Structure of pyrimidinocyclophanes in solution by NMR

The conformations and self-associative properties of novel pyrimidinocyclophanes with a substituted nitrogen atom in a spacer have been studied using several independent NMR methods (NOE, aromatic shielding effect, 2D DOSY, GIAO DFT chemical shift calculations, DNMR) in a variety of solvents, in acidic and basic media. At room temperature the title compounds in neutral solution are in slow exchange on the NMR time-scale and exist in a folded conformation. In the acidic medium protonation occurs at the bridge nitrogen, and that leads to dramatic modifications of the structure and dynamics of the compound. The protonated form exists at the equilibrium of 'folded' and 'extended' conformers. Moreover, the protonated form is prone to association and can be effectively described as a dimer at room temperature. (c) 2006 Elsevier Ltd. All rights reserved.