Peptide therapeutics have emerged as a powerful class of drugs capable of addressing challenging targets, particularly proteins involved in large and flat protein-protein interactions (PPIs). Despite their high selectivity and low toxicity, peptide- and protein-based drugs often suffer from limited stability, poor cell permeability, and low oral bioavailability. The introduction of non-natural amino acids and site-selective chemical modifications represents an effective strategy to overcome these limitations and to expand the chemical space accessible to peptide and protein therapeutics. Furthermore, site-selective protein modification provides valuable tools for investigating protein structure and function. This thesis focuses on the development of new synthetic methodologies for the generation of non-natural amino acids and for the site-selective modification of proteins. The first part of this work describes the development of a mild approach for the alpha-functionalization of glycine derivatives, enabling access to various non-natural amino acids. Using imine-based glycine surrogates bearing a leaving group on nitrogen, a broad range of nucleophiles, including sul-phur-, oxygen-, nitrogen-, and carbon-based nucleophiles, were successfully introduced in moderate to excellent yields. This methodology provides a practical route to alpha-functionalized amino acid derivatives and complements existing approaches that are limited to alkyl and aryl substitutions. The second part of this thesis addresses the challenge of achieving site-selective protein modification. Building on the reactivity of hypervalent iodine reagents, novel ethynylbenziodoxolone (EBX)-peptide conjugates bearing peptide ligands were developed for the selective functionalization of cysteine, tyrosine and tryptophan residues under mild conditions on purified proteins. Functionalization of Cys434 in the KELCH domain of KEAP1 proceeds either through direct thiol addition to the EBX reagent, forming an S-VBX adduct, or via alkyne transfer. In contrast, functionalization of tyrosine residues in the KEAP1 C434Y mutant and in the oncoprotein beta-catenin occurs exclusively through phenol addition to the EBX reagent. The peptide component serves as a recognition element, allowing targeting of specific regions on protein surfaces via non-covalent interactions between the protein and the ligand. Furthermore, the introduced functional handles enable subsequent derivatization through bioorthogonal azide-alkyne cycloaddition or palladium-mediated cross-coupling reactions. Overall, this work provides new tools for the synthesis of non-natural amino acids and for the precise chemical modification of proteins, contributing to the development of advanced strategies for peptide and protein functionalization in chemical biology and drug discovery.