Pyrylium cations/ions/salts reactions

The distribution of charge in the resonance forms (28)-(31) suggests that nucleophiles may attack at C-2, C-4 or C-6 (or at C-2 or C-4 in 1-benzopyrylium cations, and at C-l, C-3 or C-4a in 2-benzopyrylium ions) but they most commonly add at C-2 for example, attack by cyanide ion gives a 2//-pyran (37) which exists partly or wholly as the acyclic isomer (38). Steric and electronic effects in the reactants probably have a role in determining the course of the reaction of trisubstituted pyrylium salts with nucleophiles. A mixture of both 2H- and 4H-pyrans is sometimes produced, for example, from methoxide ion and 2,4,6-triphenylpyrylium perchlorate (39) no acyclic product was detected in this reaction... 

Another characteristic of electrophilic reactions of pseudoazulenes is the application of numerous cations as the electrophile, for example, diazonium salts and Vilsmeier- Haack s reagent (see Table VI), tropylium ion,135 triphenylmethyl cation,"4 pyrylium ion,119 and dithiolium ion.166 Very stable cations are formed (e.g., 120) addition of base releases the substituted pseudoazulene (see example in Eq. 10). Generally reactions of this type are thermodynamically favored (see also Section IV,B). The site of substitution... 

It should be noted that the alkyl aryl ketones 191 react with the mixture of perchloric acid and acetic anhydride under the same conditions (room temperature, 10-20 h) to give the unsymmetrical pyrylium salts 197114,115. As shown in Reference 112 this reaction is also an example of electrophilic catalysis by acylium ions, proceeding via the cationic intermediates 195 and 196 (equation 62). 

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