Hydroxamic acids Hofmann rearrangement

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 Lossen rearrangement of an hydroxamic acid under basic conditions is a variant of the Hofmann reaction in which the aroyloxy group fills the role of the bromine.314... 

Lossen rearrangement. Conversion of a hy-droxamic acid, via a Hofmann-like arrangement, to an amine containing one less carbon than the original hydroxamic acid. 

Conversions of compounds 116, 120 and 123 into IP-2-ones described above belong to Hofmann-, Curtius- and Lossen-type reactions. Therefore the parent amides, hydroxamic acids and o-aminopyridinecarboxylic acid azides, may formally be regarded as derivatives of monoaminopyridines. The above compounds rearrange into o-diamines with various degrees of ease, while benzenesulphonate 121 is transformed into intermediate isocyanate 122 under extremely mild conditions (68JOC2543). 

This reaction, known as the Lossen rearrangement, has seen but little synthetic application 2 . ito possesses no distinct advantages over the Curtius, Hofmann, and Schmidt reactions. It appears to be useful when hydroxamic acids result as primary products. The reaction has been reviewed recently. 

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