Flavanones bromination

The benzene ring of fused dihydropyrans is more easily attacked by electrophiles than that of the corresponding pyran. However, the activating effect of the carbonyl group on the 3-position of chromanones and flavanones ensures that halogenation normally occurs at that position preferentially. Thus, 3-mono- and 3,3-di-bromination is readily achieved with appropriate molar proportions of bromine but bromination of 2,3-dihydronaphtho[2,l-Z>]pyran-l-one (675) in acetic acid gave the 7-bromo derivative (676) (48JA599). 

Nitroflavone has been obtained by conversion of o-benzyloxy-2-nitro-acetophenone to the chalcone, debenzylation and cyclization to the 3-nitro-flavanone,102 and aromatization by bromination-dehydrobromination (Scheme 21).103... 

An alternative procedure to bromine itself is the use of complexed derivatives such as 2-pyrrolidone hydrotribromide (PHT), which is easier to handle. This reagent has been shown to brominate flavanones in THF, while the more vigorous conditions of hot DMSO result in concommitant dehydrohalogenation to give the flavone (Scheme 2). ... 

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