Stereochemistry of (–)-virantmycin

An NMR investigation of the importance of intramolecular hydrogen bonding in determining the conformational equilibrium of ethylene glycol in solution

“…In contrast, that of 1,9-nonanediol, not likely to have a strong intramolecular hydrogen bond, is 1.3 ppm. 30 We find that the OH shift of 2-methoxyethanol in dilute chloroform is 1.93 ppm, which brings up an interesting question. If we suppose that 1 is strongly hydrogen bonded in CHCl 3 , then unless there is a bifurcated hydrogen-bond arrangement (4) as proposed by Krueger and Mettee, 1 one should expect the favored form to be 5 with a hydrogen-bonded OH proton and a non-hydrogenbonded OH proton.…”

1H chemical shifts in NMR: Part 23, the effect of dimethyl sulphoxide versus chloroform solvent on 1H chemical shifts

“…Also, with modern spectrometers, the OH protons of alcohols and phenols can be routinely detected at such low concentrations (ca 1 mg ml 1 ) that the OH chemical shift approximates to the limiting 1 dilution value. 20 All the alcohol and phenol OH shifts recorded in the tables are these limiting values, and these chemical shifts can also be treated in the normal manner by the CHARGE programme. Owing to exchange broadening of the acid and amide protic hydrogens, these protons are difficult to detect at these low concentrations and this precludes a similar analysis for these protons.…”

Carbohydrate Hydrogen-Bonding Cooperativity − Intramolecular Hydrogen Bonds and Their Cooperative Effect on Intermolecular Processes − Binding to a Hydrogen-Bond Acceptor Molecule

“…Such exchange can easily be controlled and manipulated simply by dilution and careful solvent handling (passing CDCl 3 through dry basic alumina to remove traces of water and acid). [47] However, because of overlapping of the signals of 3-H and 4-H, the OH resonances of compound 9 could not be unambiguously assigned, either by analysis of 3 J CH,OH , or by irradiation of the 3-H or 4-H resonance signals. The relative configurations and separations of the carbohydrate H-bonding groups (hydroxy and amide groups) depend on the pyranose ring conformation.…”

“…H-bonded hydroxy groups usually show resonances that are more deshielded than those of free OH groups. [47,51] (3) The temperature dependence of the OH [52,53] and NH [54Ϫ56] resonances permits H bonds to be detected and H-bond energies evaluated. [57] In dilute samples in nonpolar organic solvents, the value of ∆δ/∆T reflects the perturbation of possible H bonds with temperature, and weak interactions with the solvent (weak H bonds in CDCl 3 or van der Waals interactions in CCl 4 or CH 2 Cl 2 ).…”