Azlactone formation

The protecting group Y of the amine is generally an alkoxycarbonyl derivative since their nucleophilicity is low. Benzyloxy- or tert-butoxycarbonyl derivatives usually do not undergo azlactone formation. 

Azlactone Formation from P-Hydroxy-a-amino Acids... 

Protected amino acids with either a free amino or carboxyl function can usually be prepared by proven methods or are even commercially available. Therefore stages (i) - (iii) may be considered as simple routine nowadays, although great care must be taken that the protected starting materials are pure enantiomers. The reactions that cause most trouble are in stages (iv), (v) and (vii). In these stages an activated carboxyl group is involved and the chiral centre adjacent to it is at peril from racemization. A typical reaction which causes epimerization is azlactone formation. With acids or bases these cyclization products may reversibly enolize and racemize. Direct racemization of amino acids has also been observed. 

The best preventive measure against racemization in critical synthetic steps (e.g. fragment condensation, see p. 239) is to use glycine (which is achiral) or proline (no azlactone) as the activated carboxylic acid component. The next best choice is an aliphatic monoamino monocarboxylic acid, especially with large alkyl substituents (valine, leucine). Aromatic amino acids (phenylalanine, tyrosine, tryptophan) and those having electronegative substituents in the /7-position (serine, threonine, cysteine) are, on the other hand, most prone to racemization. Reaction conditions that inhibit azlactone formation and racemization are non-polar solvents, a minimum amount of base, and low temperature. If all precautions are taken, one still has to reckon with an average inversion of 1 % per condensation reaction. This means, for example, that a synthetic hectapeptide contains only 0.99100 x 100% = 37% of the fully correct diastereomer (see p. 233 f.). 

These reactions are, by definition, particular to amino acids and are valuable in giving access to a wide range of heterocyclic compounds (e.g. azlactone formation Figure 4.1). 

Azlactones. Formation of dies acetic anhydride in the presence of KF Desulfonylation. Sulfonates an dry state on microwave irradiation. 

Peters applied the cooperative activation by a soft bimetallic catalyst 220, a hard Br0nsted acid, and a hard Bronsted base to the formation of highly enantio-enriched, diastereomerically pure masked a-amino acids 225 bearing adjacent tetrasubstituted and tertiary carbon stereocenters on the basis of a domino azlactone formation/Michael addition reaction starting from N-benzoylated amino acids 221 and a,P-unsaturated ketones 180 (Scheme 11.47). Since the activated catalyst was stable toward acetic anhydride, the in situ formation of azlactones 223 could be achieved through O-acylation with acetic anhydride of N-benzoylated amino acids... 

So that after reaction in an organic solvent, it is possible to remove any unreacted carbodiimide and the urea simply by washing with water. It should be noted that significant racemization, via azlactone formation of the anhydride-like intermediate, can occur during the synthesis of a large peptide (protein) molecule. However, for a smaller polypeptide, this is minimal. 

The carboxylate function will readily add to the cation, after which mixed anhydride formation will occur. This will in turn react with the amino function of a second amino acid to give peptide bond formation. Further, the mixed anhydride so formed does not accumulate in solution (its formation is rate limiting) but instead suffers immediate nucleophilic attack by the amine. Azlactone formation does not have a chance to occur, and so no significant racemization is observed during polypeptide synthesis. The mixed anhydride formed will be attacked by the second amino acid only at one of the two carbonyl functions, giving carbon dioxide and ethanol by-products. The reason for this has been discussed earlier (see peptide bond formation via acid anhydrides). 

Sodium acetate also increases the rate of azlactone formation in glacial acetic acid solutions, but no preparative application has been made of this observation. 

Carter, H. E., and C. M. Stevens Azlactone Formation in Glacial and in Aqueous Acetic Acid and Preparation of Benzoyl-a-amino-crotonic Acid Azlactone II. J. Biol. Chem. 133, 117 (1940). 

---