5-Oxazolones peptides

Carbamates can be used as protective groups for ammo acids to minimize race-mization in peptide synthesis. Raccmi/ation occurs during the base-catalyzed coupling reaction of an W-protected, catboxyl-uc ivated amino acid, and takes place in the intermediate oxazolone that foro S readily from an N-acyl protected amino... 

The azlactones of a-benzoylaminocinnamic acids have traditionally been prepared by the action of hippuric acid (1, Ri = Ph) and acetic anhydride upon aromatic aldehydes, usually in the presence of sodium acetate. The formation of the oxazolone (2) in Erlenmeyer-Plochl synthesis is supported by good evidence. The method is a way to important intermediate products used in the synthesis of a-amino acids, peptides and related compounds. The aldol condensation reaction of azlactones (2) with carbonyl compounds is often followed by hydrolysis to provide unsaturated a-acylamino acid (4). Reduction yields the corresponding amino acid (6), while drastic hydrolysis gives the a-0X0 acid (5). ... 

The last comprehensive review of the chemistry of oxazolones covered the literature through 1954. Most of the studies up to that time stemmed from either interest in the role of azlactones as precursors of a-amino acids and peptides or the monumental studies on penicillin, which, for a time, was thought to possess an oxazolone ring, rather than the correct jS-lactam moiety. 

Difficulties are often encountered in the formation of peptides from a-amino acids which lack an a-hydrogen atom, e.g. a-methylalanine, presumably because of steric hindrance. This problem is obviated by use of the oxazolone 35, an excellent reagent for the addition of a single a-methylalanyl residue to an amine or amino acid [Eq. (22)]... 

Conjugate addition of strong nucleophiles to the >C=N—C=C< moiety, followed by ring opening of the resulting saturated 5 4H)-oxazolone. Thus, 57 reacts with simple or peptidic amino acid esters [Eq. (31)]. Similarly, 62 gives 63 in methanolic 7i-propylamine, and... 

Unfortunately, in many cases the reaction is not so straightforward it becomes complicated because of the nature of the activated component. There is another nucleophile in the vicinity that can react with the electrophile namely, the oxygen atom of the carbonyl adjacent to the substituted amino group. This nucleophile competes with the amine nucleophile for the electrophilic center, and when successful, it generates a cyclic compound — the oxazolone. The intermolecular reaction (path A) produces the desired peptide, and the intramolecular reaction (path B) generates the oxazolone. The course of events that follows is dictated by the nature of the atom adjacent to the carbonyl that is implicated in the side reaction. 

COUPLING, 2-ALKYL-5(4H)-OXAZOLONE FORMATION AND GENERATION OF DIASTEREOISOMERS FROM ACTIVATED PEPTIDES... 

FIGURE 1.9 Coupling, 2-alkyl-5(4//)-oxazolone formation and generation of diastereoiso-mers from activated peptides. 

Peptide-bond formation between an A -alkoxycarbonylamino acid and an amino-containing component usually proceeds in the same way as described for coupling an Y-acylamino acid or peptide (see Section 1.9), except for the side reaction (Figure 1.7, path B) of oxazolone formation. Aminolysis of the activated component (Figure 1.10, path A) gives the desired peptide. There are three aspects of the side reaction... 

When Wa = substituted aminoacyl, that is, when Wa-Xaa is a peptide, there is a strong tendency to form an oxazolone. The 2-alkyl-5(4//)-oxazolone that is formed is chirally unstable. Isomerization of the 2-alkyl-5(4//)-oxazolone generates diaste-reomeric products. When Wa = R0C=0, there is a lesser tendency to form an oxazolone. The 2-alkoxy-5(4/7)-oxazolone that is formed is chirally stable. No isomerization occurs under normal operating conditions. Finally, when Wa = R0C=0, an additional productive intermediate, the symmetrical anhydride, can and often does form. 

NL Benoiton, FMF Chen. 2-Alkoxy-5(4/7)-oxazolones fromlV-alkoxycarbonylamino acids and their implication in carbodiimide-mediated reactions in peptide synthesis. Can J Chem 59, 384, 1981. 

FIGURE 2.14 Peptide-bond formation from chlorides of A-alkoxycarbonylamino acids. N-9-Fluorenylmethoxycarbonylamino-acid chlorides.41 The base is NaHCO, Na2C03, or a tertiary amine. The reaction is carried out in a one- or two-phase system. The latter is used to try to suppress formation of the 2-alkoxy-5(4//)-oxazolone that is generated by the action of the base on the acid chloride. The method is applicable primarily to Fmoc-amino-acid derivatives that do not have acid-sensitive protecting groups on their side chains. 

FIGURE 2.20 Intermediates and their fate in benzotriazol-l-yl-oxyphosphonium salt-mediated reactions. Indirect evidence (Figure 2.21) is not compatible with the tenet that the precursor of the peptide is the benzotriazolyl ester (path C). The evidence indicates that the peptide originates from the acyloxyphosphonium cation (path B). Conversion of this intermediate into the oxazolone (path E) can account for the epimerization that occurs during segment couphngs. 

FORMING REACTIONS TO THE COUPLING OF N-PROTECTED PEPTIDES IS DICTATED BY THE REQUIREMENT TO AVOID EPIMERIZATION 5(4H)-OXAZOLONES FROM ACTIVATED PEPTIDES... 

The chemistry of the reactions involved in coupling peptides is the same as that for coupling TV-alkoxycarbonylamino acids. However, the oxazolone that is formed by the activated peptide is chirally unstable, it is formed more readily, and there is an added impetus for it to form because the rate of bond formation between segments is lower. In addition, segments usually have to be coupled in polar solvents because they are insoluble in nonpolar solvents, and polar solvents promote the undesirable side reaction. The result is that the number of procedures actually used for coupling peptides is rather small. The methods in question are addressed below. 

Symmetrical anhydrides (see Section 2.05) There is no evidence that anhydrides of peptides exist. Methods that might be used to produce them generate oxazolones. 

P Wipf, H Heimgartner. 2. Synthesis of peptides containing a,a-disubstituted a-amino acids by the azirine/oxazolone method. Helv Chim Acta 73, 13, 1990. 

Additives increase efficiency in carbodiimide-mediated reactions by preventing intermediates from undergoing side reactions and by transforming them into activated esters that become the precursors of the peptide products. A-hydroxysuccinimide and 4-hydroxy-3-oxo-3,4-dihydrobenzotriazine are the best nucleophiles or acceptors of activated species. They trap oxazolones before they have time to isomerize. The popularity of HONSu as an additive has diminished considerably during the... 

M Goodman, L Levine. Peptide synthesis via active esters. IV. Racemization and ring-opening reactions of optically active oxazolones. JAm Chem Soc 86, 2918, 1964. 

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