Aldimines, Formation

The intervention of a metal ion in the stoichiometry of a reaction has been illustrated several times previously. Reaction is forced to completion in ester hydrolysis since the carboxylate grouping forms a more stable complex than the ester moiety does. A similar driving force underlies the formation of macrocycles and the completion of transamination by formation of the metal-Schiff base complex. The latter is particularly relevant in dilute solution and at low pH. For example, the extent of aldimine formation between pyridoxal and alanine is undetectable at the physiological pH but occurs to the extent of = 10% in the presence of zinc... 

Such intermediates are known to form between substrate and enzyme in the aldolase reaction and between pyri-doxal 5-phosphate and the amino group of enzyme or substrate in aminotransferase reactions. In the latter case, aldimine formation accounts for the high affinity of coenzyme binding to apotransaminases. 

An example of the polarographic determination of equilibrium constants can be taken from a study of ketimine and aldimine formation,... 

Metal-catalyzed Addition of Diboron Reagents to Aldimines Formation of Alpha-aminoboronate Esters... 

This unique behavior of Yb(OTf)3 was extended to a novel three-component coupling of aldehyde, amine, and several nucleophiles in organic solvents (Scheme 2-34) [71]. The aldimine formation is much faster than the nucleophilic addition to aldehydes. Synthetically, this in situ tandem transformation proved to be of significant value for the one-pot synthesis of /f-lactam skeletones (Scheme 2-35) [72],... 

Rudnick and Abeles purified proline racemase to 95% homogeneity from Clostridium sticklandii, and characterized it 92. The enzyme is composed of two identical subunits with a molecular weight of about 38000, and is independent of any cofactors or metals. Most amino acid racemases require pyridoxal 5 -phosphate, which labilizes the bond between the a-hydrogen and the chiral center by aldimine formation with the a-amino group of the substrate. However, PLP is not involved in the reaction of proline racemase acting on an a-imino acid. The enzyme also acts on 2-hydroxy-L-proline and 2-allo-hydroxy-D-proline although slowly they are epimer-ized at a rate of 2 and 5% of the rate of L-proline racemization, respectively. L-Proline and D-proline showed Km values of 2.9 and 2.5 mti, respectively1119. ... 

More recently, an alternative inactivation mechanism of KAPAS (AON synthase) by trifluoroalanine has been proposed (Figure 40)." " The authors suggested that the decarboxylation and the loss of fluoride occur immediately after the aldimine formation. The derived 2,3-unsaturated imino species is then attacked by the enzyme residue Lys236 and a water molecule, generating a gem hydroxyfluoro species which loses the last fluoride ion and forms a stable amide. 

Other Polymers.—A polymeric gel, in which salicylaldehyde and lysine residues are capable of forming an internal aldimine, may be prepared by copolymerization of iV-tf-5-methacryloylaminosalicylidene-A-e-methyacryloyl-(iS )-lysinatocopper(ii) with acrylamide and AW -methylenebisacrylamide in water with subsequent removal of copper(n) ions. The equilibrium constants for internal aldimine formation are 30-100-fold higher than that for a suitable model. The er-amino group of the L-lysine residue in the gel acts as nucleophilic catalyst in the reaction of the salicylaldehyde residue of the gel with semicarbazide, with a rate enhancement of... 

Cyclopentanone and cyclohexanone can also be determined in complex mixtures by using the formation of ketimines, from 20 per cent solutions of hexamethylenediamine, at a pH above j2.(4oi>) The aldehydes, when present, give aldimine waves at potentials about 0-2 V more positive than the ketimine waves. However, moderate concentrations of aldehydes do not interfere in the determination of ketones as (at the concentrations of hexamethylenediamine used) a substantial part of the aldimine is converted into a polarographically inactive compound which, during the course of the reaction, is competitive to the aldimine formation. For the determination of aldehydes, a 2 per cent solution of hexamethylenediamine proved to be the most suitable. 

The aldimine formation has also been applied to the determinations of cardiac glycosides which bear an aldehyde grouping in position 2. Glycine (2 m) dissolved in 1 m NaOH proved best as a supporting electrolyte. )... 

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