Conformational Analysis of a Model Synthetic Prodiginine

A conformational analysis of a synthetic model prodiginine was carried out. In solution this compound showed a strong preference for the β conformation, in which all the heterocycles are mutually cis. This conformation provided an ideal alignment of the three N–H groups for interacting with anions when the molecule is protonated. A different conformation was also detected in d6-DMSO for the mesylate salt, assigned to the α conformation, in which the C ring is engaged in an intramolecular hydrogen bond with the OMe group. The formation of a homodimer was observed in concentrated CDCl3 solutions of the neutral free base form of this prodiginine derivative. DFT calculations and the solid state structures of the hydrochloric and methanesulfonic acid salts were in good agreement with the results observed in solution. A complete study of the relative energies of different tautomers, isomers, and supramolecular complexes supported the preference for the β conformation both in water and in the gas phase.