Coupling of phenylacetaldehyde and styrene oxide with biorenewable alkenes in eco-friendly solvents

Abstract Acidic cesium salt of tungstophosphoric heteropoly acid, Cs2.5H0.5PW12O40 (CsPW), is an excellent solid acid catalyst for the reaction of phenylacetaldehyde with biorenewable monoterpenic alkenes: limonene, α-terpineol, α-pinene and β-pinene. Simultaneously with a conventional oxonium-ene cyclization to give an oxabicyclo[3.3.1]nonene compound, phenylacetaldehyde undergoes Friedel-Crafts alkylation by the monoterpenes resulting in a new product with an unusual fused tetracyclic structure, obtained in ca. 80 % yield. A combined yield for both products was up to 95 %. Styrene oxide can be also used as a starting material for these reactions to give the same products in up to 85 % combined yield. A novel one-pot tandem process thus developed involves the isomerization of styrene oxide into phenylacetaldehyde and cycloaddition of the latter to the monoterpene, with both steps being catalyzed by CsPW. The reactions were performed in green solvents diethylcarbonate or anisole, which have high sustainability ranks in modern solvent selection guides, comparable to those of ethanol and water.