5.6- Dihydro-4//-l,3-oxazinium salts

The situation was changed by the discovery of a novel approach to the synthesis of 5,6-dihydro-4//-l,3-oxazinium salts consisting in a polar 1,4-... 

As mentioned above, the polar 1,4-cycloaddition of N-acyliminium ions 14 and 18 to olefins gives 5,6-dihydro-4H-l,3-oxazinium salts 16 (Section fI,A,l), or forms the 1,3-oxazinium salts with acetylenes (Section III, A). The same ions 89 and 91 add nitriles to furnish 1,3,5-oxadiazinium salts 90 and 92, little investigated until now (65CB334 88ZOR230). Salts 92 are unstable and can dissociate at elevated temperatures with nitrile liberation. Therefore, they may serve as original reservoirs for active jV-acyliminium cations. 

The 5,6-dihydro-4/f-l,3-oxazinium cations 18 formed can be isolated as salts of stable non-nucleophilic anions (e.g. SbClg). Usually, the reaction mixtures are treated with water or with alkali solutions, and 5,6-dihydro-4//-l,3-oxazines 19 are obtained by deprotonation (equation 9, R = H). This reaction can be carried out as a two-... 

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 imines . However, the A -ethylbenzonitrilium salt 35 reacts with benzaldehyde to give the more stable Af-benzoyl-A -ethyliminium ions 36 which add to trimethylethylene to form 5,6-dihydro-4/f-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 47/-l,3-oxazinium hexachloroantimonate 38 (equation 15). 

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) . 

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