Polyenes conformation

Cohnenares, L.U., and Liu, R.S.H., ll-Methyl-9-demethylretinal and 11-methyl-9.13-didemethyhet-inal. Effect of altered methyl substitution pattern on polyene conformation, photoisomerization and formation of visual pigment analogs. Tetrahedron, 47, 3711, 1991. 

Carotenoids Many in vivo situations e.g. photosynthetic membranes lobster shells Polyene Polyene conformation polyene-polyene exciton interactions membrane potential triplet state properties "... 

In the aphidicblin synthesis in Scheme 11.23, a key bicycli intermediate is established by a mercuric ion-induced polyolefin cyclization. Steps A-D serve to construct the necessary polyene. As was discussed in Chapter 9, the stereochemistry of such cyclizations is predictable on the basis of the polyene conformation. The... 

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. ... 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

The detection of the structural effects of B—C rr-bonding in unsaturated open-chain organoboranes should prove more difficult than in boracyclo-polyenes for several reasons. First, as already mentioned, there is the conformational freedom of the open-chain boranes, which reduces B(2pz)—C(2pz) overlap (cf. 17 vs. 19). Second, there is the linear overlap of 34 versus the cyclic, Htickel aromatic overlap of 35. 

Thermal 1,5-hydrogen shifts are thus allowed and, because of the symmetry of the T.S. (39), the H atom in the product (37, x = 1) will be on the same side of the common plane of the polyene s carbon atoms as it was in the starting material (36, x = 1) this is described as a suprafacial shift. This latter point would not be experimentally verifiable in the above example, but that thermal 1,5-shifts (which are quite common) do involve strictly suprafacial migration has been demonstrated in the compound (40). This is found, on heating, to yield a mixture of (41) and (42), which are produced by suprafacial shifts in the alternative conformations (40a) and (406), respectively ... 

We will now consider the studies devoted to the next two linear polyenes 1,3,5-hexatriene and 1,3,5,7-octatetraene. First, we will present the results corresponding to geometries and conformational energies computed for these compounds. We will then discuss the computed frequencies and force fields. 

Rotational isomerism normally complicates the study of gaseous nonconjugated dienes and polyenes because many conformers appear simultaneously, and hence only few structures of free molecules in this category have been studied. 

The final entries in Table 7 concern eight-membered monocyclic dienes and polyenes. The unconjugated 1,5-cyclooctadiene was observed to have twist-boat conformation and C2 symmetry. In accordance with what is very often the case in GED studies of cyclic... 

Table 8 presents structures observed for monocyclic dienes and polyenes with rings large enough to accommodate trans C=C double bonds. In a cyclodecadiene molecule strain-free carbon skeletons can only be derived when two double bonds are diametrically placed and have the same configuration (as, cis or trans,trans). Cw,cis-Cyclodeca-1,6-diene (1,6-CDD) may exist in twelve different conformations, and it is therefore noteworthy that it almost exclusively prefers one of these, namely the one indicated in Table 8. This conformer does not have the repulsive transannular HH interactions that destabilize the corresponding saturated molecule in all conceivable conformers.

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