| Abstract: | Racemization attending the aminolysis of symmetrical (Sy) and mixed (Mx) anhydrides (An) of N-alkoxycarbonylamino acids by amino acid anions in aqueous dimethylformamide has been examined by reversed phase high-performance liquid chromatography analysis of the N-protected dipeptides produced. Racemization occurred in most cases when the anion was generated using 1 equiv. sodium hydrogen carbonate, in most cases for MxAn reactions when the base was sodium carbonate, and only in a few cases for SyAn reactions when the base was sodium carbonate. Less racemization was associated with MxAn reactions when the chloroformate of a secondary alcohol had been used for their generation. The change in chirality is explained on the basis of the formation and racemization of the 2-alkoxy-5(4H)-oxazolone, which is greater in the presence of the weaker base because of incomplete deprotonation of the amino acid zwitter-ion. |
