Dipeptide ester complexes, preparation

Table VI lists a number of dipeptide ester complexes prepared via aminolysis in Me2SO and isolated using ion-exchange chromatography many others have been obtained from similar syntheses. Crystal structures are available for A-[Co(en)2((S)-Ala-CR)-Phe)]Br3 H20 (26), obtained from reaction of A-[Co(en)2((S)-AlaOMe]3+ with (i )-PheOMe and acid hydrolysis (Fig. 1), and for A-[Co(en)2((S)-Leu-(S)-Leu OMe)]Cl3 4H20 (24), A-[Co(en)2((S,fl)-Ala-(S)-ValOMe)](C104)3 (24), and /3-[Co(trien)(Gly-GlyOEt)](C104)3 H20 (10). These show considerable variation in chelate 0-Ci-C2-N dihedral angles (0-35°) (10) and it remains to be seen whether this property is important to epimerization (at C2) in these species.

In the same line of thought, Lehn and Sirlin (148) have prepared a chiral macrocyclic molecular catalyst bearing cysteinyl residues. The catalyst complexes primary ammonium salts and displays enhanced rates of intramolecular thiolysis of the bound substrates with structural selectivity for dipeptide esters and high chiral recognition for the L-enantiomer (70 times faster) of a racemic mixture of glycylphenylalanine p-nitrophenyl esters. The representation below shows the complex between the chiral crown ether and the dipeptide glycylglycine p-nitrophenyl ester salt. 

Aminolysis of methoxy carbene metal pentacarbonyl complexes of the chromium group yields amino carbene compounds. If the aminolysis is performed with amino acid esters, amino carbene derivatives of amino acids are obtained [162,163]. The metal carbene is relatively stable under a variety of conditions but can be removed with TFA. It may therefore be regarded as an amino protecting group in peptide synthesis. The metal amino carbene amino acid may be activated and coupled to other amino acid esters. Following this idea, Weiss and Fischer prepared various dipeptides, the tripeptide (OC)5Cr(Ph)-Ala-Ala-Ala-OMe 69 [162] and the tetrapeptide (OC)5Cr(Ph)-Gly-Gly-Pro-Gly-OMe 70 (Scheme 5.35) [163]. Treatment with cone. TFA at 20 °C for 10 min. removes the metal carbene group and furnishes the free peptide esters along with Cr(CO)g. Removal of the metal carbene is also possible with 80% acetic acid (80 °C, 30 min.), which leaves Boc... 

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