1969 Volume 17 Issue 3 Pages 425-433
A ring-chain tautomerism between 3-hydroxy-6-(3'-oxoalkyl)-2(1H)-pyridone (VIII) and the alternative ring-form, 3, 6-dihydroxy-1, 2-dihydro-2-alkylindolizin-5 (3H)-one (IX), was studied by means of infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The IR spectra of VIII show the presence of a lactam-NH at 3100-3150 cm-1, a lactam-CO at 1620-1650 cm-1 and a ketone at 1700-1750 cm-1. However, the IR spectra of the ring-form (IX) lack a lactam-NH band and show only a lactam-CO band at 1620-1650 cm-1. The structures obtained from their IR spectra were also supported by their NMR spectra, in which the signal at ca. 1.45τ, due to a lactam-NH, indicates the predominance of a chain-form and also the signals at 3-4τ, due to two aliphatic OH (cis and trans), do the predominance of a ring-form. From these results, the following conclusions were made. (1) When R3=H, a ringform is the prefered, and, when R3= an alkyl, a chain-form is the prefered. However, when R2 becomes bigger than n-C5, some steric hinderance appears to keep a chain-form. (2) When R2, R3=-(CH2)n-, i.e. cyclic ketone, a chain-form is the prefered except in the case of n=4. (3) These results indicate that the ring-chain tautomerism of this type is an intramolecular, nucleophilic addition reaction of a lactam-NH to a carbonyl function.
