Nitrilium salts, synthesis

In the case of the most reactive compounds, substitution at the carbon atom of diselenonium and ditelluronium dications is also a possible pathway. For example, formation of diselenide 117 from selenoxide 115 was explained by demethylation of intermediate dication 116 with trifluoroacetate anion.126 Dealkylation of salt 118, which is stable up to —20°C, leads to formation of nitrilium salt 119. The latter is transformed to amide 120 upon hydrolysis.64 Dealkylation of intermediate diselenonium dication 122 was suggested as the key step in the oxidative synthesis of 1,2,4-diselenazolidines 123 from eight-membered heterocycles 121 (Scheme 46).127... 

Heterocyclic synthesis, from nitrilium salts under acidic conditions, 6, 95 Hilbert-Johnson reaction of 2.4-dialkoxy-pytimidities, 8, 115... 

Nitriles and nitrilium salts, heterocyclic synthesis involving. 6, 95... 

Another aspect of the reactivity of nitrilium salts has been employed in the synthesis of several heterocyclic systems. This entails their reaction with certain electron-rich functional groups, such as —OH, —SH, —NH2, etc.,27,30 according to the general scheme (67->68) yielding iminoethers (68, Z = 0), iminothioethers (68, Z = S),... 

A similar procedure, taking advantage of the nitrilium salt reactivity for the —OH grouping, permits the synthesis of several 5-oxo-pyrano[3,4-e]-l,3-oxazine derivatives (93) which Davis and Elvidge73 have recently described and in the formation of which an intermediate iV-acyl-nitrilium salt (92) has also been postulated. 

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). 

Synthesis of heterocycles with nitrilium salts and nitriles ... 

Nitriles and nitrilium salts, heterocyclic synthesis involving, 6, 95 Nitro-compounds, heteroaromatic, ring synthesis of, 25, 113 Nitrogen-bridged six-membered ring systems, 16, 87... 

The great versatility of DAMN in imidazole synthesis has already been discussed (see Section 2.1.1). It also has applications under the present heading because the reagent can be converted into amidinium salts (20) by nitrilium triflates. In the presence of varying strength bases and temperatures (20) are transformed into S-aminoimidazolcs, substituted in the 4-position by nitrile (21), cyanoformimidoyl (22) or carboxamido (23) (Scheme 2.2.8), The amidinium salts (20) are isolated as mixtures of cis and trans isomers. The yields quoted for (21)-(23) have not been optimized, and their low recoveries reflect poor solubility in the usual solvents used for extraction [26, 27]. Since nitrilium salts with R = H arc unstable, it is not possible to prepare similar imidazoles which have no 2-substituent by this method. 

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