Emodin anthrone

R = C02H, rhein anthrone R = CH2OH, aloe-emodin anthrone... 

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]. 

The formation of naphthodianthrones in nature most likely involves emodin anthrone 7 as intermediate which results from cyclisation of a linear polyketide. The latter is formed by condensation of eight acetyl CoA units. Emodin anthrone 7 is then dimerised to penicilliopsin 8 which undergoes oxidation to protohypericin 3, the direct precursor of hypericin 1 [17]-... 

Barbaloin was first isolated in 1851, but an acceptable structure was not proposed until 1952, when Miihlemann showed that condensation of aloe-emodin anthrone Ql,8-dihydroxy-3-(hydroxymethyl)anthrone, 7, R = CH2OH] with tetra-O-acetyl-a-D-glucopyranosyl bromide in acetone in... 

The biogenesis of barbaloin has not yet been studied, but its synthesis gives rise to some legitimate speculations thereon. A phenoxide ion can be alkylated on an oxygen atom or on a carbon atom, as shown in the reaction sequence below which reaction predominates depends on the phenol, the conditions employed, and the alkylating agent used. The glycosylation of the anion from aloe-emodin anthrone is an example of reaction b, whereas... 

Isobarbaloin appears to be an isomer of barbaloin, and degradative experiments give the same results as with barbaloin thus, Ldger - has shown that, on prolonged, acid treatment, isobarbaloin gives aloe-emodin and D-arabinose. It yields aloe-emodin anthrone on treatment with aqueous borax, and consumes 2 moles of periodic acid per mole. The ultraviolet spectrum of isobarbaloin is identical with that of barbaloin, showing that it, too, has an anthrone structure. The infrared spectrum of isobarbaloin shows only minor differences from that of barbaloin. From these data, it seems likely that isobarbaloin and barbaloin differ only in their stereochemistry. 

The synthesis of barbaloin from aloe-emodin anthrone and tetra-0-acetyl-a-D-glucopyranosyl bromide leads to one product only, and no isobarbaloin is formed. It seems justified to assume that this product is a /3-D-glycopyranosyl derivative in which case, one possible explanation of the isobarbaloin-barbaloin relationship is to presume that isobarbaloin is... 

Anthrones e.g., emodin anthrone (12) are probable intermediates in the biosynthesis of anthraquinones such as emodin from polyketide pathways (Fig. 6.22) and occur widely in nature. The position para to the carbonyl oxygen of the center (B) ring is sensitive to oxidation and, in many instances, oxidation products may be the major or only compounds isolated from plants which contain anthraquinones after storage (Leistner, 1985). 

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). 

Anthraquinones have been reported to be the main active components in Cassia tora, including aloe-emodin, anthrone, aurantiobtusin, chrysophanic acid,... 

In experiment 1 (Table VII), the precursors emodin (76) and emodin-anthrone (77) were labeled with the same isotope ( " C) in two different positions [at C(3) and C(ll)] by the synthetic procedures described above. After incorporation, the corresponding carbon atoms of the ergochrome EE (2) could be analyzed as acetic acid by Kuhn-Roth oxidation. The specific incorporations were derived from the activities of the two carbon atoms, determined separately by Schmidt degradation of the acetic acid. The results show that emodinanthrone (77) is incorporated into ergochrome EE (2) 4.5 times more effectively than emodin (76). 

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