Amino Acids and Peptides. XVI. Synthesis of N^G-Tosylarginyl Peptide Derivatives-Observation of Lactam Formation of Arginyl Residue

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Z-Arg (Tos) -Pro-NHNHBoc (1) and Z-Arg (Tos) -Val-NHNHBoc (2) were prepared by the DCC, DCC-HOBt, DCC-DNp, mixed anhydride and DPPA methods. In each coupling reaction, lactam formation from the N^G-tosylarginyl residue was observed, although the extent of formation was different depending not only on the carboxyl activation method but also on the kind of amino component. Addition of HOBt suppressed the formation of acylurea but did not suppress the formation of the lactam in the synthesis of 2. Addition of DNp suppressed the formation of the lactam slightly although it did not improve the yield of the target peptides. In the mixed anhydride method, a fairly large amount of the lactam was obtained in the synthesis of Z-Arg (Tos) -Pro-NHNHBoc, and a urethan-type derivative, N^α-isobutyloxycarbonyl-Pro-NHNHBoc, was also formed. However, in the case of synthesis of Z-Arg (Tos) -Val-NHNHBoc, lactam formation was suppressed compared with other activation methods and a small amount of urethan-type derivative was obtained. In both cases, the DPPA method gave a fairly good yield of the target peptide with only a trace amount of lactam formation.