| Popis: |
It was found by chromatographic, CD and NMR methods, that the thionation of piperazine-2,5-diones [cyclo(Aaa1-Aaa2) → cyclo (Aaat 1-Aaat 2 (Aaa=-NH-CHR-CO-; Aaat=-NH-CHR-CS-)] or piperazine-2,5-onthiones [cyclo(Aaat 1-Aaa2) → cyclo (Aaat-Aaat)] and, occasionally, even the spontaneous cyclization of endo-thiodipeptide esters [H-Aaat-Aaa-OR] result in enantiomeric (Aaa or Aaa=Gly) or diastereomeric mixtures of piperazine monothiones or dithiones. The diastereoisomers were separated by semipreparative HPLC and their quantitative product distribution was determined by an optimized HPLC method on Hypersil-silica column with CH2Cl2-EtOAc eluent mixtures. Isocratic RP-HPLC on ODS-Hypersil column and pre-column derivatization with 1-flu-oro-2,4-dinitrophenyl-5-L-alanine amide (Marfey's reagent) were used to monitor the racemization of Ala and Pro residues and to determine the ratio of enantiomers. Thionation of urethane protected dipeptide esters or dethionation of the corresponding endoth-iodipeptide de... |
