Rearrangements Involving Nitrogen

Besides simple condensation reactions, valence isomerization reactions, in particular, arc used for the synthesis of unsaturated, eight-membered-ring azaheterocycles. These isomeriz-ations mainly involve rearrangements of nitrogen-containing bicyclo[4.2.0]octatriene or semi-bullvalene systems. 

These rearrangement processes are characterised by a transition state in which the alkoxyl group migrates from the amide nitrogen to the carbonyl carbon (Fig. 17a) and therefore involves HEteroatom Rearrangements On Nitrogen the HERON... 

Where X is a poor leaving group, anomeric destabilization of the NX bond can lead to a novel rearrangement. The HERON (from Heteroatom Rearrangements On Nitrogen) reaction of anomeric amides was discovered in the mid-1990s and involves a concerted... 

II. Rearrangements Involving the Pivotal Nitrogen Atom in the Starting Ring... 

These reactions involve addition of the diazoester to an adduct of the carbonyl compound and the Lewis acid. Elimination of nitrogen then triggers migration. Sections B and C of Scheme 10.2 give some additional examples of pinacol rearrangements involving diazo and diazonium intermediates. 

Acylation with the acylium ion in the gas phase. An unusual experiment was performed by Seldes et aL <20010MS1069>. The N2-tautomeric form of a 5-substituted tetrazole was reacted in the gas phase with an acyl ion generated as the secondary reactive chemical by ionization plasma in a mass spectrometer. It was suggested that the mechanism of this process involved the formation of an acylated tetrazole intermediate, which could not be isolated in a condensed phase, and by rearrangement with nitrogen loss afforded an oxadiazole <20010MS1069> (cf. Section 6.07.5.2.2, Equation 16). This experiment has no preparative value but provides important information on the interaction mechanism between the neutral N-unsubstituted tetrazoles and electrophilic agents in the gas phase. 

The coupling of the betaines (352) with diazonium salts, however, may take an unusual course in that arylation results. The reaction may be rationalized by the formation of a covalent intermediate (353) which subsequently suffers a pericyclic rearrangement involving expulsion of nitrogen (81H(15)1349). 

Either formulation CCLXV or CCLXVI for methiodide A requires that the iodine catalyzed rearrangement involves migration of methyl from oxygen (in CCLXI) to nitrogen. This takes one back to earlier controversies associated with the structure of pseudostrychnine quaternary salts. More work is required. 

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