Aldehydes polyene

The color of the polymer can also be affected by inappropriate reaction conditions in the polymerization process, such as temperature, residence time, deposits of degraded polymer or the presence of oxygen. Degradation of polyesters and the generation of chromophores are thermally effected [29b, 29c, 39], The mechanism of thermal decomposition is based on the pyrolysis of esters and the formation of unsaturated compounds, which can then polymerize into colored products. It can be assumed that the discoloration takes place via polymerization of the vinyl ester end groups or by further reaction of AA to polyene aldehydes. 

Reaction of the ylide, generated from the phosphonium salt 578 (from the alcohol 577 and phosphine hydrobromide 504), with the polyene aldehyde 542 gives aleuria-xanthin acetate 579218). The methyl ester of the naturally occurring bixin 586 is formed by a combination of some carbonyl olefinations 279). The acetoxyaldehyde 580 is olefinated with methoxycarbonylmethylene-triphenylphosphorane 67 to the ( )-unsaturated ester 581. The latter is converted into the phosphonium salt 582 upon treatment with triphenylphosphine hydrobromide 504. The corresponding ylide of 582 is reacted with the dialdehyde 539 to the polyene aldehyde ester 583. The latter is reduced and converted into phosphonium salt 584. The corresponding ylide is now reacted in a third carbonyl olefination with 585 to give the methyl ester 586 279> (Scheme 98). 

An intensity enhancement has been observed for /3,y unsaturated carbonyl compounds such as 32, which is due to r-.7r interactions in the non-planar system of reduced symmetry (Labhart and Wagniere, 1959). In the series of polyene aldehydes, of the transition increases with in-... 

Figure 2.37. Absorption spectra of polyene aldehydes (by permission from Blout and Fields, 1948).

An important synthesis of polyene aldehydes due to Kuhn and Winter-stein945 is vinylogous to the above reactions. They obtained 7-phenylhepta-trienal in 50% yield by condensing cinnamaldehyde with crotonaldehyde in 70% ethanol containing piperidine acetate and 11-phenylundecapentaenal and 15-phenylpentadecaheptaenal are formed in the same way. 

FIGURE 7.15 The spectra of a series of polyene aldehydes. (From Murrell, J. N., The Theory of the Electronic Spectra of Organic Molecules, Methuen and Co., Ltd., London, 1963. Reprinted by permission.)... 

Similar insoluble oxidants were also used in the preparation of volatile, unstable polyene aldehydes during work on the plasmodial pigment fuligorubin A," while comparable solution-phase oxidants were markedly less successful (scheme 18.2). 

The formylation of polyene aldehydes and o,jS-unsubstituted ketones with the Vilsmeier reagent (DMF-POCI3) has been reported recently. For example, reaction of the ketones XXIV with the Vilsmeier reagent produces the iminium salts XXV, which are hydrolyzed to the aldehydes XXVI. The latter compounds form the conjugated ene-yne derivatives XXVII upon addition of their dioxane solution to dilute sodium hydroxide at 80-90°C... 

The synthesis of these isomers was guided by a strategy of using only stereochemically pure phosphonium salts and polyene aldehydes as intermediates. To show the importance of various chromatographic and spectroscopic methods for the analysis of such intermediates, the preparation of the (7Z)-isomer of lycopene (31), is briefly considered in Scheme 5. This isomer is a major component of the mixture of (all- )-, (7Z)- and (7Z,7 Z)-lycopene formed in the Wittig reaction of geranyltriphenylphosphonium bromide with crocetindialdehyde [10] in the presence of NaOMe in dichloromethane [18]. 

The first industrial synthesis of P,P-carotene (3) by Roche [1] followed the C19 + C2 + C19 synthesis principle [1,20]. As in the vitamin A process, the polyene chain was produced by Grignard coupling, elimination and partial hydrogenation. In addition, a new effective synthesis for polyene aldehydes has now been developed in the form of the enol ether condensation and employed industrially for the first time in the production of the C19-aldehyde 27 (Scheme 6). The enol ether condensation permits specific stepwise lengthening of conjugated aldehydes by two carbon atoms each time. Use of prop-l-enyl ethyl ether (28) (Scheme 9) gives a-methyl-branched polyene aldehydes. Chemically, the chain lengthening proceeds in three steps as follows ... 

Polyene aldehydes discolour PET from colourless to yellow, while the water can cause hydrolytic chain scission of ester linkages. The rate of thermal degradation in closed systems such as in the continuous preparation and processing of PET was formd to be about three times higher than that in an open stirred system under flowing nitrogen where acetaldehyde can escape. 

In 2006, Sato and coworkers discovered an efficient cycloisomerization of polyene aldehydes leading to the construction of 5-7 condensed carbocyclic systems resem-bhng to natural aromadendrane motifs [76]. During then-... 

Bouzbouz, S., Roche, C., and Cossy, J. (2009) Simple synthesis of conjugated all-(E)-polyenic aldehydes, ketones, and esters using chemoselective cross-metathesis and an iterative sequence of reactions Application to the synthesis of navenone B. Syrdett., 81, 803-807. 

As an example of a remote functionalized alkenyllithium compound, the alkoxy functionalized polyenyllithium 236 (prepared from 235 by bromine-lithium exchange with t-BuLi) reacted with carbonyl compounds to give after acidic hydrolysis the corresponding polyenic aldehydes 237 (Scheme 2.32) [193]. 

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