Triazoles diazoalkane cycloaddition reactions

The 1,3-dipolar cycloaddition reactions to unsaturated carbon-carbon bonds have been known for quite some time and have become an important part of strategies for organic synthesis of many compounds (Smith and March, 2007). The 1,3-dipolar compounds that participate in this reaction include many of those that can be drawn having charged resonance hybrid structures, such as azides, diazoalkanes, nitriles, azomethine ylides, and aziridines, among others. The heterocyclic ring structures formed as the result of this reaction typically are triazoline, triazole, or pyrrolidine derivatives. In all cases, the product is a 5-membered heterocycle that contains components of both reactants and occurs with a reduction in the total bond unsaturation. In addition, this type of cycloaddition reaction can be done using carbon-carbon double bonds or triple bonds (alkynes). 

D-Triazoles have been isolated from the reaction of several activated nitriles, such as cyanogen, cyanogen halides, methyl cyanoformate, and cyanic acid esters, with diazoalkanes. The reaction can formally be regarded as a 1,3-dipolar cycloaddition. The v-triazoles may be... 

Ketenes rarely produce [3+ 2]-cycloaddition products with diazo compounds. The reaction possibilities are complex, and nitrogen-free products are often obtained (5). Formation of a cyclopropanone represents one possibihty. Along these lines, the synthesis of (Z)-2,3-bis(trialkylsilyl)cyclopropanones and (Z)-2-trialkylsilyl-3-(triethylgermyl)cyclopropanones from diazo(trialkylsilyl)methanes and appropriate silyl- or germylketenes has been reported (256,257). It was found that subsequent reaction of the cyclopropanone with the diazoalkane was not a problem, in contrast to the reaction of diazomethane with the same ketenes. The high cycloaddition reactivity of diazomethylenephosphoranes also extends to heterocumulenes. The compound R2P(C1)=C=N2 (R = N(/-Pr)2) reacts with CS2, PhNCO and PhNCS to give the corresponding 1,2,3-triazole derivative (60). 

Scheme 148). The tosyl group induces ring opening, in preference to hydroxide elimination, to give a triazole. Numerous diazo esters and ketones have been prepared by diazo transfer reactions441 447 4S1 and improved yields obtained using phase-transfer catalysis.452 Diazoalkane formation via retro-1,3-cycloadditions may also be considered as diazo transfer reactions.12... 

These methods were referred to as fulminic syntheses of isoxazoles. It is important to note the similarity of these reactions with those already known transformation of diazoalkanes to pyrazoles and azides to triazoles. Recognition of these pathways opened the way for the concept of 1,3-dipolar cycloaddition. Speroni also contributed to knowledge of isoxazole chemistry by clarifying the stmcture of the Betti acids [3, 4] and conducting extensive studies on the physical properties of isoxazole and its derivatives. 

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