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Acid-catalyzed cyclization of farnesol and nerolidol

1968; Elsevier BV; Volume: 24; Issue: 2 Linguagem: Inglês

10.1016/0040-4020(68)88035-4

1464-5416

C. David Gutsche, J.R. Maycock, C.T. Chang,

Chemistry and Chemical Engineering

As the preliminary phase of a program directed to the construction of polyfunctional catalysts for specifically directed cyclizations, a re-investigation of the acid-catalyzed cyclization of farnesol and related compounds has been undertaken. To simplify the analysis of the complex mixture obtained from these reactions an assay method has been devised which, using 1,2-dimethylnaphthalene as an internal standard, allows (a) the quantitative determination of the total volatile product in terms of polyenes, farnesol, nerolidol, bisabolol, and diols and (b) the quantitative determination of the total cyclic product in terms of monocyclic compounds (via 2-(p-tolyl)6-methylheptane in the dehydrogenation product) and bicyclic compounds (viacadalene in the dehydrogenation product(. The preparations of bisabolol, a bisabolyl diol, and 2-(p-tolyl)6-methylheptane are described, these and other compounds being necessary for quantitation of the assay procedure. In the cyclization experiments the effects of changes in time, temperature, water content of the formic acid, and identity of the starting alcohol have been tested. For each run, the changes in concentration of starting material and the various products were measured as a function of time. The results from these experiments show that (a) nerolidol reacts somewhat more rapidly than farnesol (b) cis-trans and trans-trans farnesol react at the same rate and give the same ratio of products (c) the reaction rate falls off rather rapidly with increasing water content of the formic acid (d) even under very mild conditions the reactions proceed rapidly to yield, by the time that the starting alcohol is ca. 90% consumed, a product comprised mainly of monocyclic and bicyclic polyenes and mono alcohols (e) with more prolonged reaction time the concentration of diol derivatives and, probably, triol derivatives, increases (f) with more prolonged reaction time at higher temperatures the concentration of total cyclic material diminishes, probably as the result of polymerization reactions (g) the monocyclic and bicyclic products are formed concurrently rather than sequentially. These data, combined with information obtained from cyclization experiments with pure bisabolol, have led to the conclusion that the pathway to bicyclic compound involves the sequence: farnesol (or nerolidol) → bisabolyl cation → rearranged bisabolyl cation → cadinyl cation → cadinyl derivatives.