Isoprene, structural units

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-... 

Isoprene unit (Section 26 7) The characteristic five carbon structural unit found in terpenes... 

Unlike polyethylene and other simple aikene polymers, natural rubber is a polymer of a diene, isoprene (2-methyl-l,3-butadiene). The polymerization takes place by addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry ... 

In the case of an unsymmetrical diene such as isoprene, different orientations of the structural units are possible depending on which end of the diene unites with the chain radical. Of the two competing reactions (6) and (7) shown on page 241, the former would appear to be the more probable one on account of the influence of the methyl substituent in stabilizing to some extent one of the resonance hybrid structures which are shown. 

Similarly Stable and Posnjak (87) proposed cyclic formulas of four, five, or possibly more structural units for polystyrene. Lebedev (88) first assigned the cyclooctadiene structure to polybutadiene shown below before expanding the concept of the ring to include several isoprene units. 

It is interesting to note that the C. maculatus compounds show an isoprene sub-unit which bears the unsaturation and the carboxylic moiety just like callosobruchusic acid 207. A propanoate (or methylmalonate) starter would formally complete the biogenesis of the structures. 

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]. 

A number of natural monoterpene structures contain carbon skeletons which, although obviously derived from isoprene C5 units, do not seem to... 

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). 

Terpenes are a class of unsaturated hydrocarbons made up of isoprene C5 units and found in essential oils and oleoresins of plants such as conifers. The two most commonly used as solvents are turpentine and o-limonene. Their physical properties are compared with those of toluene and methylene chloride in Table 5.7. They are both immiscible with water. As can be seen in Figure 5.3, D-limonene and other small terpenes have similar molecular weights and structures to substituted cyclohexanes and toluene and are therefore to likely have solvent properties intermediate between these two VOCs. 

Some s)mthetic rubbers are superior to natural rubber in some ways. Neoprene is a s)mthetic elastomer (an elastic polymer) with properties quite similar to those of natural rubber. The basic structural unit is 2-chloro-l,3-butadiene, commonly called chloroprene, which differs from isoprene in having a chlorine atom rather than a methyl group at carbon 2 of the 1,3-butadiene chain. 

The isoprenoids are a vast array of biomolecules that contain repeating five-carbon structural units known as isoprene units (Figure 11.10). Isoprenoids are not synthesized from isoprene (methylbutadiene). Instead, their biosynthetic pathways all begin with the formation of isopentenyl pyrophosphate from acetyl-CoA (Chapter 12). 

Polybutadiene formed by high-temperature, free-radical addition polymerization is a copolymer of these three kinds of structural units. With isoprene (2-methyIbutadlene), the number of ways the unit can enter the polymer chain is still larger for example, the... 

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