30 Total Synthesis of (±)-Seychellene Based Upon Intramolecular Diels-Alder Addition of a Monocyclic Cyclohexadienone Derivative
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Seychellene (1), a sesquiterpene isolated from patchouli oil, was synthesized in racemic form by applying the intramolecular Diels-Alder addition of a monocyclic cyclohexadienone derivative as the key step. Dimethylcyclohexenone (13) was alkylated with 3-methyl-1, 5-diiodopentane in liquid ammonia using sodium amide as the base giving iodoketone (28). The reaction of two molar equiv of NBS on (28) afforded bromocyclohexadienone (33) in a moderate yield. Treatment of the latter with chromous chloride led to deconjugated ketone (35), which was then converted to conjugated ketone (34) by an acid. Treatment of (34) with dimethylamine followed by oxidation of the resulting aminoketone (36) with hydrogen peroxide gave N-oxide (37). Pyrolysis of the N-oxide caused a Cope elimination and the concurrent Diels-Alder addition of the elimination product (6) giving an adduct (7a). Hydrogenation of (7a) afforded norseychellanone (5), which has been reported to be transformed into seychellene (1).
