Xanthones and xanthenes

Silylaryl triflates, benzyne precursors, react with (thio)sahcylate esters in the presence of CsF to give (thio)xanthones. It is proposed that nucleophihc coupling of the salicylate anion with the aryne is followed by cyclisation onto the ester carbonyl group 05OL4273 . 

3-Bromo-2-styrylchromones, derived from 2 -hydroxyacetophenone in four steps, react with styrenes under Heck conditions to give 2,3-diaryIxanthones 05SL3095 . 

Tetrahydroxanthen-l-ones are obtained from the reaction of salicyl Af-tosylimines with alicyclic enones. Ring size controls the amounts of the Baylis-Hilhnan adduct which are also isolated 05SL2623 . 

Both enantiomers of 9,9 -spirobixanthene-l,l -diol have been obtained by resolution of the racemic product derived from 1-methoxyxanthone 06OBC4474 and spiro[fluorene-9,9 -xanthenes] 36 result from a one-pot reaction between fluorenone and phenol using an excess of MeS03H 06OL2787 . 

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Maeda K, Terazima M, Azumi T and Tanimoto Y 1991 CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the... 

Li JQ, Hu MY, Yao SQ (2009) Rapid synthesis, screening, and identification of xanthone-and xanthene-based fluorophores using click chemistry. Org Lett 11 3008-3011... 

Reagents (I) 4-TsPH, cat. MgS04, anhyd. CH2CI2, reflux, then BF3.0Et2, reflux 6.4.2.S Xanthones and Xanthenes... 

Azumi et al. carried out variable-field CIDNP experiments on the biradicals resulting from intramolecular hydrogen abstractions in polymethylene-linked xanthone and xanthene moieties [61a, 61b]. They also studied the temperature dependence of the mean exchange interaction for... 

Moro-Oka et al. (1976) have reported that the oxidation of 9,10-dihydroanthracene by K02 solubilized in DMSO by 18-crown-6 gives mainly the dehydrogenated product, anthracene. Under the same conditions, 1,4-hexadiene is dehydrogenated to benzene. The authors proposed a mechanism in which the superoxide ion acts as a hydrogen-abstracting agent only. The oxidations of anthrone (to anthraquinone), fluorene (to fluorenone), xanthene (to xanthone) and diphenylmethane (to benzophenone) are also initiated by hydrogen abstraction. 

The t3C resonances of xanthone (21) have been assigned on the basis of shift analysis of the structurally related compounds anthrone and xanthene (77P735) and by correlation with the component molecules diphenyl ether and benzophenone (78T1837). Delocalization of the non-bonding electron pairs at oxygen, leading to an increase in aromaticity of the pyranone ring, accounts for the deviation of the chemical shifts of C-4a, C-8a and the carbonyl carbon atom from the calculated values. 

Electrophiles attack xanthene at C-2 and sometimes also at C-7 when conditions are sufficiently vigorous. Nitric acid simultaneously oxidizes xanthene to the xanthone and thus gives 2-nitroxanthone. Friedel-Crafts acetylation of xanthene gives either 2-acetyl- or 2,7-diacetylxanthene according to the temperature (0 or 18 °C) and the proportion of acetyl chloride (74BSF2963). 

Xanthone is unreactive towards hydrazine and phenylhydrazine. The oxime is obtained by reaction of xanthione (xanthene-9-thione) with hydroxylamine, or from xanthone and hydroxylamine in pyridine. When the oxime is heated in water with phenylhydrazine, the phenylhydrazone is formed. In acid solution, xanthone reacts normally with 2,4-dinitro-phenylhydrazine but xanthone-1 -carboxylic acid (435) gives the pyridazinone (436), possibly via the hydrazone (57JCS1922). When the oxime is heated with phosphorus pentachloride it undergoes a Beckmann rearrangement to give the amide (437) (70MI22300). 

Xanthene reacts differently with alkynes to give mostly the 9-xanthylidene derivative (462) and a small amount of its cyclized analogue (463) (73RTC845). The spiran (464) is formed in 15% yield when xanthone and l,3-diacetylimidazol-2-one or its homologues are irradiated in acetone (67CB3961). 

The reduction of xanthone to xanthene has been achieved by using the Huang-Minlon modification of the Wolff-Kishner reduction (52JCS3741) by LAH (55JA5121) and by diphenyl-silane (61JOC4817). 

Intramolecular cyclisation following halogen-metal exchange in the benzonitrile derivatives 25 provides a route to xanthones and thioxanthones. Incorporation of a second aryl halide function into the benzonitrile substrate allows an anionic cascade ring-closing sequence and the formation of pentacyclic xanthene derivatives 26 <03JOC4091>. 

The anion derived from furobenzopyrandione 28 reacts with Michael acceptors regiospecifically to yield xanthones and various annulated derivatives <03OL3753> and intramolecular trapping of the anion by a carbamate side-chain leads to spiro- pyrrolidine and piperidine 9-xanthene derivatives <03TL9291>. 

Chemically xanthones (9H-xanthen-9-ones) are heterocychc compounds with the dibenzo-y-pyrone framework (1, Fig. 1). The xanthone nucleus is mun-bered according to a biosynthetic convention with carbons 1-4 being assigned to acetate-derived ring A and carbons 5-8 to the shikimate-derived ring B. The other carbons are indicated as 4a, 4b, 8a, 8b, 9, and 9a for structure elucidation pmposes [4]. 

Recently, 59 xanthones (= 9H-xanthen-9-ones) of natural or synthetic origin were investigated for their inhibitory activity toward MAO-A and MAO-B. The majority of the compounds demonstrated reversible, time-independent activities, with selectivity toward MAO-A. The most active inhibitor (1,5-dihydroxy-3-methoxyxanthone) had an IC50 of 40 nM. 3D-QSAR studies revealed the importance of an OH substituent in position 1 or 5 instead of a MeO substituent and the contrary is true for position 3, where MeO... 

The iron-catalyzed synthesis of dihydro-2H-pyrans, 2H- and 4H-chromenes, chromones, isocoumarins and xanthenes were described in a broad review on the synthesis of heterocycles (14T4827), while the synthesis of 2H-pyran and chroman derivatives, coumarins and xanthone-type compounds was described in a review of recent progress on the chemistry of dimethyl acetylenedicarboxylate (14S537). The synthesis of several polysubstituted spirooxidole-2-amino-4H-pyrans, mainly through three-component reactions, was also described in a review on the synthesis of biologically active spirooxindoles (14T9735). 

Sahin, Nieger, and Brdse have reported also the oxidative coupling of various hexahydro-xanthenols. The application of an iron complex as oxidant converted the 2-hydroxy-substituted xanthenes 1056 (Scheme 13.26) to the 3,3- t S-coupled biaryl 1057 (565). It appears that these two papers ((564) and (674)) represent the only chemical studies published on the topic of xanthone or xanthene biaiyl-coupUng. 

K. Okuma, A. Nojima, N. Matsunaga, K. Shioji, Reaction of benz)me with salicylaldehydes general synthesis of xanthenes, xanthones, and xanthols, Org. Lett 11 (2009) 169. 

Commercial PCB mixtures frequently contain impurities that may contribute to the 2,3,7,8-TCDD toxic equivalency factor. These impurities may include other PCBs, dioxins, dibenzofurans, naphthalenes, diphenyl ethers and toluenes, phenoxy and biphenyl anisoles, xanthenes, xanthones, anthracenes, and fluorenes (Jones etal. 1993). PCB concentrations in avian tissues sometimes correlate positively with DDE concentrations (Mora et al. 1993). Eggs of peregrine falcons (Falco peregrinus) from California, for example, contained measurable quantities of various organochlorine compounds, including dioxins, dibenzofurans, mirex, hexachlorobenzene, and / ,//-DDE at 7.1 to 26.0 mg/kg FW PCB 126 accounted for 83% of the 2,3,7,8-TCDD equivalents, but its interaction with other detectable organochlorine compounds is largely unknown (Jarman et al. 1993). 

Isolation of Oxidation Products. After oxygen absorption had ceased, or reached the desired value, the oxidates were poured into water. In many cases the reaction product could be removed by filtration in high yield. In this manner xanthone (m.p. 172-174°C.), was isolated from oxidations of xanthene or xanthen-9-ol thioxanthone (m.p. 208-210°C.), from thioxanthene acridine (m.p. 107-109°C.), from acridan anthracene (m.p. 216-217°C.), from 9,10-dihydroanthracene phenanthrene (m.p. 95-99°C.), from 9,10-dihydrophenanthrene pyrene (m.p. 151-152.5°C.) (recrystallized from benzene) from 1,2-dihydropyrene and 4-phenan-throic acid (m.p. 169-171 °C.) (recrystallized from ethanol) by chloroform extraction of the hydrolyzed and acidified oxidate of 4,5-methyl-enephenanthrene. 

The use of a quaternary ammonium hydroxide as a phase transfer catalyst provides an efficient conversion of xanthene (174) to xanthone under mild conditions (77TL2117), while the use of permanganate under milder conditions than those shown in Scheme 9 enables flavenes such as (175) and (176) to be converted into flavones (77BCJ3298, 78BCJ1175). The phase transfer oxidation requires strongly alkaline conditions which may render it less suitable for the oxidation of flavenes and chromenes. Xanthene has also been converted into xanthone by means of permanganate or chromic acid. 

Metal hydrides usually reduce the carbonyl to alcohol but LAH converts xanthone into a dixanthyl ether (454) or xanthene (453) according to the conditions. The latter product is also obtained by the Wolff-Kishner-Huang-Minlon reduction. Metal-liquid ammonia opens the pyran-4-one ring of chromones and flavones to form dihydrochalcones and other compounds (79JOC1494). 

The compounds were proposed to be dichloro C6-polychlorodibenzo-furans [46]. Similar observations from pulp mill samples were done by Kuehl et al. [47]. They suggested the structures of these compounds to be chlorinated xanthenes and xanthones. Buser et al. reported the occurrence of methyl-, polymethyl-, and alkyldibenzofurans in pulp mill sludge and sediments [48], Later, C5-PCBBs were detected as three dichloro and four trichloro congeners in pulp mill effluents, recipient sediment, and mussels incubated in recipient water [49]. Mass spectra of these congeners ruled out the structure of alkyl polychlorodibenzofurans and supported chlorinated alkyl bibenzyls instead, which was verified by model compound syntheses of the both structure types [50-52]. 

TCBTs are relatively persistent and non-flammable (and therefore useful in hydraulic oils). Pyrolysis in a closed system for two days in the presence of hydrogen peroxide at 300 °C produced as main components chlorinated benzo-phenones, fluorenes, fluorenones, xanthenes, and xanthones. Polychlorodiben-zofurans (PCDF) and polychlorodibenzo-p-dioxins (PCDD) were formed in much lower concentrations than from PCB product Pyralene T1 under identical conditions [84]. In pyrolysis at 450-700 °C with excess oxygen TCBTs produce more PCDFs and PCDDs. In parallel with the behavior of PCBs, PCDFs are formed in significantly higher amounts than PCDDs from PCBTs [86],... 

Xanthone (34) and thioxanthone (35) are smoothly reduced with diborane in tetrahydrofuran at 0 °C to give the correspxjnding xanthenes (36 and 37). The ease of this reaction is explained by the formation of a stable xanthonium ion (38), generated by elimination of the borate ion (Scheme 11). ... 

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