Linalool oxides reactions

Conceptually, it is interesting to speculate on the bioconversion of inexpensive secondary metabolites to others of greater value. Along these lines, Schreier and co-workers use Botrytis cinera to convert linalool to a series of other terpenoids as well as to the furanoid and pyranoid linalool oxides. Reactions of this type are good examples of converting inexpensive, available aroma chemicals to higher valued products. 

Linalool has been used to prepare a mixture of terpenes useful for enhancing the aroma or taste of foodstuffs, chewing gums, and perfume compositions. Aqueous citric acid reaction at 100°C converts the linalool (3) to a complex mixture. A few of the components include a-terpineol (34%) (9), Bois de Rose oxide (5.1%) (64), ocimene quintoxide (0.5%) (65), linalool oxide (0.3%) (66), tij -ocimenol (3.28%) (67), and many other alcohols and hydrocarbons (131). 

In the presence of acids, linalool isomerizes readily to geraniol, nerol, and a-terpineol. It is oxidized to citral by chromic acid. Oxidation with peracetic acid yields linalool oxides, which occur in small amounts in essential oils and are also used in perfumery. Hydrogenation of linalool gives tetrahydrolinalool, a stable fragrance material. Its odor is not as strong as, but fresher than, that of linalool. Linalool can be converted into linalyl acetate by reaction with ketene or an excess of boiling acetic anhydride [34]. 

Isomers of 6,7-Epoxy-linalool as Precursors of Linalool Oxides. Previously, the triol (21) had been proposed as a possible precursor of the hydroxy ethers (14) and (15), the so-called linalool oxides(Fig-ure 8). At an acidic pH (<3.5) and/or during heat treatment (e.g., steam distillation/extraction), the triol (21) had been found to be decomposed to (14) and (15) (23,24). In these previous experiments no formation of the corresponding pyranoid linalool oxides (22) and (23) was observed. We evaluated the hypothesis that linalool oxides are formed from triol (21) under natural conditions of papaya pulp (i.e. pH 5.6) and could find no formation of linalool oxides. Even in model experiments carried out at pH 3.5, only traces of linalool oxides were detected after incubation of (21) for three days. As a result of these experiments the isomers of 6,7-epoxy-linalool have to be considered as the natural precursors of linalool oxides (14, 15) as recently suggested by Ohloff et al. (25). The latter s proposal was based on earlier findings obtained in a series of chemical reactions (9). 

The odorless polyols appear to be derived from four "parent" monoterpenes - linalool, citronellol, nerol and geraniol by hydration and oxidation reactions (6,22). Figure 3 lists the polyols identified in grapes and also shows structures of components not yet found but likely to be observed should polyol formation from each parent molecule follow an analogous route. 

Damage to the flavor of "Asti Spumante" wines stored at 20 C when compared with samples held at 10 C was related to losses of linalool and glycosidic precursors of linalool, geraniol and nerol (87). Also the concentrations of a-terpineol, hotrienol, nerol oxide and the furan linalool oxides increased in wines held at ambient temperature. Di Stefano and Castino (87) concluded that low temperature storage retarded these reactions and wines so held retained the characteristic muscat flavor for years. 

Moeller synthesized tetrahydrofuran natural products (+)-linalool oxide <01OL2685> and (+)-nemorensic acid <01TL7163> by employing intramolecular coupling reactions of enol ether radical cations as well as ketene dithioacetal radical cations with oxygen nucleophiles. 

The Ti,Al-Beta shows both acidic and oxidative properties which is reflected in unwanted side-reaction. The group of Corma used the bifunctionality in the epoxidation/rearrangement of cx-terpineol to cineol alcohol and in the formation of furans from linalool.81,82 Similarly van Klaveren et al. applied Ti,Al-Beta in the one-pot conversion of styrene to phenyl acetaldehyde.83 Sato et alM solved the unwanted acid-catalyzed side reaction by neutralizing the acid site by ion exchange with alkali metals. Nevertheless the bifunctionality restricts the use of this catalyst to a limited number of reactions. 

Oxidation of the chiral (4-geranylseleno)-15-(4-toluenesulfonyl)[2.2]paracyclophane with 3-chloroperbenzoic acid gave (S )-linalool with 67% enantioselectivity39. Diethylamine was added before warming up to avoid side reactions by the generated selenic acid. On the basis of similar oxidation of paracyclophane-substituted alkyl selenides it was assumed that the asymmetric induction in the oxidation step is responsible for the modest selectivity. 

The reaction was carried out in acetonitrile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites foUowed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-6 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool. 

Oxides and peroxides can occur in many essential oils by a photochemical reaction. 1,8-Cineol and linalool monoxide can be readily separated on silica gel thin layers with 1-nitropropane-hexane (1 1, v/v), as the mobile phase.In this case, they have exhibited Rf values of 73 and 8, respectively. " Another pair of compounds, ascaridole and 1,8-cineol, can be easily separated on a silica gel layer, obtaining a value for chloroform as the mobile phase of 63 and 54, respectively. The antimony chloride reagent gives a gray color. The potassium iodide-acetic acid-starch test is usually better than ferrous thiocyanate. 

Many hydroxylated linalools [including compounds 105, 106, 108, and 110, both (Z)- and ( )-isomers], as well as the epoxides of both furanoid (109) and pyranoid (see section on pyrans) linalyl oxides, have been identified in papaya fruit (Carica papaya). At the same time, the first reported occurrence of die two linalool epoxides (112) in nature was made. These epoxides are well known to be unstable and easily cyclized (see Vol. 2, p. 165) and have been made by careful peracid oxidation of linalool. An interesting new method has now been described. While the vanadium- or titanium-catalyzed epoxidation of geraniol (25) gave the 2,3-epoxide (see above), as does molybdenum-catalyzed epoxidation with hydrogen peroxide, the epoxidation of linalool (28) with molybdenum or tungsten peroxo complexes and hydrogen peroxide led, by reaction on the 6,7-double bond, to 112. ... 

As a test reaction a mixture of cis- and trans-carveol was oxidized with TBHP. Analogous to what has previously been observed for compounds like geraniol and linalool, epoxidation was fast and selective (reaction 8). Trans-carveol was converted only to the corresponding epoxide, whereas cis-carveol gave a mixture of cis-epoxide and carvone. 

The dye-sensitized photooxygenation reaction has to be considered as a biomimetic process 451) involved in the formation of deoxylinalool oxide (132) 459), rose oxide (134) 456) and nerol oxide (135) 459) from linalool (6), citronellol (9) and nerol (8), respectively. 

In the reaction between pigments and linalool, which is a common component of perfumes, pigments such as black iron oxide and hydrated chromium oxide which... 

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