Sterically crowded cyclohexanes - 81). Synthesis,crystal structure,conformation and dynamics of pentaspiro[2.0.2.0.2.0.2.0.2.1]hexadecane,pentaspiro[3.0.2.0.3.0.2.0.3.1]- nonadecane and pentaspiro[3.0.3.0.3.0.3.0.3.1] heneicosane

The synthesis, crystal structure (7,8), conformation and dynamics of pentaspiro[2.0.2.0.2.0.2.0.2.1] hexadecane 6, pentaspiro [3.0.2.0.3.0.2.0.3.1] nonadecane 7 and pentaspiro [3.0.3.0.3.0.3.0.3.1] heneicosane 8 are described. Chair conformations have been found in the solid state (7,8) and in solution (6,7,8). The activation parameters of the chair-to-chair interconversion have been determined from bandshape analyses of exchange broadened ¹H-NMR (6,7) and¹³ C-NMR spectra (8), respectively. The results were as follows: 6: ΔH³ = 48.9 ± 0.8 kJ/mol, ΔS³ = -20.7 ± 2.8 J/mol, grd, ΔG³₂₉₈ = 55.0 ± 0.1 kJ/mol; 7: ΔH³=51.2±0.7 kJ/mol, ΔS³ = -12.0±2.4 J/mol, grd, ΔG³₂₉₈ = 54.8±0.1 kJ/mol; 8: ΔH³ - 74.2±0.6 kJ/mol, ΔS³ =-21.9 ± 1.5 J/mol, grd, ΔG³₂₉₈ = 80.7 ± 0.2 kJ/mol. On the basis of these values the barrier of inversion of the still unknown hexaspirane 5 is predicted to exceed 160 kJ/mol.