Metal-ammonia reduction—XIV : Fluoranthene: Correlation of primary product structure with HMO theoretical prediction

Reduction of fluoranthene with lithium in liquid ammonia affords the previously undetected theoretically predicted product 3,10b-dihydrofluoranthene (3) in good yield, accompanied by minor amounts of the 2,3-dihydro isomer (2) which is shown to arise via base-catalyzed isomerization of 3. Analogous reductive methylation of fluoranthene with methyl bromide affords either 10b-methyl-3,10b-dihydrofluoranthene (4) or 3,10b-dimethyl-3,10b-dihydrofluoranthene (5), as the major product, depending upon use of lithium or sodium. Mechanism and stereochemistry of these transformations are discussed. NMR spectral analysis of the 3,10b-dihydrofluoranthene derivatives is reported; these compounds are distinguished by an unusual 1,4-cyclohexadiene ring system locked in a relatively rigid flattened boat conformation.