SYNTHESIS OF A POLYCYCLIC PI-CONJUGATED SYSTEM CONTAINING AN AZULENE UNIT BY THE FLASH VACUUM PYROLYTIC METHOD .2. SYNTHESIS AND PARATROPIC PROPERTIES OF 3H-CYCLOPENT[A]AZULEN-3-ONE AND ITS METHYL-DERIVATIVES

3H-Cyclopent[a]azulen-3-one and two methyl derivatives, which have a new tricyclic pi-conjugated system, were synthesized starting from 9-methoxycarbonyl-1,2-dihydro-3H-cyclopent[a]azulen-3-one by the application of flash vacuum pyrolysis in the final step. All proton signals of these compounds were observed at higher fields than those of the referenced 1,2-dihydro compound, but there are no significant differences between the carbon magnetic resonances of 3H-cyclopent[a]azulen-3-ones and the reference compound. These high field shifts on H-1 NMR were appreciated in terms of an induced paramagnetic ring current caused by the contribution of a 12-pi-electron system in the periphery of this molecule. 3H-cyclopent[a]azulen-3-one was stable in the crystalline state, but gradually changed in solution to a hexacyclic compound by decarbonylative dimerization, while the 2-methyl derivative was very stable both in the crystalline state and in solution. Although the reaction of 3H-cyclopent[a]azulen-3-one with cyclopentadiene immediately gave a [4+2] cycloaddition product, the reaction with dimethyl acetylenedicarboxylate gave no products. These chemical behaviors are consistent with the results of Huckel molecular orbital calculations.