Aldimine bond

Breaking of the internal PLP-lysine aldimine bond by reaction with carbonyl reagents, hydroxylamine and aminooxyacetate was also accompanied by a strong diminution of the positive CD and by a reduction of the dissymmetry factor.44 It has been inferred that the... 

Diagrammatic representation of bovine rhodopsin embedded in the disk membrane. Only three of the seven a-helical segments are shown. The only point of attachment between retinaldehyde and opsin is the indicated aldimine bond. [Modified and reproduced, with permission, from D, F. O Brien, The chemistry of vision. Science 218,961 (1982), 1982 by the American Association for the Advancement of Science.]... 

The chelation of copper(II) with the Schiff base obtained from salicylaldehyde and glycine stabilizes the aldimine bond to such an extent that the metal chelate remains undissociated at pH 3, whereas the Schiff base itself is dissociated at this hydrogen ion concentration (76, 155). In this case, the formation of the bicyclic chelate ring system completely counteracts the electron-withdrawing effect of the metal ion. 

During maturation or aging of the living animal, collagen fibers strengthen and are further stabilized primarily by complex covalent bonds. Lysine, hydroxylysine, and histidine residues are heavily involved in the formation of these covalent bonds, that is, aldimine bonds between lysine and lysine or hydroxylysine (Balian and Bowes, 1977 Belitz et ah, 2004 Engel and Bachinger, 2005 Eyre and Wu, 2005 Nelson and Cox,... 

All the fibril-forming collagen t) es (t) e I, II, III, V, XI, XXIV, and XXVII collagens) are cross-linked through a mechanism based on the reactions of aldehydes derived from some lysine (or hydroxylysine) side chains. Histidine might also participate in the formation of a trivalent cross-link by reacting with an aldimine bond formed between a lysine aldehyde and hydroxylysine residue. 

Pyridoxal phosphate (PLP) acts as an amino carrier in transamination reactions. It is covalently bound to the a-amino group of a lysine residue in the enzyme by a Schiff base or aldimine bond, that is, by a carbon-nitrogen double bond between the a-amino group of the lysine and a carbon of PLP (see p. 641 of text). The enzyme catalyzes a... 

The preference of the nitrogen atom of the fragment =C(SR)NH2 to condense with an aldehyde rather than a ketone was also clearly shown in the case of a template reaction (Eq. 2.42) [105]. The course of this reaction implied hydrolysis of the aldimine bond of the starting complex, with the subsequent condensation of the aldehyde formed by the nitrogen atom and of the acetylacetone ketogroup by the isothiosemicarbazide nitrogen atom. 

It is remarkable that the photosensitivity of the insects, Crustacea, arthropods, cephalopods, together with the vertebrates and even the halophilic bacteria, is based on a common chemistry. In each organism, the pigment responsible for absorption of light consists of a retinaldehyde (Cl) prosthetic group coupled by a protonated aldimine bond to a membrane protein. The reactions following light absorption always involve cis-trans isomerization, a process that sometimes leads to deprotonation of the aldimine bond. 

If, however, hydrogen is present in the a-position of the iV-alkyl substituent, 2-alkyl-oxaziridines are easily decomposed by alkali. Base attack on this H atom effects 1,2-elimination at the C—N bond. From (86) and aldimine (87) forms, and a mixture of ammonia and two carbonyl compounds is finally obtained, one of them containing the carbon atom of the original oxaziridine ring, the other originating from the IV-alkyl group (57JA5739). 

Iminium salts can be made in a number of ways such as protonation of enamines (7), alkylation of aldimines and ketimines (5,9), cleavage of a covalent bond in a... 

Whereas vicinal hydroxy, mercapto, and hydroselenoaldimines of azoles strongly prefer the aminomethylene tautomeric form (Section II,E,2), their metal chelates 380 are characterized by.pronounced equalization of bond lengths within the chelate ring, which makes their structures similar to those expected for the aldimine tautomeric type. 

Depending on the steric bulk of the nitrogen bonded substituent, the reaction of 4 with pyridine-2-aldimines can also proceed by an intermolecular insertion of the silylene into the C-H bond of the acyclic CN group [19]. 

A ferrocenyl-based aldimine chelated dimethylzinc with its imino and oxygen atom to produce the tetrahedral dimethylzinc complex 44. Both zinc-carbon bonds are equidistant (1.974(2) A) and the zinc-oxygen donor bond (2.381(2) A) is substantially longer than the zinc-nitrogen counterpart (2.213(2) A) (Scheme 37).91... 

Ketimines were hydrogenated faster than aldimines, and electron-donating groups accelerated the rate of hydrogenation. The OH and RuH bonds are regenerated by hydrogen transfer to the unsaturated 16-electron Ru complex from isopropanol, generating acetone (Scheme 7.13). 

Other substrates with C=N double bonds frequently used in organic synthesis, such as aldimines and ketimines, will be considered in Section 10.7.5. 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

Furthermore, when trimethylsilylacetylene 40 was used as an alkyne in the [IrCl(cod)]2-catalyzed reaction, propargyUc amines (where the alkyne was added to the double bond of imine) were obtained (Equation 10.7) [21, 23]. It is probable that the reaction proceeds through oxidative addition of the terminal C—H bond of alkyne to the Ir complex, followed by the insertion of imine to the resulting Ir-H complex. The crosscoupling reachon of trimethylsilyl (TMS)-acetylene with aldimines took place by [IrCl(cod)]2, leading to the corresponding adducts (Equahon 10.8) [24]. 

Also known as Morita-Baylis-Hillman reaction, and occasionally known as Rauhut-Currier reaction. It is a carbon—carbon bond-forming transformation of an electron-poor alkene with a carbon electrophile. Electron-poor alkenes include acrylic esters, acrylonitriles, vinyl ketones, vinyl sulfones, and acroleins. On the other hand, carbon electrophiles may be aldehydes, a-alkoxycarbonyl ketones, aldimines, and Michael acceptors. 

The reversal of this process could potentially occur with reprotonation from either face of the C=N double bond, and a mixture of aldimines would result, leading to generation of a racemic amino acid. This accounts for the mode of action of PLP-dependent amino acid racemase enzymes. Of course, the enzyme controls removal and supply of protons this is not a random event. One important example of this reaction is alanine racemase, employed by bacteria to convert L-alanine into o-alanine for cell-wall synthesis (see Box 13.12). 

In transamination and racemization reactions, we have seen loss of a proton from the aldimine, i.e. breaking of bond a. Let us now consider the two... 

Breaking of bond b accounts for PLP-dependent decarboxylations. Decarboxylation of the intermediate aldimine is facilitated in the same way as loss of a proton in the transamination sequence. The pro-tonated nitrogen acts as an electron sink, and the conjugated system allows loss of the carboxyl proton. 

It is interesting to see how different products are formed according to which of the three different bonds is cleaved in the aldimine derived from an amino acid and PLP. There is one other point to ponder though. What determines the type of cleavage that occurs The answer must lie in the enzyme and... 

A new cinchona alkaloid-derived catalyst has been developed for the enantioselective Strecker reaction of aryl aldimines via hydrogen-bonding activation. For reference, see Huang, J. Corey, E. J. Org. Lett. 2004, 6, 5027-5029. 

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