Linear and Cyclic Enkephalin Pseudopeptides. Use of RP-HPLC to Separate and Identify Diastereomers Formed during Synthesis

Synthesis of diketopiperazines with on-resin N-methylation and cyclative release

Enantiomerically pure N-methylated diketopiperazines (DKP) can be obtained by treating a N-methylated resin-bound dipeptide with 20% piperidine in dimethylformamide via a process known as cyclative release. N-methylated resin-bound dipeptides can be formed from N-methylated precursors or N-methylation can be selectively performed on the resin. When on-resin N-methylation was performed on the C-terminal side of the dipeptide, diastereomers were formed. Yet the cyclative release is shown to be a stereoselective process, as seen using preformed N-methylated amino acids. The procedure was also applied to synthesize the pseudodiketopiperazine cyclo(Phepsi[CH2NH]Leu). When comparing nonmethylated, monomethylated and bismethylated derivatives, we find that N-methylation results in a dramatic increase in solubility.

Determination of the chiral purity of dipeptide isosteres containing a reduced peptide bond by gas chromatographic analysis

The stereochemical purity of psi (CH2-NH) dipeptides has been determined using gas chromatography-mass spectrometry. Different structures were found due to the derivatization procedures. A selective preparation of the linear bistrifluoroacetylated derivative and the monotrifluoroacetylated lactam makes it possible to monitor the chiral purity of the pseudodipeptides synthesized. Racemization occurring during peptide hydrolysis can be differentiated from racemization during the synthesis by using deuterium labelling. The method allows the optimization of the synthesis protocols and will be useful for further monitoring of the chiral purity of the pseudopeptides synthesized.