The Crystal and Molecular Structure of an Octacycloheneicosane Inert Towards Lewis Acid Isomerization

Abstract
Hydride transfer reduction (with 96% H2SO4 and methylcyclohexane) of norbornadiene trimer (1) gives exo, exo‐octacyclo[8.8.1.13,6.112,15.0 2,9.04,8.011,18.013,17]heneicosane (4). The X‐ray structural details of 4 and those calculated by force field methods are in excellent agreement. Further rearrangement of 4 to more stable C21H28 isomers (e.g., 2 or 3) does not take place even with highly active Lewis acid catalysts. This exceptional behavior is attributed mechanistically to energetically unfavorable “bottlenecks”, i. e., intermediates with quaternary carbons and highly strained structures lying along the rearrangement pathway, which cannot be achieved under the reaction conditions without decomposition taking place.

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