Terpenes hydroxylation

Celik, A., Flitsch, S.L. and Turner, N.J. (2005) Efficient terpene hydroxylation catalysts based upon P450 enzymes derived from actinomycetes. Organic Biomolecular Chemistry, 3, 2930-2934. 

Sterols. Sterols (4) are tetracycHc compounds derived biologically from terpenes. They are fat-soluble and therefore are found in small quantities in fats and oils. Cholesterol [57-88-5] (4a) is a common constituent in animal fats such as lard, tallow, and butterfat. The hydroxyl group can be free or esterified with a fatty acid. 

Modena and colleagues47 have developed use of some chiral, non-racemic terpene alcohols as directing groups for highly diastereoselective m-chloroperbenzoic oxidation of sulfides into sulfoxides. Specifically the isobornyl vinylic sulfides 8 undergo hydroxyl-directed oxidation to give a 9 1 ratio of diastereomeric sulfoxides (equation 11). 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

The initial hydroxylation in the degradation of some terpenes the ring methylene group of camphor by Pseudomonas putida (Katagiri et al. 1968 Tyson et al. 1972 Koga et al. 1986), and the isopropylidene methyl group of linalool by a strain of P. putida (Ullah et al. 1990). 

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

Kleindienst, T.E., Harris, G.W., Pitts, Jr., J.N. (1982) Rates and temperature dependences of the reaction of hydroxyl radical with isoprene, its oxidation products, and selected terpenes. Environ. Sci. Technol. 16, 844-846. 

The terpenes are simple lipids whose base unit is isoprene. Oxygen-containing ter-penes are called terpenoids, and terpenes with hydroxyl groups are called terpenols. Terpenes are further classified based on the number of isoprene imits in the molecule as shown in Table 22.6. Examples of terpene molecular structures are presented in Figure 22.18. 

Reactions of the hydroxyl radical dominate the removal of hydrocarbons. However, several other reactants make significant contributions, including hydroperoxy radical, ozone, and oxygen atoms. (This conclusion depends on the hydrocarbon being considered it is claimed that some terpenes in air are attacked mainly by ozone. )... 

Figure 5.7. There are many examples now known of the synthesis of NPs via matrix pathways (see also Figure 9.3). However, a nice example of the benefit of such flexibility was revealed when a mutant of spearmint that had smelled more like peppermint was studied.A comparison of the terpenes in both plants revealed that the single gene mutation had not resulted in a single chemical change but multiple changes, in the mutant plant, a hydroxyl group was added to the 3-position of the cyclohexene ring of limonene while the wild-type hydroxylated the 6-position. Some of the other wild-type tailoring enzymes in the mutant did not discriminate fully between the 3- and 6-hydroxylated products so a new family of NPs were produced which gave the mutant plant an odour of peppermint.

Terpenoids are synthesised by the condensation of a series of isoprene (2-meth-ylbuta-1,3-diene) units, followed by enzymatic cyclisation by a terpene cyclase, and subsequent modification such as hydroxylation, and are grouped on the basis of their carbon chain length. Monoterpenes and sesquiterpenes consisting of ten and 15 carbon atoms, respectively, are ranked among the most important aroma compounds. Despite their diversity, all terpenoids are synthesised... 

Bilobalide (40) was isolated from the leaves by Wsinges ef al. [41], and is a sesquiterpene lactone, which has a Cert-butyl group and two hydroxyl groups in its chemical structure [42]. The ginkgo terpenes described above (bilobalide and ginkgolides) seem to be unique constituents to Ginkgo biloba because they have never been found in any other plants. 

In addition to being oxidized by the hydroxyl radical, alkenes may react with the N03 radical as has been described by several investigators (52, 56, 66). Listed in Table I are some of the organic nitrates that have been predicted to be produced via reaction of OH and N03 with isoprene and pro-pene. Analogous compounds would be expected from other simple alkenes and from terpenes such as a- and (3-pinene. Other possible organic nitrates may be produced via the oxidation of aromatic compounds (53, 54) and the oxidation of carbonaceous aerosols (67). Quantitative determination of these species has not been made in the ambient atmosphere. 

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