(±)-1-[(1R*,2R*,8aS*)-1,2,3,5,6,7,8,8a-Octahydro-1,2,8,8-tetramethylnaphthalen-2-yl]ethan-1-one (5) was identified as a minor (ca. 5%) but very powerful (5 pg/l (air)) constituent of the important perfumery synthetic Iso E Super®. Its structure was assigned by NMR spectroscopy and established by a stereoselective synthesis starting from α-ionone (10). Diastereoselective conjugate addition of Me2CuLi to 10 was followed by a haloform reaction, esterification, and isomerization of the C=C bond by treatment with NaOCl (Schemes 3 and 4). The resulting allyl chloride 17 was ozonized and transformed into the trimethyl(vinyl)octahydrocoumarin 20 by diastereoselective Grignard reaction with ethynylmagnesium chloride, and subsequent Lindlar hydrogenation. Ireland-Claisen rearrangement of 20 followed by methylation with MeLi afforded the target molecule 5 that was identical with the material isolated from commercial Iso E Super®.