Synthesis of Fragrance Ingredients by Tandem Hydroformylation‐Cyclization of Limonene Catalyzed by Rhodium Complexes and Pyridinium p‐Toluenesulphonate

The rhodium-catalyzed hydroformylation of limonene (1) in the presence of PPh3 or P(O-o-tBuPh)3 as auxiliary ligands and pyridinium p-toluenesulfonate, as an acid co-catalyst, gave two diasteroisomers of 4,8-dimethyl-bicyclo[3.3.1]non-7-en-2-ol (3) in nearly quantitative yield. Limonene is a cheap natural product obtained commercially from citrus fruits, in particular, as a sub-product of the orange juice industry; whereas alcohol 3 is an expensive perfume ingredient. Alcohol 3 is formed through the hydroformylation of 1 giving a corresponding aldehyde (2) followed by the intramolecular carbonyl ene reaction resulting in cyclization of the aldehyde. The cyclization step is highly stereoselective with only one diasteroisomer of 3 being formed from each of two diasteroisomers of 2. The use of the P(O-o-tBuPh)3 ligand not only remarkably accelerates the hydroformylation step compared to the system with PPh3, but also increases significantly the cyclization activity of the catalytic system.