Incorporation of the non-natural amino acid into peptides to create the new functionalized peptides have been employed as a useful tool in the area of bioorganic and medicinal chemistries. To date, several ɑ-amino acid analogs in which the carboxylic acid moiety is replaced with other acidic functional groups, such as sulfonic acid, phosphonic acid, boronic acid, and tetrazole etc., have been developed. However, only a few examples of the incorporation of the above analogs into the middle portion of peptides have been reported mainly due to their instability of the corresponding amide-like bonds.
　Recently, we have developed a novel amino acid analog bearing a 4-hydroxy-2,3-dioxocyclobut-1-enyl (Sq) group (ɑ-amino squaric acid, ɑ-Asq).^{1, 2}  The Sq group is characterized by its planar structure, acidic OH group, and electron deficient property, so it was employed as a potent carboxylic acid surrogate in medicinal chemistry. We anticipated that α-Asq could be employed for the flexible peptide incorporation due to a highly electron deficient property of the Sq moiety and the chemically stable nature of the resulting N-Sq amide-like bond.
　In this report, we would like to describe our efforts toward the incorporation of α-Asq into peptides on solid-phase. To achieve this goal, we employed the α-Asq-containing dipeptide ester as an α-Asq-incorporating unit. As this application, we created the novel α-Asq-containing peptide library and evaluated the inhibitory activity toward carboxypeptidase Y.