Nitronate structures mechanisms

Formation of nitrones can be achieved in the first stage of a Krohnke type reaction in which p-n trosodi methy 1 an dine reacts with 2-oo-bromoacetylphenoxathiin in alkaline medium (336). The synthesis of a series of cyclic nitrones of structure (182) has been achieved by regioselective, and by an unusual [3 + 2] cycloaddition of a-nitrosostyrenes (181) to 1,3-diazabuta-l,3-dienes (180) (Scheme 2.64) (337a). Theoretical studies of the substitution effect at the imine nitrogen on the competitive [3 + 2] and [4 + 2] mechanisms of cycloaddition of simple acyclic imines with nitrosoalkenes have been reported (337b). 

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

If the alkene is already joined on to the nitrone by a covalent bond so that the dipolar cycloaddition is an intramolecular reaction, one particular outcome may be dictated by the impossibility of the alternatives. Here is a simple case where an allyl group is joined to the same ring as in the previous example. The product has a beautifiiUy symmetrical cage structure and the mechanism shows the only way in which the iryolecule can fold up to allow a 1,3-dipolar cycloaddition to occur. 

The reaction of a lithium ester enolate (146) with a nitrone (147) to yield a fi-hydroxylamino acid ester (149) has been recently investigated by Domingo, Merino and coworkers, using DFT (B3LYP/6-31G ) methods, to gain insight on the molecular mechanism. The proposed transition structure (148) shown in equation 42 derives from attack of the most nucleophilic center of enolate 146 on the most electrophilic center of... 

The theoretical prediction (endo 96%, Table 6) for the reaction of H-nitrone with maleonitrile, where the steric hindrance to endo-orientation is possibly further reduced with respect to the reaction of 1-pyrroline-l-oxide, is very reasonable. The optimised transition structures reported in Figure 2 give evidence of a well concerted mechanism and offer some qualitative insight on the interactions between the reaction partners. 

Reaction of an aldehyde with N-methylhydroxylcunine in the presence of an acid catalyst in aqueous solution leads to the formation of a structure known as a nitrone, as shown in equation 7.75. Experimental evidence indicates that the reaction occurs in two steps, with the second step showing general add catalysis. Propose a mechanism for the reaction. 

The stability of ARs is conditioned by tautomeric conversions and depends on the chemical structure of substituents at the N atom, temperature and solvent [5,9]. The general mechanism of AR decay is a disproportionation reaction with formation of nitrones and hydroxylamines. ARs having primary or secondary alkyl groups are short-lived species because they easily undergo disproportionation by the scheme ... 

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