Polyenes, higher

The organization of Part Two is according to structural type. The first section, Chapter Seven, is concerned with the synthesis of macrocyclic compounds. Syntheses of a number of heterocyclic target structures appear in Chapter Eight. Sesquiterpenoids and polycyclic higher isoprenoids are dealt with in Chapters Nine and Ten, respectively. The remainder of Part Two describes syntheses of prostanoids (Chapter Eleven) and biologically active acyclic polyenes including leukotrienes and other eicosanoids (Chapter Twelve). 

The metathesis of acyclic alkadienes and higher polyenes may involve both inter- and intramolecular processes. An example of an intermolecular reaction is the conversion of 1,5-hexadiene into 1,5,9-decatriene and ethene ... 

The Tz orbitals of butadiene (Scheme 18) qualitatively obtained from the orbitals of ethylenes are also supported by the electronic spectra of polyenes. The HOMO of butadiene is higher that the HOMO of ethylene since the former is the out-of-phase combination of the latter. The LUMO of butadiene is the in-phase combination of the LUMOs of ethylene and lies lower than the LUMO of ethylene. The energy gap between the HOMO and the LUMO is smaller in butadiene. In fact, the wavelength ( m ) is longer for butadiene (217 nm) than for ethylene (165 nm). The wavelength increases with the chain length of the polyenes. 

In order to exhibit provitamin A activity, the carotenoid molecule must have at least one unsubstituted p-ionone ring and the correct number and position of methyl groups in the polyene chain. Compared to aU-trans P-carotene (100% provitamin A activity), a-carotene, P-cryptoxanthin, and y-carotene show 30 to 50% activity and cis isomers of P-carotene less than 10%. Vitamin A equivalence values of carotenoids from foods have been recently revised to higher ratio numbers (see Table 3.2.2) due to poorer bioavailability of provitamin A carotenoids from foods than previously thought when assessed with more recent and appropriate methods. 

As in the polyenes case, we can see that these values increase with the lengthening of the polymeric cMin. We observe the good agreement between our values and those obtained by Williams[12] for the three first compounds of the series, whereas for the higher polymers our values are lower. After checking of the calculations, it seems to us that an error could have occur in Williams computations. 

Beyond N=7, the longitudinal hyperpolarizability of cumulenes increases and becomes higher than the corresponding polyenes values for 8[Pg.307]

Many substituted thiophenes have also been electrochemically polymerised [19,54,399-405] (Table 4) as have thiophene dimers [21,37,55,251,400,406], trimers [21, 83,407], and tetramers [256,406], with the thiophene dimer giving rise to higher quality films than does the monomer [37, 395,408]. Several polycyclic monomers including a thiophene ring have also been polymerised [408-416], as have a series of compounds consisting of two thiophene rings linked by a polyene chain (Fig. 23c). The polymerisation of dithieno-thiophene (Fig. 23d) results in a polymer which shows remarkable similarity to polythiophene in its properties [409,410,414],... 

In order to investigate how HC1 affects the polyene sequences we (38) have investigated the effect of trifluoroacetic acid (TFA) on the polyenes introduced chemically into PVC using the method of Shindo (59). TFA was chosen since its concentration in solution can be controlled and set at higher levels than is possible with HC1 thus allowing the possibility of producing high concentration of intermediates which can be detected. 

Higher molecular weight dienes and polyenes which are solid and can be crystallized make it possible to study their structure by X-ray diffraction. This, of course, will give information only about the crystalline form (see discussion on steroids and antifungal molecules, Section VII). 

In general, one can use a variety of oxidation techniques to form derivatives of dienes and higher polyenes for their analysis however, the information obtained with conjugated systems is muddled by the complexity of products. Also, it is obvious that since the oxidized derivatives contain different functionalities, e.g. epoxides, alcohols, acids etc., the analytical techniques employed should also be variable (see also Scheme 2). 

Due to steric hindrance by cis substituents, coplanarity and conjugation of cis polymers is hindered. As a result the absorption of trans polyenes occurs at higher wave lengths compared with the absorption of cis polyenes. The same effect is observed in cis trans stilbenes. 

The higher conjugated unsaturated systems will be mentioned later in connexion with the polyenes and the carotenoids (p. 233). 

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