4- Acylmethyl-3-azapyrylium salts

Recently, the surprising ability of 4-alkyl-substituted 3-azapyrylium salts to undergo acylation at the 4-alkyl group by acylium cations was discovered157. This reaction gives 4-acylmethyl-3-azapyrylium salts 385 (equation 110). By comparing the spectral characteristics of 385 with those of model compounds 388 and 391 (equation 111) it was shown157 that salts 385 exist preferably in the enol form 386. This conclusion is confirmed also by AM 1 quantum-chemical calculations. 

However, it is most surprising that alkyl-substituted 3-azapyrylium salts can undergo formylation and acylation at the side chain. These reactions are examples of the interaction of heterocyclic cations with cationoid electrophiles. Some of these processes are presented in Scheme 12 (90KGS134 91KGS265 94TH1). It is assumed that such transformations are possible due to the intramolecular deprotonation of the methyl group in 79 to form charged anhydro base 80 (see structure 39). The 4-acylmethyl-3-... 

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