A comparison of the enamino carbonyl conjugation efficiency for hydrogen bonding formation in pyridone and dihydropyridone systems

The crystal structures of two compounds containing enaminone heterodiene sy stems and forming intermolecular hydrogen bonds N-H ... O are reported: 1) 3-ethoxycarbonyl-2-methyl-4-pyridone (hereafter ETPY) and 2) 3-ethoxycarbon yl-2-phenyl-6-methoxycarbonyl-5,6-dihydro-4-pyridone (hereafter EPPY). The crystal packing is controlled by intermolecular hydrogen bonds N-H ... O=C connecting the heteroconjugated enaminone groups in infinite chains, in ETP Y crystals the intermolecular hydrogen bond involves the heterodienic pathw ay with the highest pi-delocalization that is effective for a very short N ... O distance of 2.701(9) Angstrom (average from two molecules in the asym metric unit). Probably due to the steric hindrance, the hydrogen bond in EP PY is formed following the heterodienic pathway that involves the ester C=O group, although pi-delocalization along this pathway is less than that alo ng the pyridone-part pathway resulting in a longer N ... O distance of 2.88 6(3) Angstrom.