The 1-Methylene Derivatives of [7](2,6)Pyridinophane. Preparative and Conformational Studies

The 1-exo-methylene derivatives (IV, V and VII) of [7](2,6)pyridinophane (I) are synthesized by Wittig’s reaction of [7](2,6)pyridinophan-1-one (III) or by the dehydration of the corresponding carbinol (VI). The C-4 protons are all shielded magnetically to varying extents, depending on their respective 1-substituents. The shielding effect is ascribed to the anisotropy of the pyridine ring. The signals of the C-4 protons are temperature-dependent, and the energy barriers for the conformational changes are estimated to be 8.7 for III, 9.1 for IV, 9.1 for V and 9.8 kcal/mol for VII.