UNEXPECTED PYRYLIUM TO PYRYLIUM DOMINO TRANSFORMATION. SYNTHESIS OF PYRANO[3,4-<i>c</i>]PYRAN-7-IUM CATION AND ITS RECYCLIZATION TO 2,7-NAPHTHYRIDINE DERIVATIVE
Keywords:naphthyridine, pyrylium salts, triethyl orthoformate, domino process, recyclization
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.