The β-p-Nitrobenzyl Ester to Minimize Side Reaction during Treatment of Aspartyl Peptides with Methanesulfonic Acid

When Boc-Asp (OBzl)-Ser (Bzl)-OBzl was deblocked with methanesulfonic acid-anisole to obtain free dipeptide, a few degree of α to β shift was observed. Boc-Asp-(OBzl)-Thr (Bzl)-OBzl had similar property. When β-carboxyl group was protected with p-nitrobenzyl group stable to methanesulfonic acid-anisole, no detectable α to β shift was observed. Boc-Asp (ONb)-Ser (Bzl)-Asp (OBzl)-Pro-Arg (MBS)-ONb was treated with methanesulfonic acid-anisole, followed by catalytic hydrogenation or treatment with zinc powder in acetic acid for the cleavage of p-nitrobenzyl group to give Asp-Ser-Asp-Pro-Arg.