Ureas Lossen rearrangements

The hydroxamate anion 566 is invoked as intermediate in the formation of urea 568 from the iV-tosylated /3-lactam 564 by a Lossen rearrangement in the presence of a secondary amine 565 (equation 252). Apparently, attack of amine in an S/v2 fashion at the C(4) carbon of the /3-lactam 564 opened the ring to form hydroxamate anion 566, which then initiated a facile Lossen rearrangement to isocyanate 567 trapped with another molecule of base to give 568. 

The acyl azide undergoes a rearrangement similar to the Hofmann rearrangement (method 446) and to the Lossen rearrangement (method 448). This step is carried out in inert solvents like benzene and chloroform to give the isocyanate directly or in solvents like alcohol and water which will react with the isocyanate to form urethanes and ureas. 

This reaction was first reported by Lossen in 1872. It is a thermal or alkaline conversion of hydroxamic acid into an isocyanate via the intermediacy of its O-acyl, sulfonyl, or phosphoryl derivative. In the presence of water, amine, or alcohol, the isocyanate is converted into amine, urea or urethane, respectively. Therefore, this reaction is generally known as the Lossen rearrangement. Occasionally, it is also referred to as the Lossen reaction, Lossen degradation, or Lossen transformation. ... 

The use of azides in the Curtius reaction is hazardous and the utility of the Hoffman and Lossen rearrangements is limited to preparation of aliphatic isocyanates, as aqueous media are employed (aromatic isocyanates react readily with water to form substituted ureas). Tertiary butyl hypochlorite can be used for non-aqueous Hoffman rearrangements but is costly. In practice mostly phosgenation of a primary amine is commercially important ... 

Hydroxamic acid 3 can be converted to activated hydroxamate 2 via reaction with a variety of electrophilic reagents. The Lossen rearrangement is then initiated by exposure of 2 to base or heat resulting in anion 10. Through a fast subsequent transformation via cyclic intermediate 12, isocyanate 1 is generated. This isocyanate 1 can be further converted to urea 13 or amine 15. 

A few decades later, a similar reaction of the tricyclic substrate, N-sulphonyloxy-2,3-norbom-5-enedicarboximide 25 was reported to undergo Lossen rearrangement under basic conditions to yield urea 26. ... 

Aromatic A-hydroxamate 27 can be transformed to cyclic urea 28 through DMF assisted Lossen rearrangement in excellent yield. ... 

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