Isoprene units, chemical shifts

The chemical shifts due to the monosulfidic crosslinks are influenced not only by the position on the monomeric unit to which it belongs, but also by the position of the carbon atom of the monomeric unit on the other side of the bridge. The shielding parameters of monosulfide substitution on the individual carbons of the isoprene unit have also been determined. It is shown that resolvable polysulfidic crosslink resonances exist in all positions of the backbone carbons while monosulfidic crosslinks appear only between C-l and C-4 carbons with detectable intensity [18]. 

Figure 11.17 Relationship between length of isoprene-units and chemical shifts of 13C-NMR signals reflecting the arrangement of isoprene-units, observed at 50 MHz [88]...

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). 

Figure 3. Chemical shifts of the signals characteristic of the arrangement of the isoprene units (8, 13).

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. 

The PMR spectra obtained showed the effect of the lithium on the chemical shift of the protons on the terminal monomer unit in a series of polydienes, including butadiene, isoprene eind 2,3-dimethylbutadiene, and the effect of solvents on these spectra. 

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