Limonene, selective

Limonene (15) can be isomerized to terpiaolene (39) usiag Hquid SO2 and a hydroperoxide catalyst (/-butyl hydroperoxide (TBHP)) (76). Another method uses a specially prepared orthotitanic acid catalyst with a buffer such as sodium acetate (77). A selectivity of about 70% is claimed at about 50% conversion when mn at 150°C for four hours. 

Several generalities can be formulated regarding selective reduction of polyolefins. Usually the least hindered double bond is hydrogenated pre ferentially (123), and, with steric hindrance about equal, the most strained bond will be reduced first. Exocyclic olefins are reduced more easily than those in the ring (R)-(+ )-Limonene, 190 g, was shaken with W-4 Raney nickel (12 g) under hydrogen at atmospheric pressure. After 31.9 1 of hydrogen had been absorbed, the solution was filtered. Essentially, pure (R)-( -i- )-carvomenthene was obtained in 96% yield (58). 

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

A tandem hydroformylation/carbonyl ene reaction can be observed in cases, in which substrates with at least two isolated oleftnic bonds are hydro-formylated at only one double bond selectively. Thus hydroformylation of limonene with PtCkCPPlH /SnCk/PPlH or PtCl2(diphosphine)/SnCl2/PPh3 gives a mixture of two diastereomeric alcohols upon carbonyl ene reaction of the intermediate aldehyde, (Scheme 36). Best results are achieved with a PtC Cdppb) complex. The mechanism of the final intramolecular cycli-zation step resembles an acid catalyzed carbonyl ene reaction [89]. 

The combination of Con-salts with NaBPLt is a selective reducing agent of a disubsti-tuted side-chain olefinic double bond in the presence of a trisubstituted endocyclic double bond, which is demonstrated in the reduction of limonene (equation 29)93 97. 

A new stereoselective epoxidation catalyst based on a novel chiral sulfonato-salen manganese(III) complex intercalated in Zn/Al LDH was used successfully by Bhattacharjee et al. [125]. The catalyst gave high conversion, selectivity, and enantiomeric excess in the oxidation of (i )-limonene using elevated pressures of molecular oxygen. Details of the catalytic activities with other alkenes using both molecular oxygen and other oxidants have also been reported [126]. 

The catalyst obtained via anion exchange with the commercial resin Amberlite lRA-900 showed excellent selectivity in epoxidation of acid-sensitive natural terpenes and allylic alcohols [73-75]. The selectivity of 92% at 83% conversion was attained in epoxidation of a-pinene and limonene. The catalytic activity of the reused catalyst was completely maintained after several catalytic cycles, and the filtrate was catalytically inactive [75]. 

Figure 11.6 indicates approximate /-values for some of the components in peppermint oil on a BPX-5 column this column selects mainly on the basis of molecular weight and shape. For example p-pinene has the same molecular weight as limonene but has a more compact shape and thus a lower /-value. Menthyl acetate has a higher /-value than menthol because of its higher molecular weight. 

Limonene is a liquid with lemon-like odor. It is a reactive compound oxidation often yields more than one product. Dehydrogenation leads to / -cymene. Limonene can be converted into cyclic terpene alcohols by hydrohalogenation, followed by hydrolysis. Nitrosyl chloride adds selectively to the endocyclic double bond this reaction is utilized in the manufacture of (—)-carvone from (+)-limonene (see p. 61). 

Selective conversion of pinene, 3-carene, and limonene or dipentene to terpineol, without terpin hydrate formation, is also used. Addition of organic acids (weak acids require catalytic amounts of mineral acids) produces terpinyl esters, which are subsequently hydrolyzed to terpineol, sometimes in situ. 

The preferred industrial method of carvone synthesis utilizes the selective addition of nitrosyl chloride to the endocyclic double bond of limonene. If a lower aliphatic alcohol is used as solvent, limonene nitrosochloride is obtained in high yield. It is converted into carvone oxime by elimination of hydrogen chloride in the presence of a weak base. Acid hydrolysis in the presence of a hydroxylamine acceptor, such as acetone, yields carvone [88]. 

The same rational P450cam mutants which have already been described for limonene and pinene oxyfunctionalisations were also successfully applied to valencene. In whole-cell biotransformations -nootkatol and nootkatone formed as main products with up to 25% overall yield, corresponding to activities of up to 9.9 nmol (nmol P450) min [201]. Higher activities (up to 43 min ) but lower selectivities than those with P450cam were obtained with mutants derived from Bacillus megaterium P450 BM3. 

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