The minor anthraquinones of Xanthoria parietina(L.) Beltram, the chlorination of parietin, and the synthesis of fragilin and 7-chloroemodin (‘AO-1’)

Abstract

Extraction of the lichen X. parietina has yielded the anthraquinones emodin (II), teloschistin (III), and parietinic acid (V), as well as parietin (I). Chlorination of parietin (I) with chlorine (1 mol.) gave 5-chloroparietin (XVI), with chlorine (2 mol.) gave chiefly 4,5-dichloroparietin (XVII), and with an excess of chlorine gave 4,5,7-trichloroparietin (XVIII). Dechlorination of 4,5,7-trichloroparietin (XVIII) with an excess of sodium dithionite gave parietin (I), and with a deficiency of sodium dithionite gave, after methylation, tri-O-methylemodin and 4,5,7-trichlorodi-O-methylparietin. Dechlorination of 4,5,7-trichloroparietin (XVIII) with hydrazine hydrate and palladium–charcoal, after methylation, furnished di-O-methylfragilin (XIX). Partial demethylation of di-O-methylfragilin (XIX) yielded fragilin (IX), and complete demethylation of the former compound afforded 7-chloro-emodin (‘AO-1’)(VIII).