ABSTRACT
Perchlorates have been used extensively, since pyrylium Perchlorates tend to be sparingly soluble, however all Perchlorates should be treated with CAUTION: Perchlorates, particularly dry Perchlorates can decompose explosively. 2,4,6-Triphenylpyrylium undergoes exchange at the 3- and 5-positions in hot deuterioacetic acid, but the process probably involves not protonation of the pyrylium cation, but formation of an equilibrium concentration of an adduct, with acetate added to C-2, allowing enol ether protonation and thus exchange. The transformation represents a good method for preparing the nitrogen heterocycles, providing the pyrylium salt can be accessed in the first place. The initial adduct exists as one of a number of ring-opened tautomeric possibilities, depending upon conditions; it is probably the amino-dienone which recloses to give the nitrogen heterocycle. The addition of hydride to pyryliums takes place mainly at an a-position, generating 2H-pyrans which rapidly open to form the isolated products, dienones best extracted immediately into an organic solvent; the minor products are the isomeric 4H-pyrans.
