Limonene hydroperoxide

In citrus oils, limonene and y-terpinene are also attacked in the presence of light and oxygen. Car-vone and a series of limonene hydroperoxides are formed as the main aroma substances. 

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. 

Peroxides, hydroperoxides [1, 2] e.g. (photo)-axidation products of limonene [3]... 

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 quite different limonene conversion is the dehydrogenation into p-cymene, thus giving a green aromatic. Pd-catalysts give yields of >95% [18]. p-Cymene can be oxidized to the hydroperoxide, which can be rearranged to p-cresol, a valuable chemical. 

More recently, Bachi and coworkers extended and adapted the TOCO reaction to the synthesis of 2,3-dioxabicyclo[3.3.1]nonane derivatives hke 228 (Scheme 52) ° ° . As detailed in Scheme 53a, the bridged bicyclic hydroperoxide-endoperoxides hke 229 are obtained, from (S )-limonene (227), in a 4-component one-operation free-radical domino reaction in which 5 new bonds are sequentially formed. Particular experimental conditions are required in order to reduce the formation of by-products 230 and (PhS)2, and to favor the critical 6-exo-ring closure of peroxy-radical 231 to carbon-centered radical 232206 chemoselective reduction of bridged bicyclic hydroperoxide-endoperoxides... 

Dimethylcyclohexene (209) is converted to two tertiary hydroperoxides, 210 (12%) and 211 (88%).85,135 1-Methylcyclohexene (30) gives rise to 45% of the tertiary hydroperoxide 31 and to 55% of a non-resolved mixture of secondary hydroperoxides, 52.135 Since the ratio tertiary/secondary hydroperoxides is the same as with limonene (16) and carvomenthene (19), a similar product distribution among the secondary hydroperoxides 32 as was found for 16 and 19 has been assumed. No product distribution has been reported for the photosensitized oxygenation of 1-methylcyclopentene, 213.85,123... 

Electrochemical reduction of the ozonization products from monoterpenes, i.e., />-meth-l-ene, (-l-)-limonene, (+ )-a// /ia-pinene, (+)-car-3-ene, provides the corresponding double-bond cleavage products in 45-70% yields57. The electrolysis of the acetyloxy hydroperoxide 28 derived from p-menth-l-ene 27 is carried out in an Ac0H/H20(6/1 v/v)— AcONa— (Pb/Pb) system at —1.1 to —1.4V vs. SCE, 2.0 to 2.2 A/dm2 in a divided cell to give the corresponding keto-alcohol 29 in 70% yield (Scheme 3-10). 

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>. 

A considerable number of chiral natural compounds has been investigated under photooxygenation conditions, mostly terpenoids and steroids. Of these compounds, ( + )-limonene (1) has been utilized as a popular probe for singlet oxygen in view of the fact that it gives a characteristic pattern of regio- and stereoisomeric allylic hydroperoxides which were reduced to the alcohols 2-4 as products22-24. 

Various chemical processes of limonene, which lead to the obtainment of useful chemicals and some analytical methods, are based on these reactions. Many flavor chemicals are synthesized from limonene by reaction with water, sulfur and halogens, or hydrolysis, hydrogenation, boration, oxidation and epoxide formation (Thomas and Bessiere, 1989). Hydroperoxides have also been studied and isolated because of their effect on off-flavor development in products containing citrus oil flavoring agents (Clark et al., 1981 Schieberle et al., 1987). Hydration of d-limonene produces alpha-terpineol, a compound that gives off an undesirable aroma in citrus-flavored products. It is also possible to produce alpha-terpineol and other useful value-added compounds... 

Clark, Jr., B.C., Jones, B.B. and Iacobucci, G.A. 1981. Characterisation of the hydroperoxides derived from singlet oxygen oxidation of (+)-limonene. Tetraeldmn. 37(suppl 1) 405—409. 

One of the easiest direct routes to 1-oxygenated menthanes is by direct oxidation of limonene (285). Autoxidation gives a mixture of 1- (540) and 2-hydroperoxides (541), and all of these have now been separated and characterized.Photochemical addition of methanol to give the (3-terpineol... 

Previous studies indicated that the structure of the alkyl hydroperoxide in molybdenum catalyzed epoxidations has only a minor effect on the rate and selectivity [10]. Hence, we were initially surprised to observe that PHP failed to give the expected epoxidation of cyclohexene (1) and limonene (2) in the presence of a molybdenum catalyst (Tablel). Epoxidation of limonene with TBHP as oxidant, in contrast, gave the epoxide of the more highly substituted double bond in 84% selectivity, consistent with nucleophilic attack of the olefin on the alkylperoxomolybdenum(Vl) [3,5]. We tentatively concluded that this low reactivity of PHP is a result of steric hindrance in the putative alkylperoxomolybdenum(VI) intermediate. This prompted us to carry out a systematic investigation [8] of steric effects of the alkyl substituents in the alkyl hydroperoxide on the rate of molybdenum catalyzed epoxidations. 

Terpenes. Another class of compounds which holds considerable promise as inhibitors of cancer are the terpenes. D-limonene, which is a component of citrus oils, is the terpene which probably has been studied most extensively as an Inhibitor of carcinogenesis. Early studies reported that D-limonene, a mixture of D-limonene with its hydroperoxide, and orange oil were similar in their ability to prevent subcutaneous tumors induced by benzo(rst)pentaphene (DBP) (56) but that spontaneous and DBP-lnduced lung adenomas were reduced in mice fed D-llmonene but not in those fed orange oil or the hydroperoxide of llmonene. 

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