17O NMR of Enamino-Diesters: Intramolecular Hydrogen Bonding in 5-Alkylaminomethylene-2,2-dimethyl-1,3-dioxane-4,6-diones

Natural abundance 17O NMR spectra of a series of 5-alkylaminomethylene-2,2-dimethyl-1,3-dioxane-4,6-diones, recorded in acetonitrile solution, are reported. The δ(17O) values correlate well with those of 4-alkylaminobut-3-en-2-ones and with the pKa values of amines. The effects of the N-alkyl groups on the 17O shift values is diminished, owing to the resonance effects of the alkoxy groups. The shift difference (DδHB) between the two carbonyl groups is mainly attributed to the intramolecular hydrogen bonding and depends on the donor property of the amino group.