Urethanes peptide synthesis

FMF Chen, Y Lee, R Steinauer, NL Benoiton. Mixed anhydrides in peptide synthesis. Reduction of urethane formation and racemization using A-methylpiperidine as tertiary amine base. J Org Chem 48, 2939, 1983. 

WD Fuller, MP Cohen, M Shabankareh, RK Blair, M Goodman, FR Naider. Urethane-protected amino acid A-carboxy anhydrides and their use in peptide synthesis. J Am Chem Soc 112, 7414, 1990. 

WD Fuller, M Goodman, FR Naider, Y-F Zhu. Urethane-protected a-amino acid /V-carboxyanhydndes and peptide synthesis. Biopolymers 40, 183, 1996. 

RB Merrifield, AR Mitchell, JE Clarke. Detection and prevention of urethane acylation during solid-phase peptide synthesis by anhydride methods. J Org Chem 39, 660, 1974. 

The protection of amino groups of amino sugars benefits particularly from the use of new blocking groups introduced for peptide synthesis. In this context, light-sensitive urethans and amides that can be utilized for the protection of amino groups in amino sugars are of particular interest in saccharide synthesis and modification. 

In Section 7.3, the subject of amino acid analysis is covered including a brief description of hydrolytic techniques, column preparations, and data analysis 7,8 This is followed by a discussion of racemization assays (Section 7.4). These areas of analysis are critically important for researchers in the field of peptide synthesis 9-11 In Section 7.4, a systematic approach is outlined for the study of racemization and mechanisms are discussed for epi-merizations at asymmetric sites. A comparative study is presented for determining the intrinsic rates of racemization, based upon urethane-protected V-carboxy an hydrides (UN-CAs). These approaches are extremely important for understanding the tendencies of novel amino acids and other building blocks to racemize. 

Zang, J.Y., Beckman, E.J., Piesco, N.P., and Agrawal, S., A new peptide-based urethane polymer synthesis, biodegradation, and potential to support cell growth in-vitro. Biomaterials 21 1247-1258, 2000. 

In a parallel study, Wipf and Fritch11041 have shown that also urethane-protected (Boc), and even amino acid segments, are tolerated as acyl compounds on the aziridine nitrogen. The best results were obtained with alkylcopper reagents derived from CuCN and an alkyl-lithium in the presence of boron trifluoride-diethyl ether complex. Some 6-alkylated compounds (11-15%) were isolated as well. This work was extended to a solid-phase procedure that resulted in resin-bound alkene isosteres that could immediately be used in further peptide synthesis.11051 For this purpose, the 2-nitrophenylsulfonyl (oNbs) group was used for nitrogen protection and aziridine activation. It could be readily cleaved with benzenethio-late, which was compatible with the acid-sensitive Wang linker used. 

The different cleavage conditions for the above urethane protecting groups have enabled so-called orthogonal protection strategies to be developed, which in turn enable selective deprotection to be performed on different amines present in the same molecule. For example in peptide synthesis, the N-Boc group could be cleaved selectively using TMSOTf, followed by aqueous work up. 

Peptide synthesis. In the above-mentioned synthesis of urethanes the carboxylic acid azide may be the intermediate, and this possibility prompted the Japanese chemists to investigate the usefulness of diphcnylphosphoryl azide in peptide synthesis. Indeed the reagent allows coupling of acylamino acids or peptides with amino acid esters or peptide esters in the presence of a base in high yield and with practically no racemiza-tion. The new method is compatible with various functional groups. 

The use of ethoxycarbonyl-protected amino acids as well as methoxycarbonyl derivatives for peptide synthesis was reported in 1903 by Fischer, although the protecting groups in related peptide derivatives could not be cleaved without affecting the peptide bonds,f since urethanes derived from aliphatic primary alcohols are about as stable as the amide bond. Thus, this type of carbamate can only be used for reversible protection of amino groups, at least in... 

In addition to phosphonium salts, which are widely used for the activation of urethane-protected amino acids in peptide synthesis (see Section 3.7), other organophosphorus reagents derived from phosphinic, phosphonic, and phosphoric adds have proved to be efficient coupling reagents. 

Although the first application of acid chlorides in peptide synthesis was reported in the 1930s, the tendency for urethane-protected amino acid halides to convert into oxazol-5(4//)-ones, which are less reactive intermediates, has sometimes negated the benefits from employing this kind of active species. 

The preparation of acyl azides has received considerable attention due to the value of these compounds as synthetic intermediates. In the Gurtius rearrangement for example, acyl azides arc converted into isocyanates, urethans, ureas and amines and this aspect of the chemistry of acyl azides is considered in detail in a later chapter. The use of acyl azides in peptide synthesis has increased the scope of general... 

Several groups are useful for protecting amino groups, where the deprotection is a reductive elimination, Eq. (12), in which Y is an urethane and X a halogen [80,180]. Thus, for example, iodoethoxycarbonyl, trichloroethoxycarbonyl, and tribromoethoxycarbonyl are of practical importance in peptide synthesis. 

With the exception of glycine, all proteinogenic a-amino acids are chiral. The prevention of racemiza-tion in the case of the most commonly used a-protected amino acids is a prerequisite of successful peptide synthesis. W-acylamino acids might lose their stereochemistry according to the mechanism shown in Scheme 1. The proper choice of the protection group (e.g. urethane-derived), activation method and additives has to be made carefully. 

In peptide synthesis the use of a suitable protection for the N-terminal amino group is required not only to prevent the formation of a complex mixture of oligo- and cyclo-peptides, but an additional demand on the functionality applied for this purpose is that it should prevent possible racemization of the activated amino acid. Racemization usually takes place via an intermediate oxazolone (7) that forms readily from A -acyl-protected amino acids (Scheme 2). This side reaction can be mostly suppressed by using a carbamate as an N-terminal-protecting group. Therefore, nearly all blocking functions currently applied in this field are of the urethane type. 

It is not only benzyl-type urethanes which are widely used for the blocking of amino groups. This goal may also successfully be achieved by the application of alkyl urethanes. Among these, the f-butoxycar-bonyl group (t-BOC or Boc), introduced in 1957, deserves special mention. Today it probably is the most frequently used amino-protecting function in peptide synthesis. The t-BOC group can be easily... 

Chen, F. M. F. Steinauer, R. Benoiton, N. L., Mixed Anhydrides in Peptide Synthesis. Reduction of Urethane Formation and Racemization Using N-Methylpiperidine as the Tertiary Amine Base. J. Org. Chem. 1983, 48, 2939 Bock, M. G. DiPardo, R. M. Evans, B. E. Rittle, K. E. Veber, D. F. Frei-dinger, R. M., An Expedient Synthesis of 3-Amino-l,3-dihydro-5-phenyl-2H-l,4-benzodiazepin-2-one. Tetrahedron Lett. 1987, 28,939. 

Another means of overcoming the sterically very demanding coupling reactions is to reduce the bulkiness of the residues, inherent to the presence of both the C"ra-disubstitu-tion, and the urethane protecting group. This has been demonstrated by the use of a-azi-do acids in which the azide is the precursor of the amino function [113]. These monomers can be activated as acid chlorides, and their preparation is also compatible with the presence of the side chain protecting group used in Fmoc-based peptide synthesis. This approach has been exploited in the solid-phase synthesis of a-aminoisobutyric acid (Aib)-rich peptides [114]. 

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