Dienals, formation

The enantiopure tricarbonyl(dienal)iron complex 61 suitably transfers chirality in the piperidine ring formation. Condensation to the Schiff base is followed by the intramolecular Mannich reaction catalyzed with p-TSA. The piperidine was converted to dienomycin C (62) in five additional steps <99EJOC1517>. 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Kosugi, H Kato, T., and Kikugawa, K. 1988. Formation of red pigment by a two-step 2-thiobarbi-turic acid reaction of alka-2,4-dienals. Potential products of lipid oxidation. Lipids 23 1024-1031. 

Reactions of vinyl halides with acrolein acetals and secondary amines lead to the formation of minor amounts of dienal acetals and major amounts of aminoenal acetals (equation 35).s3 The diene product retains the sterochemistry in the vinyl halide while the aminoenal acetal loses it through equilibration of the ir-allylpalladium intermediate. 

Pyrylium behaves as the synthetic equivalent of the pentadienal cation in its reactions with organometallic reagents, allowing the stereoelective formation of 2Z,4 -dienals. This approach to the synthesis of retinoids using a variety of pyrylium salts has been most successful (95JCS(P1)2385). 

Aldehyde Formation. Several investigators observed a marked dominance of hexanal in the volatile products of low-temperature oxidation. At the higher temperatures, however, 2,4-decadienal was the major aldehyde formed (19,20,21). Both aldehydes are typical scission products of linoleate hydroperoxides. Swoboda and Lea (20) explained this difference on the basis of a selective further oxidation of the dienal at the higher temperature, while Kimoto and Gaddis (19) speculated that the carbon-carbon bond between the carbonyl group and the double bond (Type B) is the most vulnerable to cleavage under moderate conditions of autoxidation, while scission at the carbon-carbon bond away from the olefinic linkage (Type A) is favored under stress such as heat or alkali. 

Volatile compounds generated by model systems of zeln, corn amylopectin and corn oil extruded at barrel temepratures of 120°C and 165°C were analyzed by GC and GC/MS. The largest quantities of lipid oxidation products were detected in systems containing all three components. In each system, the quantity of 2,4-deca-dienal was low relative to the quantities of hexanal, heptanal and benzaldehyde. Identification of the Maillard reaction products, 2-methyl-3(or 6)-pentyl-pyrazine, 2-methyl-3(or 6)-hexylpyrazine and 2,5-di-methyl-3-pentylpyrazine, suggested that lipid-derived aldehydes might be involved in the formation of substituted pyrazines. 4-Methylthiazole was identified as a major decomposition product of thiamin when corn meal containing 0.5% thiamin was extruded at a final temperature of 180°C. 

The pH-independent reaction of cyclopentadiene oxide 12 in water is clearly different from that of 13.97 This reaction yields 33% of 3-cyclopentenone, 35% of cis-2,4-pentadienal and 35% of a mixture of cis and trans 1,2- and 1,4-diols (Scheme 28). When the reaction is carried out in D20 instead of H20, no deuterium is incorporated into the ketone product. Thus, 1,2-hydrogen migration is required for this reaction just as it is in the rearrangement of arene oxides to phenols. The mechanisms of product formation in this reaction are not fully understood. The observation that the diol mixture is similar to that from the acid-catalyzed hydrolysis of 12 suggests that an allylic carbocation may be involved in the diol-forming reaction. Ketone (96) and dienal (97) products are potentially formed either by stepwise or concerted mechanisms, and there is insufficient evidence to rule out either one. There is a significant normal salt effect on this pH-independent... 

The true extended aldol reaction, the combination of an extended enolate in the y-position, with an aldehyde or ketone, can best be realised by combining a silyl enol ether 54 with an acetal 70 under Lewis acid catalysis.20 The Lewis acid, usually TiCl4, catalyses the formation of the oxonium ion 71 which adds in the y-position to the silyl enol ether [cf. 64] to give the adduct 72 from which the remaining OMe group can be removed with base to give the dienal 73, the extended aldol product. 

Formation pathways have been discussed by Mulders (1973c), who proposed detailed schemes for 3-methyl- (J.13) and 5-methylthiophene-2-carbaldehyde (J.14), the precursors being mercaptoacetalde-hyde and 2-butenal the former product has been identified upon boiling cysteine with a-diketone (Kobayasi and Fujimaki, 1965a). Mottram (1991) proposed a mechanism of formation for long-chain alkylthiophenes from a-y-unsaturated dienals. 

A pathway leading to the formation of pyridines by aldolization of aldehydes, reaction with ammonia and intramolecular cyclization has been proposed by Vernin (1981). Mottram (1991) explained the formation of 2-alkylpyridines by reaction of ammonia on dienals. 

Three total syntheses of racemic N-methylmaysenine (133b) have been described. The synthesis by Corey et al. [223] is based on the following key steps the formation of the C9-C10 bond by reaction of the corresponding 2-lithiodithiane with a dienal bearing the benzene nucleus (separation of diastereoisomers is required). Then, a dienoic acid was formed in order to carry out the lactamization of the resulting amino acid, which finally was submitted to carbamoylation and dithiane cleavage (Scheme 61). 

More generally, it has been reported that four-, five-, six-, eight-, and twelve-membered rings of bicyclic carbonates can be opened. An example of the six-membered ring opening is given in Eq. 21 [46]. In the reaction, oxidative addition of the allylic C-0 bond toward Pd(0) species followed by decarboxylation affords a palladacycle intermediate. The subsequent p-carbon elimination results in the formation of a dienal. 

A similar synthetic sequence on the enantiomer 226S provided 227S. Oxidation, phosphonate formation [135], and acetal hydrolysis provided enone phosphonate 230. Condensation with the racemic dienal 231 yielded diastereomeric tetraenes 232, which corresponded to the C12-C25 fragment of pimaricin 54. 

Peixoto S, Ngueyen TM, Crich D, Delpech B, Marazano C. One-pot formation of piperidine- and pyrrolidine-substituted pyridinum salts via addition of 5-alkylamino-penta-2,4-dienals to N-acyliminium ions Applications to the synthesis of (+/-)-nicotine and analogs. Org Lett 2010 12(21) 4760-3. 

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