Versatile synthesis of cadalene and iso-cadalene from himachalene mixtures: Evidence and application of unprecedented rearrangements

From a mixture of α-, β- and γ-himachalenes extracted from waste wood of Atlas cedar (Cedrus atlantica), cadalene (1,6-dimethyl-4-isopropylnaphthalene) and iso-cadalene (1,6-dimethyl-3-isopropylnaphthalene) were produced in two steps with up to 71% ± 5% yield through the ar-himachalene intermediate using I₂ and/or AlCl₃ as reagents. The selectivity is shown to sharply depend on the operating conditions: while I₂/AlCl₃ in dichloromethane promotes the formation of cadalene, the formation of iso-cadalene is favored in the presence of AlCl₃ in cyclohexane. The bicyclic aromatic compounds were thus obtained through unique rearrangements involving sequential CC bond cleavage/formation and hydride transfer processes. In the absence of AlCl₃ or I₂, dihydrocurcumene was also found to be formed with up to 70% selectivity. A tentative mechanism is proposed and discussed.