Photochemical Curtius

A third approach to 3-amino-/3-lactams is by Curtius rearrangement of the corresponding acyl azides. These are readily prepared from r-butyl carbazides, available via photochemical ring contraction of 3-diazopyrrolidine-2,4-diones in the presence of f-butyl carbazate (c/. Section 5.09.3.3.2). Thus treatment of (201) with trifluoroacetic acid followed by diazotiz-ation gives the acyl azide (202) which, in thermolysis in benzene and subsequent interception of the resulting isocyanate with r-butanol, yields the protected 3-amino-/3-lactam (203) (73JCS(P1)2907). 

Curtius rearrangement Thermal or photochemical rearrangement of acyl azides to give isocyanates. 116... 

Interest in the photochemical generation and fate of acyl nitrenes has again been expressed. In benzoyl, p-methoxybenzoyl, and m-fluorobenzoyl azides, Curtius rearrangement of the photochemically generated acyl nitrenes to the corresponding isocyanates competes with formation of insertion products in halogen-free solvents.78 In halogenomethanes, the yields of isocyanates are... 

Acyl azides are well-known compounds. Their role in the thermal Curtius rearrangement, a reaction that apparently does not involve a nitrene, will be discussed in subsection 8.3.2. Photochemical decomposition of acyl azides elidts nitrene reactivity. In particular, intramolecular C-H-insertion reactions have been observed, but not usually in high 5deld. ... 

Since Theodor Curtius reported the synthesis of ethyl diazoacetate in 1883, Buchner had investigated its reactions with carbonyl compounds, alkenes, alkynes, and aromatic compounds for more than 30 years.His extensive contributions in this area resulted in two reactions named in his honor the Buchner-Curtius-Schlotterbeck reaction (formation of ketones from aldehydes and aliphatic diazo compounds) and the Buchner reaction. The prototypical example of the latter involves the thermal or photochemical reaction of ethyl diazoacetate with benzene to give (via norcaradiene 7) a mixture of four isomeric cycloheptatrienes 8-11. Initially, Buchner believed that a single norcaradiene product 7 was generated from this reaction, but later, he realized that the hydrolysis of the product afforded a mixture of four isomeric carboxylix acids. The norcaradiene formulation persisted until 1956 when Doering reinvestigated this reaction. ... 

Curtius rearrangement (Section 18.14) The thermal or photochemical decomposition of an acyl azide to give an isocyanate. 

The synthesis of 4,5-difluoroimidazole has also been described by Kirk using a photochemical Schiemann reaction (Scheme 59) [74]. Curtius rearrangement of the acyl azide, prepared in two steps from ethyl 4-fluoroimidazole-5-carboxylate via hydrazide, afforded the key intermediate Boc-protected 5-fluoroimidazol-4-carboxylate. In situ Boc deprotection followed by diazotization of the amine and photolysis of the diazonium tetrafluoroborate afforded 4,5-difluoroimidazole. 

An alternative route has been developed in Marquez laboratory to prepare the methano-carbocyclic thymidine 127 (see Vol.28, p.286). A key step involved the highly stereoselective formation of 126 by addition of diazomethane to the a,P unsaturated nitrile, itself accessible from a known intermediate in two steps. Photochemical loss of nitrogen from 126 was followed by a Curtius sequence to establish an amine from which the thymine ring was formed. The aristeromycin analogue 128 has also been prepared, using chemistry reminiscent of earlier work on analogues of this type (Vol.28, p.285-6), and bicyclic compounds of type 129 have been prepared as racemates. ... 

Iminophosphonium Salt Formation. Irradiation of azi-dotris(dimethylamino)phosphonium hexafluorophosphate in acetonitrile at 254 nm using a Rayonnet photochemical reactor for 15 h at 25 °C gives the iminophosphonium salt with release of nitrogen (eq 3). This is the first example reported of a Curtius-type rearrangement involving a charged atom. ... 

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