Conjugated dienes defined

Conjugated diene (Section 10 5) System of the type C=C—C=C in which two pairs of doubly bonded carbons are joined by a single bond The tt electrons are delocalized over the unit of four consecutive sp hybridized carbons Connectivity (Section 1 6) Order in which a molecule s atoms are connected Synonymous with constitution Constitution (Section 1 6) Order of atomic connections that defines a molecule... 

The lag-phase measurement at 234 nm of the development of conjugated dienes on copper-stimulated LDL oxidation is used to define the oxidation resistance of different LDL samples (Esterbauer et al., 1992). During the lag phase, the antioxidants in LDL (vitamin E, carotenoids, ubiquinol-10) are consumed in a distinct sequence with a-tocopherol as the first followed by 7-tocopherol, thereafter the carotenoids cryptoxanthin, lycopene and finally /3-carotene. a-Tocopherol is the most prominent antioxidant of LDL (6.4 1.8 mol/mol LDL), whereas the concentration of the others 7-tocopherol, /3-carotene, lycopene, cryptoxanthin, zea-xanthin, lutein and phytofluene is only 1/10 to 1/300 of a-tocopherol. Since the tocopherols reside in the outer layer of the LDL molecule, protecting the monolayer of phospholipids and the carotenoids are in the inner core protecting the cholesterylesters, and the progression of oxidation is likely to occur from the aqueous interface inwards, it seems reasonable to assign to a-tocopherol the rank of the front-line antioxidant. In vivo, the LDL will also interact with the plasma water-soluble antioxidants in the circulation, not in the artery wall, as mentioned above. 

The result for 1,3-butadiene is —15.7 1.9 kJmol-1, a value comparable to conventional expectations save the sign22. We can now define the difference of the value for an arbitrary conjugated diene of interest and this 15.7 kJmol-1 as E z (equation 18). 

RP-HPLC with nonaqueous solvents and UVD at 246 nm was developed for the determination of low level POVs of vegetable oils. These measurements are specific for conjugated diene peroxides derived from vegetable oils with relatively high linoleic acid content. These measurements may be supplemented by nonspecific UVD at 210 nm and ELSD for detection of all eluted species. The elution sequence of the triglycerides in a nonaqueous RP-HPLC is linearly dependent on the partition number of each species, Vp, which is defined as = Nq — 2Ni, where Nq is the carbon number and is the double bond number. In the case of hydroperoxides = Nq — 2Nd — Vhpo, where Vhpo is the number of hydroperoxyl groups in the molecule (usually 1 for incipient POV). For... 

As discussed in the last sections, addition of sodium methoxide-alkenylborane complexes to copper bromide-dimethyl sulfide at 0 °C immediately gives conjugated dienes. However, if the temperature is lowered to —15 °C, the dark blue-black complex formed is stable and can be trapped by allylic halides to afford a stereochemically defined synthesis of (4 )-1,4-dienes (Eq. 93) 148). 

An interesting example of Pd-catalyzed oxidation of a 1,3-conjugated diene is shown in Scheme 20. This reaction type produces stero-defined 1,4-functionalized dienes, which are useful synthetic building blocks and reaction intermediates. Benzoquinone oxidation of an ) -allyl-Pd intermediate is a key step in releasing the organic product. ... 

In the presence of a fluoride ion generating reagent such as tris(diethylamino)sulfonium difluorotri-methylsilicate (TASF, 99) and a palladium catalyst, vinyltrimethylsilane couples with vinyl and aryl iodides." For substituted vinylsilanes, the fluorodimethylsilyl group is particularly effective in the coupling reaction, which takes place with retention of configuration of both substrates (Scheme 17). Silicon-based C—C bond formation is as useful as that which employs organoborons " for the synthesis of stereo-defined conjugated dienes. 

Generation of stereo- and regio-defined alkenylmetals via hydrometallation or carbometallation of alkynes followed by cross-coupling (Scheme 1-11) is a synthetically attractive notion for preparing arylated alkenes, conjugated dienes, and conjugated enynes. Its feasibility was demonstrated in 1976 [14,15] in the prototypical examples shown in Schemes 1-4 and 1-5. In these processes, hydroalumination was employed for generating the required alkenylmetals. 

Yao, Q. Rapid Assembiy of Structuraiiy Defined and Highiy Functionaiized Conjugated Dienes via Tethered Enyne Metathesis. Org. Lett. 2001,3, 2069-2072. 

Alkenyl-alkenyl coupling offers a versatile synthetic method of stereo-defined conjugated diene systems. Both (Z, )-and (Z, Z)-conjugated alkadienyl carboxy-lates 95 and 96, which are present in a range of natural products, were obtained by coupling (Z)-and ( )-heptenylboronic acids 92 and 93 with (Z)-2-bromovinyl octanoate (94) [84]. 

The soybean oil was stripped of tocopherols by short-path molecular distillation. The relative potency was based on inhibition of hydroperoxides measured by conjugated diene formation at optimal concentrations of about 100 ppm for a-tocopherol, about 300 ppm for y-tocopherol. The optimal concentration for 5-tocopherol was not clearly defined and the antioxidant activity increased with concentration up to -1900 ppm. 

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