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The synthesis of cyclo (c)-cys-ser, c-cys-asp and c-cys-try via cyclisation of the corresponding cystin-dipeptidesters and subsequent reduction of the disulfides of the diketopiperazines with mercapto-ethanole is described. The ir-spectra of the cyclo-dipeptides and their disulfides were studied with regard to interactions of the functional groups. The spectra indicate a strong hydrogen bridge between the OH-groups of c-cys-ser-disulfide and interactions between the SH and other functional groups of the above mentioned diketopiperazines. The kinetics of the reaction of cysdiketopiperazines with N-ethylmaleimide and p-nitro-phenylacetate were studied. The SH-groug of cys-diketopiperazines has a greater nucleophilic reactivity towards p-nitro-phenylacetate than was expected on the basis of the pK SH -values.

Synthese und Eigenschaften unsymmetrischer Diketopiperazine des Cysteins / Synthesis and Properties of Unsymmetrical Diketopiperazines of Cysteine