Open polyenes

Carotenoids are a class of lipophilic compounds with a polyisoprenoid structure. Most carotenoids contain a series of conjugated double bonds, which are sensitive to oxidative modification and cis-trans isomerization. There are six major carotenoids (ji-carotenc, a-carotene, lycopene, P-cryptoxanthin, lutein, and zeaxanthin) that can be routinely found in human plasma and tissues. Among them, p-carotene has been the most extensively studied. More recently, lycopene has attracted considerable attention due to its association with a decreased risk of certain chronic diseases, including cancers. Considerable efforts have been expended in order to identify its biological and physiochemical properties. Relative to P-carotene, lycopene has the same molecular mass and chemical formula, yet lycopene is an open-polyene chain lacking the P-ionone ring structure. While the metabolism of P-carotene has been extensively studied, the metabolism of lycopene remains poorly understood. 

Electrocyclic processes are also observed with ionic systems, both cationic and anionic. The decisive factor in the stereochemistry is the number of electrons in the delocalized polyenic system. Thus the mode of ring opening or ring closure depends on the n electrons in the open polyene. 

Other retinals, in which the ionone ring was modified by an additional double bond, 38 [189,190], ring contraction, 39 [70], or a retinal with open polyene chain, 40... 

For even, open polyenes, nucleophilic addition occurs preferentially at the terminal position for odd, open polyenes, attack occurs at the terminal positions only if the metal fragment is strongly electron withdrawing. 

Rule 3. Open polyenes terminal addition in all cases. Open polyenyls usually terminal attack, but nonterminal if L M is electron-donating. 

Rule 3 For even open polyenes (e.g. fy -butadiene) nucleophilic attack at the terminal carbon atom is always preferred, for odd open polyenyls attack at the terminal carbon atom occurs only if the attached metal containing group is strongly electron-withdrawing. 

The three rules successfully correlate a large body of experimental data on the site of nucleophilic attack on cationic 18-electron complexes. They do not always work rule 3 in particular is not very well defined in relation to odd open polyenes. Rule 2 seldom fails, but several exceptions to Rule 1 are now known. Rule 1, for example breaks down in relation to the cation Sodium... 

Because it is always easier to look first at the open polyene system, we will consider the Jt molecular orbitals for hexatriene (Fig. 20.17). The HOMO for the thermal reaction is >3, and that for the photochemical reaction, 4>4. 

G, gave the ring-opened polyene compounds [FeMn G8H7(OEt)Ar (GO)6] 13. Analogous reactions have also been reported for the Re system." ... 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

Chemists in the early 1900s believed that the only requirement for aromaticity was the presence of a cyclic conjugated system. It was therefore expected that cyclooctatetraene,. as a close analog of benzene, would also prove to be unusually stable. The facts, however, proved otherwise. When cyclooctatetraene was first prepared in 1911 by the German chemist Richard Willstatter, it was found not to be particularly stable but to resemble an open-chain polyene in its reactivity. 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

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. 

C—C a bond to be cleft (Fig. 7.34). The LU of the C—C a part will conjugate with the HO of the n part of the ground-state polyene moiety in case of reaction, so that the orbital symmetry relations clearly determines the direction of bond fission. The direction of change is indicated by arrows. In this manner, in the thermal opening, the (4 )-chain will be... 

Fig. 7.34. The steric pathway of the ring-opening of cyclic polyenes. (In case ii) the consideration of the SO—HO interaction does not change the conclusion)...

This type of reaction, whether it involves the cyclisation of a polyene, as here, or the ring-opening of a cyclic compound to form a polyene, is known as an electrocyclic reaction. 

Henderson, J. D. et al., Chem. Eng. News, 1983, 61(33), 2 An opened 100 mg ampoule of the salt was stored dark in a desiccator at —20°C between occasional samplings during a few months. A few minutes after return to storage, the ampoule contents decomposed violently. It was supposed that peroxide formation (and perhaps polymerisation of the polyene) was involved in the incident. See Other PEROXIDISABLE COMPOUNDS... 

FIGURE 29. Energies of first (triangles) and second transition (squares) of radical cations i-Bu—(CH=CH) — Bu-i (filled symbols) and some carotenoid polyenes (open symbols) as a function of 1 In... 

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