Nitroalkanes furoxans

Nitrolic acids, which can be isolated from sources such as nitroalkanes and aldoximes, are also probable intermediates in a wide variety of furoxan preparations involving nitric and nitrous acid, and nitrogen oxides. Diacylfuroxans have been synthesized via nitrosation of methyl ketones for example acetone reacts with anhydrous dinitrogen tetroxide or nitric acid/sodium nitrite to give diacetylfuroxan. 

Dehydration of primary nitroalkanes with phenyl isocyanate or acetic anhydride in the presence of catalytic triethylamine affords nitrile oxides, which may be trapped as their 1,3-dipolar cycloadducts or allowed to dimerize to the corresponding furoxans. Other dehydrating agents that have been used include diketene, sulfuric acid and, when the a-methylene group is activated by electron-withdrawing groups, boron trifluoride in acetic anhydride, trifluoroacetic anhydride with triethylamine, and nitric acid in acetic acid. 

Apart from a few stable species, nitrile oxides exhibit a high tendency toward dimerization to furoxans for this reason, several methods have been developed for the in situ generation of such dipoles in the presence of the desired acceptor. The most common procedures involve thermal or base-mediated deydrohalogenation of hydroxymoyl halides, oxidation of aldoximes, and dehydration of primary nitroalkanes <1984CHEC(6)1>. 

In certain cases, 1,3-dipoles containing fluorocarbon groups will participate in cycloadditions to unsaturated hydrocarbons (Scheme 18143 and Eqs. 38144 and 39145 147). The reaction of the nitroalkane derivative 57 probably involves the nitrile oxide 58 as intermediate, since the furoxan product (59) corresponds to a dimer.144 Furoxans have also been obtained... 

Owing to their tendency to dimerize to furoxans (1,2,5-oxadiazole 2-oxides), nitrile oxides 5 are usually generated in situ, i.e., in the presence of suitable dipolarophiles such as alkenes, alkynes, etc., from stable precursors such as aldoximes 12 (X = H) or from primary nitroalkanes 13 (Scheme 2) [5,57-67]. Generation of nitrile oxides 5 from aldoximes 12 (X = H) involves either direct oxidation or halogenation of aldoximes 12 (X = H) to hydroximoyl halides 12 (X = Cl or Br) followed by dehydrohalogenation [5,57-67,79,80]. Alternatively, nitrile oxides 5 are conveniently generated via dehydration of primary nitroalkanes 13 [ 17,38,39,65,66,81-95]. This review covers the literature in the last 10-15 years pertaining to the chemistry of isoxazoHnes synthesized from primary nitroalkanes 13. 

Addition of Cu salts to the base allows nitroalkanes to undergo condensations with dipolarophiles it is reasonable to relate this catalytic effect to the known existence of complexes with nitronates [92]. Formation of such complexes might affect conversion of nitro compound into a species prone to cycloaddition and possibly catalyzes cycloaddition. Dehydration to nitrile oxide must be considered, too. In fact, furoxans can be detected in the presence of dipolarophile, while in their absence furoxans can be prepared, at least from nitroacetates or other activated nitro compounds [83]. The Mn nitronate from ethyl nitroacetate has been reported to generate the corresponding nitrile oxide [81]. 

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