Furoxans intramolecular nitrile oxide cycloadditions

As was mentioned earlier, furoxans are often encountered as unwanted byproducts in nitrile oxide cycloadditions. There are, however, some efforts to exploit this facile C C forming dimerization for synthesis. In one case, an intramolecular hw(nitrile oxide) cycloaddition was used for a synthesis of biotin (322a). More recently, the intramolecular dimerization was employed for the construction of medium- and large-size rings. This was feasible if one of the two nitrile oxide functionahties was relatively hindered and stable (see Section 6.1.4). Unsymmetrical... 

Some routes of chemical transformations of nitrile oxides connected with the problem of their stability were briefly discussed in Section 1.2. Here only two types of such reactions, proceeding in the absence of other reagents, viz., dimerization to furoxans and isomerization to isocyanates, will be considered. All other reactions of nitrile oxides demand a second reagent (in some cases the component is present in the same molecule, and the reaction takes place intramolecularly) namely, deoxygenation, addition of nucleophiles, and 1,3-dipolar cycloaddition reactions. Also, some other reactions are presented, which differ from those mentioned above. 

DFT studies of the intramolecular ene-like (or the so-called 1,3-dipolar ene) reaction between nitrile oxides and alkenes show that this reaction is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate. The competitive reactions, either the intramolecular [3 + 2] cycloaddition between nitrile oxides and alkenes or the intermolecular dimerization of nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar ene reaction if the tether joining the nitrile oxide and alkene is elongated, or if substituents such as trimethylsilyl are absent (425). 

---