Generation methods nitroalkane dehydration

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>. 

Mioskowski and co-workers reported the use of Burgess salt, DAST, ox-alyl chloride and phosphorus oxychloride for dehydration of nitroaUcanes 13 among which they found DAST to be the best reagent [87]. Recently, a microwave-assisted generation of nitrile oxide 5 under the catalytic influence of 4-(4,6-dimethoxy[l,3,5]triazin-2-yl)-4-methylmorpholinium chloride (DMTMM)/DMAP [88] has been described. However, di-f-butyl dicarbonate (Boc20)/DMAP, a method reported by Basel and Hassner, allows the dehydration of nitroalkanes 13 under much milder conditions (Scheme 4) [89]. In fact, this has become an efficient method because of the innocuous nature of the side products (f-BuOH and CO2), which simplifies the purification of the ultimately desired isoxazoHne product. 

Giacomelli et al. have generated nitrile oxides from nitroalkanes 13 using DMTMM (4-(4,6-dimethoxy[ l,3,5]triazin-2-yl)-4-methylmorpholinium chloride) 70 as the dehydrating agent in the presence of DMAP in acetonitrile and reacted them with alkenes 68 to afford isoxazolines 69 (Scheme 21) [88]. The authors found microwave irradiation conditions to be superior to the conventional room temperature method. 

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