Tricyclic Quaternary Ammonium Salts Derived from Cinchona Alkaloids

Tricyclic systems with quaternary bridgehead nitrogen atoms are rare but an interesting class of compounds. Chiral quinuclidine derivative salts with fused five and six-membered rings (X-ray) were obtained via modification of Cinchona alkaloids. The ease of ring formation was dependent on its size, while even mild activation sufficed to close the five membered ring. On the other hand the systems with fused benzene and a six-membered ring formed atropisomers separated by a barrier of ca. 15 kcal/mol, whose interconversion was studied by DFT and NMR.