A NOVEL CYCLOOCTA[D]-BARRELENO[D]PYRIDAZINE REARRANGEMENT

As part of the valence tautomeric system cyclooctatetraene (2) reversi ble arrow bicyclo[4.2.0]octa-2,4,7-triene (3), the monocyclic polyene (2) reacts as a dienophile with the diazadiene 3,6-bis(trifluoromethyl )-1,2,4,5-tetrazine (1) to form after N-2-elimination two products: Th e 1:1-adduct 6, a cycloocta[d]pyridazine, and the tetracyclic bis-Diel s-Alder cycloadduct 9. Depending on solvent polarity and temperature, compound 6 displays interesting novel rearrangements. On refluxing in nonpolar solvents like toluene, compound 6 yields the barreleno[d]pyri dazine 13 in high yield. Compound 13 can be oxidized with 4-phenyltria zolinedione 14 to the barreleneo[d]pyridazine 15. When the more polar nitromethane is used as a solvent, heating of 6 at 90 +/- 2 degrees C surprisingly leads to the dihydrocycloocta[d]pyridazine 18 in a multis tep reaction including proton-shift tautomerism and [1,5]-sigmatropic rearrangement. The structures of the compounds 15 and 18 were assigned by the spectral data and verified by X-ray crystallography.