From Nitrones

Nitronates derived from primary nitroalkanes can be regarded as a synthetic equivalent of nitrile oxides since the elimination of an alcohol molecule from nitronates adds one higher oxidation level leading to nitrile oxides. This direct / -elimination of nitronates is known to be facilitated in the presence of a Lewis acid or a base catalyst [66, 72, 73]. On the other hand, cycloaddition reactions of nitronates to alkene dipolarophiles produce N-alkoxy-substituted isoxazolidines as cycloadducts. Under acid-catalyzed conditions, these isoxazolidines can be transformed into 2-isoxazolines through a ready / -elimination, and 2-isoxazolines correspond to the cycloadducts of nitrile oxide cycloadditions to alkenes [74]. 

Carbon alkyUdon of simple nitronate anions is aiso possible by the reaction with iV-snbsd-tiited pyridiruums, as exemplified in Eq. 5.41. Such types of reacdons are classified as Spj 2 reacdons, in which electron transfer reacdons from nitronate anions to pyridiruums are involved as key steps." ... 

Name from nitron genes (Greek = saltpeter-forming). 

Scheme 4.38. Enantioselective formation of [Mactams from nitrones and alkynes.

Oxazines are prone to hydrogenolysis since the relatively weak N-O bond is easily cleaved. This reaction has often been employed for the transformation of this cycle (generally obtained from nitrones) into amino alcohols in a stereocontrolled manner. For example, reaction of 57 with hydrogen and palladium on charcoal as catalyst (Equation 1) furnished the expected substituted pyrrolidine 58 in moderate yields <2003EJ01153>. 

Similar facial selectivity has been observed in nitronates derived from chiral vinyl ethers (69), as well as from nitronates prepared with a chiral Lewis acid, which lack any bias from a chiral auxiliary (66). Even in the absence of a substituent at C(4), as in the nitronate 287, there remains a high facial selectivity upon the addition of a dipolarophile (Eq. 2.28) (84). Both RHE and B3LYP calculations for the approach of a dipolarophile to the nitronates 289 and 290 show at least a... 

Further examples of intramolecular 1,3-dipolar cycloadditions (cf. Section 10.21.9.2.2(ii)) include the synthesis of an isoxazolidine from nitrone 193 (Equation 128) <1997J(P1)1805>, and a pyrazolo heterocycle from the nitrile oxide generated in situ from the oxime 194 (Equation 129) <1999TL3535>. 

This 1,3-dipolar cydoaddition not only gave excellent results but was also found to be very general with regard to the nitrone component. Several types of aryl- and alkyl-substituted nitrone have been applied successfully. Irrespective of the substitution pattern, high diastereomeric ratios and enantioselectivity were obtained (see Scheme 8.9, products 35a,d,f,g). Variation of the N-alkyl group is also possible. As can be see from Scheme 8.9 (see, e.g., products 35a-c), the reactions also proceed well when an N-allyl and N-methyl-substituted nitrone is used. Acrolein, 32b, and crotonaldehyde, 32a, were used as the aldehyde component. It is noteworthy that this reaction is also suitable for use on a larger scale, as has been demonstrated by the 25 mmol-scale preparation of endo-35a (98% yield, 94% ee) starting from nitrone 31a and crotonaldehyde. 

Table 3 Compounds 34 and 31 prepared from nitrones 33 by heating in different solvents...

Scheme 9. Routes for the synthesis of Nuphar alkaloids from nitrones.

Reactive free radicals also react with the nitrogen of nitroso groups, forming a nitroxide one atom closer to the trapped radical than is the case with nitrone spin traps. This results in ESR spectra containing more chemical structural information. While nitroso spin traps provide radical identification, the resultant adducts are often less stable than those derived from nitrone traps. In particular, nitroso traps are unreliable for oxygen-centered radicals even in vitro. 

Agawa and his co-workers have reported a series of investigations of the reactions of cumulenes with oxaziridines. Reaction with isocyanates led to oxadiazolidinones 86 45,70 same product obtained from nitrones and isocyanates. Detailed mechanistic information is not available and the classification of these reactions as cyclo-... 

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