Oxidation of emodin anthrone and stereochemistry of emodin bianthrone

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Abstract
Synthetic emodin bianthrone is separated into meso- and (±)-diastereomers which are identified by means of an asymmetric p.m.r. shift reagent. They behave differently on mild oxidation. Both give protohypericin accompanied by an anthraquinone fraction but, whereas the major anthraquinone from the meso is emodin, the (k) gives chiefly the bianthraquinonyl, skyrin. This difference is accounted for in terms of the geometry of oxidative coupling. Oxidation of emodin anthrone in alkaline dimethyl sulphoxide gives a l,4-anthraquinone as major product. The scope of this novel oxidation has been extended to other anthrones and it has been utilized in the controlled synthesis of purpurin derivatives. It may involve anthrslnol tautomers as intermediates.