Isoprene terpene formation

This book covers homogeneous gas-phase kinetics important in the atmosphere, which has been almost established, and provides the solid scientific bases of oxidation of trace gases and oxidant formation. Nevertheless, unresolved problems remain, for example, unsatisfactory reproduction of observed OH/HO2 mixing ratio by model simulation under certain conditions, and oxidation mechanisms involving isoprene, terpenes and other biogenic hydrocarbons, and anthropogenic aromatic hydrocarbons. Therefore, descriptions of these topics are not completed in the book. Heterogeneous reaction chemistry is not covered well except for the chemistry on polar stratospheric clouds (PSCs) and reactive uptake coefficients of selected... 

Other catalytic applications of HBF4 include alkene isomerizations, alkylation of alcohols with diazoalkanes, preparations of substituted pyridines, hydrolysis of a-hydroxyketene or a-(methylthio)ketene thioacetals to a,p-unsaturated thioesters, and terpene formation from isoprenic precursors. ... 

Some representative Claisen rearrangements are shown in Scheme 6.14. Entry 1 illustrates the application of the Claisen rearrangement in the introduction of a substituent at the junction of two six-membered rings. Introduction of a substituent at this type of position is frequently necessary in the synthesis of steroids and terpenes. In Entry 2, formation and rearrangement of a 2-propenyl ether leads to formation of a methyl ketone. Entry 3 illustrates the use of 3-methoxyisoprene to form the allylic ether. The rearrangement of this type of ether leads to introduction of isoprene structural units into the reaction product. Entry 4 involves an allylic ether prepared by O-alkylation of a (3-keto enolate. Entry 5 was used in the course of synthesis of a diterpene lactone. Entry 6 is a case in which PdCl2 catalyzes both the formation and rearrangement of the reactant. 

Fig. 7.1 Isoprenoid derivatives (a) formation of a terpene from 2 isoprene units (b) structure of lycoprene - a tetraterpene showing the isoprene units.

Terpenes are formed by the head to tail joining of isoprene units. In nature, this is accomplished via the formation of isopentenyl pyrophosphate (IPP) from mevalonic acid, followed by the polymerization of the isopentenyl-pyrophosphate. 

Reaction of an zinc-copper couple on alumina (N2) in the presence of isoprene results in formation of 2, 3, and 4 in the approximate ratio 2 2 1. The product (2) is a natural terpene, karahaenone. The zinc oxyallyl cation (a) is... 

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). 

Dnring the formation of the terpenes, the isoprene nnits are linked in a head to tail fashion. The nnmber of nnits incorporated into a particnlar terpene serves as a basis for the classification of these componnds, as shown below ... 

Head-to-tail bonds between isoprene units are vastly more common in nature than are the alternative head-to-head or tail-to-tail patterns. Figure 5.4 shows structural formulas of five more terpenes, all derived from two isoprene units. Geraniol has the same carbon skeleton as myrcene. In the last four terpenes of Figure 5.4, the carbon atoms present in myrcene and geraniol are cross-linked to give cyclic structures. To help you identify the points of cross linkage and ring formation, the carbon atoms... 

Terpene compounds have natural occurrence in plants found as major components of most of the plant essential oils. Based on their structural and functional properties, terpene compounds have been classified according to their basic structural unit isoprene containing five carbons, fii the formation of terpenes, prenyldi-phosphate serves as a precursor. The terpene compounds exist in the form of mono-, sesqui-, hemi-, di-, tri-, and tetraterpenes. The mmioterpenes containing two isoprene units are responsible to cmistruct the major portion of all the essential oils. These compounds work as carbure, alcohol, aldehyde, ketone, ester, ether, peroxyde, and phenols [58]. The sesquiterpene compounds contain three isoprene units, and the functional properties are very close to monoterpene compounds. 

Two units of isoprene combine to form terpenes, Cjo compounds such as geraniol. Three isoprene units form sesquiterpenes, C15 compounds such as famesol. Diterpenes are C20 compotmds, triterpenes are C30 compounds, and so on. Myriad terpenoid compounds are known, and most can be traced to derivatives of isoprene as starting material. Most terpenes follow the isoprene rule, which dictates the head-to-t ul formation described earlier. AH of the terpenes shown in Figure 12.76 do not stricdy follow the pattern, but each has at least one head-to-tail connection. 

Acetyl Coenzyme A 993 Fats, Oils, and Fatty Acids 994 Fatty Acid Biosynthesis 997 Phospholipids 999 Waxes 1001 Prostaglandins 1002 Nonsteroidal Antiinflammatory Drugs (NSAIDs) and COX-2 Inhibitors 1004 Terpenes The Isoprene Rule 1005 Isopentenyl Diphosphate The Biological Isoprene Unit 1008 Carbon-Carbon Bond Formation in Terpene Biosynthesis 1008 24.10 The Pathway from Acetate to Isopentenyl Diphosphate 1011 Steroids Cholesterol 1013 Vitamin D 1016... 

It turns out that they are chemically related to each other, as you can imagine from the chemical structures shown in Fig. 12.1. They are called terpenes and terpenoids. They can be regarded as derivatives from a five-carbon compound called isoprene (2-methyl butadiene). Two isoprene molecules combine to form mono-terpene (ten-carbon compound). The fragrant oils mentioned above are all the derivatives of mono-terpene. Terpenes are derived from a common metabolic intermediate of glucose, acetyl-CoA (coenzyme A). By the way, a tri-terpene (which three terpene molecules combine to form) called squalene leads to the formation of steroids, and if you connect a large number of isoprene in a linear fashion, you will get natural rubber (Chap. 5). 

It is remarkable that the biogenesis of the hop resins is almost complete before the formation of essential oil components starts (76,77). The isoprene residues are coupled to terpenes when an excess is present with respect to the biogenesis of the hop bitter acids. 

The biosynthetic pathways leading to the formation of (mono)terpenes by different organisms have been systematically investigated in the past [1 ]. As already discussed in the preceding chapter, the biosynthesis sequence conunences with formation of the two key building blocks isopentenyl diphosphate (IPP, 1) and dimethylallyl diphosphate (DMAPP, 2) either via the mevalonate or the mevalonate-independent pathway [1, 3, 4]. These two compounds represent the activated isoprene units that can be assembled and... 

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