Base hydrolysis amino acid esters

Simple esters cannot be allylated with allyl acetates, but the Schiff base 109 derived from o -amino acid esters such as glycine or alanine is allylated with allyl acetate. In this way. the o-allyl-a-amino acid 110 can be prepared after hydrolysis[34]. The Q-allyl-o-aminophosphonate 112 is prepared by allylation of the Schiff base 111 of diethyl aminomethylphosphonates. [35,36]. Asymmetric synthesis in this reaction using the (+ )-A, jV-dicyclohex-ylsulfamoylisobornyl alcohol ester of glycine and DIOP as a chiral ligand achieved 99% ec[72]. 

Table 6.3 Effect of Metal Coordination on Rate Constants for Base Hydrolysis of Amino Acid Esters at 25 °C...

Essentially three different routes can be considered for the base hydrolysis of an amino acid ester in the presence of a metal ion (equations 8-10). In general terms hydrolysis of the monodentate N-coordinated ester (equation 8) would be expected to be somewhat similar to base hydrolysis... 

In order to assess the magnitudes of the catalytic effects of metal ions on the hydrolysis ol a-amino acid esters, it is necessary to have kinetic data on the base hydrolysis of such compounds... 

The chemistry involved can be illustrated by a specific example. Using a ligand such as ethylenediaminemonoacetate,55 ternary complexes with amino acid esters can be formulated as either (5) or (6). The ester ligands in the ternary complexes of edma undergo base hydrolysis some 103 times faster than the free unprotonated esters, although considerably lower effects occur with methyl L-histidinate (Table 5). The magnitude of these effects suggests that the amino acid... 

Activation parameters have been determined58 for the hydrolysis of a-amino acid esters in mixed ligand complexes (Table 6). For base hydrolysis of the complex [Cu(nta)-(NH2CH2C02Et)], AH = 20.5 kJ mol-1 and AS = -138 J K 1 mol-1. Catalysis in this system is primarily due to a substantial decrease in AH (by ca. 21 kJ mol1) compared with the free ligand. A detailed discussion of the activation parameters is available.58... 

An interesting correlation has been observed53 between the formation constant XCuL of the metal complex and its catalytic activity in a mixed ligand with an amino acid ester. Large values of XCUL (equation 13) lead to lower base hydrolysis rates in the ternary complex. The Lewis... 

Table 9 Rate Constants fcOH f°r the Metal Ion-promoted Base Hydrolysis of some Bidentate Amino Acid Ester Species...

The coordination of optically active amino acids and their methyl esters to nickel(II) complexes of l,2-bis(2-(5)-aminomethyl-l-pyrrolidinyl)ethane (24 R = H) and l,2-bis(2-(S)-N-methyl-aminomethyl-l-pyrrolidinyl)ethane (24 R=Me) has been studied.98 Some amino acidate ions coordinate stereoselectivity, as do their methyl esters, so that base hydrolysis of the esters proceeds stereoselectively. 

The base hydrolysis of cn-amino-acid esters is catalysed by transition metal ions, and the following reactions contribute to the mechanism ... 

When amines are snbstitnted for hydroxide as the base, there is no change in the rate of hydrolysis. This is becanse amines do not catalyze the hydrolysis of amino acid esters instead they prefer to add directly to the carbonyl carbon to form the corresponding amides. In this reaction the rate-limiting step is not the addition of amine, bnt rather the deprotonation of the coordinated amine by another base (eqnation 8). The of the tetrahedral intermediate is approximately 7 since almost any nonsterically hindered base with a pTTa above 7 can deprotonate it. " ... 

Base hydrolysis of amino acid esters 61-4.2.1.2 Monoamino esters... 

A method for the synthesis of optically active hydroxylamines from optically active primary amines with retention of configuration is based on the conversion of the amines to corresponding benzylimines followed by oxidation to 3-phenyloxaziiidines (74). Subsequent hydrolysis leads to hydroxylamines (Scheme 16). This sequence has been extended to the synthesis of optically active N-hydroxy-a-amino acid esters. ... 

Enzymic resolution is also generally useful. At first sight it is of restricted applicability, since most of the classical methods are based on the selectivity of a proteinase for catalysing the hydrolysis of the l enantiomer of an A-acyl derivative of a DL-amino acid (Equation (6.7)) or of a DL-amino acid ester. The normal substrates for these enzymes are derivatives of particular coded amino acids. 

This is based on observations by Grant and Alburn 164 that trypsin-catalyzed hydroxylaminolysis of amino acid esters was favored over hydrolysis in frozen reaction mixtures (for a review see Hansler and Jakubke 163 ). In 1990 Schuster et... 

Protein Hydrolysates. Instead of ethyl hippurate, a peptic hydrolysate of ovalbumin was used as substrate for the resynthesis reaction (64). This substrate (300 mg) was dissolved in water, adjusted to pH 6.0 with NaOH and to 0.9 ml with additional water. An amino acid ester was added to produce a 22.2mM solution and the mixture preincubated at 37°C for 15 min. Papain (3 mg), dissolved in 0.1M L-cysteine (0.1 ml), was combined with the above-mentioned preincubation mixture and incubation carried out at 37°C. After 2 hr, 0.1N NaOH (10 ml) was added to stop the enzymatic reaction and the resulting solution allowed to stand for 3 hr to hydrolyze completely the remaining amino acid ester as well as the ester group from the peptide product. The free amino acid produced from the base-catalyzed hydrolysis of the amino acid ester was determined with an amino acid analyzer. The amount of the amino acid incorporated was obtained by subtracting the determined value from the initial concentration of amino acid ester. The data obtained with the same L-amino acid esters as used in the model experiment (above) are plotted along the ordinate of Figure 3. An excellent correlation is found between the data from the model experiment and those from this experiment using a protein hydrolysate. In Table III data are shown for the extent of covalent incorporation after 2 hr of various amino acid ethyl esters into the protein hydrolysate. There is a close relationship between... 

The poly(siloxane) polymers are usually prepared by the acid or base hydrolysis of appropriately substituted dichlorosilanes or dialkoxysilanes, or by the catalytic polymerization of small ring cyclic siloxanes [71-75]. The silanol-terminated polymers are suitable for use after fractionation or are thermally treated to increase molecular weight and in some cases endcapped by trimethylsilyl, alkoxy or acetyl groups [76,77]. Poly(siloxanes) synthesized in this way are limited to polymers that contain substituent groups that are able to survive the relatively harsh hydrolysis conditions, such as alkyl, phenyl, 3,3,3-trifluoropropyl groups. Hydrosilylation provides an alternative route to the synthesis of poly(siloxanes) with labile or complicated substituents (e.g. cyclodextrin, oligoethylene oxide, liquid crystal, amino acid ester, and alcohol) [78-81]. In this case... 

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