Acid chlorides, Curtius rearrangement, acyl azides from

Carboxylic acid chlorides may be converted to isocyanates via the Curtius rearrangement of the acyl azide. A solution of tetrabutylammo-nium azide (prepared by CH Cl extraction from aqueous sodium azide and tetrabutylammonium hydroxide) in toluene or benzene reacts readily with acid chlorides to give acyl azides. These are heated in solution to afford the isocyanates and nitrogen (Eq. 6.63) [101]. 

Warren, J. D., Press, J. B. Trimethylsilylazide/KN3/18-crown-6. Formation and Curtius rearrangement of acyl azides from unreactive acid chlorides. Synth. Commun. 1980, 10, 107-110. 

The Curtius rearrangement, like the Hofmann rearrangement, involve migration of an -R group from the G-O carbon atom to the neighboring nitro gen with simultaneous loss of a leaving group. The reaction takes place on heat ing an acyl azide that is itself prepared by nucleophilic acyl substitution of m acid chloride. 

The sodium azide pathway (Pathway A), Figure 4, begins with a thionylchloride treatment of the free acid to form the acyl chloride. Subsequent treatment with sodium azide may involve a non-aqueous environment (1,2-dimethoxy ethane, "dry method"), or an aqueous medium ("wet method"). The organic azide is recovered from the reaction mixture and converted into the isocyanate by the Curtius rearrangement. This may be accomplished in solid form ("dry method"), or in a non-aqueous solvent like dioxane or DMF ("solution method"). 

Lactone 286 was transformed into aminomonocarba-disaccharide 288 using a five-step procedure involving the Curtius rearrangement of acyl azide 287 formed from the appropriate acid chloride (Scheme 3,43), ... 

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