Monoterpenes oxidation products

The second reaction that we wish to describe is that of the aerobic oxidation of a-pinene, which is a bicyclic monoterpene with an oxidizable double bond. The use of such a renewable feedstock in the production of chemicals is considered as the first step towards greening the life cycle of chemical products. In this context, oxidation a-pinene, which occurs widely in the plant kingdom, may be viewed as an important reaction because its oxidation products... 

In the cyclopentanoid monoterpene family, compounds containing an iridoid-structure exhibit various bioactivities in Nature. For example, dehydroiridodiol, isolated from dry leaves of the cat-attracting plant Actinidia polygama Miq., is known to be an attractant for the male adults of the Chrysopidae and shows activity in amounts as small as lO"" pg [35]. Dehydroiridodial, a more oxidized product, was isolated as a pungent principle of Actinidia polygama Miq. and was characterized by T. Sakan et al. in 1978 [36]. 

The cases where terpene metabolism has been studied In Insects are very few Indeed. Certain Ips and Dendroctonus bark beetles convert monoterpenes such as a-plnene, 6-plnene and myr-cene to oxidation products, some of which have pheromonal activities ( 5, 3A, 35). A Dendroc tonus bark beetle s cytochrome... 

As in the discussion of the acyclic monoterpene ketones (see 1.1.3) instead of the sesquiterpene ketones, the structurally related compounds neryl-(176) and geranylacetone (169) will be reviewed. Indeed, as mentioned before, neryl- and geranylacetone are the norsesquiterpene -analogues of 6-methyl-5-hepten-2-one, which is the /1-oxidation product of nerol and geraniol, just like neryl- and geranylacetone are the bioconversion products of famesol (see 2.1.1). 

The early literature on monoterpene biotransformation was highly influenced by the approach used in steroid biotransformations and mainly focused on terpenoids accumulated by fungal strains which do not mineralize the substrate but partly oxidize it by fortuitous cometabolism. These studies often resulted in the accumulation of a mixture of different products in low yields and at low concentrations [1]. Several bacteria which completely mineralize monoterpenes have been described more recently. It has become obvious from the later studies that multiple pathways are involved in the degradation of monoterpenes in many of these microorganisms, and consequently it has been difficult to obtain mutants allowing the accumulation of partially oxidized products. 

Oxidation of monoterpenes with hydrogen peroxide and PdCl2 as catalyst in acetonitrile has been reported [79]. Besides the normal Wacker oxidation products, aldehydes, ketones, and other oxidation products such as epoxides and glycols are formed. 

In addition to the important role biogenic terpenes play in gas-hase chemistry, their impact also extends to heterogeneous air chemistry. Although Went (1960) linked the formation of the blue haze over coniferous forests to the biogenic emission of 20 monoterpenes over 40 years ago, it was not until recently that terpenes received their due attention with respect to their role in secondary organic aerosol (SOA) formation. O Dowd et al. (2002) reported that nucleation events over a boreal forest were driven by the condensation of terpene oxidation products. Formaldehyde (HCHO) is a high-yield product of isoprene oxidation. The short photochemical lifetime of HCHO allows the observation of this trace gas to help constrain isoprene emissions (Shim et al. 2005). 

Conifer resin, which is a mixture of monoterpenes and diterpenes is an important protective compound against bark beetles and other conifer herbivores. The volatile monoterpenes emanating from a specific tree is often the cue for bark beetles to find a tree where the tree defenses could be compromised from abiotic or biotic stresses. The synchronized mass attack is the strategy bark beetles use to reduce the effects of resin-based defenses in conifers. Aggregation hormones released by bark beetles are oxidized monoterpenoids such as ipsdienol, ipsenol and verbenol. It is believed that these compounds can be oxidation products of host plant monoterpenes such as myrcene. Recently it has been observed that most of the monoterpenoid aggregation pheromone components are biosynthesized de novo in bark beetles [7]. 

The content of oxygenated compounds increased considerably during storage (43). Monoterpene alcohols, as oxidized products of monoterpenes, showed the most significant increase. In contrast, linalool (both cis and tram) was found to decrease at 20°C after... 

Chamber (Caltech Indoor Chamber Facility) experiments have also been carried out by Lee et al. to monitor the time evolution and yields of gas-phase VOCs produced by the ozonolysis of ten different terpenes [197] six monoterpenes (a-pinene, (3-pinene, 3-carene, terpinolene, a-terpinene and myrcene), two sesquiterpenes (a-humulene and p-caryophyllene) and two oxygenated terpenes (methyl chavicol and linalool). Several gas-phase oxidation products were identified including formaldehyde, acetaldehyde, formic acid, acetone, acetic acid, nopinone and pinonaldehyde. 

A monoterpene found in the volatile oil of bitter orange peel Citrus aurantium L. subsp. amara) and lemon peel [Citrus limon (L.) Burm.]. (7 )-(- -)-isomer is the dominant enantiomer in most plant that produce it, however, both enantiomers (and the racemic form dipentene) are found in different ratios and amounts in many plant products, such as volatile oils of rosemary, lavender, lemon grass, eucalyptus, and others [65], The use of this compound as a fragrance is widespread. The two optical forms of limonene, differ in the intensity of odor, which is stronger in case of (-)-limonene [the odor quality is more turpentine than orange as is for (/ )-(- -)-isomer] [66,67], (7 )-(- -)-Limonene, although not allergenic itself, forms a number of oxidation products which are frequent skin sensitizers. 

Experimental procedures have been described in which the desired reactions have been carried out either by whole microbial cells or by enzymes (1—3). These involve carbohydrates (qv) (4,5) steroids (qv), sterols, and bile acids (6—11) nonsteroid cycHc compounds (12) ahcycHc and alkane hydroxylations (13—16) alkaloids (7,17,18) various pharmaceuticals (qv) (19—21), including antibiotics (19—24) and miscellaneous natural products (25—27). Reviews of the microbial oxidation of aUphatic and aromatic hydrocarbons (qv) (28), monoterpenes (29,30), pesticides (qv) (31,32), lignin (qv) (33,34), flavors and fragrances (35), and other organic molecules (8,12,36,37) have been pubflshed (see Enzyp applications, industrial Enzyt s in organic synthesis Elavors AND spices). 

Gas-phase products from the reactions of ozone with the monoterpenes (-)-p-pinene and (+)-sabinene included the ketones formed by oxidative fission of the exocyclic C=C bonds as well as ozonides from the addition of ozone to this bond (Griesbaum et al. 1998). 

Finally, we mention here recent progress made in the Bayer-Villiger oxidation. Zeolite Sn-Beta (1.6 wt.% Sn) was found to be an excellent catalyst [25], Thus, the monoterpene dihydrocarvone gives - with Sn-Beta and H202 - exclusively the lactone (Scheme 5.7), whereas m-chloroperbenzoic acid and Ti-Beta/H202 give the epoxide as the main product. 

Whereas some species oxidize host terpenes more randomly, producing an array of rather unspecific volatiles with little information, others use highly selective enzyme systems for the production of unique olfactory signals. However, apart from transformations of monoterpene hydrocarbons of host trees, oxygenated monoterpenes may well be biosynthesized de novo by the beetles (see below). 

Oxygenated monoterpenes which are found in almost every bark beetle species attacking coniferous trees, include czs-verbenol 246, frans-verbenol 247, and myrtenol 248, representing primary products of allylic oxidation of the host terpene a-pinene 45. Further oxidation of 247 or 248 leads to the... 

Regioselective [4-1-2] cycloadditions to Cjq are also possible with 2,3-dimethyl-buta-1,3-diene (4) and with the monoterpene 7-methyl-3-methylideneocta-l,6-diene (5, myrcene) [22]. These monoadduct formations proceed under mild and controlled conditions. Most of these addition products of 1,3-butadiene derivatives (e.g. 4, 5, 8-12) are unstable against air and light [25]. The dihydrofuUerene moiety in the Diels-Alder adducts act as a 02-sensitizer and promotes the oxidation of the cyclohexene moiety to the hydroperoxide. Reduction of the hydroperoxide with PPhj yields the corresponding allylic alcohols [25]. 

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