Pyrylium ions, hydrolysis

It will be observed that most syntheses yield pyrylium salts in which positions 2,4, and 6 are substituted. Since according to formulas Ib-lc these positions have a partial positive charge, it can readily be understood why electron-donating substituents (hydroxy, alkoxy, alkyl, or aryl) in these positions stabilize the pyrylium salts. Only three pyrylium salts which do not have substituents in either a-position have been reported and few unsubstituted in y or in one a-position they are less stable toward hydrolysis, and in the case of perchlorates they explode more easily, than 2,4,6-trisubstituted compounds. In fact, the former are secondary, the latter tertiary carbonium ions. This fact also explains why the parent compound (1) was prepared only in 1953. 

Pyrylium phosphonic acids (60a) have been prepared by the reaction of triethyl phosphite with pyrylium salts followed by oxidation of the product (either before or after hydrolysis) with the triphenylmethyl carbonium ion. This approach has been used to obtain the analogous thiopyrylium phos-phonates (60b) and the cyclopropene and tropylidene derivatives (61) and (62). The enamine phosphonates (63) are readily accessible from addition... 

Pyrylium perchlorate 38 forms colourless crystals which are prone to hydrolysis and decompose explosively at 275°C. It can be obtained from A -acceptor substituted pyridinium ions (see p 272), e.g. by the SO3 complex 35, on treatment with NaOH followed by addition of HCIO4. Initially, the orange-coloured sodium salt of 5-hydroxypenta-2,4-dienal (glutaconic dialdehyde in the enol form) 36 is formed which subsequently cyclizes via the red protonated species 37 [7] ... 

---