Anthraquinone-1-acetic acid, cyclization

Xanthones in higher plants are also formed by this mixed pathway, though the polyketide chain originates from a benzoic acid instead of the usual cinnamic acid. Cyclization now affords a benzophenone (691), rather than a chalcone, which subsequently cyclizes to the xanthone, a route used with considerable success in the laboratory (Scheme 280). Other xanthones are derived only from acetate, through ring opening, decarboxylation and cyclization of an anthraquinone precursor. 

From the established carbon-carbon connectivities in 304a, it was proposed that a possible precursor of the bicyclo[7.3.1]enediyne substructure could be C-14 chains such as 329 or 330, or their biogenetic analog, which can be connected with the anthraquinone moiety through a Friedel-Crafts type acylation reaction to form the C-8-C-9 bond and can also cyclize to the bicycle structure through two condensation reactions to form the C-3-C-4 and C-7-C-8 bonds. Introduction of an additional acetate unit at C-5 followed by oxidative degradation to form the carboxyl group and O-methylation from methionine should occur at some point. Tentatively, a heptacar-bonyl acid such as 331 was proposed as a possible precursor of the anthraquinone moiety. Moreover, an enediyne chain such as 329 is also conceivable as a precursor of the anthraquinone portion [332]. 

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