ar-Himachalene

The reaction of dehydrogenation on the natural mixture of himachalene is already known [1]. A convenient synthesis of arylhimachalene was investigated. The three isomeric sesquiterpenes a-, b- and g-himachalene, constituents of the essential oil of Atlas cedar (cedrus atlantica), were dehydrogenated with Raney-nickel/activated charcoal. The reaction gives a good yield and high selectivity. A three-necked flask equipped with a stirring bar and a long condenser, and containing 150 mL of a mixture of a-, b- and g-himachalene, 5 g of activated charcoal and 18.5 g of Raney nickel was heated. The temperature was maintained at 250°C for 5 days. After cooling, hexane was added. The mixture was filtered through silica gel column and the hexane evaporated. The product was purified by fractional distillation under reduced pressure (T=100°C, P=10^-1 mm Hg), yield: 83%. The purity of arylhimachalene was determined by gas chromatography (95%).

¹H NMR (CDCl₃): 7.34 (d, J⁴=1.61Hz, H₂), 7.27 (d, J³=7.84Hz, H₅), 7.13 (dd, J⁴=1.61Hz, J³=7.84Hz, H₄), 3.41 (m, H₇), 2.46 (s, H₁₂), 1.94, 1.42 (m, m, H₉), 1.92, 178 (m, m, H₁₀), 1.91, 1.42 (m, m, H₈), 1.57 (s, H₁₄), 1.49 (s, H₁₃), 1.49 (s, H₁₅).

¹³C NMR (CDC_l3): 148.19 (C₁), 141.72 (C₆), 135.45 (C₃), 128.06 (C₂), 127.05 (C₄), 125.96 (C₅), 41.63 (C₁₀), 40.02 (C₁₁), 37.04 (C₈), 34.98 (C₇), 34.48 (C₁₄), 30.27 (C₁₅), 24.57 (C₉), 21.73 (C₁₂), 21.56 (C₁₃).