The conformations and the ring inversions of some 8- and 10-methyl-substituted 8,9,10,11-tetrahydro-7H-cycloocta[de]naphthalenes and their 9-oxo derivatives.

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The IR and NMR studies on the conformations of several 8- and 10-methyl-substituted 8,9,10,11-tetrahydro-7H-cycloocta[de]naphthalenes (cis-8,10-dimethyl (3a), trans-8,10-dimethyl (4a), 8,8-dimethyl (5a), 8,8,10-trimethyl (6a), and 8,8,10,10-tetramethyl (7a)) and their 9-oxo derivatives (3b–7b) have been described. It was found that compounds 3–7 exist exclusively in a boat conformation in the ground state, the peri ring of which is somewhat distorted (puckered) as a result of the steric repulsion between the interior benzyl protons; the molecules of 3a and 3b are rather strongly puckered, while those of 7a and 7b exist in a less puckered form than the conformations of the other methylated compounds. In the latter case, a novel type of the reflex effect was found arising from the 1,3-synaxial CH3–CH3 steric interaction and the interior benzyl H–H interaction. Moreover, it was revealed that free energy barriers (ΔG\newq) to boat inversion for compounds 5a, 5b, 7a, and 7b are markedly increased (ΔΔG\newq=4.8–5.9 kcal/mol) as compared with the unmethylated compounds. These results were discussed in terms of the boat-to-boat interconversion process in which the pseudorotations of the peri bonds are involved. Some discussion was also made on the spectral and conformational features of the eight-membered pericyclized naphthalene ring system.
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