Chemical studies on Mexican plants used in traditional medicine, III: New 4-phenylcoumarins from Exostema caribaeum

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-methoxy-3',4'-dihydroxy-4-phenylcoumarin [1a] 7,4',5'-trihydroxy-4-phenyl-5,2'-oxidocoumarin [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.