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

From a mixture of alpha-, beta- and gamma-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 I-2 and/or AlCl3 as reagents. The selectivity is shown to sharply depend on the operating conditions: while I-2/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 C-C bond cleavage/formation and hydride transfer processes. In the absence of AlCl3 or I-2, dihydrocurcumene was also found to be formed with up to 70% selectivity. A tentative mechanism is proposed and discussed. (C) 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.