Stereoselective Synthesis of <i>trans</i>‐Decalins via Intramolecular Ene Reactions; Synthesis of the Enantiomerically Pure Cadinane‐Sesquiterpene Veticadinol

Knoevenagel condensation, intramolecular ene reaction, demethoxycarbonylation, Prins reaction, intramolecular alkylation and Grignard reaction--starting from (R)-citronellal 2 and dimethyl malonate 1, these reaction steps lead stereoselectively to veticadinol 3. A new catalyst was used for the ene reaction, namely FeCl3 on Al2O3, which is readily available and gives very good selectivities as well as high yields.