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

A large number of hydroxylated 2,3‐diaryl‐9 H ‐xanthen‐9‐ones have been synthesised by two different approaches, starting either from 3‐bromo‐2‐methyl‐4 H ‐chromen‐4‐one or from ( E )‐3‐bromo‐2‐styryl‐4 H ‐chromen‐4‐ones. The former method involves Heck reactions between 3‐bromo‐2‐methyl‐4 H ‐chromen‐4‐one and styrenes, leading to ( E )‐2‐methyl‐3‐styryl‐4 H ‐chromen‐4‐ones; these condensed with benzaldehyde to give ( E , E )‐2,3‐distyryl‐4 H ‐chromen‐4‐ones, which led to the desired 2,3‐diaryl‐9 H ‐xanthen‐9‐ones under reflux in 1,2,4‐trichlorobenzene. 3‐Bromo‐2‐styryl‐4 H ‐chromen‐4‐ones were obtained either by aldol condensations between 3‐bromo‐2‐methyl‐4 H ‐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‐9 H ‐xanthene‐9‐ones were obtained in one‐pot transformations involving Heck reactions between ( E )‐3‐bromo‐2‐styryl‐4 H ‐chromen‐4‐ones and styrenes, followed by electrocyclisation and oxidation processes. The 2,3‐diaryl‐3,4‐dihydro‐9 H ‐xanthene‐9‐one intermediates were also isolated under these conditions, and so when 5‐methoxy‐2‐styryl‐4 H ‐chromen‐4‐ones were used as starting materials the 1‐hydroxy‐6,7‐diaryl‐9 H ‐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‐9 H ‐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‐9 H ‐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)