Abstract
The cyclohepta-3,5-diene ring in the title compound, C 14 H 12 N 2 O 6 , is the first example of a 'free' ring to be characterized by crystal structure analysis. All previous examples are either bridged, fused to other rings or have exocyclic conjugative substituents attached to the ring sp 2 -C atoms. All but one of these examples adopts the C 2 -symmetric (twist) conformation, despite the fact that experimental gas-phase and computational results show a C s -symmetric (envelope) conformation for the free ring. The free cyclohepta-3,5-diene in the title compound clearly adopts a C s -symmetric conformation with C1 displaced from the plane of the other six ring atoms. The ring is forced to adopt the C 2 -symmetric conformation in more complex systems so as to minimize unfavourable intramolecular non-bonded repulsions.
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