Unexpected pyrylium to pyrylium domino transformation. Synthesis of pyrano[3,4- c ]pyran-7-ium cation and its recyclization to 2,7-naphthyridine derivative

4-Ethyl-3-formyl-2,6-diphenylpyrylium perchlorate was obtained from 2,6-diphenylpyrylium perchlorate in three steps. Its reaction with triethyl orthoformate was accompanied by rearrangement of the initial pyrylium ring and led to pyrano[3,4- c ]pyran-7-ium perchlorate system through a domino process involving ethanol addition-elimination. A plausible mechanism of the reaction is suggested on the basis of DFT quantum-chemical calculations. Reaction of the obtained pyrano[3,4- c ]pyran-7-ium perchlorate with ammonium acetate led to a 2,7-naphthyridine derivative without a skeletal rearrangement.