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Heteropoly Acid Catalysts for the Synthesis of Fragrance Compounds from Biorenewables: Cycloaddition of Crotonaldehyde to Limonene, α-Pinene, and β-Pinene



The interaction of widespread monoterpenes limonene, α-pinene, and β-pinene with crotonaldehyde using silica-supported tungstophosphoric heteropoly acid H3PW12O40 and its acidic cesium salt Cs2.5H0.5PW12O40 as solid acid catalysts in dichloroethane solutions results in cycloaddition, which gives the same fragrant oxabicyclo[3.3.1]nonene product in a high yield. The product is likely to be formed through an α-terpenyl carbenium ion intermediate, which is generated from monoterpene protonation and undergoes nucleophilic attack by crotonaldehyde. Both H3PW12O40 and Cs0.5H0.5PW12O40 are efficient and truly heterogeneous cycloaddition catalysts.