Here, we report a peptide aspartic acid side-chain benzyl ester as a useful precursor that can be efficiently converted into various functional groups, including acid, amide, carbonyl hydrazide, carbonyl azide, or thio ester groups, without other protection for the peptide. With this strategy, we synthesized a series of novel branched and cyclic arginylglycylaspartic acid peptides through successive peptide C-terminal ligation and side-chain ligation based on a side-chain carbonyl azide or thio ester.