Polyene structures

This method of diene formation with definite E and Z structures has wide synthetic applications [518], particularly for the syntheses of natural products with conjugated polyene structures. Bombykol and its isomers (650 and 651) have been prepared by this method[5l9]. The synthesis of chlorothricolide is... 

There is a great deal of uncertainty as to the mechanism of PVC degradation but certain facts have emerged. Firstly dehydrochlorination occurs at an early stage in the degradation process. There is some infrared evidence that as hydrogen chloride is removed polyene structures are formed (Figure 12.18). 

The Z,Z,Z,Z,Z-isomer is required by geometry to have bond angles of 144° to maintain planarity and would therefore be enormously destabilized by distortion of the normal trigonal bond angle. The most stable structure is a twisted form of the , Z,Z,Z,Z-isomer. MO (MP2/DZd) calculations suggest an aromatic stabilization of almost ISkcal for a conformation of the , Z,Z,Z,Z-isomer in which the irmer hydrogens are twisted out of the plane by about 20°, but other calculations point to a polyene structure. ... 

Neither compound exhibits properties that would suggest aromaticity. The NMR spectra are consistent with polyene structures. Both compounds are thermally unstable and revert back to dihydronaphthalenes ... 

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

Carotenoids are known to be sensitive to oxygen, and are said to be unstable in presence of air. Because of their polyene structures, carotenoids are susceptible to reactions with the so-called ROS that may be radicals (02", HO ) or non-radicals (H2O2, O2). 

Singlet oxygen is more reactive than triplet oxygen toward polyene structures in PVC (95). Also, unlike triplet oxygen, singlet oxygen reacts preferentially with the shorter polyene... 

As most of the nitroxyl spin-labelled synthetic derivatives of conjugated polyenes are light yellow crystals, the bond lengths were determined by X-ray crystallography38. The spectroscopic method used to measure the conformation is electron nuclear double resonance (ENDOR). It is beyond the scope of the present review to explain the method38 but the authors of the pertinent paper conclude that ENDOR is an accurate non-crystallographic method to determine polyene structures in solution. 

VII. SELECTED EXAMPLES OF MULTI-PARAMETER ANALYSIS FOR DIENES AND POLYENES STRUCTURE DETERMINATION... 

Raman microspectroscopy [3] allows the observation of the transformation of a polyene structure to a carbon one. The formation of conjugate polyene units under the conditions of chemical dehydrochlorination of the polymer was confirmed by the presence of characteristic narrow peaks at 1,107 and 1,490 cm in the Raman spectra. The products obtained by thermal treatment at elevated temperatures are highly disordered sp -carbon materials, in which the porous structure has developed upon subsequent gasification (Fig. 4.3). 

TGA data confirm transformation of the polyene structure into the carbon one. Thus isothermal heating at 200°C resulted in almost complete dehydrochlorination of the polymer, and intense weight loss occurs only at 300°C (Fig. 4.2, curve 2). 

While initial aging did cause some loss in volume resistivity, it was no more severe in unstabilized than in stabilized compositions. The most startling effect was the marked increase in volume resistivity under severe aging conditions. This may be due to volatilization loss of DOP or cross-linking of polyvinyl chloride, both of which could reduce migration of ionic impurities, or it may involve adsorption of ionic impurities by the dark-colored, conjugated, polyene structure which forms by loss of hydrogen chloride. In any case it deserves further study. 

Since the resulting polyene structure is sensitive to strong base, inacrocyclization is carried out with a variant on the Hortwr-Wads worth-Emwons reaction involving lithium chloride and DBU. 

The Wollcnherg method was used to construe the sensitive -polyene structure of roflamycoin A variation on the Homer-WaJsworth-Lmmons reaction was selected for the maerocyclization, since the possibility for macrolacto-ni/ation — an obvious approach for a luctnne — is ruled out due to steric hindrance from the isopropyl group in the position (C-35) adjacent to the lactone functionality... 

A polyaldolic condensation of acetone can be induced by triflic acid to give a solid resin with a polyenic structure resembling poly(methylacetylene) having some functional groups946 (289). 

In the open-ring isomer, free rotation is possible between the ethene moiety and the aryl group. Therefore, the open-ring isomer is non-planar and the it-electrons are localized in the two aryl groups. On the other hand, the closed-ring isomer has a bond-alternative polyene structure and the jt-electrons are delocalized throughout the molecule. These geometric and electronic structural differences result in some differences in their physical properties. For example, the closed-... 

---