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

Abstract

In the crystalline N,N'-dimethylated uracil derivatives Za, b, the kinetically stabilized enol group forms an H-bond with 0-C(4), as demonstrated by increased shielding of specifically labelled 2a and 2b in the I70-NMR spectra (~lS('~o)(C(4)-0) = -30 ppm); absence of dilution and solvent effects show that the H-bridge is intramolecular, forming an eight-membered chelate ring. The (apparent) shielding effect dS("O) in Za,h 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.Introduction. -In spite of their lower thermodynamic stability, simple nonconjugated enols are often not too unstable under appropriate conditions (absence of catalysts, aprotic solvents) and can sometimes even be isolated, before being tautomerized to the more stable carbonyl compounds [2]. Even vinyl alcohol has a lifetime of several min at room temperature in MeCN solution 131. The reason for this sluggishness of ketonisation is that the rate-determining step, uiz. the transfer of a proton to C(a), is not a rapid reaction [4]; this contrasts with the reactions of the same enols with halogen or similar electrophiles, which are nearly diffusion-controlled [5]. Increased kinetic stability is

A Solid-State 17O Nuclear Magnetic Resonance Study of Nucleic Acid Bases

“…Nuclear magnetic resonance (NMR) spectroscopy of 1 H, 13 C, and 15 N nuclei has become an indispensable technique for studying nucleic acid structures. , Despite the fact that oxygen is also an abundant element in nucleic acid molecules, only scattered studies can be found in the literature that employ 17 O ( I = 5 / 2 ) NMR to investigate molecules related to nucleic acids. − The lack of experimental 17 O NMR data is a direct consequence of the fact that it is intrinsically difficult to obtain high-resolution NMR spectra for quadrupolar nuclei. For 17 O NMR, two major factors contribute to the practical difficulty.…”

Synthetic methodologies in organic chemistry involving incorporation of [17O] and [18O] isotopes

“…Zhuo et al . reported the synthesis of 1,3‐dimethyl‐[4‐ 17 O]‐uracil and 1,3,6‐trimethyl‐[4‐ 17 O]‐uracil from unlabeled uracils.…”

Hydrogen-Bonding in Cyclic 2-(3-Oxo-3-phenylpropyl)-Substituted 1,3-Diketones: 17O-NMR Spectra and X-Ray Structure Determination

“…We previously observed eight-membered-rings formed by intramolecular Hbonding between the side chain enolic OH and the uracil C(4)O in 1,3-dimethyl-5-(3-cyano-3-hydroxypropan-3-en-1-yl)uracil [10]. Gellman and co-workers [11] have reported intramolecularly H-bonded eight-membered rings for diamides.…”