Formation of diketopiperazine

Formation of Diketopiperazines. Esters of a-amino acids can be readily prepared by refluxing anhydrous alcoholic suspensions of a-amino acids saturated with dry HQ. Diketopiperazines are formed by heating the alcohohc solution of the a-amino acid ester. 

Cyclic structures can form as a result of side reactions. One of the most common examples is the formation of diketopiperazines during the coupling of the third amino acid onto the peptide chain (Fig. 7). Intramolecular amide bond formation gives rise to a cyclic dipeptide of a six-membered ring structure, causing losses to the sequence and regeneration of the hydroxyl sites on the resin. The nucleophilic group on the resin can lead to fiuther unwanted reactions [14]. 

This preparation is analogous to the formation of diketopiperazines from dipeptide esters. In a similar approach, morpholine-2,5-diones were obtained by acidolytic cleavage of N-(bromoacety 1)am ino acids from Wang resin (Entry 3, Table... 

The early work of Wessely suggested the occurrence of some side reactions which led to the formation of diketopiperazines (77) or hydan-toins (27). These reactions may put an upper limit to the degree of polymerisation which may be obtained in poly-additions of NCA s but it is more probable that these products arise from an unsuccessful initiation (see p. 40) rather than from a termination. Their participation in the process decreases the yield of C02 evolved in the polymerisation. 

Trityl resins are particularly suitable for immobilization of nucleophilic substrates such as acids, alcohols, thiols, and amines. They are quite acid-sensitive and are cleavable even with acetic acid this is useful when acid-labile protecting groups are used. The stability of trityl resin can be tailored by use of substituted arene rings, as shown by chlorotrityl resin, which furnishes a more stable linker than the trityl resin itself. Steric hindrance also prohibits formation of diketopiperazines during the synthesis of peptides. Orthogonality toward allyl-based protective groups was demonstrated in the reverse solid-phase peptide synthesis of oligopeptides [30] (Scheme 6.1.4). 

The procedure described is essentially that of Shioiri and Yamada.4 Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis.5 It has been used for racemlzation-free peptide syntheses,4 6 7 thiol ester synthesis, a modified Curtius reaction,6 9 10 an esterification of a-substituted carboxylic add,11 formation of diketopiperazines, an alkyl azide synthesis,13 phosphorylation of alcohols and amines,14 and polymerization of amino acids and peptides.15 Furthermore, diphenyl phosphorazidate acts as a nltrene source3 and as a 1,3-dipole.16 17 An example In the ring contraction of cyclic ketones to form cycloalkanecarboxyllc acids 1s presented 1n the next procedure, this volume. 

Fig. 1.7. The formation of diketopiperazine (I) and linear peptide (II), as catalysed by the MIP artificial enzymes of Patrikeev et al. [27].

The formation of diketopiperazines (DKP) is also possible when heating occurs before the ion formation in the mass spectrometric analysis of amino acids. These compounds will further form characteristic ions by the scheme ... 

The formation of diketopiperazine and Schiff bases as by-product in SPPS is reported [165. 167]... 

Fig. 5.39 Possible variations in molecular environment for isoleucine (ile after Mitterer Kriausakul 1984).With increasing diagenesis, hydrolytic cleavage will move interior amino acids to terminal positions in peptide chains and ultimately to free amino acids.The N-terminal unit has a free amino group, whereas the C-terminal analogue has a free carboxyl group.These two varieties of terminal groups are interconverted via the formation of diketopiperazines. HR = high epimerization rate LR = low epimerization rate HE = high epimerization extent LE = low epimerization extent.

Scheme 17.3 The formation of diketopiperazine (DKP) not only reduces the total yield but can also lead to increased impurities due to the de-loaded benzyl alcohol s involvement in side reactions.

Table 17.1 Comparison of coupling agents used to suppress the formation of diketopiperazine (DKP).

Scheme 70. Intramolecular attack by the side-chain amino group in cephalosporins in the pH range of 6-9, leading to formation of diketopiperazine degradants.

Brasiliamides are comprised of two phenylpropane moieties and acetates. The plausible biosynthetic pathway of brasiliamides starting with the formation of diketopiperazine ring is outlined in Fig. (23). Phenylalanine and 3-methoxy-4,5-methylenedioxyphenylalanine are combined into a diketopiperazine (111). Reduction of ketones, dehydration reaction, and rearrangement of the double bond lead to intermediates 112 and 113. Acetylation of the two compounds 112 and 113 leads to brasiliamide C (106) and brasiliamide B (105), respectively. The two compounds 112... 

Peptoid oligomers are synthesized on a Rink amide resin (50 /Ltmol scale) to avoid diketopiperazine formation. In order to suppress the formation of diketopiperazine during the synthesis of peptoids with a C-terminal carboxylic group, one can use the 2-chloro tritylchloride resin [155]. Following Fmoc removal, the resin is bromoacylated by successively adding a solution of bromoacetic acid (83 mg, 600 tmol, 12 equiv.) in DMF (830 fiL) and 200 fiL of DIG (103 /xL, 660 tmol, 13 equiv.) in DMF (170 >L) to the resin. The reaction mixture is shaken for 30 min at room temperature. A double coupling is performed systematically. The resin is then filtered and washed three times with 2 mL of DMF. The nucleophilic substitution step... 

Diketopiperazine (DKP), a cyclic dipeptide easily formed by the cyclodimerization of amino acid esters. The formation of diketopiperazines is an undesired cycliza-tion reaction by incorporation of the third amino acid by stepwise SPPS. The free amino group of the resin-bound dipeptide can attack the peptide-resin anchorage intramolecularly, resulting in the formation of diketopiperazine [J. R. Spencer etal., Int. J. Pept. Protein Res. 1992, 40, 282]. 

Thus far, no evidence of a diketopiperazine b anion has been reported, possibly because of the role of basic residues (which are not amenable to negative mode ionization) in the formation of diketopiperazine structures. 

Schemes Chemical inactivation of rHuMGDF by cyclization and cleavage of the first two iV-terminal amino acids, Ser and Pro. The degradation leads to the formation of diketopiperazine and des(Ser,Pro)rHuMGDF...

---