1,3-Oxazinium ions/salts reactions

The heterocyclic systems of 1,3-oxazinium ions are widely used in the organic synthesis . Thus, 5,6-dihydro-4/f-l,3-oxazines 40, being a cyclic derivative of 1,2-aminoalcohols, can serve as Cj-synthones for the preparation of aldehydes, ketones and carboxylic acids . The 47/-l,3-oxazines 47 can undergo oxidative dehydrogenation by treatment with the trityl salts or benzoquinone to form the 3-azapyrylium salts 48, which then give various jV-acylenamine derivatives 49-52 by reaction with water or with active methylene comjKJunds (equation 18) . 

However, 2,4,6-trisubstituted pyrylium salts with certain active methyl and methylene compounds undergo ring fission and subsequent cyclization to benzenoid products. 2,4,6-Triphenylpyrylium ion (261 Z = O) in this way forms 2,4,6-triphenylnitrobenzene (299) with nitromethane and the substituted benzoic acid (300) with malonic acid, the latter reaction involving a decarboxylation. In reactions of this type, 1,3-oxazinium salts react with active hydrogen compounds to give pyridines (Scheme 25). 

Monocyclic 1,3-oxazepines 419 can be prepared by reaction of aliphatic diazo compounds with 1,3-oxazinium perchlorates 418 (Scheme 194) <1974S187>. Tetra- and penta-phenyl-1,3-oxazepines 421 (R = H or Ph) have been obtained by the reaction of azide ion with pyrylium salts 420 (Scheme 195) <1978H(11)331>. 

The assumption that the azomethine 34 is formed by an intermolecular hydride transfer from aldehyde 30 to the nitrilium salt 31 is not confirmed because the different Schiff bases 34 obtained carry the group R which are originated from the aldehyde 30. In this way, the process described in equation 14 can be reversed and applied as an enamide synthesis by acylation of imines2,3. However, the 7V-ethylbenzonitrilium salt 35 reacts with benzaldehyde to give the more stable Af-benzoyl-7V-ethyliminium ions 3647,49 which add to trimethylethylene to form 5,6-dihydro-4i/-l,3-oxazinium salt 37. On the other hand, the reaction of ions 36 with phenylacetylene leads to another type of 1,3-oxazinium heterocycle, namely to 4i/-l,3-oxazinium hexachloroantimonate 38 (equation 15). 

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