Emodin polyketide derivative

Leistner in Germany and Burnett in Great-Britain have performed much work on the biosynthesis of anthraquinones [5,23-28]. Two different pathways for the biosynthesis of anthraquinones in plants exist either the polyketide pathway or the shikimate pathway [5], Anthraquinones synthesised according to the former pathway are substituted in both ring A and C. Examples are emodin and chrysophanol. These polyketide-derived anthraquinones are found in Rhamnaceae, Polygonaceae and Leguminosae... 

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). 

Emodin (11) and helminthosporin (9) (Fig. 6,1) (see also Chapter 5) are derived from acetate-malonate precursors. An enzyme that provides strict substrate control, emodin 1-0-methyltransferase, was purified 89-fold (Hutchinson, 1986). Chrysophanol (57), found in Rumex (Polygonaceae) and Rhamnus (Rhamnaceae) species, appears to result from folding of the polyketide chain in one arrangement (58), whereas aloesaponarin (59) seems to involve a second form (60) (Fig. 6.13). Both chrysophanol and aloesaponarin occur in the roots of Aloe saponaria (Liliaceae) (Leistner, 1981). Under special conditions, callus cultures of Rhamnus species... 

Emodins with both aromatic rings hydroxylated are synthesised exclusively by the polyketide pathway as octaketides. Biosynthesis of bianthrones and other condensed emodin derivatives occurs by one-electron oxidation of anthrone derivatives (anthranols) to radicals, which are joined to form bianthrones. 

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