Fragrances Through Hydrochlorination of Terpenes

Among the reactions that have enriched terpene chemlstry, perhaps the simplest and one of the outstanding is hydrochlorination. This reaction has been used for structural elucidation, for derivatization to identify and/or purify terpenes and for synthesizing perfumery chemicals. How the application of hydrochlorination on terpenes has unearthed the treasure caves of fragrances is a vibrant chapter worthy of portrayal.

At the onset, it is of interest to touch upon routes to fragrances which utilise isoprene (I), the fundamental terpene unit, now available prolifically as a petrochemical. Here, the key reaction is the mono-hydrochlorination of the hydrocarbon with aqueous or gaseous hydrogen chloride to give prenyl chloride [1-chloro-3-methyl-2-butene] (2), the latter is obtained in 86% yield by mono-hydrochloroinating (I) admixed with cuprous chloride. Condensation of (2) with acetone at moderate temperature in presence of sodium hydroxide and an amine type catalyst affords methylheptenone (3) EthynyIation of (3) with acetylene and sodium hydroxide in N-methyl pyrrolidone solution without pressure gives dehydrolinalool (4). Dehydrolinalool is a very versatile intermediate. By catalytic hydrogenation with 2% Pd/Al2O3 in propanol, it gives 99% linalool (5a) and the latter can be rearranged to geraniol and nerol (6a); it condenses with acetone to give ψ-ionone (7) and the ionones (8a). With vanadium catalyst, it rearranges to citral (9). The pivotal role of this aldehyde as such and in the synthesis of aroma chemicals is well documented.

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