Citrus fruits, limonene

Loss of a proton from the tertiary carbocation formed m this step gives limonene an abundant natural product found m many citrus fruits Capture of the carbocation by water gives a terpmeol also a known natural product... 

Crude turpentine is distilled to obtain refined products used in the fragrance and flavour industry. Only the unsaturated mono- and bicyclic terpenes are of interest for resin production. These are mainly a-pinene, p-pinene and dipentcne (D,L-limonene) (Fig. 17). D-Limonene is obtained by extraction of orange peel in citrus fruits. 

Cold-pressed essential oils from the peel are some of the most important by-products recovered during the processing of Citrus fruits. The presence of limonene in the aqueous discharges, with its antimicrobial activity [1], decreases the effectiveness of the waste treatment system and increases the time necessary for the biological breakdown of the organic matter produced in the peel oil recovery system [2,3]. Additional recovery of essential oils from waste water would increase industry s returns and reduce the pollution problems associated with the disposal of waste water [4,5]. Several methods for reducing the levels of residual essential oils in the aqueous effluent have been developed over the years [6-11]. 

Wattenberg LW and Coccia JB. 1991. Inhibition of 4-(methylnitrosamino)-l-(3-pyridyl)butanone carcinogenesis in mice by D-limonene and citrus fruit oil. Carcinogenesis 12 115-117. 

Main constituent of citrus fruit peels oils, common in many other essential oils. (+)-limonene orange, lemon, grape oils ... 

Limonene is a monoterpene that occurs in citrus fruits. Two enantiomers of limonene produce two distinct flavours (—)-limonene is responsible for the flavour of lemons and (+)-limonene for orange. Similarly, one enantiomeric form of carvone is the cause of caraway flavour, while the other enantiomer has the essence of spearmint. 

The fungal bioconversion of limonene was further studied [82]. Penicillium sp. cultures were isolated from rotting orange rind that utilised limonene and converted it rapidly to a-terpineol. Bowen [83] isolated two common citrus moulds, Penicillium italicum and P. digitatum, responsible for the postharvest diseases of citrus fruits. Fermentation of P. italicum on limonene yielded cis- and frans-carveol (93) (26%) as main products, together with cis- and from-p-mentha-2,8-dien-l-ol (110) (18%), (+)-carvone (94) (6%), p-mentha-1,8-dien-4-ol (111) (4%), perillyl alcohol (100) (3%), p-menth-8-ene-1,2-diol (98) (3%), Fig. (17). Conversion by P. digitatum yielded the same products in lower yields. The two alcohols />-mentha-2,8-dien-1 -ol (110) and p-mentha-1,8-dien-4-ol (111) were not described in the transformation studies where soil Pseudomonads were used [69]. 

The systematic name for the —CH=CH2 group is ethenyl. Provide a systematic name for limonene, which is found in lemons and other citrus fruits... 

Isomerization of the double bond of geranyl pyrophosphate from to 7 produces neryl pyrophosphate. As shown in Figure 28.3, the carbocation that is formed from neryl pyrophosphate can cyclize to a new carbocation that contains a six-membered ring. Nucleophilic addition of water to this carbocation produces a-terpenol, whereas loss of a proton produces limonene, a monoterpene with a lemonlike odor that occurs in citrus fruits. Further transformations lead to other monoterpenes, such as menthol,... 

The effects of biochemical use are at various folds, but the major direct substation of biochemicals for petrochemicals is found at the upstream of chemical production processes. For instance, the oil distraction from crops is far less harmful than the oil extraction from petroleum. In the case of phenol, making phenol from plants is estimated to generate 80 percent less pollutants than making phenol from petroleum. Another example is limonene. Limonene derived from citrus fruit... 

It may seem strange to classify a type of bond as a functional group, but you will see later that C=C double bonds impart reactivity to an organic molecule just as functional groups consisting of, say, oxygen or nitrogen atoms do. Some of the compounds produced by plants and used by perfumers are alkenes (see Chapter 1). For example, pinene has a smell evocative of pine forests, while limonene smells of citrus fruits. 

Biosolvents or green solvents —as they are sometimes called—are derived from agrochemicals (see Figure 12.24). Some of the better known biosolvents are limonene (derived from citrus fruits), methyl soyate (produced from soy), and ethyl lactate (produced mainly from com). 

Tidd has clarified the role of pyrophosphates in terpene biogenesis by measuring the hydrolysis rate of isopentenyl pyrophosphate and related pyrophosphates over the physiological pH range. Potty and Bruemmer, continuing their search for enzymes causing transformations of terpenes in citrus fruits, have discovered a system that reduces ( + )-limonene [but not (— )-limonene] in the orange. 

The product here is limonene—a terpene of the peel of citrus fruits. One enantiomer occurs in lemon peel—the other in orange peel. See Chapter45. 

The effects of D-limonene and citrus oils on 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone-in-duced neoplasia of the lungs and forestomach of female A/J mice has been investigated. D-Limonene and the citrus fruit oils given orally 1 h prior to 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone,... 

Braddock, R.J. and Cadwallader, K.R. 1995. Bioconversion of citrus 4-limonene, in Fruit Flavors Biogenesis, Characterisation and Authentication, ACS Symp. No. 596. R.L. Rouseff and M.M. Leahy, eds. Washington, DC American Chemical Society, pp. 142-148. 

Citrus fruits contain peel oil, the essence from which oil is obtained during concentration of the juice process. Citrus oils are characterized by a high percentage of terpene hydrocarbons (limonene, C10H16), which contribute little to aroma. The unique characteristics of limonene are its relative insolubility in dilute alcohol and its susceptibility to oxidation, causing off-flavor production. If the monoterpenes are removed, the resulting oil is called terpene-free or terpeneless oil. Aldehydes, esters, and alcohols are the main contributors to the aromas of citrus oil. These compounds are relatively polar and soluble in water therefore, they are satisfactory for applications in food and beverage. 

D-Limonene Another class of low-temperature HTF is based on naturally derived terpenes such as D-limonene. U.S. Patent 3,597,355 describes o-limo-nene as being particularly preferred among all the monocycloterpenes because of its characteristic properties such as low viscosity at low temperatures. D-Limonene is the major component in the oil of citrus fruit and is present in trace quantities in orange juice. It is recovered in commercial quantities by distilling orange oil obtained from citrus peels. Being derived from the citrus industry, o-limonene is considered a safe and environmentally friendly HTF, and hence it is preferred in many food and pharmaceutical processes. However, the melting point of D-limonene is about —78°C. Below this temperature, it becomes a thick white gel like substance that is impossible to pump. Therefore, the use of o-limonene is limited to... 

The Diels-Alder is such a powerful reaction that it is good tactics to take advantage of it wherever possible. Vig chose to disconnect limonene (32) (a natural odour principle found in most citrus fruits) by a one carbon Wittig step because that revealed a para Diels-Alder product (27). 

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