Racemization, dipeptides

Lehn and Sirlin found a very high degree of enantiomeric differentiation in the reaction of [327] with racemic dipeptide p-nitrophenyl esters [Gly-(DL)-Phe-OPNP]. Gly-(L)-Phe-OPNP reacts at approximately the same rate as the achiral Gly-Gly-OPNP, but the D-enantiomeric dipeptide ester is converted at a... 

Gagne and coworkers utilized this combination to discover enantioselec-tive receptors for (-)-adenosine [12]. A racemic dipeptide hydrazone [( )-pro-aib] generated a stereochemically diverse DCL of n-mer. The dimers were composed of two chiral (DD/LL) and one achiral isomer (DL), the four trimers (DDD, LLL, DDL, and LLD), the tetramers of four chiral and two achiral isomers, etc. Two techniques were used to measure the enan-tio-imbalance that was caused by the enantioselective binding of the chiral analyte to the enantiomeric receptors (Fig. 5.11). Since the unperturbed library is optically inactive, the optical enrichment of each library component could be measured by a combined HPLC optical rotation detection scheme (laser polarimeter, LP). LP detection differentiated unselective binding (amplification but not optical enrichment) from enantioselective recognition of the analyte (amplification and optical enrichment). In this manner the LL dimer (SS) of the dipeptide was amplified and identified as the enantioselective match for (-)-adenosine. 

Synthetic pathways towards the dimethylene isosteres comprise several disconnections, indicated by the essential bond forming reactions (Scheme 2). Several methods yield racemic dipeptide analogues. These are usually incorporated into the peptide sequence and the resulting epimeric peptides are separated. When either R1 or R2 = H, asymmetric syntheses towards the required enantiomer are available. When both R1 and R2 H, only the reduction of the i )[CH=CH] precursor yields homochiral compounds. As many co-amino acids (R1 = R2 = H) are commercially available, their synthesis needs not be discussed here. 

Among the first reported synthetic methods for alkene isosteres, a sigmatropic rearrangement of oxidatively activated allylic selenides to provide Boc-protected allylic amines was used for the synthesis of the D,L-Tyn i[is, CH=CH]Gly isostereJ711 The method resulted in a racemic dipeptide isostere, and only a Gly residue at the C-terminus is possible. It is no longer competitive compared with more recent methods using rearrangement of allylic tri-chloroacetimidates. 

The organs of various animals, such as the dog and rabbit, would thus appear to differ in their power of making use of synthetical polypeptides, but the animal organism as a whole is not so selective as the enzyme of the pancreas which hydrolyses the racemic dipeptide asymmetrically in the body the racemic compound is completely burnt up, since no dipeptide composed of the optically active variety of the amino acid not occurring in a protein could be isolated. Further, the animal organism is able to utilise polypeptides not hydrolysed by pancreatic juice, so that if such polypeptides are present in the protein, they can still be utilised by the body although unaffected by trypsin. 

If, however, the /7-nitrophenyl ester of iV-henzoyl-L-leucine is treated with 1-methyl-piperidine in chloroform for 30 min and then coupled with glycine ethyl ester, the dipeptide isolated is almost completely racemic. Furthermore, treatment of the p-nitrophenyl ester of iV-benzoyl-L-leucine with 1-methylpiperidine alone leads to the formation of a crystalline material, C13H15NO2, having strong IR bands at 1832 and 1664 cm . Explain these observations, and suggest a reasonable stmcture for the crystalline product. 

First, they compared CSPs 1 and 3 prepared by the two-step solid-phase methodology with their commercially available counterparts (CSPs 2 and 4) obtained by direct reaction of the preformed selector with a silica support. Although no exact data characterizing the surface coverage density for these phases were reported, all of the CSPs separated all four racemates tested equally. These results shown in Table 3-3 subsequently led to the preparation of a series of dipeptide and tripeptide CSPs 5-10 using a similar synthetic approach. Although the majority of these phases exhibited selectivities lower or similar to those of selectors built around a single amino acid (Table 3-3), this study demonstrated that the solid-phase synthesis was a... 

Fig. 3-7. Evaluation of a focused library of 71 DNB-dipeptide CSPs for enantioseparation of the test racemate 8. (Reprinted with permission from ref. [86], Copyright 1999, American Chemical Society.)...

Based on chiral functional monomers such as (15), MICSPs can be prepared using a racemic template. Thus, using racemic A-(3,5-dinitrobenzoyl)-a-methylbenzy-lamine (16) as template, a polymer capable of racemic resolution of the template was obtained [67]. Another chiral monomer based on L-valine (17), was used to prepare MIPS for the separation of dipeptide diastereomers [68]. In these cases the configu-... 

The very first investigations on this topic indicated that racemization can be monitored in micro reactors and that the degree of racemization seems not to be higher than in conventional organic synthesis. For dipeptide formation from the penta-fluorophenyl ester of (Ji)-2-phenylbutyric acid and (S)-a-methylbenzylamine, racemization of 4.2% was found [158]. At higher concentration (0.5 M instead of 0.1 M), a higher degree of racemization was found (7.8%). 

OS 26] [R 4] [P 18] For dipeptide formation from the pentafluorophenyl ester of Boc-D-alanine and (S)-a-methylbenzylamine an extent of racemization of 5.6% was found [86]. This experiment also served to demonstrate monitoring of the racemization of an a-amino acid used in peptide synthesis. 

Racemization studies in the synthesis of the tripeptide Z-Gly-Phe-Gly from Z-Gly-Phe and Gly-OC2H5 revealed that in THF at room temperature such racemization occurred to the extent of about 5%, in DMF at -10 °C, however, less than 0.5%. 53 103 In the synthesis of Boc-Val-Tyr-OC2H5 (50%) from Boc-Val and Tyr-OC2H5 with CDI, a small amount of 0-acylation of tyrosine (4%) also occurred in the dipeptide. 11] A V -Carbonyldibenzimidazole was found inferior to CDI in the synthesis of peptides because of poorer yields and more rigorous reaction conditions needed. 53... 

Application of amino acid silyl esters or A-silyl amino acid silyl esters as amino components is very convenient in peptide synthesis with CDI, because the resulting peptide silyl esters are easily hydrolyzed to dipeptides during the usual work up. They need not be saponified in a separate step, as would be the case with the corresponding alkyl esters. Furthermore, no racemization occurs with this method.tl8],tl9]... 

Another -activation of amino acids for peptide synthesis is achieved by preparing sulfenamides from sulfenylimidazoles. A sulfenylimidazole is formed in situ from the sulfenyl chloride (prepared from the disulfide and chlorine) and imidazole, which reacts further with an amino acid ester to give a sulfenamide in high yield. Conversion of such sulfenamides with IV-acyl amino acids by means of triphenylphosphine affords dipeptides with racemization of less than 0.5%.[481... 

Christensen16 isolated the dipeptide valylvaline from completely hydrolyzed gramicidin. This worker later showed that he had isolated a racemic mixture of D(-)-valyl-D(-)-valine and L(+)-valyl-L(4)-valine rather than dipeptides containing one d and one L-residue.17... 

NL Benoiton, K Kuroda, ST Cheung, FMF Chen. Lysyl dipeptide and tripeptide model systems for racemization studies in amino acid and peptide chemistry. Can J Biochem 57, 776, 1979. 

The development of chiral separation methods for dipeptides is of relevance for purity controls, for checking racemization processes in peptide syntheses, and for the investigation of peptide and protein hydrolysates. Since their introduction as chiral... 

Cyclo(Gly-Gly) was the first cyclic dipeptide synthesized and elucidated, since then a wide variety of members of the cyclic dipeptide family have been synthesized by various methodologies. The cyclic dipeptides were initially prepared by the action of ammonia on the free dipeptide esters, liberated from the corresponding amine salts. The long duration of exposure to ammonia, required by the free dipeptide esters to effect cyclization, lead to extensive racemization when employing optically pure linear dipeptide precursors. Early cyclic dipeptide synthetic methods have involved the following ... 

Sequence inversion and racemization have been associated with uncatalyzed formation of the cyclic dipeptides and has been shown to greatly complicate the kinetics of formation. Cyclic dipeptide formation, by uncatalyzed processes, is rapid enough to pose an apparent threat to the stability of proteins and a possible rationale for the posttranslational N-acetylation of proteins that have been observed in higher organisms. The rate of DKP formation will also depend on the carbonyl ester protecting groups or the structures of the peptide-resin linkage in the solid-phase mode. Furthermore, cyclization is a concentration-independent reaction and demands the use of dilute solutions. ... 

Figure 3.25 Racemic dynamic combinatorial library targeting (-)-adenosine. Left Racemic porline containing dipeptide library building block. Middle Various enantiomeric oligomers formed through reversible hydrazone exchange. Right The SS enantiomer selected upon equilibration with (-)-adenosine.

The Phe-Gly isostere (Z)-5 was prepared starting with the aldehyde (7b) (Scheme 2). The A/-Boc protected dipeptide isostere (Z)-5 was obtained as a racemate. 

The authors applied the same synthetic strategy to racemic 4-alkyl -(iodo-methyl)-2-phenyl-5(4/ )-oxazolones 266 and obtained a diastereomeric mixture of oxazolines 267 and 268 (Scheme 7.86). The diastereoisomers were separated chromatographicaUy and then converted into dipeptides incorporating an a-alkyl-serine residue. ... 

Racemic cw-l-amino-2-phenylcyclohexanecarboxylic acid 693 can be prepared by Diels-Alder reaction of (Z)-4-benzyhdene-2-phenyl-5(47/)-oxazolone 621 and butadiene in an analogous manner. Coupling A-tert-butyloxycarbonyl-L-proline with 693 yielded diastereomeric dipeptides that were separated chromatographi-cally. The behavior of the individual dipeptides was studied as a means to effect p-tum modulation by such cyclohexane analogues of phenylalanine. ... 

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