Angular Distortions at Tetracoordinate Carbon Planoid Distortions in α,α′‐Bridged Spiro[4.4]nonanes and [5.5.5.5]Fenestranes

Structures of α,α′‐bridged spiro[4.4]nonanes and [5.5.5.5]fenestranes, obtained from X‐ray analysis and MNDO calculations, are investigated in terms of symmetry‐deformation coordinates. The central C(C) 4 fragments of these molecules show strong preference for compression, which is due to opening of opposite bond angles. The planoid deformations and strain energies of the fenestranes are discussed.