Compounds with bridgehead nitrogen. Part 64 —conformational equilibria in the 6‐ p ‐nitrophenylperhydrodipyrido[1,2‐ c : 2′,1′‐ e ]imidazoles

The effect of 6‐substitution on the conformational equilibria of the perhydrodipyrido[1,2‐ c :2′,1′‐ e ]imidazoles has been investigated. r ‐6H, t ‐11a, t ‐11b‐6‐ p ‐Nitrophenylperhydrodipyrido[1,2‐ c :2′,1′‐ e ]imidazole was found to exist as a pair of enantiomeric non‐interconverting N ‐outside‐ cis‐syn‐trans conformers, whereas the r ‐6 H , c ‐11a, c ‐11b and r ‐6 H,c ‐11a, t ‐11b isomers adopted the trans‐syn‐trans conformations. Both syn isomers exhibit restricted rotation of the 6‐ p ‐nitrophenyl substituent at room temperature and below.