Studies on synthesis of 10,11-dihydro-5H-dibenzo[a,d]cycloheptene derivatives. III. Stereostructures of isomeric 8,9-dimethoxycarbonyldibenzo[2,3:5,6]bicyclo[5.2.0]nonan-4-ones determined by 1H-1H and 13C-{1H} nuclear Overhauser effects in nuclear magnetic resonance spectroscopy.