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

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Abstract
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‐4‐ones 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‐diaryl‐3,4‐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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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