Synthesis of C-ring substituted xanthones from the [4+2] cycloaddition reaction of vinylchromones and acyclic enamines

A novel approach to the synthesis of C-ring substituted xanthones utilising the [4+2] cycloaddition reactions of enamines with aromatically substitute d vinylchromones has been developed. 1-Methyl-, 1-ethyl-, 2-methyl- and 2-e thyl-substituted xanthones are obtained in a one-pot synthesis from the rea ction of pyrrolidine enamines derived from acetone, butan-2-one, propanal a nd butanal respectively, when taken as solvent incorporating a catalytic am ount of pyrrolidine. Some 1-methylidene- and 1-methylidene-2-methyl-substit uted tetrahydroxanthones were also obtained and these compounds are propose d to be intermediates in the reaction, since they undergo facile conversion to 1-methyl- and 1,2-dimethyl-xanthones. Further evidence for the proposed reaction pathway was obtained from the isolation of a 1-pyrrolidino-2,2-di methyl substituted tetrahydroxanthone from the reaction between a vinylchro mone and the pyrrolidine enamine of 2-methylpropanal.