Studies on lactam formation during coupling procedures of N alpha-N omega-protected arginine derivatives

We evaluated the quantity of delta-lactam generated during the synthesis of arginine-containing dipeptides using Z-Arg(Tos)-OH, Boc-Arg(Tos)-OH, Fmoc-Arg(Boc)2-OH and Fmoc-Arg(Pmc)-OH and assayed several carboxyl-activating procedures for coupling the protected arginines to different amino components. We observed significant amounts of delta-lactam during the synthesis of Z-Arg(Tos)-methyl ester and Z-Arg(Tos)-amide, as well as of Boc-Arg(Tos)-chloromethyl ketone. The mixed anhydride coupling procedure and the di-Boc-protecting guanidino group induced more delta-lactam formation than any other coupling or NG-protection method. The amide, benzyl, 4-(NO2)-benzyl and methyl alpha-carboxyl-protected amino acids generated more delta-lactam than did those protected by tertbutyl or N2H2-Boc. So far it has not been possible to propose a general mechanism for delta-lactam formation or a process that completely abolishes it. Therefore, this side reaction should be considered almost inevitable. Its minimization requires examination of arginine-containing peptides in each specific synthesis.