The Lossen rearrangement involves the generation of an isocyanate via thermal or base-mediated rearrangement of an activated hydroxamate which can be generated from the corresponding hydroxamic acid. Activation of hydroxamic acid can be achieved through O-acylation, O-arylation, chlorination, or O-sulfonylation. Such hydroxamic acids can also be activated using polyphosphoric acid, carbodiimide, Mitsunobu conditions, or silyation. The product of the Lossen rearrangement, an isocyanate can be subsequently converted to a urea or an amine resulting in the net loss of one carbon atom relative to the starting hydroxamic acid.