Chemical Studies on Mexican Plants Used in Traditional Medicine, III: New 4-Phenylcoumarins from Exostema caribaeum

Abstract
Investigation of the MeOH extract of Exostema caribaeum (Rubiaceae) led to the isolation of three new 4-phenylcoumarins. Their structures, 5-O-.beta.-D-galactosyl-7-methyoxy-3''-4''-dihydroxy-4-phenylcoumarin [1a] 7,4'',5''-trihydroxy-4-phenyl-5,2''-oxido-coumarin [2a] and 7,4''-dimethoxy-5''-hydroxy-4-phenyl-5,2''-oxido-coumarin [3a] were elucidated by spectral methods and chemical transformations. It was also demonstrated that 4-phenylcoumarins undergo oxidative cyclization under basic conditions in the presence of air to give 4-phenyl-5,2''-oxido-coumarins.

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