Monoterpenes, acyclic

Group A Monoterpenes (acyclic, monocyclic, and bicyclic) and plant oils. 

The second example features R cartilagineum, a cold-water species that enjoys a wide distribution (Gabrielson and Scagel, 1989). Patterns of variation for halogenated monoterpenes have been reported for this species collected from several locations that include Atlantic, Pacific, and Antarctic waters. Both cyclic and acyclic... 

Table 5.4 Occurrence of acyclic and cyclic polyhalogenated monoterpenes in Plocamium cartilagineum...

Acyclic diperoxyketals, 14 287 18 457 Acyclic diyne metathesis (ADIMET), 26 951 Acyclic monoterpenes, aroma chemicals, 3 237... 

In order to obtain lactones from natural alkenols, we investigated the cyclocarbonylation of monoterpenic alcohols. The catalytic precursor is [PdCl2L2] in the presence of a slight excess of tin chloride and phosphine ligands. Dihydromyrcenol, a representative acyclic terpene containing a termi-... 

While earlier it was generally thought that the acyclic monoterpene alcohols 242-245 are derived from the host tree s oleoresin component, myreene [453], more recent results clearly show that at least in some species they are produced denovo [454-456]. 

For Instance, many terpene derivatives mimic insect hormone actioa Juvabione (15) is the classical example of a juvenile hormone (JH) mimic that prevents egg maturation in Pyrrhocoris bugs. Aromatic terpene ethers (16), methylene dloxyphenyl terpene ethers (17), and other farnesyl derivatives also have JH activity and the latter ones (18) also cause sterility in Pyrrhocoris. For the most part JH active terpenes are among the sesquiterpenes but several monoterpenes also have insect sterilizing effects (19. 20). The acyclic monoterpene cltral reduces the fertility of rats by causing follicular degeneration (21). 

IPP and DMAPP lead to geranylpyrophosphate (GPP), which is an immediate precursor of monoterpenes. The formation of nerylpyrophosphate (NPP) from GPP gives rise to a wide range of acyclic, cyclic, bicyclic or tricyclic skeletons. Reactions like rearrangement, oxidation, reduction and hydration via various terpene cyclases result in the formation of numerous terpene derivatives. Condensation of GPP and IPP leads to farnesylpyrophosphate (FPP), the immediate precursor of sesquiterpenoids. Likewise, FPP and IPP are conducive to diterpenoids. 

Monoterpenes are formed from two attached isoprene 3 units 2,6-dimethyloc-tane as the simplest skeleton. Thus, they can be acyclic or linear like -myrcene 4, ( )-/l-ocimene 5, (Z)-/l-ocimene 6, and allo-ocimene 7 (Structure 4.3). Or they can be cyclic, meaning ring-forming, such as in the simplest form like p-menthane 8 or p-cymene 9. Monocyclic 8, 9, bicyclic (5-3-carene 10 and tricyclic tricyclene 11 type monoterpenes are found in essential oils [1-4, 6-14, 16-23,38,39, 42, 47, 48]. 

Monoterpenes occur in plants in various structural forms some are cyclic while the others are acyclic. They also contain various types of functional group, and depending on their functional groups they can be classified as simple hydrocarbons, alcohols, ketones, aldehydes, acids or phenols. Some examples are cited below. 

The monoterpenes are subdivided into three groups acyclic, monocyclic and bicyclic (there is only one tricyclic terpene tricyclene). Each group contains hydrocarbon terpenes, terpene alcohols, terpene aldehydes, ketones, oxides etc... 

Z-Tagetone (82a) -Tagetone (82b) Z-Ocimenone (83a) -Ocimenone (83b) Fig. (13). Acyclic monoterpene ketones tagetone and ocimenone... 

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). 

An early example is the fascinating rearrangement802 of acyclic monoterpenes geraniol 195 and nerol 196 to a stable observable oxonium ion in HSO3F—SO2—CS2 at —78°C, which represents yet another dramatic example of altered course of terpene rearrangement in superacid medium. Careful quenching of this ion led to the isolation of 3/3,6a,6a/3-trimethyl- s-perhydrocyclopenta[b]furan 197 in excellent yield (Scheme 5.76). 

The halogenated acyclic marine monoterpenes are often considered to be the biogenetic precursors of the alicyclic monoterpenes that are presented in this section. Many of the preceding algae species also contain cyclic monoterpenes. As was the case in preceding sections only newly characterized compounds are numbered and the reader is referred to the first survey for structures of previously isolated compounds (7). 

Stierle, D. B., Wing, R. M., and Sims, J. J., Marine natural products XVI. Polyhalogenated acyclic monoterpenes from red alga Plocamium of Antarctica, Tetrahedron, 35, 2855, 1979. 

Terpenes are built out of Cs-isoprene units. There is a variety of terpenes ranging from Cs-terpenes which are called hemiterpenes to tetraterpenes with C40 atoms. There are also terpenes with more Cs-units, such as natural rubber, which is a polyterpene. Terpenes are also classified by the number of carbon cycles in the molecule, e.g., monoterpenes can have up to two cyclic systems in one molecule. Examples of some monoterpenes are shown in Scheme 2 myrcene is acyclic,... 

Terpenes are a class of naturally occurring olefins found in many plants. They are biosynthetically derived from isoprene, CsHg, and therefore consist of Cs-building blocks and are found in both cyclic and acyclic forms. An important subgroup is the monoterpenes, which consist of two isoprene units and are of the molecular formula Ci0H16. Of the monocyclic monoterpenes, by far the most industrially relevant is limonene, which is abundantly present as a fragrance in various household products. From the bicyclic monoterpenes, a-pinene, p-pinene, camphene, and carene are of industrial relevance and are also mostly used in the fragrance industry [44]. 

Among the other terpenes that have been used as substrate in the MTO-catalyzed oxidation reactions are the bicyclic monoterpenes carene and camphene, the acyclic monoterpene myrcene, fi-citronellene (the reduced form of myrcene), and the bicyclic sesquiterpene (i-caryophyllene (Fig. 6). 

Acyclic monoterpene alcohols are present in many essential oils and can be isolated by distillation, although nowadays, they are often synthesized from other sources. Geraniol and linalool (Fig. 7) are the most important monoterpene alcohols and together with nerol are the primary products in terpene biosynthesis [44]. a-Terpineol is a frequently used cyclic monoterpene alcohol in the fragrancing industry because of its typical lilac odor and is also partly responsible for the typical... 

Croteau, R., Felton, M. and Ronald, R.C. (1980a) Biosynthesis of monoterpenes conversion of the acyclic precursors geranyl pyrophosphate and neryl pyrophosphate to the rearranged monoterpenes fenchol and fenchone by a soluble enzyme preparation from fennel (Foeniculum vulgare). Archives of Biochemistry and Biophysics 200(2), 524-533. 

Figure 3.2 Monoterpenes (C10H16). (A) Myreene, an acyclic monoterpene. (B) a-Pinene, a dicyclic structure. (C) d-Limonene, a monocyclic structure. Courtesy Spiring Enterprises Ltd.

Different acyclic monoterpenes are made up of two isoprene units, but the positions of the three double bonds vary. 

Myreene and ocimene are not arranged in ring structures they are acyclic. However, many monoterpenes link up to form rings or cyclic structures, for example in limonene. 

V.B. (2007) Oxidation of acyclic monoterpenes by P450 BM-3 monooxygenase influence of the substrate B/Z-isomerism on enzyme diemo- and regio-selectivity. Tetrahedron, 63, 9413-9422. 

Monoterpenoids are responsible for fragrances and flavors of many plants and thus their products are used in perfumery and as spices. To date over 1,500 monoterpenoids are known, and these constitute acyclic, monocyclic, and bicyclic monoterpenoids (32), which occur in nature as hydrocarbons, alcohols, aldehydes, and carboxylic acids and their esters. Several acyclic monoterpenoid hydrocarbons are known, and these include trienes such as 3-myrcene (Cl), a-myrcene (C2), (Z)-a-ocimene (C3), ( )-a-ocimene (C4), (Z)-3-ocimene (C5), and ( )-P-ocimene (C6). 3-Myrcene and 3-ocimene are constituents of basil (Oci-mum basilicum, Labiatae) and bay (Pimenta acris, Myrtaceae), pettitgrain (Citrus vulgaris, Rutaceae) leaves, strobiles of hops (Humulus lupulus, Cannabaceae), and several other essential oils. Unsaturated acyclic monoterpene alcohol constituents of plants and their derived aldehydes play a signihcant role in the perfume industry. Some common acyclic monoterpene alcohols and aldehydes include geraniol (C7), linalool (C8) (a... 

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