Dibenzo xanthones

The preparation of xanthones (9//-dibenzo[/Mpyran-9-ones) is illustrated by Schemes 35 and 36 <1997SL1081, 2005OL4273, CHEC-III(7.08.19)600>. 

Both xanthones and thioxanthones are available through CsF-promoted coupling of arynes with salicylates and thiosalicylates. The synthesis proceeds through sequential intermolecular nucleophilic substitution and intramolecular electrophilic cyclisation (Scheme 39) <07JOC583>. The Pd-catalysed annulation of 3-iodoflavone with the same triflate results in the dibenzo[a,c]xanthone (Scheme 39) <07JOC223>. 

Chemically xanthones (9H-xanthen-9-ones) are heterocychc compounds with the dibenzo-y-pyrone framework (1, Fig. 1). The xanthone nucleus is mun-bered according to a biosynthetic convention with carbons 1-4 being assigned to acetate-derived ring A and carbons 5-8 to the shikimate-derived ring B. The other carbons are indicated as 4a, 4b, 8a, 8b, 9, and 9a for structure elucidation pmposes [4]. 

The bixanthenyl 55 which possesses the ring system of the secalonic acids has been obtained from the 7-bromoxanthene via the arylboronic ester and a subsequent Suzuki coupling <04JOC6830>. The anionic polycondensation of acetonaphthones 56 with homophthalates gives dibenzo[Z7j]xanthones and offers an approach to the naturally occurring hypoxyxylerone system <04SL2693>. 

---