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

Abstract

Abstract The interaction of widespread monoterpenes limonene, α‐pinene, and β‐pinene with crotonaldehyde using silica‐supported tungstophosphoric heteropoly acid H 3 PW 12 O 40 and its acidic cesium salt Cs 2.5 H 0.5 PW 12 O 40 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 H 3 PW 12 O 40 and Cs 0.5 H 0.5 PW 12 O 40 are efficient and truly heterogeneous cycloaddition catalysts.