Efficient Syntheses of New Polyhydroxylated 2,3-Diaryl-9H-xanthen-9-ones

A large number of hydroxylated 2,3-diaryl-9H-xanthen-9-ones have been synthesised by two different approaches, starting either from 3-bromo-2-methyl-4H-chromen-4-one or from (E) -3-bromo-2-styryl-4H-chromen-4-ones. The former method involves Heck reactions between 3-bromo-2-methyl-4H-chromen-4-one and styrenes, leading to (E)-2-methyl-3-styryl-4H-chromen-4-ones; these condensed with benzaldehyde to give (E,E)-2,3-distyryl-4H-chromen-4-ones, which led to the desired 2,3-diaryl-9H-xanthen-9-ones under reflux in 1,2,4-trichlorobenzene. 3-Bromo-2-styryl-4H-chromen-4ones were obtained either by aldol condensations between 3-bromo-2-methyl-4H-chromen-4-one and benzaldehydes, or through Baker-Venkataraman rearrangements of 2-acetylphenyl cinnamates, followed by one-pot bromination/cyclisation with phenyltrimethylammonium tribromide. The 2,3-diaryl-9H-xanthene-9-ones were obtained in one-pot transformations involving Heck reactions between (E)-3-bromo-2-styryl-4H-chromen-4-ones and styrenes, followed by electrocyclisation and oxidation processes. The 2,3-diaryl3,4-dihydro-9H-xanthene-9-one intermediates were also isolated under these conditions, and so when 5-methoxy-2-styryl-4H-chromen-4-ones were used as starting materials the 1-hydroxy-6,7-diaryl-9H-xanthene-9-ones were also observed. The second method is a general one, because it allowed the synthesis of a great number of 2,3-diaryl-9H-xanthen-9-ones with several substitution patterns, whereas the first one is limited to certain derivatives. The last step in the synthesis of hydroxylated 2,3-diaryl-9H-xanthen-9-ones was the cleavage of the hydroxy protecting groups with boron tribromide. The structures and stereochemistry of all new compounds were established by NMR studies. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)