Reactions of Methyl-Substituted Cyclopentanones with Lithium Aluminum Hydride and Methyllithium. Structural Determinations and Proton Nuclear Magnetic Resonance Study of the Reaction Products

Pour methyl-substituted cyclopentanones were reacted with lithium aluminum hydride in tetrahydrofuran and with methyllithium in ether at 0°C. In general, secondary trans alcohols were formed preferentially with lithium aluminum hydride, while tertiary cis alcohols were obtained as the major product with methyllithium. In 2,5-di-methyl- and 2,2,5-trimethylcyclopentanones, the original preference of the cis alcohols was reversed when methyllithium was replaced with methylmagnesium iodide. The structures of the reaction products were determined by comparison with an authentic trans alcohol which was prepared from the hydroboration-oxidation of the corresponding olefin. 1H NMR studies of the resultant alcohols are presented. The chemical shift of 1-H or 1-Me decreases as the number of methyl substituents increases; these chemical shifts are smaller in the trans alcohol series than those in the cis alcohols. An enormously large decrease in chemical shift, 1.25 ppm, was observed when 1-H was shielded by two cis-methyl groups at C-2 and C-5. The same phenomenon was observed for the 1-Me group. The vicinal coupling constants of 1-H were found to be 4.5 Hz for the cis alcohols and 8.0 Hz for the trans alcohols in the 2,5-dimethyl- and 2,2,5-trimethylcyclopentanol systems.