Synthesis of two allergenic constituents of propolis and poplar bud excretion.

The prenyl ester and the phenylethyl ester of caffeic acid, formed in the bud excretion of poplar species, were shown recently to be the major contact allergens in bee-glue. An unambiguous synthesis of these compounds, based on the reaction of caffeic acid with 1-bromo-3-methyl-2-butene and with beta-bromoethylbenzene, respectively, is reported. The synthetic products confirm the previously described structures of the natural products and allow further testing of their allergenic properties.