Preparation and thermal rearrangement of trans-3-[1-methyl-2-(2-methyl-1-propenyl)cyclopropyl]-2-cyclohexen-1-one. A synthesis of (±)-[β-himachalene

Publisher Website
Google Scholar
Abstract
A total synthesis of the sesquiterpenoid (.+-.)-.beta.-himachalene [a Cedrus deodara constitutent] (2) is described. Treatment of 5,5-dimethyl-2-vinyl-1,3-dioxane (24) with bromoform and aqueous sodium hydroxide in the presence of a phase-transfer catalyst afforded the dibromocyclopropane 25. When the latter substance was allowed to react (tetrahydrofuran-hexamethylphosphoramide, -95.degree. C) with n-butyllithium in the presence of methyl iodide, a mixture of the epimeric products 26 (87-93%) and 27 (7-13%) was produced in high yield. Compound 26 was converted via a 2-step sequence (hydrolysis with 88% formic acid, 26 .fwdarw. 28; Wittig reaction with isopropylidenetriphenylphosphorane, 28 .fwdarw. 16) into the bromocyclopropane 16, which was transformed into the cuprate reagent 17. Reaction of 3-iodo-2-cyclohexen-1-one (4) with reagent 17, followed by thermolysis (xylene, reflux) of the resultant product 18 (the title compound), afforded, in quantitative yield, the dienone 12. Methylation of 12 furnished compound 29 which, upon hydrogenation in the presence of tris(triphenylphosphine)chlororhodium, gave the ketone 32. Conversion of compound 32 into the corresponding enol phosphate 33, followed by reduction (lithium, ethylamine-tetrahydrofuran, tert-butyl alcohol) of the latter material, provided 2.