The thermal or Lewis acid catalyzed intramolecular ene reaction of the Knoevenagel adduct 3 from citronellal and dimethyl malonate affords the two diastereoisomeric dimethyl (5-methyl-2-trans-isopropenylcyclohexyl)malonates 4a and 4b. Anhydrous iron(III) chloride on basic alumina proved to be the best catalyst giving, at - 78°C, the two diastereoisomers 4a and 4b in a ratio of 98.8:1.2 in 92% yield.