The resulting azide of the above step is then the subject of a Curtius rearrangement.

On Curtius rearrangement acyl azides yield isocyanates.

He published the Curtius rearrangement in 1890/1894 and also discovered diazoacetic acid, hydrazine and hydrazoic acid.

The reaction is related to the Curtius rearrangement except that in this reaction the azide is protonated and hence with different intermediates.

It is synthesised by oxidative coupling of 4-amino-3-(azidocarbonyl)furoxan followed by Curtius rearrangement and further oxidation.

Particularly worth mentioning is the Curtius rearrangement whereby the acid azide spontaneously rearranges to the isocyanate functional group upon heating in an aprotic solvent.

The synthesis uses such reactions as the Mannich reaction, Pictet-Spengler reaction, the Curtius rearrangement, and chiral rhodium-based diphosphine-catalyzed enantioselective hydrogenation.

In one variation called the Darapsky degradation (A. Darapsky, 1936), a Curtius rearrangement takes place as one of the steps from an α-cyanoester to an amino acid.

Alkyl or aryl acyl chlorides react with sodium azide in aqueous solution to give acyl azides, which give isocyanates in the Curtius rearrangement.

The primary solvolyis reaction product was derivatized by reduction with lithium aluminium hydride and oxidation with sodium permanganate to the dicarboxylic acid followed by a Curtius rearrangement expelling carbon dioxide.

Azides easily extrude diatomic nitrogen, a tendency that is exploited in many reactions such as the Staudinger ligation or the Curtius rearrangement or for example in the synthesis of γ-imino-β-enamino esters.

The Curtius rearrangement (or Curtius reaction or Curtius degradation), as first defined by Theodor Curtius, is a chemical reaction that involves the rearrangement of an acyl azide to an isocyanate.

The Curtius rearrangement may be thought of as a two-step process, the first step being the loss of nitrogen gas, forming an acyl nitrene (2), and the second step being the rearrangement of acyl nitrenes by migration of R-group to form the desired isocyanate (3).

Saponification of the methyl ester 11 with LiOH was followed by a Curtius rearrangement of the resulting acid 12 with diphenylphosphoryl azide in refluxing toluene to afford isocyanate intermediate, which its treatment with NaOMe/MeOH forms the corresponding carbamate 13 in 82% yield.