Synthesis and diatropicity of a phenanthrene annelated trans-dimethyldihydropyrene: a novel molecule to indicate the high .pi.-bond order of the 9,10 bond of phenanthrene
syn-8,16-Dimethylphenanthro[9,10:1',2'][2.2]methacyclophane-1,9-diene: the first derivative of 8,16-dimethyl[2.2]methacyclophanediene failing to undergo spontaneous valence isomerization to the dihydropyrene system