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.