Synthesis of Tricyclic Ketones with Sesquiterpene Skeletons by Acid-Catalyzed Rearrangement ofβ-Monocyclofarnesol

Starting from dihydro-β-ionone (6) a mixture of three tricyclic ketones with sesquiterpene skeletons 14, 15, and 16 was prepared by Wittig-Horner reaction with triethyl phosphonoacetate, Red-Al® reduction, acid-catalyzed rearrangement of the resulting β-monocyclofarnesol (7), alkaline hydrolysis of the formates 8 – 10, and subsequent molybdenium-catalyzed oxidation. The mechanistic background of the acid-catalyzed rearrangement of β-monocyclofarnesol (7) is discussed in detail. The resulting tricyclic ketones 14 – 16 exhibit intense woody odor notes with peppery vetiver or camphoraceous cedarwood aspects.