The chemistry of fungi. Part LXX. Synthesis of some xanthones

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Abstract
Oxidation with hexacyanoferrate(III) of 2,3′,4,6-tetrahydroxybenzophenone (2; R1= OH, R2= R3= H) gave 1,3,7-trihydroxyxanthone (3; R1= OH, R2= R3= H) by a para-intramolecular coupling; other 2,3′-hydroxylated benzophenones reacted similarly. The same reagent and 2,4,5′-trihydroxy-2′-methoxy-3-methylbenzophenone (7; R1= R2= Me, R3= H) formed 6-hydroxy-4a-methoxy-5-methylxanthen-2(4aH),9-dione (12) which was aromatised (i) by hydrochloric acid to 4-chloro-2,6-dihydroxy-5-methylxanthone (11) and (ii) by zinc and acetic acid to 2,6-dihydroxy-5-methylxanthone (3; R1= R2= H, R3= Me). Other 2,5′-hydroxylated benzophenones behaved similarly. Whereas hexacyanoferrate(III) produced 1,4,6-trihydroxy-5-methylxanthone (14; R2= H, R1= Me) from 2,2′,4,5′-tetrahydroxy-3-methylbenzophenone (7; R1= R3= H, R2= Me), the same ketone with 2,3-dichloro-5,6-dicyano-p-benzoquinone gave 2-(2,4-dihydroxy-3-methylbenzoyl)-p-benzoquinone (8), which rapidly formed 1,4,6-trihydroxy-5-methylxanthone (14; R1= Me, R2= H) by an internal Michael addition. Similar results were obtained with cognate benzophenones.