Among the numerous existing chemical motifs, alkenes, alkynes, enol ethers and enamides, with an unsaturated carbon-carbon bond, are versatile functional groups that are found in many natural products and bioactive compounds. They are widely used as valuable building blocks for the design and synthesis of complex organic molecules. Therefore, the development of new strategies to introduce those motifs into molecules is highly desirable. Traditionally, the treatment of human disease has been dominated by small-molecule drugs, but new therapeutic agents, such as peptide conjugates, have emerged as promising next generation of medications. Besides, the development of new therapeutics to cure diseases strongly relies on the study and understanding of biological processes. In this regard, the functionalization of peptides and proteins is of high interest and spite of significant achievements, the need for more efficient and diverse strategies remains present. Because of their low toxicity, high functional group tolerance, and stability in biocompatible media, hypervalent iodine reagents are valuable tools for late-stage peptide and protein functionalization. In this context, the first goal of my PhD was to develop new hypervalent iodine reagents to access valuable enamide and enol ether motifs. To this end, the synthesis of stereodefined enamide- and enol ether-based vinylbenziodoxolone reagents (N- and O-VBX) was achieved. The N- and O-VBXs were obtained by stereoselective addition of sulfonamide and phenol nucleophiles onto the alkynyl triple bond of ethynylbenziodoxolone reagents (EBX). The reaction tolerated a broad range of functional groups and stable VBXs reagents could be isolated in high yields. The second objective of this thesis was to study the reactivity of the newly developed VBX reagents. The enhanced reactivity of the hypervalent bond allowed their use as formal vinyl cations in presence of nucleophiles and in palladium catalyzed cross-coupling reactions at room temperature. In addition, a new mode of reactivity of hypervalent iodine was described. The O-VBX reagents were shown to be synthetic equivalents of oxy-allyl cation precursors. The new transformation, promoted by an excess of base, was working especially well with phenol nucleophiles, leading to aryl enol ethers bearing an allylic ether with complete E-stereoselectivity. In absence of external nucleophiles, the 2-iodobenzoate group of the reagent could be transferred, leading to aryl enol esters with still complete E-stereoselectivity. Finally, this chemistry was applied to the late-stage functionalization of peptides and proteins. Taking advantage of the ability of phenol nucleophiles to react with EBXs, we developed a selective tyrosine bioconjugation methodology. Under physiological conditions, tyrosine-containing peptides and proteins could react with EBXs to give stable O-VBX bioconjugates. The methodology tolerated a broad range of functional groups, with the exception of the cysteine side-chain. Complex peptides of different sizes and proteins were successfully labelled with high chemo- and site-selectivity. The installed hypervalent iodine bioconjugates could be further functionalized by palladium-catalyzed Suzuki-Miyaura cross-coupling reactions and strain-promoted azide-alkyne cycloadditions. The potential of the doubly-orthogonal functionalization was further demonstrated in a cellular uptake experiment.