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 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.

Spontaneous rearrangement releases a carboxylate anion to produce the isocyanate intermediate.

Finally, the respective amine and CO2 are generated by abstraction of a proton with a base and decarboxylation.