Dibenzo- y-pyrone

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]. 

Methoxy-6-bromobenzoic acid and phloroglucine boiled 1 min. in 1 N NaOH, containing a little CuS04 —d-methoxyA -dihydroxy-dibenzo-a-pyrone. Y 70%. (F. e. with lower yields s. E. Lederer and J. Polonsky, Bl. 1948, 831.)... 

Fused benzene rings aid nucleophilic attack on pyridines, pyridinium and pyrylium ions, and pyrones the loss of aromaticity involved in the formation of the initial adduct is less in monobenzo derivatives and still less in linear dibenzo derivatives than in monocyclic compounds. For the same reason, the tendency for this initial adduct to re-aromatize is less for benzopyridines. Fused benzene rings also influence the point of attack by nucleophilic reagents attack rarely occurs on a carbon atom shared with a benzene ring. Thus, in linear dibenzo derivatives, nucleophilic attack is at the y-position (157). 

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