Hydroxamic acids, Lossen degradation

The Hofmann-degradation of acid amides (60-80%, via N-bromoamides) as well as the Lossen-degradation of hydroxamic acids by [l,2,l]-elimination of HBr, or H20, respectively, leads to isocyanates (Scheme 4), which usually yield primary amines RNH2 by solvolysis 36). 

Known examples are the partial Hofmann degradation of pyridine-2,3-dicarbox-amides,134 the Lossen degradation of pyridine-2,3-bis(hydroxamic) acid,135 this giving both possible products, i.e. 3-hydroxypyrido[2,3-(/]pyrimidine-2,4(l//,3//)-dione and 3-hydroxy-pyrido[3,2-d]pyrimidine-2,4(l//,3//)-dione as a 5 1 mixture, and the conversion of a primary carboxamide function into an intermediate isocyanate in the 2-position by oxidation with lead(IV) acetate.136... 

An example is the partial Lossen degradation of pyridine-3,4-bis(hydroxamic) acid, this giving both possible products, i.e. pyrido[4,3-[Pg.204]

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. ... 

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