The Lossen rearrangement is the conversion of a hydroxamic acid

Hydroxamic acid

A hydroxamic acid is a class of chemical compounds sharing the same functional group in which an hydroxylamine is inserted into a carboxylic acid. Its general structure is R-CO-NH-OH, with an R as an organic residue, a CO as a carbonyl group, and a hydroxylamine as NH2-OH. They are used as metal...

 (1) to an isocyanate

Isocyanate

Isocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...

 (3) via the formation of an O-acyl, sulfonyl, or phosphoryl intermediate hydroxamic acid O-derivative (2) and then conversion to its conjugate base. Here, 4-Toluenesulfonyl chloride

4-Toluenesulfonyl chloride

4-Toluenesulfonyl chloride is an organic compound with the formula CH3C6H4SO2Cl. This colourless, malodorous solid is a reagent widely used in organic synthesis...

 is used to form a sulfonyl O-derivative of hydroxamic acid.

The isocyanate can be used further to generate ureas in the presence of amines (4) or generate amines in the presence of H₂O (5).
The mechanism below begins with an O-acylated hydroxamic acid derivative that is treated with base to form an isocyanate that generates an amine

Amine

Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 and CO₂

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas in the presence of H₂O.