NMR of Terminal Oxygen. Part 14. Kinetically stabilized simple enols containing methylated uracil groups: Application of a 17O-NMR test of H-bonding

In the crystalline N,N′-dimethylated uracil derivatives 2a , b , the kinetically stabilized enol group forms an H-bond with O? C(4), as demonstrated by increased shielding of specifically labelled 2a and 2b in the ¹⁷O-NMR spectra (Δδ(¹⁷O)(C(4)—O) ? ?30 ppm); absence of dilution and solvent effects show that the H-bridge is intra-molecular, forming an eight-membered chelate ring. The (apparent) shielding effect Δδ(¹⁷O) in 2a, b is larger than that in salicylamide. The strong H-bond explains why the enols 2 , in spite of the absence of steric hindrance, are kinetically stabilized.