Oxygenation pattern anthraquinones

Note that many other natural anthraquinone structures are not formed via the acetate pathway, but by a more elaborate sequence involving shiki-mate and an isoprene unit (see page 158). Such structures do not contain the characteristic meta oxygenation pattern, and often have oxygenation in only one aromatic ring (see page 164). 

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). 

During their work, Kogl and Postowsky 432) isolated emodin (288) together with a second pigment which they named dermocybin. Structural studies by Birkinshaw 102) established dermocybin as either the 6- or 7-O-methyl ether of l,5,6,7,8-pentahydroxy-3-methyl-9,10-anthraquinone ( nordermocybin ) which was synthesised, thus establishing the oxygenation pattern in dermocybin itself. 

Anthraquinones occur as their oxygenated derivatives, with the substitution pattern reflecting their origins from the polyketide or shikunate pathway [13,18-20]. They are typically rather polar and, in many cases, their polarity is increased by their glycosidation. Although dimeric forms are known, most of the compounds identified are monomeric and have a relatively simple substitution of the central nucleus. Figure... 

---