Condensation, xanthone

Condensation, isatin, 5, 71 Condensation, phenylurea, 3, 93 Condensation, xanthone, 7, 84... 

Analogue routes from 4-fluoropyridine (88JHC81), 2-chloroquinoline (89JHC1589), 2-chloropyrazine (88S881), and 3,6-dichloropyridazine (90JHC1377) lead, respectively, to condensed aza-xanthones 155-158 (Scheme 44). 

Dihydrofuran (used as the vinyl ether), condenses with 2-acetyl-1,4-benzoquinone to give tetrahydrofuro[2,3-6]benzofuran (144), which is found in natural benzofuranoid xanthones (sterigmatocystin).372... 

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

Dihydroxyxanthones.1 KF in DMF is a more efficient catalyst than K2C03 for condensation of methyl salicylates with chlorobenzoquinones to provide phenoxybenzo-quinones (1). The products on reduction (Na2S204) and cyclization (H2S04) afford 1,4-dihydroxy xanthones (2) with high regioselectivity (equation I). 

For example, condensation of diphenyl ether with phosgene under Frledel-Crafts conditions gives 4,4 -diphe-noxy benzophenone as the major product (Ref. 13). We developed an improved process which leads to a very pure product with low content of xanthone [Scheme 16] ... 

Likewise the reactive methyl groups in 2-methylchromone (19) and 2,6-dimethyl-4-pyrone are also susceptible to condensation with vinamidinium salts (7), as are their vinylogues (compare Section B. 1.). The colored condensation products (42) smoothly cyclize to xanthones (43) ... 

The standard methods for the synthesis of xanthones are via the benzophe-none 17 and diaryl ether intermediates 18 (Fig. 7). The intermediate ben-zophenone derivatives 17 can be obtained by condensation between an ortho-oxygenated benzoic acid and an activated phenol, in the presence of phosphorus oxychloride and zinc chloride (a) [44]. This intermediate is also accessible through condensation by the Friedel-Crafts acylation of appropriately substituted benzoyl chlorides with phenolic derivatives (b) [45]. Then the oxidative or dehydrative processes cause the cyclization of 2,2 -di-oxygenated ben-zophenone to xanthone (c) [46]. 

Previous researchers have suggested [6,8] that benzophenones are biosynthesized by condensation of metabolites from the shikimate pathway, forming the A-ring, and the acetate-malonate pathway, creating the B-ring. This produces the basic 13-carbon benzophenone skeleton, Fig. (1). Support for this biosynthetic pathway includes the isolation of benzophenone synthase from Centaurium erythraea [9] and research by Atkinson etal. [10] who examined benzophenones as intermediates in the synthesis of xanthones. (Xanthone biosynthesis is reviewed by... 

Furthermore, the analysis of the pyrolysis residue of PC obtained at 400°C, after 1 hour of isothermal heating, showed the presence of several consecutive xanthone units indicating that at this temperature the isomerization and condensation processes are quite extensive. ... 

Benzophenones are derived from several pathways. In higher plants, most benzophenones arise by cyclization of a polyke-tide chain of three malonates added to a hydroxybenzoic acid precursor (Manitto, 1981). Cyclization occurs via a Claisen condensation mechanism (Fig. 10.14). These compounds serve as precursors for xanthones in many plants. In some instances [e.g., in the heartwood of Symphonia globulifera (Clusiaceae)], the benzophenone and the corresponding xan-thone co-occur [in this case maclurin (33) and 1,3,5,6-tetra-hydroxyxanthone (34)] (Weiss and Edwards, 1980). 

Xanthones are known in free-living fungi, and recent studies indicate that they are rather common in lichens too. Unlike the fungal xanthones, many lichen xanthones have one or more nuclear chlorine substituents. The fundamental structure of the known lichen xanthones could be derived directly by linear condensation of seven acetate and malonate units with one orsellinic acid-type cyclisation. The two rings are joined by a ketonic carbon and by an ether-oxygen arising from cyclodehydration. Xanthones include arthothelin, concretin,... 

Thermolysis of diphenyl carbonate (as a model system for bisphenol A-polycarbon-ate) at 360 °C results in the formation of significant amounts of carbon dioxide, phenol, xanthone, diphenyl ether, and phenyl o-phenoxybenzoate. These products are formed by the transformation of diphenyl carbonate to o-phenoxybenzoic acid, which in turn forms xanthone (by self-condensation), diphenyl ether (by decarboxylation) and phenyl o-phenoxybenzoate (by esterification with DPC) [673],... 

Route A involves the condensation of appropriately substituted phloro-glucinol derivatives with substituted orsellinic acid derivatives in the presence of anhydrous zinc chloride and phosphorus oxychloride. The analogous condensation in the presence of trifluoroacetic anhydride yielded the xanthone (150) (C-acylated product) as a minor byproduct together with the major aryl ester (151) (0-acylated product). However trifluoroacetic anhydride has become the reagent of choice (route B) since the use of 0-methyl and 0-benzyl protecting groups has enabled good yields of the xanthones to be obtained. 

Elix, Musidlak, Sala and Sargent 128) have reported a novel synthetic route to xanthones which used an Ullmann reaction to produce a diaryl ether and its subsequent condensation to yield the xanthones (route C). This route is exemplified by the synthesis of thiophaninic acid (136) (Scheme 16). 

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