Polyene moiety

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... 

While intermolecular hydroamination of simple alkenes remains a great challenge as of now, addition of amines to alkynes, allenes, and dienes proceeds more easily. However, the addition of an amine to a polyene moiety may not necessary result in the formation of a new stereocenter, if an imine is generated (Scheme 11.18). 

As can be seen from the above examples, the double elimination method is highly efficient and practical for polyene synthesis because labile polyenic moieties are protected as stable p-alkoxy or 5-halo sulphones until the final stage of the synthesis, when the desired skeleton and double bond system are generated in a single step. It should also be emphasized that the desulphonylation process is particularly suitable for synthesis of acid-sensitive retinoids and carotenoids because the reaction is performed under basic conditions. 

The traws-cumulene (205) underwent photochemical trans/cis isomerization to give a 1 1 trans/cis mixture. Mycolactone A/B (206) upon visible light irradiation led to a mixture of four products via the photochemical [4ti + 27i] cycloaddition of the polyene moieties. The structures of the products were elucidated by determining the structures of the four photoproducts of the tetraenoate (207). ... 

The last example in this section is the total synthesis of RK-397 (83) by Sammakia et al. [71]. RK-397 (83) is an oxopolyene macrolide, which was isolated from a strain of soil bacteria. This natural product presents several synthetic challenges, including particularly the installation of the highly sensitive polyene moiety. Sammakia and coworkers successfully tackled this obstacle through a selective alkene cross metathesis reaction at a late stage (Fig. 24). By screening a number of reaction conditions, they found that treatment of alkene (84) and 2,4,6-hexatrienal with Grubbs I catalyst smoothly afforded polyene product (85) in 72% yield as a 4 1 mixture of /Z isomers. 

Partial catalytic hydrogenation of citreoviridin followed by oxidation afforded methyl pyruvate and an oily methyl ester (3). The uv spectrum of citreoviridin (388 nm, e 48,000) suggested that the pyrone chromophore was conjugated with a polyene moiety. The partial structure of citreoviridin was therefore proposed as 4. 

Spectrofluorimetric measurements have shown that degradation conditions can influence the nature of the fluorescent species in degraded PVC (100). On the basis of such measurements, a photooxidized polymer appeared to contain long polyenes, while a cyclohexadiene moiety seemed likely to be present in a sample that had been degraded thermally at 180°C in air (100). The latter result tends to support the involvement of cyclohexadiene... 

Even simple dienes and polyenes are difficult to classify in comparison with alkenes. Whereas bromination, oxidation and reaction with tetranitromethane (TNM) can identify the number of double bonds and their location in the molecular structure, conjugated double bonds produce very complex mixtures. Furthermore, many of the tests based on 7r-complexation can also apply for aromatic moieties. An example is the TNM 7r-complex which is yellow with benzene and orange with naphthalene and the tests are therefore non-specific. 

To link the two half moieties of the molecule, a Julia-Kocienski olefmation was carried out between the C19 building block 59 (again prepared by syn-SN2 -substitu-tion of a propargylic oxirane with DIBAH) and the C20 building block 60, formed via oxidation of 58 with Mn02 (Scheme 18.19). Although this reaction initially led to the formation of the Z-isomer as the major product, the latter was readily isomerized at room temperature to the desired all-trans-polyene peridinin (6). 

Hirota and coworkers41 reported a planar structure of new polyene macrolide antibiotic YS-822A (65), which they isolated. XH and 13C NMR spectra of 65 showed a number of broad and overlapping signals, but the 1H-1H and 13C- H COSY spectra implied the existence of a mycosamine moiety and several other partial structures. The connectivity of these partial structures was established by extensive 2D NMR experiments, including homonuclear Hartmann-Hahn and heteronuclear multiple-bond connectivity measurements, which led to the determination of the gross planar structure of 65. 

Li and coworkers49 reported a molecular motion of /1-carotene and a carotenopor-phyrin dyad (composed of a porphyrin, a trimethylene bridge and a carotenoid polyene) in solution. Internal rotational motions in carotenoid polyenes and porphyrins are of interest because they can mediate energy and electron transfer between these two moieties when the pigments are joined by covalent bonds. Such internal motions can affect the performance of synthetic model systems which mimic photosynthetic antenna function,... 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex44. Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

Zerovalent transition metal carbonyl moieties may act as electron acceptors, and thus activate coordinated polyene ligands toward nucleophilic attack. Reaction of (C.411<5 )-Fe(CO)3 with KBHEt3 (—80 °C) proceeds via attack at a coordinated carbon monoxide to generate the anionic iron-formyl species 185 (Scheme 47)184. Upon warming to... 

Whereas additions of carbon radicals to alkene moieties are the best characterized homolytic additions, carbon radicals are known to add to a wide range of unsaturated systems. These include polyenes, alkynes, arenes, heteroarenes, carbon monoxide,isonitriles, °° ° nitriles, ° imines and derivatives, ° ° aldehydes,nitrones, and thiones. ° Many of these reactions, such as addition of an alkyl radical to a carbonyl group, ° are thermodynamically unfavorable and readily reversible, and they form the basis of composite group-transfer reactions discussed below. 

Polyene cyclization in terpene and steroid synthesis is critically dependent on the terminator in order to generate useful functionalities for further modification of the products. Allyl- and propargylsilanes have proven their value in facilitation of the cyclization and generation of an exocyclic methylene and allene, respectively. Thus, a concise approach to albicanyl acetate [126] and the rapid construction of a tetracyclic precursor of steroids [127] are sufficient to demonstrate the concept. Again, a comparison of the substrates with a silyl group with those having a simple alkyl moiety is very enlightening. 

However, the same sequence of downfield shifts, namely NR < S < O < Se, is observed. Thus, at least the heterocyclic moiety appears to be aromatic. The ratio of the vicinal coupling constants 75 6 / 745 (0.70-0.74) is significantly less than for compounds of the Kekule series (ca. 0.9), much higher than observed for a polyenic TT-system (cyclohexa-1,3-diene displays a value of 0.52), and close to the... 

---