Double bonds, keto conjugated

Conjugation has a great influence on the structure of aliphatic and alicyclic compounds. Thus, the existence of an amino form has been established (in addition to the extreme case of aromatic amines17) for all compounds where the double bond is conjugated with a carbonyl group (or its equivalent),12 with esters12,18 and nitriles of a,/8-un-saturated /8-amino acids,519,20 and with /8-amino-ketones. The /3-keto-esters ethyl 2-cyclopentanone- 1-carboxylate and ethyl 2-cyclo-hexanone-l-carboxylate exist as mixtures containing 95% of the keto... 

The lipids of some plant species contain special FA not found in other plant families (see Table 11.6). They may contain a double bond in a different position (Diedrich and Henschel, 1991a), trans double bonds or conjugated double bonds, and acetylenic bonds (Diedrich and Henschel, 1991b), or are substituted with a keto, epoxy, or hydroxy group or even alicyclic or aromatic substituents. 

Thermal oxidation of unsaturated fats is accompanied by considerable isomerization of double bonds, leading to products containing trans double bonds and conjugated double bond systems. Non-volatile decomposition products are formed by thermal oxidation, hydrolysis and cyclization. A large number of hydroxy, alkoxy-substituted, epoxy and keto compounds are produced by oxidation, free fatty acids and mono- and diacylglycerols by hydrolysis, and cyclic monomers by cyclization of polyunsaturated fatty acids in heated fats. Aldehydo glycerides (also referred to as core aldehydes ) and keto glycerides... 

Figure 3.6 Top Keto-enol tautomerization in acetone. The equilibrium for this molecule strongly favors the keto-tautomer. Bottom The tautomeric equilibrium for 2,4-pentanedione (acetylacetone) favors the enol form because the double bond is conjugated with the remaining carbonyl group and the hydrogen of the enol can form a hydrogen bond with the carbonyl oxygen that creates a very stable six-membered ring.

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

Reprotonation then produces a new imine (keti-mine), and also restores aromaticity in the pyridine ring. However, because of the conjugation, it allows protonation at a position that is different from where the proton was originally lost. The net result is that the imine double bond has effectively moved to a position adjacent to its original position. Hydrolysis of this new imine group generates a keto acid and pyridoxamine phosphate. The remainder of the sequence is now a reversal of this process. This now transfers the amine function from pyridoxamine phosphate to another keto acid. 

Unsaturated epoxides are reduced preferentially at the double bonds by catalytic hydrogenation. The rate of hydrogenolysis of the epoxides is much lower than that of the addition of hydrogen across the carbon-carbon double bond. In a, -unsaturated epoxides borane attacks the conjugated double bond at -carbon in a cis direction with respect to the epoxide ring and gives allylic alcohols [660], Similar complex reduction of epoxides occurs in a-keto epoxides (p. 126). 

In an interesting reaction, -santonin was reduced with lithium in liquid ammonia so that the lactone was hydrogenolyzed to an add and one of the double bonds conjugated with the carbonyl was reduced. The other double bond as well as the keto group did not undergo reduction [1091]. 

Molecules in which the enolic double bond is in conjugation with another double bond. Some of these are shown in Table 2.1. As the table shows, carboxylic esters have a much smaller enolic content than ketones. In molecules like acetoacetic ester, the enol is also stabilized by internal hydrogen bonding, which is unavailable to the keto form ... 

In neutral medium A16-, A17(20)-, and A20-olefins are hydrogenated over palladium in preference to the double bonds of A4-3-ketones.67 The double bonds of A4-3-ketones and A16-20-ketones are reduced in preference to A5-,93 A7-48 155 and A9(I -olefins.2 169 The double bond of a A14-16-ketone is saturated before a 5-ene.70,163 In basic medium the carbonyl conjugated double bond is preferentially saturated in all cases92 presumably because of the strong adsorption of the enolate anion (see section II-C). Debromination (9a-bromo 11-ketone) occurs before saturation of the A4-3-keto double bond14 over Raney nickel but hydrogenation of benzyl ethers takes place concurrently with the hydrogenation of this double bond over palladium in neutral medium.96... 

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