Isoprene, isomeric chain unit

Factors Affecting the Isomeric Chain Unit Structure in Organolithium Polymerization of Butadiene and Isoprene... 

Morton M, Rupert JR. Factors affecting the isomeric chain unit structure in organolithium polymerization of butadiene and isoprene. In Bailey FE Jr editor. Initiation of Polymerization. ACS Symposium Series. Volume 212. Washington (DC) American Chemical Society 1983. p 283. 

For example, the ion of [M-69]+, which is observed in the tandem mass spectra of lycopene, neurosporene, and y-carotene but not a-carotene, p-carotene, lutein, or zeaxanthin, indicates the presence of a terminal acyclic isoprene unit. Elimination of a hydroxyl group or a molecule of water, [M-17]+ or [MH-18]+, from carotenoids such as astaxanthin or zeaxanthin is characteristic of the presence of a hydroxyl group. Also, tandem mass spectrometry can be used to distinguish between isomeric carotenoids such as a-carotene and p-carotene, or lutein and zeaxanthin. For example, the ring of a-carotene containing the double bond that i s not conj ugated to the rest of the polyene chain shows unique retro-Diels-Alder fragmentation to form the ion of [M-56]+. In a similar manner, isomeric lutein and zeaxanthin differ by the... 

In the case of isoprene, low molecular weight model compounds (dimer, trimer, etc.) can be prepared in benzene to produce oligomeric analogues of the polyisoprenyllithium active in polymerization [195]. The NMR spectra of such oligomers show that cis and trans forms of the lithium bearing terminal unit occur, and that one predominates at room temperature [195, 196]. It is probably the cis form, although this is difficult to establish without doubt. Except for the one unit chain, the cis—trans ratio varies reversibly with temperature. Transfer to tetrahydro-furan-rich mixtures at low temperatures shows that isomerization occurs when the solution is warmed to —40°C, probably to the trans form. This is the stable form in such solvents at all temperatures. The NMR spectra are basically the same in both hydrocarbon and ether solvents. Only the resonance due to the proton on the y-carbon is shifted upfield in polar... 

The chemical shifts of the characteristic carbon signals in acyclic terpenes, polyprenols, and cis-trans isomerized poly-isoprenes are plotted in Fig. 3. Here, the chemical shifts are correlated using the w C-5 methyl carbon signal at 17.66 ppm as an internal standard (except for isomerized polyisoprenes) in order to compensate for the effect of solution concentration. It is clear that these chemical shifts are independent of the chain length of the compounds and can be used for the determination of the arrangement of isoprene units as well as the terminal units in various isoprenoid compounds (8). 

As shown in Figure 1.7, the polymerization of monomers with two double bonds (e.g., butadiene and isoprene) leads to polymer chains with a residual double bond per monomer unit cis-trans isomerism is possible. Two important polymers that show this type of isomerism are 1,4-polybutadiene and 1,4-polyisoprene. The regularity of the trans configuration makes this type of isomer more crystalline, with a higher melting point compared to the cis configuration. 

The basic skeleton of isoprenoids may be modified by the introduction of a wide variety of chemical groups, by isomerization, shift of double bonds, methyl groups, etc. Hence a bewildering number of chemical structures arises. In addition compounds derived from other biogenic pathways may contain isoprene residues. For instance the K vitamins (D 8.1), ubiquinones (D 8.3), chlorophylls (D 10.1), plastoquinones, and tocopherylquinones (D 22.4) have isoprenoid side chains with up to ten isoprene units. Polyketides (D 3.3), alkaloids (D 8.4.2), and coumarins (D 22.2.2) may be substituted by dimethylallyl groups. The terpene residues are attached to nucleophilic sites, such as active methylene groups and phenolic oxygen atoms. 

First, by a process of electron transfer, the molecule can be linked into the chain through carbon atoms 1 and 4, the remaining C=C double bond being between carbon atoms 2 and 3. This repeat unit is called 1,4 and there are two possible isomeric forms, cis-1,4 with the carbon atoms of the double bond both on the same side of the backbone chain, and trans-1,4 with the carbon atoms of the double bond on opposite sides of the chain. A third alternative, with the butadiene linked into the chain through carbon atoms 1 and 2, has the C=C double bond hanging off the chain as a pendant vinyl group. This incorporation is known as 1,2. Figure 2-7 shows these three possibiHties. Isoprene can be incorporated in four different ways. 

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