclization.

[reaction: see text] A new route toward puupehenone-related bioactive metabolites from (-)-sclareol, based on the palladium(II)-mediated diastereoselective cyclization of a drimenylphenol, is described. Utilizing this, the first enantiospecific synthesis of the antitumor and antimalarial (-)-15-oxopuupehenol, together with improved syntheses of (+)-puupehenone, (+)-puupehedione, and (+)-15-cyanopuupehenone, were accomplished.

[reaction: see text] Condensation of a benzopyranonphthalide with Michael acceptors provides an efficient, general method for regiospecific preparation of xanthones as well as linear and angular polycyclic aromatic systems containing a xanthone fragment.