Fused and Bridged Tetracyclic C13 and C14 Adamantanes. Synthesis of Methyl-2,4-ethano-, 1,2-Trimethylene-, 2,4-Trimethylene-, and 1,2-Tetramethyleneadamantane

Article abstract of DOI:10.1021/jo00349a019

Carbocation isomerization of methyltetracyclododecanes 1 and 2 gives an equilibrium mixture (5:3:2) of 1-, 2-, and 8-methylethanoadamantanes 15-17.By contrast, a tetracyclotridecane (3) which does not contain a methyl group rearranges under mild conditions to a mixture (97:3) of 1,2- and 2,4-trimethyleneadamantane (18 and 19).Under more vigorous conditions, the two sets of products interconvert, but determination of an equilibrium composition is hampered by extensive fragmentation with the formation of alkyladamantanes.Isomerization of a tetracyclotetradecane precursor (34) leads to 1,2-tetramethyleneadamantane (36) as the sole product.The products of rearrangement (15-19 and 36) are consistent with the results of empirical force field calculations.It is suggested that such calculations might be relied upon to predict the most stable isomers when kinetic barriers or extensive side reactions prevent an accurate experimental determination of the equilibrium composition.Possible rearrangement mechanisms for the conversions 3 -> 18/19 and 34 -> 36 were analyzed by means of the isomerization graph composed of all isomers connected by 1,2 alkyl shifts.Plausible pathways involving eleven steps for isomerization of 3 into 18 and eight steps for the isomerization of 34 into 36 are identified.