Xanthones, hydroxy

Xanthone, 2-hydroxy-1,3,4,7-tetramethoxy-molecular dimensions, 3, 624 Xanthone, 1,3,7-trihydroxy-synthesis, 3, 837 Xanthone-1-carboxylic acid reactions... 

XVII XVII, 1st Supplement (combined with Volumes XVIII and XIX) XVII, 2nd Supplement 1933 1934 1952 2359-2503 2359-3031 2359-2503 One Cyclic Oxygen (S, Se or Te). Stem nuclei Furan, 27. Thiophene, 29. Hydroxy compounds Furfuryl alcohol, 112. Carbonyl compounds Butyrolactone, 234. Furfural, 272. 2-Aeetyl-thio-phene, 287. Xanthone, 355. Succinic anhydride, 404. Phthalic anhydride, 469. 

The spectra of xanthones are characterized by the carbonyl resonance at low field which is generally unaffected by substituents (cf. Figure 6). However, a 1-hydroxy group is able to form a hydrogen bond with the carbonyl group with the result that the carbonyl carbon atom is deshielded. 

A xanthone also results from the cyclization of 2 -nitro-6-hydroxy-2,4-dimethoxyben-zophenone, which is the sole product from a photo-Fries rearrangement of 3,5-dimethoxyphenyl 2-nitrobenzoate (81T209). Ring closure is effected in boiling piperidine and appears to involve elimination of nitrous acid together with ether cleavage. 

Several oxidants have been used to form xanthones from hydroxybenzophenones by oxidative coupling at ortho and para positions to an activating hydroxy group. It should be emphasized that much of this work has been carried out on a small scale and is of limited preparative value. 

Phenol oxidative coupling occurs specifically para to the 3 -hydroxy group in the photolysis of 2,4,6,3, 4 -pentahydroxybenzophenone, affording a reasonable yield of the xanthone (66JCS(C)175). 

Figure 4.46 (a) Charge transfer in excited states of hydroxy- and amino-aromatics, (b) Xanthone offers two potential protonation sites... 

Benzynes generated from silylaryl triflates 803 react with ortho-hydroxy benzoates via a tandem nucleophilic coupling-electrophilic cyclization process to afford xanthones in good yield (Scheme 224) <2005OL4273>. 

Both the hetero and carbonyl oxygen atoms of maltol, 3-hydroxy-2-methylpyran-4-one, are replaced by sulfur on treatment with Lawesson s reagent. While the ketone —> thione conversion can be achieved with various sulfur reagents, the heteroatom exchange is unusual and appears specific for Lawesson s reagent (Equation 179). This reaction does not occur with either related chromones or xanthones <2006CC206>. 

The key intermediate in the total synthesis of furaquinocin was obtained in good yield by a reductive Heck reaction that proceeded with a sterically hindered base pentamethylpiperidine (PMP) <02JA11616>. A new hypothesis for the major skeletal rearrangement (anthraquinone —> xanthone —> coumarin) that occurs in the complex biosynthesis of aflatoxin Bi was proposed. To test this hypothesis, an intermediate 11-hydroxy-O-methylstergmatocystin (HOMST) was synthesized as shown below. The key transformation in this synthesis involved the treatment of an ester-aldehyde with Pr3SiOTf, which smoothly produced a mixed acetal. Direct reduction with DIBAL-H led to the aldehyde. The desired product was eventually obtained via several steps as shown <02JA5294>. 

Capsaicin (= qms-8-Methyl-Js- [(4-hydroxy-3-methoxyphenyl) methyl] -6-nonenamide tem-8-Methyl-A-vanillyl-6-nonenamide) (vanilloid phenolic) l,7-Dihydroxy-2,3-dimethoxyxanthone (xanthone)... 

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