Lossen rearrangement related hydroxamic acids

In the Lossen reaction a hydroxamic acid derivative (usually an 0-acyl derivative) is deprotonated by base, and rearranges via migration of the group R to give an isocyanate 2. Under the usual reaction conditions—i.e. aqueous alkaline solution—the isocyanate reacts further to yield the amine 3. The Lossen reaction is closely related to the Hofmann rearrangement and the Curtins reaction. 

The fourth related rearrangement reaction is the Lossen reaction, which generally occurs by base treatment of 0-substituted hydroxamic acids which possess electron-withdrawing functions at the oxygen atom (e.g. O-acylhydroxamic acids), giving amines via isocyanates (equation 6). Preliminary 0-activation (e.g. O-acylation) of hydroxamic acids is essential for a smooth rearrangement, otherwise it will not occur, ilie Lossen reaction is not as useful as the other three rearrangements since hydroxamic acids are not readily available. 

Related hydroxamic acids such as thiohydroxamic acids can also undergo Lossen rearrangement. Under thermal condition, thiohydroxamic acids 16 can generate thioureas 17. ... 

Since leaving group R COO leaves as R COO , the rearrangement is facilitated by the presence of electron-withdrawing groups in meta or para positions. Of these four related processes the Lossen rearrangement is the least useful in the synthesis of amines because of the unavailability of hydroxamic acids. 

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