Carbonyl conjugated chains

The process under consideration could be discussed in terms of ring-chain tautomerism of nitronates (378=381) (see the lower part of Scheme 3.215), the more so that such examples were documented for proton analogs of similar SENA (492). However, both nitronates ((378) and (381)), which were prepared by independent syntheses, are quite stable, and therefore their isomerization in the presence of a silyl Lewis acid should involve ring-chain tautomerism of cations. Evidently, cyclization of nitronate (378) is attributed to high electrophilicity of the carbon atom of the carbonyl group, provided that this group is not involved in the conjugation chain. 

A vast range of natural sugars exempMly these cyclic addition products. A typical sugar exists predominantly in the form of a hemiacetal or hemiketal in solution, although this is an equilibrium reaction, and the open chain carbonyl form is always present to a small extent (<1%). The formation of a six-membered cyclic hemiacetal from glucose is achieved by attack of the C-5 hydroxyl onto the protonated carbonyl (conjugate acid). 

Two basic approaches have been taken. The first consists in grafting organometallic donor and acceptor groups, such as ferrocene [as in [84] (Calabrese et al, 1991)] ruthenium derivatives [as in [83] (Whittall et al, 1996)] and tungsten carbonyl, instead of their organic counterparts on tt-conjugated chains. Quite successful in this respect, although not truly org nometallic, are zwitterions based on borate donors and ammonium acceptors [86] (Lambert et ah, 1996) and Lewis acid complexation as in [85] (Kammler et al, 1996). 

Peroxide-induced lignin decolorization is achieved by hydroperoxide ion attack of the carbonyl carbon of the quinones and ketones, which form a part of long conjugated chains (and contribute to the color) in the lignin. In this way the conjugation is interrupted, which reduces the intensity and wavelength of the contributing color [24] (e.g., Eqs. 15.38 and 15.39). 

Further conjugation by a 76 double bond has only a small influence in shifting this absorption towards the bottom of the frequency range. This is well shown in the sterol series [9], and in examples quoted by Cromwell [6]. This is not unexpected, as Blout, Fields and Karplus[16] have shown that the frequency of an a(3-unsaturated aldehydic carbonyl group is not lowered by more than a few wave numbers even by long conjugated chains of up to seven double bonds. 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Conjugated dienes sucb as butadiene and its open-chain analogues can act as 17 ligands the complexes are u.sunlly prepared from melal carbonyl complexes by direct replacement of 2CO by the diene. Isomerization or rearrangement of the diene may occur a.s indicated schematically below ... 

Lengthening the side chain produces the antidepressant maprotiline (73), which has a topological relationship to the clinically useful tricyclic antidepressants. The requisite acid is constructed by conjugate addition of the carbanion of anthrone (64) to acrylonitrile, followed by hydrolysis to give 70. Reduction of the carbonyl group with zinc and ammonia gives anthracene 71 by dehydration of the intermediate... 

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