Limonen-1,2-oxide systems

In the limonene oxidation reaction, various immobilized systems have been tested as catalyst. The use of MTO supported on poly(vinylpyridine) polymers however turned out to be a poor system for the limonene oxidation since both the conversions and the selectivities were quite a bit lower than the optimal non-immobilized system [56]. These systems were also used in combination with ionic liquids, and here, a similar result as with the non-immobilized system was found, yielding mostly diepoxide, with the best-performing catalyst MTO/PVP-25% in [BMIM][BF4], giving 92% of diepoxide at full conversion [61]. 

The EO of Crithmum maritimum (=Cachrys maritima, Apiaceae, rock samphire) comprises limonene and y-terpinene with an amount of 22.3% and 22.9%, respectively, as the major components. Two different test methods (TBA assay and a micellar model system where the antioxidative activity in different stages of the oxidative process of the lipid matrix was monitored) were used. Both assays explain the very high activity of this EO. In the TBA assay BHT and a-tocopherol were used as positive standards and the oil showed a better capacity than those substances. Comparable results were obtained by the micellar method system where the EO acts as a protector of the oxidation of linoleic acid and inhibits the formation of conjugated dienes (Ruberto et al., 2000). The modification of LDL by an oxidative process for instance can lead to atherosclerosis. Natural antioxidants such as P-carotene, ascorbic acid, a-tocopherol, EOs, and so on are able to protect LDL against this oxidative modification. y-Terpinene proved itself to be the strongest inhibitor of all used authentic compounds for the formation of TBARS in the Cu -induced lipid oxidation system (Grassmann et al, 2003). So, the addition of y-terpinene to food can possibly stop the oxidative modification of LDL and reduce the atherosclerosis risk. 

The activity of the FePeCli6-S/tert-butyl hydroperoxide (TBHP) catalytic system was studied under mild reaction conditions for the synthesis of three a,p-unsaturated ketones 2-cyclohexen-l-one, carvone and veibenone by allylic oxidation of cyclohexene, hmonene, and a-pinene, respectively. Substrate conversions were higher than 80% and ketone yields decreased in the following order cyclohexen-1-one (47%), verbenone (22%), and carvone (12%). The large amount of oxidized sites of monoterpenes, especially limonene, may be the reason for the lower ketone yield obtained with this substrate. Additional tests snggested that molecular oxygen can act as co-oxidant and alcohol oxidation is an intermediate step in ketone formation. 

A synthesis of the 2,3-dioxabicyclo[3.3.1]nonane system 24, which is present in the antimalarial yingzhaosu, involves an initial thiol-limonene co-oxygenation reaction followed by reduction of a hydroperoxide by PPh3 and a sulfenyl —> sulfonyl oxidation <02T2449>. 

These catalysts have been tested in the stereoselective epoxidation of R-(+)-limonene and (-)-a-pinene. Here only the epoxidation of (-)-a-pinene as depicted in Figure 4 is considered. The oxidant applied in the reaction is somewhat similar to the one introduced by Mukaiyama et al. and was favored over the system used by Jacobsen and coworkers14. 17. The major benefit of this system is that undesirable salt formation can be avoided by the use of environmentally benign molecular oxygen at RT instead of NaOCl as oxidant at 0°C. 

Allylic oxidations of the menthadiene system are still of interest because it would be economically useful to have a really cheap method for obtaining carvone from limonene. One of the more effective methods recently described makes use of the chromium trioxide-pyridine complex in methylene chloride which Dauben et al. found to give 36% of carvone (130) and 33 % of isopiperiten-one (129). Perhaps if the same technique were applied in the oxidation of... 

The complexes [M(acac)2], [M = Cu(ll) and Co(ll)] anchored onto an AC functionalized with hexanediamine were shown to be efficient catalysts in the oxidation of pinane without leaching of the active phase [93a]. Furthermore, limonene was oxidized by [Co(acac)2] anchored onto several acid-oxidized carbons modified with diamines (ethylenediamine, tetramethylenediamine, hexam-ethylenediamine, and dodecamethylenediamine) [93b]. The reaction was carried out in a batch reactor at 333 K in the solvent system acetone r-butanol, with... 

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