Limonene chemistry

Scheme 56 summarizes Mori s synthesis of (S)-vesperal (38), the female sex pheromone of the longhorn beetle (Vesperus xatarti) [85]. (F)-Limonene yielded (S)-38 by utilizing organoselenium chemistry. 

The three main platform molecules employed in terpene chemistry are a-pinene and / -pinene, which are extracted from turpentine oil (350000 t a-1) a co-product of paper pulp industry, and limonene extracted from citrus oil (30000 t a-1). 

I. As discussed in this chapter, a relatively new area in indoor air pollution is that of hydroxyl radical chemistry. However, the importance of indoor OH chemistry (as well as 03 and N03 chemistry) is determined by the rates of the reactions compared to the rate of air exchange. An OH concentration of 7 X 105 cm-3 has been reported in an indoor air environment by Weschler and Shields (1997a). Assess the importance of the OH reaction for the removal of limonene indoors compared to its removal by reaction with 03. The 03-limonene rate constant is 2 X 1CT16 and the OH-limonene rate constant is 1.7 X 10-l() cm3 molecule-1 s-1. Take the 03 concentration to be 20 ppb. How do these compare with a typical air exchange rate of 0.75 h-1 used in these experiments ... 

In summary, D-limonene has been exploited with interesting results in polymer chemistry however, it remains to be seen whether its chirality can be used to induce similar effects in small molecule syntheses. Many of the benchmark reactions e.g. Diels-Alder, Michael addition) used in the alternative solvent field are reactions of olefinic substrates and therefore could not be performed successfully in a terpene solvent. 

For a review of the chemistry of limonene, see Verghese, J Perfum Essent Oil Rec 1968,... 

As can be seen from the few examples cited above SET processes are now fairly common in organic photochemistry. One of the areas where considerable study has taken place is the process referred to as a photo-NOCAS. Within this framework Albini and coworkers have shown that the products formed from the reaction of 2,3-dimethylbut-2-ene with 1,4-dicyanobenzene are compounds (22)- 25). The reaction was brought about using phenanthrene as the initial light absorber. This technique leads to cleaner reactions than those where the 1,4-dicyanobenzene is irradiated directly. The solvent system used is methanol/ acetonitrile and products (24) and (25) are the result of solvent incorporation. A further example of photo-NOCAS chemistry has been reported by Arnold and coworkers.Typical of the examples studied is the reaction illustrated in Scheme 2. The cyclization of the dienes (26) was also examined. This specific example deals with the generation of radical cations from (/ )-(+)-a-terpineol and (/ )-(+)-limonene with 1,4-dicyanobenzene as the electron accepting sensitizer. In another detailed study on reactions of this type the factors that control the regiochemistry in photo-NOCAS processes have been assessed. ... 

Carvone is the principal odour component of spearmint oil. Both the oil and synthetic 1-carvone are used as ingredients in mint flavours. The synthetic material is made from d-limonene, which is the major component of orange oil and therefore is available as a by-product of orange juice production. Quest International is the world s major producer of 1-carvone. The classical chemistry used to produce 1-carvone is shown in Figure 4.19. The chirality of the carvone is crucial to the odour, since the enantiomeric d-carvone has an odour reminiscent of dill or caraway rather than spearmint. It is therefore important that any... 

Furuta T, Yoshii H, Miyamoto A, Yasunishi A, Hirano H. 1993. Effects of water of inclusion complexes of D-limonene and cyclodextrins. Supramolecular Chemistry 1 321-325. 

Interestingly, a cascade alkoxyl radical fragmentation-peroxidation-hydrogen abstraction reaction occurs in some cases when a hemiacetal is treated with DIB/I2 under oxygen pressure. This reaction may have interesting applications in synthetic organic chemistry. We have used it in a one-step synthesis of A and A rings of the tetranortriterpene limonene and related compounds (Eq. 19, Scheme 6) [48]. 

Having seen how the basic principles of carbocation chemistry can explain our three chemical puzzles, it is worthwhile revisiting some of the reactions mentioned in previous chapters to see how the same factors were at work in those cases. Firstly, we can look at the key step in the conversion of limonene to carvone which we encountered in Chapter 4, Figure 4.13. This is shown again in Figure 5.36, but now we can understand why the observed selectivity is found. The nitrosyl cation is very electron deficient and seeks out the more electron rich double bond of limonene (5.29). This is the trisubstituted double bond in the ring. The addition takes place entirely on C2 since the resulting carbocation is tertiary and hence more stable than the secondary carbocation which would be formed if the reaction were to occur in the opposite direction. Therefore only carbocation (5.60) is formed. 

Mitiku, S.B., Ukeda, H., and Sawamura, M., Enantiomeric distribution of alpha-pinene, beta-pinene, sabinene, and limonene in various citrus essential oUs, in Food Flavors arul Chemistry Advances of the New Millennium, Spanien, M., Shahidi, R, Parliament, T.H., Mussian, C., Ho, C.T., and Contis, E.T., Eds., The Royal Society of Chemistry, Cambridge, U.K., p. 216, 2001. 

In the early days of organic chemistry, each new compound was given a name that was usually based on its source or use. Examples (Figs. 1.13 and 1.14) include limonene (from lemons), a-pinene (from pine trees), coumarin (from the tonka bean, known to South American natives as cumaru), and penicillin (from the mold that produces it, Penicillium notatum). Even today, this method of naming can be used to give a short, simple name to a molecule with a complex structure. For example, cubane (p. xxvii) was named after its shape. 

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