Abstract
Recent studies suggest that the main contact allergen of propolis is dimethylallyl ester of caffeic acid (compound 1). We synthesized compound 1 together with its O-methyl derivatives, eg, dimethylallyl ester of ferulic acid (compound 2) and dimethylallyl ester of 3,4-dimethoxycinnamic acid (compound 3) having protected hydroxylic functions. The synthesis of compounds 1 and 2 was affected through reactions of caffeic and ferulic acid, respectively, with dimethylallyl bromide. Compound 3 was prepared by methylation of 1. The structure and purity of these products were confirmed by analytical and spectroscopic data. Moreover, thin-layer chromatography examination of poplar bud extracts and propolis samples showed the presence of compounds 1 and 2, but not of product 3. We patch tested nine subjects sensitized to propolis with these compounds. Three of them reacted to compound 1, but no reaction was seen to products 2 and 3. These data support the hypothesis that sensitizing activity of ester 1 could be related to the presence of free hydroxylic groups.
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