Nitrile ylides nitro compounds

Enolates of aldehydes, ketones, and esters and the carbanions of nitriles and nitro compounds, as well as phosphorus- and sulfur-stabilized carbanions and ylides, undergo the reaction. The synthetic applications of this group of reactions will be discussed in detail in Chapter 2 of Part B. In this section, we will discuss the fundamental mechanistic aspects of the reaction of ketone enolates with aldehydes md ketones. 

The two major methods of preparation are the cycloaddition of nitrile oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamines. Additional methods include reaction of /3-haloketones and hydroxylamine, the reaction of ylides with nitrile oxides by activation of alkyl nitro compounds from isoxazoline AT-oxides (methoxides, etc.) and miscellaneous syntheses (62HC(i7)i). 

Nitrile oxides are often generated by the dehydration of nitro compounds by reagents such as phenyl isocyanate. Azomethine ylides can be generated by the pyrolysis of aziridines or by the prototopic isomerization of imines upon heating. 

Ylides can cyclopropanate unsaturated systems which are susceptible to Michael additions, i.e. a,jS-unsaturated ketones, esters, amides, nitriles, sulfones, sulfonamides, and nitro compounds. Enhancement of electron withdrawal from the carbon-carbon double bond facilitates the reaction. The reaction is non-stereospecific. The intermediacy of zwitterions has generally been accepted, and hence the stereochemistry of the product may be predictable on the basis of the stepwise mechanism. Namely, the Michael addition of the ylide will occur predominantly from the less hindered side of the double bond in a given molecule and the subsequent cyclization will take place in the conformation which minimizes the non-bonded repulsions. 

Nowadays a broad range of different 1,3-dipoles, ozone, azides ° and diazoalkanes on the one hand as well as dipoles like nitrones, nitro compounds, carbonyl ylides, nitrile oxides, nitrile imines and ylides on the other hand, are well-established. The addition of these 1,3-dipoles to an alkene is one of the most frequently used cycloaddition reactions in organic synthesis. ... 

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