Chloroacetyl amino acids

In 2001, 3-unsubstituted 4-alkyl-4-carboxy-2-azetidinones have been reported to be prepared by base-assisted intramolecular alkylation of /V-benzvI -/V-chloroacetyl amino acid derivatives [180]. /V-benzyl or /V--methoxvbenzvI) amino acid derivatives in THF, treated with propylene oxide and chloroacetyl chloride afforded the /V-chloroacetyl amino acid derivatives. The treatment of the latter in CH3CN with CS2CO3 (or NaH) produced the intramolecular cyclization of 4,4-disubstituted (3-lactams, (Scheme 81). 

The /V-alkyl-/V-chloroacetyl amino acid derivatives [180] have been reported to undergo the base-promoted cyclization to 3-lactams [194]. The stereoselectivity, due to memory of chirality, was highly dependent on the substituents of the starting amino acids. The amino acid side-chain (R3) appeared to be the principal stereodirecting element, offering additional support for the explanation that the memory of chirality was caused by a hindered rotation around the C-N bond (Scheme 87). 

The substrate specificity of acylase is very broad, and a wide range of pro-teinogenic and nonproteinogenic Af-acetyl and A-chloroacetyl amino acids are transformed by the enzyme. The enzyme membrane reactor (Figure 4) is operated continuously as a recycle reactor, and the enzyme is retained by a UF hollow-fiber membrane (MWCO 10000). 

The main application of the enzymatic hydrolysis of the amide bond is the en-antioselective synthesis of amino acids [4,97]. Acylases (EC 3.5.1.n) catalyze the hydrolysis of the N-acyl groups of a broad range of amino acid derivatives. They accept several acyl groups (acetyl, chloroacetyl, formyl, and carbamoyl) but they require a free a-carboxyl group. In general, acylases are selective for i-amino acids, but d-selective acylase have been reported. The kinetic resolution of amino acids by acylase-catalyzed hydrolysis is a well-established process [4]. The in situ racemization of the substrate in the presence of a racemase converts the process into a DKR. Alternatively, the remaining enantiomer of the N-acyl amino acid can be isolated and racemized via the formation of an oxazolone, as shown in Figure 6.34. 

An example is the intramolecular stereoselective alkylation of (carbonylamino)acetonitrile derived from 1-arylethanamines as chiral inductors. Thus, base-catalyzed cyclization of [alkyl-(chloroacetyl)amino]acetonitriles led to /1-lactams which have been transformed into (R)- or (S)-aspartic acid. Optical purity ranged from 21 to 67%65. 

Cyclization of a dipeptide having a hydroxylamine unit at the N-terminus is the second general method for the synthesis of 1-hydroxypiperazine-2,5-diones. Thus, Japanese workers have reported that the N-bromoacetyl derivatives (210) obtained from the corresponding dehydro amino acid esters, on treatment with hydroxylamine, give low yields of 1-hydroxy-3-alkylidenepiperazine-2,5-diones (211) (78BCJ550). The corresponding iodo compounds lead to better yields, whereas the chloroacetyl derivative does not cyclize under these conditions. 

A general approach towards the asymmetric synthesis of amino acid derived 4-alkyl-4-carboxy-2-azetidinones has been described [192], The (+)- or (-)-lO-(N, Af-dicyclohexylsulfamoyl)isobomeol was used as chiral auxiliary in the intramolecular cyclization of /V-(/>methoxybenzyI)-/V-chloroacetyl Phe and Ala derivatives for the stereocontrolled base-catalyzed construction of the (1-lactam ring (Scheme 85). 

Reaction of potassium 9-fluoro-3-methyl-10-(4-methylpiperazin-l-yl)-2,3-dihydro-7H-pyrido[l,2,3-de][l,4]benzoxazine-6-carboxylate with 2-[3-(2,4-dichlorophenoxy)propyl]-3-[(2-chloroacetyl)amino]quinoxa-lin-4(3H)-one gave ester derivative of 2,3-dihydro-7H-pyrido[l,2,3-de] [l,4]benzoxazine-6-carboxylic acid. 3-Amino-2-(aryloxymethyl)quinox-alin-4(3H)-ones were N-acylated with 9-fluoro-3-methyl-10-(4-methyl-piperazin-l-yl)-2,3-dihydro-7H-pyrido[l,2,3-de][l,4]benzoxazine-6-car-boxylic acid chloride in the presence of K2C03 in boiling benzene for 6 h (07MI71). 

Radiolabeled 3-demethyl-3-chloroacetylthiocolchicine with a l4C label in the chloroacetyl moiety (DCTC) was found to be a potent inhibitor of tubulin polymerization and of colchicine binding to tubulin. The reaction was 80-90% inhibited in the presence of saturating, amounts of known antitubulin compounds such as podophyllotoxin, combretastatin A-4, and colchicine itself. The tubulin /3 subunit was labeled 5-6 times faster than the a subunit. Cyanogen bromide digestion of the /3 subunit which had reacted covalently with DCTC indicated that at least three positions in /3-tubulin had reacted with DCTC. Purification and amino acid sequencing of these peptides are in progress (138). 

I(chloroacetyl)amino]-(x-(methoxyimino)-4-thiazole-acetic acid... 

The N-hydroxy amino acid derivatives are likely to be applicable to other metalloproteases. Thermolysin is inhibited irreversibly at pH 7.2 by ClCH2CO-DL-HOLeu-OCH3 where HOLeu is N-hydroxyleucine (47). The inhibition reaction involves coordination of the hydroxamic acid functional group to the active-site zinc atom of the enzyme. This then places the chloroacetyl group adjacent to Glu-143, an essential catalytic residue of thermolysin (see Figure 9). An ester linkage is formed and the enzyme is inactivated irreversibly. This reagent also inactivated two neutral metalloproteases from B. subtilis, but reacted only very slowly with carboxypeptidase A (t1/2 > 3 d). 

However, 0.5 g p-thiocresol and 0.5 mL m-cresol were used as scavengers as recommended in the literature for chloroacetylated peptide compounds (33). The crude peptide was purified by RP-HPLC and, after lyophilization, was characterized by analytical HPLC, mass spectrometry, and amino acid analysis. The pure product was obtained at 65% yield. 

Chloroacetylated oligotuftsin H-[Thr-Lys-Pro-Lys(ClAc)-Gly]4-NH2 was dissolved in 0.1 M Tris-HCl buffer (pH 8.2) in a peptide concentration of 1 mg/ mL. The 20-mL tube used for the reaction was filled up fully and closed tightly. The epitope peptide synthesized by the procedure presented in Subheading 3.1.1. (H- LKN1cADPNRFRGKDL22C-NH2) was added to the mixture in 1-2 mg aliquots every 15 min (see Note 5). In this way low concentrations of the epitope peptide prevented disulfide bond formation. The coupling was followed by analytical HPLC that showed the incorporation of the epitope peptide. The reaction was completed in 2 d using two equivalents of epitope peptide to each chloroacetyl group (see Note 6). The branched chimeric peptide was purified on RP-HPLC (60% yield) (see Note 7) and after lyophilization the purified product was characterized by analytical HPLC, amino acid analysis and mass spectrometry. 

Table 2. Intramolecular cycloaddition of chloroacetylated alkynes 5a-g derived from amino acids. ...

It is well known that thermal polycondensation of ha-logenoacetates leads to polyglycolide through the elimination of a metal halide salt, which can be considered as the driving force of the polycondensation reaction. The method has recently applied to get PEAs constituted by an alternating sequence of glycolic acid and co-amino acid units (Figure 8.3a) [20-24]. Monomers be easily synthesized by the reaction of chloroacetyl chloride with the appropriated (o-amino acid and by a subsequent neutralization with a... 

L-Homoserine (2-amino-4-hydroxybutyric acid) [672-15-1] M 119.1, m 203 , [a] +18.3 (in 2M HCl), pKj st(i) 2.1, pK Est(2) 9.3. Likely impurities are V-chloroacetyl-L-homoserine, V-chloroacetyl-D-homoserine, L-homoserine, homoserine lactone, homoserine anhydride (formed in strong solutions of homoserine if slightly acidic). It cyclises to the lactone in strongly acidic solution. It crystallises from water by adding 9 volumes of EtOH. [Greenstein Winitz The Chemistry of the Amino Acids J. Wiley, Vol 3 pp 2612-2616 1961, Beilstein 4 IV 3187.]... 

The C-7 hydroxyl group is in many ways the most accessible functional group on the taxane ring system, and is second in reactivity only to the C-2 hydroxyl group in the side chain. For this reason its chemistry has been studied fairly extensively, and many simple derivatives have been prepared. Thus the acetyl derivative 4.1.1.1 (140,141), benzoyl derivative 4.1.1.2 (142), glutaiyl derivative 4.1.1.3 (143), amino acid derivatives 4.1.1.4-4.1.1.6 (144), chloroacetyl derivative 4.1.1.7 (145), and a docosahexaenoic acid derivative 4.1.1.8 (146) have all been prepared and found to have comparable activity to taxol. 

There are considerable differences in the hydrolysis rates of different amino acids. If the rate is too low for practical purposes, then the chloroacetyl derivatives of the racemates can be applied as substrates instead of the acetyl derivatives. Of course, it is often worthwhile to recover the unchanged D-acylamino acid and hydrolyze it with aqueous acid to produce the D-enanthiomer of the amino acid. The unnatural D isomers are frequently used as building components in studies of structure-activity relationships, in the preparation of hormone analogs resistant to the action of proteolytic enzymes and in the synthesis of microbial peptides. 

The above facts strongly support the unusual role of N-hydroxy-amino acids as amino acid antagonists. Powers stated that the methyl ester of chloroacetyl-N-hydroxyleucine (50) 108) and the formyl-N-hydroxyleucyl-alanyl-glycine amide (51) 109) are the first specific non-reversible termolysine inhibitors. They act through coordination of the zinc atom of this metalloprotein, as has been determined by X-ray crystallography 110). Compound (51) is also an inhibitor of elastase 111). 

Aminoacylases catalyze the hydrolysis of A-acyl amino acid derivatives, with the acyl groups preferably being acetyl, chloroacetyl, or propionyl. Alternatively, the corresponding A-carbamoyl- and A-formyl derivatives can be used [132], Enzymes of the amino acylase type have been isolated from hog kidney, and from Aspergillus or Penicillium spp. [133-135]. The versatility of this type of enzyme has been demonstrated by the resolution of racemic iV-acetyl tryptophan, -phenylalanine, and -methionine on an industrial scale using colunm reactors (Scheme 2.15) [136, 137]. 

---