Amino esters, coordinated

Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

Coordinated a-amino amides can be formed by the nucleophilic addition of amines to coordinated a-amino esters (see Chapter 7.4). This reaction forms the basis of attempts to use suitable metal coordination to promote peptide synthesis. Again, studies have been carried out using coordination of several metals and an interesting early example is amide formation on an amino acid imine complex of magnesium (equation 75).355 However, cobalt(III) complexes, because of their high kinetic stability, have received most serious investigation. These studies have been closely associated with those previously described for the hydrolysis of esters, amides and peptides. Whereas hydrolysis is observed when reactions are carried out in water, reactions in dimethyl-formamide or dimethyl sulfoxide result in peptide bond formation. These comparative results are illustrated in Scheme 91.356-358 The key intermediate (126) has also been reacted with dipeptide... 

A two-point fixation of amino acids and amino esters in non-ionic forms via simultaneous metal-coordination and hydrogen bonding interactions was observed with RhCl(npOEP) which in CDC13 was found to extract 1 mol of free amino acids such as phenylalanine or leucine from water at pH 7 to form adducts irreversibly [284], the amino group forming a stable Rh-N bond. 

An ion-pair derived from the substrate and solid NaOH forms a cation-assisted dimeric hydrophobic complex with catalyst 39c, and the deprotonated substrate occupies the apical coordination site of one of the Cu(II) ions of the complexes. Alkylation proceeds preferentially on the re-face of the enolate to produce amino acid derivatives with high enantioselectivity. However, amino ester enolates derived from amino acids other than glycine and alanine with R1 side chains are likely to hinder the re-face of enolate, resulting in a diminishing reaction rate and enantioselectivity (Table 7.5). The salen-Cu(II) complex helps to transfer the ion-pair in organic solvents, and at the same time fixes the orientation of the coordinated carbanion in the transition state which, on alkylation, releases the catalyst to continue the cycle. 

An enantioselective one-pot, three-component imino-Reformatsky reaction has been reported.20 Combining a benzaldehyde, an aniline, and an alkyl bromoacetate ester, (g) ees of up to 92% have been achieved in the /i-amino ester product, using a recyclable A-methylcphedrinc as auxiliary. A nickel(II) salt and dimethylzinc are employed the latter serves as dehydrating agent, reductant, and coordinating metal. 

The results could be interpreted by assuming exclusive coordination of the N-acyldehydroamino acid moiety with the rhodium complex in which the rest of the molecule, i.e. the a-amino ester moiety, is located in the outer sphere of the chiral coordination site this may be the reason why virtually no double asymmetric induction was observed. However, a simple asymmetric hydrogenation using dppb as achiral ligand (Entry 10) disclosed preferential formation of Bz-( )Phe-( )Phe-OMe with 24.4% asymmetric induction, which is consistent with the result using Ac-APhe-( )Phe-OH as substrate (Entry 20). Accordingly, it seems that the results of using DIOPs are rather exceptional. In this context, we further looked at the effect of the chiral center on the catalytic asymmetric induction... 

The complexation of the amino esters to Ru(II) did not give any chiral recognition. For example, reaction of Ru(apap-P)(MeCN)i with 10 equiv. of rac-valine or -leucine esters yielded a racemic mixture of the corresponding bis (amino ester) complexes DD DL LL = I 2 1) however, chiral recognition to 52% (for leucine methyl ester) was observed for the complexation of the rac-leucine esters to Ru(apap-P)CO , and the oxidation of the amino esters by the chiral Ru(apaP P)(0)2 (or perhaps, more precisely, coordination of amino ester to the Ru imino ester complex)... 

Three-component Petasis reactions among salicylaldehydes, amines, and organo-boronic acids have been catalysed with a chiral thiourea BINOL catalyst to prepare alkylaminophenols with yields and ee values of up to 92% and 95%, respectively. Formation of amino esters Et02CCH(Ar)NR2 by Petasis combination of Et02CH0, R2NH, and ArB(OH)2, catalysed by Cu(I), is believed to involve transmetallation from boron to copper from which Ar is delivered intramolecularly to the iminium group of the coordinated intermediate. ... 

There are a few documented examples of studies of ligand effects on hydrolysis reactions. Angelici et al." investigated the effect of a number of multidentate ligands on the copper(II) ion-catalysed hydrolysis of coordinated amino acid esters. The equilibrium constant for binding of the ester and the rate constant for the hydrolysis of the resulting complex both decrease in the presence of ligands. Similar conclusions have been reached by Hay and Morris, who studied the effect of ethylenediamine... 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

LDL (apo B-lOO, E) receptors occur on the cell surface in pits that are coated on the cytosolic side of the cell membrane with a protein called clathrin. The glycoprotein receptor spans the membrane, the B-lOO binding region being at the exposed amino terminal end. After binding, LDL is taken up intact by endocytosis. The apoprotein and cholesteryl ester are then hydrolyzed in the lysosomes, and cholesterol is translocated into the cell. The receptors are recycled to the cell surface. This influx of cholesterol inhibits in a coordinated manner HMG-CoA synthase, HMG-CoA reductase, and, therefore, cholesterol synthesis stimulates ACAT activ-... 

Amino acid esters act as chelates to Co111 for example, the /3-alanine isopropyl ester is known as both a chelate and as an /V-bonded monodentate,983 and the mechanism of hydrolysis of the ester, which is activated by coordination, to yield chelated /3-alanine has been closely examined. 

The amino acid complexes [TcNCl(L)(PPh3)] (HL=L-cysteine, L-cysteine ethyl ester, cysteamine) have been prepared from [TcNC PPlfj ] or ASPI14 [TcNCU]/PPh3 [88]. The crystal structure of the L-cysteine ethyl ester complex 21 shows a Tc=N bond length of 1.605(3) A and the Tc atom displaced by 0.594(1) A above the square basal plane [88]. Other structurally characterized examples are the square pyramidal 22 with ONSP coordination and Tc=N 1.611(3) A [81], and 23 with NSPC1 coordination and Tc=N 1.615(7) A [89],... 

---