The protected carboxamide function of N^α, N^ca-di-tert-butyloxycarbonyl-ω-carbamoyl-α-amino acids worked well with nucleophilic reagents. Applying this novel reactivity, we developed an efficient synthetic route to N^α-tert-butyloxycarbonylhomoglutamic acid and its derivatives, including N^α-tert-butyloxycarbonylhomoglutamic acid δ-benzyl ester, from N^α, N^ca-di-tert-butyloxycarbonylhomoglutamine.