Anthrone dimers

Biosynthesis of Anthraquinones Distribution of Anthraquinones Biological Functions of Anthraquinones Economic Importance and Uses Anthrones and Anthrone Dimers Hypericin and Antihrone Dimers References... 

Hypericin (77), a complex anthrone dimer, is found in approximately half of the nearly 200 species of the genus... 

Quinone diazides can also be obtained by the diazo group transfer reaction of 4-tosyl azide. For example, 9-diazo-10-anthrone (2.55) is formed from anthrone (2.54) if the reaction is carried out in an ethanol-piperidine mixture. On the other hand, if ethanol is replaced by pyridine, dimerization with loss of molecular nitrogen takes place and the azine 2.56 is isolated (Scheme 2-32 Regitz, 1964 Cauquis et al., 1965). In the preceding discussion tosyl azide was shown to be an electrophilic reagent. It therefore seems likely that it is not the anthrone 2.54 but its conjugate base which reacts with tosyl azide. 

Treatment of diarylphthalins, obtained by reduction of diarylphthalides, with sulfuric acid results in rearrangement to yield a mixture of benzo[c]furans and anthrones (34JA1406). When diarylphthalins (441) were reacted with sulfuric acid the benzo[c]furans (442) were isolated. A dimeric benzo[c]furan (443) was obtained when the phthalin (444) was treated with acetic anhydride or phenyl isothiocyanate. 

Aromatic natural products of polyketide origin are less prevalent in plants compared with microorganisms. The majority of the plant constituents that contain aromatic stmctures are known to arise from the shikimate pathway (see below). Unlike those derived from the shikimate pathway, aromatic products of the polyketide pathway invariably contain a meta oxygenation pattern because of their origin from the cyclization of polyketides. Phenolic compounds such as chrysophanol-anthrone (Bl), and emodin-anthrone (B2), and the anthraquinones, aloe-emodin (B3) and emodin (B4) (Fig. 2), are products of the polyketide pathway and are found to occur in some plants of the genera Cassia (Leguminosae) (21), Rhamnus (Rhamnaceae) (22), and Aloe (Liliaceae) (23). The dimer of emodin-anthrone (B2), namely hypericin, (B5) is a constituent of the antidepressant herbal supplement, St. John s wort (Hypericumperforatum, Hy-pericaceae) (24). 

Little is known about the biosynthetic origin of hypericin(s) they are related to the anthranoid metabolism and emodin anthrone is possibly their precursor. Synthesis in vitro of hypericin following alkaline dimerization of emodin and oxidation of its reduction derivative, emodine anthrone, has in fact been demonstrated [35,36]. 

Oxidative dimerization of anthrone and its derivatives can be effected by oxygen,286 aromatic nitro compounds,287 diazomethane,288 or oxygen in the presence of palladium.289 The following procedure is for the use of nitrobenzene 287... 

Under certain conditions, oxanthrone can also be formed during the hydrogenation process (Scheme 14.3). Oxanthrone is not reoxidized in the autoxidation process, and further hydrogenation of oxanthrone leads to anthrone, tetrahy-droanthrones, and dianthrones (the latter via oxidative dimerization of anthrone)... 

The red-violet toadstool Cortinarius rufoolivaceus (Basidiomycetes) contains violet dimers of torachry-sone 8-O-methyl ether and a 1,2- or 1,4-anthracene-quinone, known as the R. They are formed from yellow-green precursors by atmospheric oxidation the precursors contain an anthracene or, respectively, an anthrone unit in place of the anthracenequinone. The main pigments are R. A C32H2gO, Mr 556.57, dark red amorphous powder, mp. 303-307 °C, [alp +320° (CH3OH)) and R. B C32H28O9, Mr 556.57, red amorphous powder, mp. 260 °C, [ajp +240° (CH3OH). ... 

In addition to conversion to anthraquinones, compounds such as emodin anthrone can be coupled by free-radical processes to produce a series of complex dimers such as hypericin (77) and fagopyrin (78) (Fig. 6.20) (Brockman and Lack-ner, 1979 Scott, 1967). 

Oxidative dimerisation at the dihydroanthracenone level may be followed by enzymatic or chemical modifications which transform portions of the molecule into anthrone or anthraquinone moieties. The majority of th( dimers is present in the form of their di-, tri- or tetra-O-methyl ethers, in which case the pigments become more stable as the methylation increases. Ultimately, bianthraquinones may result and these compounds too are conveniently included in this section. 

The therapeutic application of some drugs is due to 1,8-dihydroxy-anthraquinones they contain. The single compounds may occur in the quinone, anthranol or anthrone form, or possibly as glucosides or dimers [195,227]. As a rule, mixtures of many compoimds of this class occur in the plants. Since their activities are not identical, the analytical determination of the individual compounds is attempted. 

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