Given the occurrence and diversity of nitrogen-containing molecules, the development of new amination methods is of significant importance. Indeed, a recent study shows that 60% of the FDA approved drugs contain a nitrogen heterocycle. Undoubtedly, novel methodologies arising for uncommon intermediates for the incorporation of nitrogen atoms are needed to access more complex molecules. The present document focus on the development of new methods for the formation of C-N and N-N bonds for the synthesis of acyclic and heterocyclic products. Isocyanates are useful synthons and reactive intermediates. To overcome their toxicity and instability, blocked (or masked) isocyanates have been developed: an equilibrium generates the isocyanate in-situ, allowing for safer precursors and better control over the concentration of the reactive isocyanate. This strategy enables the development of new reactivity, particularly for heteroatom-substituted isocyanates. However, reactions of oxygen-substituted isocyanates (O-isocyanates) remained severely underdeveloped. In Chapter 2, bench-stable N-oxy-carbamates and N-oxyureas are reacted under basecatalysis or thermal conditions to form the corresponding O-isocyanate intermediate in situ. In the first part of this chapter, a survey was performed and optimum experimental conditions for the controlled formation of O-isocyanate intermediates from the block precursors were found. Gratifyingly, the known side-reactions of O-isocyanates (trimerization and 1,2-shift) were avoided and different nucleophiles and substituents were studied for the controlled formation of N-oxyureas via substitution reaction of blocked O-isocyanates. Cascade reactions provided the opportunity to further develop this controlled reactivity of O-isocyanates. Herein, the first cascade-reaction of O-isocyanates is portrayed using - and -aminoester as the partners for the synthesis of hydantoin and dihydrouracil derivatives (>30 examples). Moreover, the conditions were modified to perform the reaction with -alcohol and - thioesters. Finally, evidence for the O-isocyanate intermediate is provided.