Electrochemical Chlorination of Physcion - An Approach to Naturally Occurring Chlorinated Secondary Metabolites of Lichens

The electrochemical chlorination of physcion (1,8-dihydroxy-3-methoxy-6-methylanthracene-9,10- dione; 1) in AcOH and CH(2)Cl(2) was investigated by cyclic voltammetry and prep.-scale electrolysis. This approach provided access to a number of diverse biologically and pharmacologically interesting chlorinated secondary metabolites of lichen. Unlike the only previous literature report on the 'classical' chlorination of physcion (1), which allowed the preparation of 4-chlorophyscion (2b), 4,5-dichlorophyscion (3b), and 2,4,5-trichlorophyscion (4), the present procedure also gave fragilin (= 2-chlorophyscion; 2a) and 2,4-dichlorophyscion (3a), alongside the previously obtained 2b, 3b, and 4. All of these compounds, except for 2a, were isolated by column chromatography and medium-pressure liquid chromatography (MPLC) and characterized by spectral data. The preparative electrolysis with a 2 F . mol(-1) charge consumption in AcOH and 10 F . mol(-1) in CH(2)Cl(2) may have a practical synthetic utility, since the thus obtained product mixtures can be readily fractioned by column chromatography to afford pure 2b and 4, respectively. The regioselectivity of the reaction is explained by the resonance stabilization of the corresponding arenium cations - potential products of an electrophilic attack of a 'positive' Cl species on the physcion molecule.