The sesquiterpenoid nootkatone and the absolute configuration of a dibromo derivative

Nootkatone, or (4R,4aS,6R)-4,4a,5,6,7,8-hexahydro-4,4a-dimethyl-6-(1-methylethenyl)naphthalen-2(3H)-one, C(15)H(22)O, a sesquiterpene with strong repellent properties against Formosan subterranean termites and other insects, has the valencene skeleton. The dibromo derivative (1S,3R,4S,4aS,6R,8aR)-1,3-dibromo-6-isopropyl-4,4a-dimethyl-1,2,3,4,5,6,7,8-octahydronaphthalen-2-one, C(15)H(24)Br(2)O, has two independent molecules in the asymmetric unit, which differ in the rotation of the isopropyl group with respect to the main skeleton. The C-Br distances are in the range 1.950 (4)-1.960 (4) A. Both independent molecules form zigzag chains, with very short intermolecular carbonyl-carbonyl interactions, having the perpendicular motif and O...C distances of 2.886 (6) and 2.898 (6) A. These chains are flanked by intermolecular Br...Br interactions of distances in the range 4.067 (1)-4.218 (1) A. The absolute configuration of the dibromo derivative was determined, from which that of nootkatone was inferred.