Lossen rearrangement  The Lossen rearrangement is the conversion of a hydroxamate ester to an isocyanate. Typically O-acyl, sulfonyl, or phosphoryl O-derivative are employed.The isocyanate can be used further to generate ureas in the presence of amines or generate amines in the presence of H2O. The mechanism below begins with an O-acylated hydroxamic acid derivative that is treated with base to form an isocyanate that generates an amine and CO2 gas in the presence of H2O. The hydroxamic acid derivative is first converted to its conjugate base by abstraction of a hydrogen by a base. Spontaneous rearrangement kicks off a carboxylate anion to produce the isocyanate intermediate. The isocyanate in the presence H2O hydrolyzes and then decarboxylation via abstraction of a hydrogen by a base generates an amine and CO2 gas. Hydroxamic acids are commonly synthesized from their corresponding esters.