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 substituted vinylchromones has been developed. 1-Methyl-, 1-ethyl-, 2-methyl- and 2-ethyl-substituted xanthones are obtained in a one-pot synthesis from the reaction of pyrrolidine enamines derived from acetone, butan-2-one, propanal and butanal respectively, when taken as solvent incorporating a catalytic amount of pyrrolidine. Some 1-methylidene- and 1-methylidene-2-methyl-substituted tetrahydroxanthones were also obtained and these compounds are proposed 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-dimethyl substituted tetrahydroxanthone from the reaction between a vinylchromone and the pyrrolidine enamine of 2-methylpropanal.