Formation of aminosuccinyl derivative from β-phenacyl aspartyl peptides catalyzed by sodium thiophenoxide

In the solid-phase synthesis of cholecystokinin 30–33, Trp-Met-Asp-Phe- amide, the β-phenacyl ester was used to protect the β-carboxyl of aspartyl residue. The ester was cleaved, on the solid support, with a 1 M solution of sodium thiophenoxide in DMF, prior to ammonolysis. The product, after purification by countercurrent distribution, was identified as a mixture of isoasparaginyl and aspartyl peptides. A study of the deprotection step, with sodium thiophenoxide, on a model peptide (t-butyloxycarbonyl-β-phenacyl-aspartyl-phenylalanineamide) showed the rapid formation of the aminosuccinyl derivative, catalyzed by this reagent.