Isoprene structure

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. 

FIGURE 10-22 Some other biologically active isoprenoid compounds or derivatives. Isoprene structural units are set off by dashed red lines. In most mammalian tissues, ubiquinone (also called coen-... 

The sesquiterpenes, diterpenes, and poly terpenes can be considered most simply from a structural point of view as products composed of multiple units of isoprene. Structures currently assigned to some of the sesquiterpenes may have to be revised as the study of terpene chemistry advances, since many of these have been based on the validity of the isoprene rule for terpene structures and exceptions to this rule are becoming known (37). 

Figure 1.25 The isoprene rule for classifying compounds as terpenes. To apply the rule one ignores the double bonds (see myrcene) and in some cases it is also necessary to distort the isoprene structure (see a-pinene)...

The isoprenoid structure hypothesis corresponds to an average of three methyl substitutions along the carbon chain. By comparison, the equivalent H-abstraction reaction of the 2,6,10-tri-methyl-dodecane, with a regular tri-isoprene structure, becomes ... 

Many naphthenes contain the biogenic isoprene structure (see below) which can be used as biogenic markers for identifying petroleum origin. 

Chen, H.Y., Field, JJi., 1967. Configuration of isoprene structure in butyl rubber by time-averaging high resolution nuclear magnetic resonance. J. Polym. Sci. Polym. Lett. Ed. 5 (6), 501-503. 

Type of NR Isoprene structure Major commercial applications... 

In general, infrared spectroscopy can be used to investigate the chemical bonds of the NR matrix and the chemical links of the inorganic filled added to the matrix. For infrared spectrum of composites and nanocomposites based on NR, it is possible to identify all of the vibration band characteristics of the poly(cw-l,4-isoprene) structure being principally two main sets of bonds. The first set around 3000 cm and the second set around 1500 cm For the inorganic filler, it is expected to identify bands mainly between 800 and 200 cm that it can be attributed mainly to the metal-oxygen bonds. 

It was Harries who (by ozonolysis) demonstrated that rubber contained a repeating isoprenic structure which was capable of chain expansion by addition of isoprene emits. Since he was unable to detect end groups, Harries proposed ever expanding "rings" of isoprene. Conforming somewhat to the wisdom of the day, he expounded the idea that the rubber-like properties were caused by aggregation of the rings. 

Today s environmental concerns demand clean reaction processes that do not use harmful organic solvents.Water is without doubt the most environmentally friendly solvent. NR latex is exuded from the Hevea tree as an aqueous emulsion therefore, it would be desirable to modify the NR latex. Many chemical reactions, such as hydrogenation, epoxidation, chlorination,graft copolymerization and oxidative degradation have been performed on the reactive double bonds of the isoprene structure along the molecular chain. 

First, dissociation of the lithium alkyl association [Structure (29)] takes place, followed by activation by complexing of the monomer lithium alkyl with the c/ -isoprene [Structure (31)]. For insertion in a second step, a dimer alkyllithium is necessary [Structures (32) and (33)]. 

They are also natural polyisoprenes and are isomers of natural rubber. They are extracted from sheets of plants in the family of sapotaceae (Southeast Asia and Equatorial America). The polymer chains are constructed from repeating units of 1,4-trans isoprene structure. Contrary to natural rubber, they partially crystallize spontaneously according to two conformations one is 2i helical (a-form), the other one is totally transplanar and is equivalent to a li helix ( -form). The mechanical properties of these materials are determined by the values of the transition temperatures. Amorphous zones undergo a glass transition at —60 C and the two crystalline forms (a and P) melt at 64°C. The corresponding polymers are thus thermoplastics and acquire (after vulcanization) a highly elastic behavior beyond 70°C. 

In the formation of elastomers from diolefin monomers such as butadiene or isoprene, there are a number of possible structures. As the control of these structures is critical in obtaining optimum properties, this area has received great attention from the synthesis chemist. The possible isoprene structures are ... 

Synthetic polymers are obtained by use of Zeigler-Natta catalysis, which leads to a poly(cis-l,4-isoprene) structure. These materials are usually strengthened by vulcanization with either sulfur or a peroxide treatment. This process involves the generation of crosslinkages which are either sulfur or oxygen bridges. 

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