5 -Oxazolones 2-alkoxy

COUPLING OF N-ALKOXYCARBONYLAMINO ACIDS WITHOUT GENERATION OF DI AST E REOI SOM E RS CHIRALLY STABLE 2-ALKOXY-5(4H)-OXAZOLONES... 

FIGURE 1.10 Coupling of IV-alkoxycarbonylamino acids without generation of diastereoi-somers. Chirally stable 2-alkoxy-5(4fl)-oxazolones. 

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. 

ALKOXY-5(4H)-OXAZOLONES AS INTERMEDIATES IN REACTIONS OF A/-ALKOXYCARBONYLAMINO ACIDS... 

FIGURE 1.18 2-AI koxy-5(4/7)-oxazol ones as intermediates in reactions of IV-alkoxycarbo-nylamino acids.22 After removal of the symmetrical anhydride from a reaction mixture containing Boc-valine and ethyl-(3-dimethylaminopropyl)-carbodiimide hydrochloride, the filtrate contained a novel activated form of Boc-valine (20% yield) that was established to be the 2-alkoxy-5(4H)-oxazolone. Slow addition of Boc-valine to ethyl-(3-dimethylamino-propyl)-carbodiimide hydrochloride in dilute solution gave a 55% yield. Petrol = petroleum ether, bp 40-60°. 

V-alkoxycarbonylamino add (Figure 1.10, path B) did not occur without immediate expulsion of the alkyl group, giving the amino-acid Af-carboxyanhydride (see Section 7.13). 2-Alkoxy-5(47/)-oxazolones are now recognized as intermediates in coupling reactions and are products that are generated by the action of tertiary amines on activated A-alkoxy carbonyl amino adds (see Section 4.16).20 22... 

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. 

More /V-acylurea is generated if tertiary amine is present because the latter removes any protons that might prevent the rearrangement (see Section 2.12). The two intramolecular reactions also occur to a greater extent when interaction between the O-acylisourea and the /V-nucleophile is impeded by the side chain of the activated residue. This means that more 2-alkoxy-5(4//)-oxazolone and /V-acylurea are generated when the activated residues are hindered (see Section 1.4). A corollary of the above is that the best way to prepare an /V-acylurea, should it be needed, is to heat... 

That being said, it must be recognized that the evidence that the (V-acy I isourea is the precursor of the 2-alkoxy-5(4//)-oxazolone is only circumstantial because experiments starting from the former have yet to be achieved. The oxazolone could theoretically come from the symmetrical anhydride. The latter generates 2-alkoxy-5(4//)-oxazolone in the presence of tertiary amines (see Section 4.16) even dicy-clohexylcarbodiimide (DCC) was basic enough to generate 2-t< rt-butoxy-5(4 )-oxazolone from Boc-valine anhydride. However the weight of evidence points to O-acylisourea as the precursor of the 2-alkoxy-5(4//)-oxazolone. In the absence of. V-nucleophile, such as in the preparation of esters, the major precursor of product is the symmetrical anhydride.7,8... 

NL Benoiton, FMF Chen. Preparation of 2-alkoxy-5(4//)-oxazolones from mixed anhydrides of /V-alkoxycarbonylamino acids. Ini J Pept Prot Res 42, 455, 1993. 

Just as protonation of the O-acylisourea enhances its electrophilicity, and consequently its consumption, it can be postulated that the reactivity with nucleophiles of any 2-alkoxy-5(4//)-oxazolone that is formed is enhanced by its protonation. Thus, it is reasonable to assert that a beneficial effect of HOBt in improving efficiency in couplings of /V-alkoxycarbonylamino acids is protonation of the oxazolone (see Section 2.25) that facilitates its consumption.432... 

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. 

The existence of a protonated oxazolone has been demonstrated indirectly by a simple experiment. When p-nitrophenol was added to an excess of 2-alkoxy-5(4//)-oxazolone in dichloromethane, a yellow color appeared. The color persisted until all the p-nitrophenol had been consumed by the oxazolone. The anion of p-nitro-phenol is yellow. The explanation for the color of the mixture is the presence of the p-nitrophenoxide anion that was generated by abstraction of the proton by the oxazolone. In summary, protonation of the O-acylisourea suppresses the side reaction of oxazolone formation as well as the side reaction of A-acylurea formation and accelerates its consumption by enhancing its reactivity and generating an additional good nucleophile that consumes it. Protonation of the oxazolone suppresses epimerization by preventing its enolization and also increases the rate at which it is consumed.4 68 78 79... 

NL Benoiton. 2-Alkoxy-5(47f)-oxazolones and the enantiomerization of IV-alkoxy-carbonylamino acids. Biopolymers (Pept Sci) 40, 245, 1996. 

FIGURE 4.5 Enantiomerization of a residue by base-induced enolization of the 2-alkoxy-5(4//)-oxazolone formed during coupling of iV-alkoxycarbonylamino acids. 

FIGURE 4.14 Reactions of activated A-alkoxycarbonylamino acids in the presence of tertiary amine. Acyl halides and mixed and symmetrical anhydrides generate 2-alkoxy-5(4/7)-oxazolone in the presence of tertiary amine. Aminolysis of 2-alkoxy-5(47f)-oxazolone in the presence of E N led to partially epimerized products. OAct = activating group. 

FIGURE 4.15 Enantiomerization during aminolysis of 2-alkoxy-5(4A)-oxazolones in the presence of Et3N.24 Percentage -d-l- Epimer formed in reaction with H-Lys(Z)-OBzl.HCl/A-methylmorpholine in CH2C12. 

FIGURE 7.6 Decomposition of a mixed anhydride (A) to the 2-alkoxy-5(4//)-oxazolone and the alkyl carbonate.9 The latter is in equilibrium with the anion whose reaction (B) with a second molecule of anhydride produces pyrocarbonate and the acid anion whose reaction (C) with a third molecule produces the symmetrical anhydride. The oxazolone eventually reacts with the alcohol to give the ester. (D) Acyloxonium ions formed by reaction of the anhydride with dimethylformamide and tetrahydrofuran. 

FIGURE 7.34 Decomposition of the symmetrical anhydride of A-methoxycarbonyl-valine (R1 = CH3) in basic media.2 (A) The anhydride is in equilibrium with the acid anion and the 2-alkoxy-5(4//)-oxazolone. (B) The anhydride undergoes intramolecular acyl transfer to the urethane nitrogen, producing thelV.AT-fcwmethoxycarbonyldipeptide. (A) and (B) are initiated by proton abstraction. Double insertion of glycine can be explained by aminolysis of the AA -diprotected peptide that is activated by conversion to anhydride Moc-Gly-(Moc)Gly-0-Gly-Moc by reaction with the oxazolone. (C) The A,A -diacylated peptide eventually cyclizes to the IV.AT-disubstituted hydantoin as it ejects methoxy anion or (D) releases methoxycarbonyl from the peptide bond leading to formation of the -substituted dipeptide ester. 

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