Iso E Super, one of the most successful synthetic aroma chemicals in the history of perfumery, was originally introduced in the mid 1970s by International Flavors and Fragrances Inc. Iso E Super possesses a woody-amber odor and is used globally in many fragrance applications. Among these are fine perfumes and detergents. Iso E Super is obtained according to Figure 1, which includes a condensation reaction between acetaldehyde and methylethyl ketone, a Diels-Alder reaction between methyl pentenone (MPO) and myrcene, and an acid catalyzed cyclization of the Diels-Alder adduct (DAA). All reactions involved are not very selective, and the cyclization is accompanied by secondary isomerizations, so the final product is a complex mixture of isomeric ketones.
A typical GLC profile for Iso E Super is given in Figure 2. Peaks with greater than 1 percent concentration are labeled with letters A to H. In commercial versions, these isomers are usually present in the following concentrations: A, 3-6 percent; B, 40-60 percent; C, 4-9 percent; D, 11-18 percent; E, 1-2 percent (unreacted DAA, minor isomer, and also a coeluting peak E*- a product of double bond isomerization of DAA); F, 2-4 percent (unreacted DAA, major isomer); G, 2-4 percent; and H, 9-14 percent.
Surprisingly, until now the structures of only two constituents of Iso E Super have been established: the major isomer (peak B) and one of the minor isomers G. The absence of adequate data regarding other isomers may have been due to the difficulties in their separation and purification. Givaudan researchers have found that the predominant isomer B has a weak odor with a threshold of about 500 ng/l of air, and, most interestingly, that the amber odor note of the product is imparted primarily by the minor isomer G, which has a very low odor threshold value of approximately 5 pg/l of air. In other words, the odor of ketone G is a hundred thousand times stronger than that of B. In this connection, it became a very interesting and challenging problem to isolate and evaluate other components of Iso E Super.