Polyenes, reduction

Perhaps the most notable example of this chemistry is in the production of vitamin A [68-26-8] where the P-ionyUdenacetaldehyde is condensed with the ester-yhd to obtain the polyene ester. Reduction then yields vitamin A (see Vitamins). 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

Clotrimazole and other azole derivatives have a different mode of action than the polyenes, eg, amphotericin B. The latter biad to the ergosterol present ia the membranes of yeasts and fungi, but azole derivatives inhibit the cytochrome P-450 dependent biosynthesis of ergosterol (8—11). This inhibition not only results in a reduction of ergosterol, but also in an accumulation of C-14 methyl sterols. They disturb membrane permeabiUty, inhibit cell rephcation, and are basically responsible, in combination with the reduction of ergosterol levels, for the antifungal action. 

Reduction of the nonaromatic polyene [12]annulene, either electrochemically or with lithium metal, generates a 14-7c-electron dianion" ... 

Selectivity depends importantly on the catalytic metal. A number of selectivity series have been determined for simple olefins, and the presumption is that the sequence holds for more complex polyenes as well. Selectivity for the reduction of allene to propylene declined with metal in the order palladium... 

Our results fit also with a previous investigation (9) on polyenes based on a version of the 2h-lp Cl scheme restricted to the virtual one-electron states generated by a minimal basis. In our case, however, the fragmentation of lines into satellites is much more pronounced. The reason lies in the size-consistency of the ADC[3] approach (as contrasted with the size-inconsistency of any truncated form of Cl (27d), in the full handling of the virtual space, and (10) in the inclusion of correlation corrections to the reference ground state, leading to (37) a net reduction of the quasi-particle band gap of conjugated polymers. 

It should be noted, on the other hand, that a symmetry reduction is predicted even in molecules II, VIII and XIII whose peripheral skeletons correspond to 4n-l-2 cyclic polyenes. The transannular bonds in these molecules are different in nature from those mentioned above. For example, the introduction of the transannular bonds between atoms 2 and 8 and between 3 and 7 of cyclododecapentaene to form bowtiene (II) (Fig. 3) brings about the splitting of the top filled degenerate orbitals of the unperturbed system into two levels, one with its energy raised and... 

Reductive elimination from 2-en-l,4-diol derivatives has been used to generate 1,3-dienes. Low-valent titanium generated from TiCl3-LiAlH4 can be used directly with the diols. This reaction has been used successfully to create extended polyene conjugation.305... 

The reductive coupling/silylation reaction was extended to more complicated polyenes, such as the triene-substituted cyclopentanol 73, which cyclizes to provide 74 with a 72% yield and 6 1 dr after oxidation (Eq. 10) [44], The reaction is chemoselective the initial insertion occurs into the allyl substituent, which then inserts into the less hindered of the two remaining olefins, leaving the most hindered alkene unreacted. 

Dynamic nuclear polarisation (DNP) enhanced 15N CP MAS NMR has been exploited by Mark-Jurkauskas et al.79 in the studies of intermediates of the bacteriorhodopsin photocycle. The data for L intermediate were similar to those found for 13-ds,15-anti retylidene chloride, while those for K intermediate were similar to those of acid blue bacteriorhodopsin in which the Schiff base counterion was neutralised (Table 3). The 15N chemical shifts observed have shown that for bacteriorhodopsin, the Schiff base in K intermediate state loses contact with its counterion and establishes a new one in L intermediate state. The proton energy stored at the beginning in the electrostatic modes has been transformed to torsional modes. The transfer of energy is facilitated by the reduction of bond order alternation in the polyene chain when the counterion interaction is initially broken and is driven by the attraction of the Schiff base to a new counterion. 3D CP MAS experiments of NCOCX, NCACX, CONCA and CAN(CO)CA types have been used in studies of proteorhodopsin.71... 

F. Sato developed titanium (Il)-based c/s-reduction of alkynes as shown in Scheme 5 [14], and the method was applied to the synthesis of pheromones by Kitching (Scheme 6) [15]. This titanium (Il)-based reaction gives pure (Z)-alkenes. Kitching summarized the contemporary methods for the synthesis of skipped polyynes and their reduction to skipped polyenes [15]. 

Polyarenes, reduction reactions, 49 Polyenes, alkene to alkane reductions, 41-45... 

TABLE 4. Dienes and polyenes through reductive deoxygenations... 

TABLE 22. Polyenes through reductive coupling with zinc... 

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