Carboxyanhydride method

Similarly, reversed-phase HPLC can be used as an Eilternative to the racemization test for amino acids as developed by Manning and Moore (115). Rivier and Burgus (109) have suggested the use of L-phenylalanine, coupled via the N-carboxyanhydride method to a hydrolysate, to monitor racemization during synthesis, although other hydrophobic L-amino acids should also prove equally effective. The use of /eri-butyloxycarbonyl-L-amino acid-Af-hydroxysuccinimide esters in the separation of enantiomeric amino acids and diastereoisomeric peptides has been described (110). Ultimately, these methods may not prove as versatile as the use of chiral stationary phases made by stereoselective control of the bonding process or, alternatively, with surface-active reagents similar to the D-... 

A quite different approach with respect to the support backbone was taken by Bleeder and Pfaender using pol)water-soluble support [37, 38, 103]. They synthesized the modd peptide Ala-Trp-Ile-Arg in aqueous phase by appUcation of the iV-carboxyanhydride method. The peptide was attached through an arginine as anchoring group for the peptide to the polymer... 

Currently, a-amino acids are prepared by several routes such as by the fermentation of glucose, by enzyme action on several substances and by the hydrolysis of proteins. Many methods for synthesising the polymers are known, of which the polymerisation of A -carboxyanhydrides is of particular interest, as it yield-products of high molecular weight (Figure 18.24). 

Kramer JR, Deming TJ (2010) General method for purification of alpha-amino acid-N-carboxyanhydrides using flash chromatography. Biomacromolecules 11 3668-3672... 

The most general and frequently used method to synthesize long chains of block copolypeptides for vesicle assembly is successive ring opening polymerizations of a-amino acid-Ai-carboxyanhydride (NCA) monomers [18, 20, 21, 39-51]. NCA monomers are readily prepared from commercially available amino acids, most commonly through direct phosgenation [52]. 

Pyridine (1) is a weak and good base and good solvent for effecting aminolysis of acyl fluorides (see Section 7.12) and for preparing Boc-amino-acid iV-carboxyanhydrides (see Section 7.14) and activated esters by the carbo-diimide method (see Section 7.7), especially the esters of Boc-amino acids, as it prevents decomposition of the activated residue (see Section 7.15). It is the preferred base for aminolysis of acyl fluorides in dichloromethane. 

Observations Until a method not requiring A-carboxyanhydride for the formation of... 

Ab A) measured by CO evolution method as a function of time for an Afl 400 sodium methoxide initiated polymerization of y-benzyl-L glutamate-N-carboxyanhydride... 

The polymerisation of a-aminoacid A-carboxyanhydride, which is accompanied with the elimination of carbon dioxide, constitutes a convenient method for preparing high molecular weight polypeptide. This polymerisation, with the use of a-aminopropionic acid (alanine) A-carboxylic acid anhydride as a monomer, was also carried out in the presence of coordination catalysts such as group 2 and 3 metal alkyls [168-174] or their combinations with water, secondary amine or alcohol [168,173] and yielded polyalanine (Table 9.2). 

An alternative route for the generation of enantiopure oligopeptides has been elucidated recently by our group. The method comprises the self-assembly of racemic or non-racemic thio-esters or N-carboxyanhydrides of a-amino acids into either 2-D or 3-D crystalline architectures followed by lattice-controlled reactions. 

The N-Carboxy-Amino Acid Anhydride Method. Since the isopeptide bond of e-methionyllysine was hydrolyzed readily by intestinal aminopeptidase and the released amino acid was biologically available, we decided to further increase the amount of covalently attached methionine through a polymerization reaction. The most suitable amino acid derivative for this approach is the N-carboxyanhydride or Leuchs anhydride. 

Synthesis and Polymerization of Leuchs Anhydrides. The N-carboxy-a-amino acid anhydrides, referred to as Leuchs anhydrides or as NCAs, are synthesized either from N-alkoxycarbonyl derivatives of a-amino acids or from free amino acids. Cyclization of the amino acid derivative by SOCl2 or similar reagents was described first by Leuchs in 1906, but direct synthesis involves treating the a-amino acid or its hydrochloride with phosgene as shown in Figure 4. This method, commonly termed the Fuchs-Farthing method, is the one that is used most frequently since the N-carboxyanhydride may be freed easily from the carbamyl chloride, isocyanate derivative, and hydrogen chloride by crystallization. 

The stepwise addition of N-carboxy a-amino acid anhydrides to amino acids or peptides in organic solvents is well documented (for review see Refs. 69 and 70). More recently the method has been applied to peptide synthesis in aqueous solutions (71,72,73). Novel syntheses of N-substituted carboxyanhydrides of a-amino acids have been carried out (74,75) and the crystal molecular structures of several of them have been determined (76,77,78). 

For efficient peptide bond formation acid halides azides 2,b l and Leuchs anhydridesb l were employed as the first activated species in peptide synthesis. Since then, besides considerable improvements to the azide (see Section 3.1) as well as the V-carboxyanhydride 7 procedure d (sgg Section 3.4.3), the methods have evolved over decades along a few basic principles as outlined in Scheme 2. The symmetrical the mixed carboxylic acid 4,b 5s] and the carbonic acid anhydrides were developed and remain useful despite the... 

Acylation of anoino acids or peptides with Z-protected amino acids can be performed with aU known methods including the a-amino acid Al-carboxyanhydrides (NCAs), since protocols for the preparation of the highly reactive Z-protected NCAs are now available (see Section 3.4.2).t l Moreover, since formation of oxazol-5(4//)-ones is very slow with urethane-type protecting groups, racemization, at least by this mechanism, is largely suppressed (see Sections 1.2.1.2 and 7.4).[ 5.n,52]... 

Scheme 6 The Leuchs Method for Preparation of a A -Carboxyanhydride Reaction of a A -Benzyloxycarbonyl-Protected Amino Acid with an Acid Halide Generating Agent (X-A) Followed by Cyclization To Give the NCA and a Benzyl Halide ...

Scheme 11 The Bailey Method for the Synthesis of Peptides from Ai-Carboxyanhydrides at Low Tempera-...

In order to prepare Asp- or Glu-containing peptide fragments, in which side-chain carboxy groups are unblocked, one method is to introduce aspartic or glutamic acid into the peptide through its A-carboxyanhydride. Another method is to introduce aspartic or glutamic acid with the side chain protected as the benzyl ester and then to remove the benzyl ester by YiJ Pd and finally convert the C-terminal methyl ester into the corresponding hydrazide. 

Methods employing pre-activated species have also been utilized, but they need to be highly activated, such as the active esters OPfp,t l OObbt, and OTdo.bh The latter two are useful for color-based monitoring of acylation reactions. Also included in this class are Fmoc amino acid Al-carboxyanhydrides,t l acid chlorides,and acid fluorides. 1 1 The latter can also be formed in situ using TFFH.b ... 

Blecher and Pfaender used polyethylenimine (PEI) of molecular weight 30000 as a water-soluble support for the synthesis of a model tetrapeptide This polymer is designed for use in combination with the N-carboxyanhydride coupling method. 

Blacklock,T. J. Shuman, R. F. Butcher, J. W Shearin,W. E.,Jr. Budavari, J. Grenda,V., Synthesis of Semisynthetic Dipeptides Using N-Carboxyanhydrides and Chiral Induction on Raney Nickel. A Method Practical for Large Scale. J. Org. Chem. 1987, 53, 836 Pradhan, A. A. Vera, J. H., Effect of acids and bases on the solubility of amino acids. Fluid Phase Equilibria 1998, 152, 121. 

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