Monoterpenes formation

The relationship of cyclic monoterpenes, e.g. 4.3) and 4.53), to geranyl pyrophosphate 4.41) is an obvious one. The tram double bond in 4.41) means that 4.41) cannot cyclize directly to give monoterpenes such as 4.53), and neryl pyrophosphate 4.51) may be more directly involved in biosynthesis. [A cell-free preparation of Mentha piperita has been obtained which will catalyse the conversion of neryl pyrophosphate 4.51) into a-terpineol 4.3) [65].] The conversion of geraniol 4.2) into nerol 4.49) is well known and involves a stereospecific proton removal from C-1 loss of a proton indicates that the aldehyde 4.50) is involved in double-bond isomerization [66]. Initiation of cyclic monoterpene formation can be seen as... 

Figure 2 Overview of the pathway leading to monoterpene formation in plants.

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

Elimination of water with the aid of sulfunc acid [9 14] and formation of the corresponding monoterpene hydrocarbons... 

Figure 27.10 Mechanism of the formation of the monoterpene limonene from geranyl diphosphate.

The activity of the FePeCli6-S/tert-butyl hydroperoxide (TBHP) catalytic system was studied under mild reaction conditions for the synthesis of three a,p-unsaturated ketones 2-cyclohexen-l-one, carvone and veibenone by allylic oxidation of cyclohexene, hmonene, and a-pinene, respectively. Substrate conversions were higher than 80% and ketone yields decreased in the following order cyclohexen-1-one (47%), verbenone (22%), and carvone (12%). The large amount of oxidized sites of monoterpenes, especially limonene, may be the reason for the lower ketone yield obtained with this substrate. Additional tests snggested that molecular oxygen can act as co-oxidant and alcohol oxidation is an intermediate step in ketone formation. 

The biosynthesis of monoterpenes, the major components of peppermint essential oils, can be divided into four stages (Fig. 9.4). Stage 1 includes the formation of isopentenyl diphosphate (IPP) and dimethylallyl alcohol (DMAPP). In plants, two separate pathways are utilized for the synthesis of these universal C5 intermediates, with the cytosolic mevalonate pathway being responsible for the formation of sterols and certain sesquiterpenes, and the plastidial mevalonate-independent pathway being involved in the biosynthesis of isoprene, monoterpenes, certain sesquiterpenes, diterpenes, tetraterpenes, as well as the side chains of chlorophyll and plastoquinone.16 In peppermint oil gland secretory cells, however, the mevalonate pathway is blocked and the biosynthesis of monoterpenoid essential... 

Chrysanthemic acid (1) consists of ten carbons, suggesting that it is a monoterpene. The cyclopropane ring of the acid moiety is a feature of pyrethrins. Rivera et al. isolated chrysanthemyl pyrophosphate synthase (CPPase or alternatively referred to as chrysanthemyl diphosphate synthase) underlying the formation of chrysanthemyl pyrophosphate (16) containing a cyclopropane ring from two molecules of dimethylallyl pyrophosphate (15) (DMAPP) and the gene thereof [21]. They found that the reaction involves the cF-2-3 cyclopropanation of DMAPPs in a non-head-to-tail manner. 

Ealdt J, Arimura G, Gershenzon J, Takabayashi J, Bohlmann J (2003) Functional identification of AtTPSOS as ( )-P-ocimene synthase a monoterpene synthase catalyzing jasmonate- and wound-induced volatile formation in Arabidopsis thaliana. Planta 216 745-751... 

Chen F, Ro DK, Petri J, Gershenzon J, Bohhnann J, Pichersky E, Tholl D (2004) Characterization of a root-specific Arabidopsis terpene synthase responsible for the formation of the volatile monoterpene 1,8-cineole. Plant Physiol 135 1956-1966... 

The compounds geranyl diphosphate, farnesyl diphosphate, and geranylgeranyl diphosphate are biochemical precursors of monoterpenes, sesquiterpenes, and diterpenes respectively, and virtually all subsequent modifications of these precursors involve initial formation of an allylic cation through loss of diphosphate as the leaving group. 

The formation of cyclic terpenoids involves intramolecular electrophilic addition, and this can be exemplified by the following monoterpene structures, again with all reactions being enzyme controlled. 

Hydroxy-3-methylcarbazole (52) could also function as a precursor for the formation of carbazole alkaloids with a C23 skeleton as depicted in Scheme 3.6 (370). Insertion of a Cio unit, viz., geraniol at C-1 of 2-hydroxy-3-methylcarbazole (52) would yield mahanimbinol (mahanimbilol) (56). The geranyl monoterpene unit could undergo various transformations, thus giving rise to isomeric carbazole alkaloids with a C23 skeleton, such as mahanimbine (139), cyclomahanimbine (murrayazolidine, curryanin) (151), and bicyclomahanimbine (160). Therefore, mahanimbinol (mahanimbilol) could be considered as the representative member of the carbazoles with a C23 skeleton. The occurrence of 2-hydroxy-3-methylcarbazole (52),... 

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]. 

Conifers tend to be sources of a variety of larger terpenoids, a major component of which are the Cl(l monoterpenes (Table 6.24). The structures of some of these are shown in Fig. 6.22. The mechanisms of the formation of these hydrocarbons in plants are closely linked, so that emissions of more than one monoter-pene often occur together. For example, geranyl... 

Chew, A. A., and R. Atkinson, OH Radical Formation Yields from the Gas-Phase Reactions of 03 with Alkenes and Monoterpenes, J. Geophys. Res., 101, 28649-28653 (1996). 

Hakola, H., J. Arey, S. M. Aschmann, and R. Atkinson, Product Formation from the Gas-Phase Reactions of OH Radicals and 03 with a Series of Monoterpenes, J. Atmos. Chem., 18, 75-102 (1994). 

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. 

Cineole 42 as well as 1,4-cineole 43 are cyclic ethers (Structure 4.10). All including ascaridol 44 are bicyclic oxygenated monoterpenes. Their formation can be seen in Fig. 4.3. 

Head-to-tail rearrangement of four isoprene units results in the formation of diterpenes (C20H32), as seen also in Fig. 4.2. Diterpenes are generally found in resins, e.g. pimaric acid and abietic acid. Some diterpenoids are also constituents of essential oils, e.g. phytol [3, 7-14, 37, 52, 53]. Like sesquiterpenes, diterpenes are heavier than monoterpenes therefore, they require more energy to go to the vapour phase. For this reason, longer distillation times are necessary for their recovery. The DNP lists 118 different structural types for diterpenoids [37]. Important diterpenes found in essential oils will be detailed. Some representatives of volatile diterpenes are as in Structure 4.32. 

Gershenzon, J. and Croteau, R. (1993). Terpenoid biosynthesis the basic pathway and formation of monoterpenes, sesquiterpenes and diterpenes. In Lipid Metabolism in Plants, ed. T. S. Moore Jr, pp. 339-388. Boca Raton, FL CRC Press. 

Experiments were also carried out at 80 and lOO C. According to our observations at these high temperatures, solid- phase chemical transformations may take place between certain flavor constituents and cyclodextrin hydroxyls/monoterpene alcohols and phenolic compounds appear as a result of a solid-phase transacetylation of terpeneaoetates and phenyl-acetates with the simultaneous formation of cyclodextrin-acetates/. Long term heat treatments of cyclpdextrin-flavor complexes should not be run above 6o°C in order to avoid such phenomena. 

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