Heteropoly acid catalysts in upgrading of biorenewables: Cycloaddition of aldehydes to monoterpenes in green solvents

•Heteropoly acids are an environmentally benign alternative for common acid catalysts.•Monoterpenes are an abundant renewable feedstock for flavor and fragrance industries.•CsPW and HPW/SiO2 catalyze the addition of aldehydes to terpenes in green solvents.•The reactions give fragrant oxabicyclo[3.3.1]nonene compounds in good yields.•Organic carbonates and 2-methyltetrahydrofuran replace toxic chlorinated solvents.

Acidic Cs salt of tungstophosphoric heteropoly acid, Cs2.5H0.5PW12O40 (CsPW), is excellent solid acid catalyst for liquid-phase cycloaddition reactions of biomass-based and easily available from essential oils monoterpenic compounds, such as limonene, α-terpineol, α-pinene, β-pinene and nerol, with aldehydes, including benzaldehyde, crotonaldehyde as well as biomass-derived cuminaldehyde and trans-cinnamaldehyde. The reactions give oxabicyclo[3.3.1]nonene compounds potentially useful for the fragrance and pharmaceutical industries in good to excellent yields. The process is environmentally benign and can be performed in biomass-derived solvent 2-methyltetrahydrofuran and eco-friendly “green” organic solvents such as dimethylcarbonate and diethylcarbonate under mild conditions at low catalyst loadings without leaching problems. The solid CsPW catalyst can be easily separated from the reaction media and low-boiling solvents can be removed by distillation. Silica-supported H3PW12O40 also demonstrated good performance in these reactions.

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