Retro-aldol degradation

On treatment of glutaraldehyde with 2-hydroxy-3-nitropropionic acid, only 2-nitrocyclohexane-l,3-diol could be isolated in 10% yield, indicating that, under the conditions used, a retro-aldol degradation takes place with liberation of nitromethane. However, when using methyl 2-methoxy-3-nitropropionate, which cannot undergo a retro-nitro-methane addition", products (101) and (102) are obtained in yields of 7 and 13% respectively... 

Aqueous solutions of unsaturated carbonyls are hydrolyzed under alkaline conditions to produce additional carbonyl-containing compounds. Alkaline conditions traditionally used to accelerate this reaction cascade can be replaced with elevated temperatures and pressures without greatly affecting the overall hydrolysis of these unsaturated carbonyls. Water-mediated retro-aldol degradation of alpha/beta unsaturated carbonyls appears to be significant as a means to thermally-generate flavor-active carbonyls, as well as lead to deterioration of character-impact compounds possessing these features. 

In two recent reports on retro-aldol degradation of (El,Z)-2,6-nonadienal to (Z)-4-heptenal and ethanal ( 5), and (E,Z)-2,4-decadienal to (Z)-2-octenal, hexanal and ethanal (6), it was demonstrated that this reaction could be thermally-driven at neutral pH. 

Table 1. Some Important Precursors of Alpha/Beta Unsaturated Carbonyls in Food and their Retro-Aldol Degradation Products...

Overall, this paper provides a basis for water-mediated retro-aldol degradation chemistry contributing to thermally-induced volatile flavors. Although lipids provide the principle starting materials for the generation of alpha/beta unsaturated carbonyls in... 

Retro-Aldol Degradation of Alpha/Beta Unsaturated... 

Josephson, D.B. and Lindsay, R.C. Retro-Aldol degradation of unsaturated aldehydes role in the formation of ci5 -4-heptenal from frani-2,cw-6-nonadienal in fish, oyster and other flavors. J. Am. Oil Chem. Soc. 64, 132-138 (1987). 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

Figure 2. Mechanism for the Formation of 2-Pentenal, Propanal, 2-Propanone and Ethanal from the Thermal Degradation and Retro-Aldol Condensation of Linolenic Acid.

The alkadienals could be formed from the autoxidation of PUFA and may contribute desirable arenas to freshly prepared foods (19). Further degradation of alkadienals often increased undesirable flavors. Josephson and Lindsay demonstrated that 2,4-decadienal could produce 2-octenal and ethanal (20) and 2,6-nonadienal could produce 4-heptenal and ethanal (21) via retro-aldol condensation mechanisms. Hsieh et al. (22) reported that iscmers of various alkadienals and alkatrienals gave green, greasy and oxidized fish oil odors in crude menhaden fish oil. 

One possible mechanism for the degradation of 2,4-decadienal is the retro-aldol mechanism proposed by Josephson and Lindsay (8) for the oxidation of a,B-unsaturated fatty aldehydes. In their investigation of the degradation products of 2,4-decadienal, Josephson and Lindsay (9) identified 2-octenal and hexanal as primary products. Both 2-octenal and hexanal were identified in the present systems. 

Scheme 4.9 Kinetic resolution of 19, a precursor to the natural product epothilone C, was accomplished by degrading the unwanted stereoisomer by an antibody-catalyzed retro-aldol reaction.

When sweet potato (Ipomoea batatas) is infected with the fungus Fusarium solani it produces ipomeamarone (31) and a number of furans (32)—(35) which are pulmonary toxins and which may be degraded mono- or sesqui-terpenoids. The discovery of 4-hydroxymyoporone (36) as a phytoalexin of sweet potato has led to the suggestion72 that the C9 furans may be formed from this by retro-aldol condensation. Radioactive (36) was transformed by F. solani into (32)—(35), and control... 

Based on these results, it might be concluded that, because of the high concentrations of carbohydrate degradation products in the HHP treated system, retro-Aldol reactions are favored, which could then lead to the formation of higher amounts of, in particular, the cyclopentenones. 

This segment was also obtained from anhydrodeacyltautomycin 5, formed by degradation of tautomycin 1, as shown in Scheme 13 (14). When we treated 1 with triethylsilyl triflate and 2,6-lutidine at 3°C, the silyl enol ether 74 was obtained. Considering this finding, we decided to mask the C2 carbonyl as an olefin before the retro-aldol reaction to prevent C3 epimerization. 

However, these findings were before the development of our current method to perform AAA reaction and complete the macrocycle. Hence, after Aaron had completed the synthesis of ustiloxin D, it seemed the most obvious route to ustiloxin A was to introduce iodine functionality on the dopa moiety at a late stage in the ustiloxin D synthesis, then incorporate the sulfinylnorvaline moiety. However, iodination of protected ustiloxin D 58 or the acyclic precursor 57 resulted in degradation through a retro-aldol reaction and destruction of the p-hydroxydopa residue and/or other pathways. 

Unsaturated aldehydes with a double bond conjugated to the carbonyl group are easily degraded during the deep frying process (Formula 3.95). Addition of water results in the formation of a 3-hydroxyaldehyde that is split by retro aldol condensation catalyzed by heat. Examples of this mechanism are the degradation of (E,Z)-2,6-nonadienal to (Z)-4-heptenal and acetaldehyde, as well as the cleavage of 2,4-decadienal into 2-octenal and acetaldehyde. 

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