Solution and solid-state studies on the molecular conformation of mono- and disubstituted pyridine amidoesters: the role of characteristic C-H...O and N-H...O interactions

Molecular conformations of the model amidoester 2,6-pyridine di(ylaminosucc inamic acid methyl eater) (2) and its picrate 2PICR H2O have been studied b y X-ray diffraction, FT-IR and NMR techniques. In the solid state, two diff erent conformations have been found for 2 and its N-protonated form. The co nformation of 2 is stabilised by two intramolecular hydrogen bonds involvin g amide group oxygens and pyridine ring protons with the formation of two s ix-membered rings (CH...O distances of 2.3 and 2.3 Angstrom). The molecular conformation of 2PICR is stabilised by two intramolecular hydrogen bonds f ormed by the above-mentioned oxygens and pyridine H-atom in the system of t wo six-membered rings. According to FT-IR spectra taken in the solid state and in solution at different concentrations, four carbonyl groups of 2 are involved in a complicated pattern of inter- and intramolecular hydrogen bon ds with some possibility of dimer formation. The H-1 NMR spectra for 2 and the monosubstituted compound 4 along with the measurement of NOE effects al low us to observe the conformational change on protonation of the pyridine N-atom. The pattern of intramolecular interactions observed in the solid st ate is conserved to a large extent in solution at lower concentrations. Sin ce amide and ester C=O groups are involved in intra- and intermolecular int eractions, changing molecular conformation, lack of complexing properties, and reactivity in aminolysis reactions can be rationalised. (C) 1998 Elsevi er Science B.V. All rights reserved.