Pyridine-induced solvent shifts

Johnson and co-workers have utilized the pyridine-induced solvent shifts (d =5cdci3 C5H5N p p ni.) of some model cyclohexanones in the determination of the cyclohexanone ring stereochemistry of cycloheximide (75) and some of its derivatives. For their model... 

The carbonyl plane rule is also applicable to pyridine-induced solvent shifts in some ketones. However, the reference plane is displaced so that it passes through the a-carbons rather than the carbonyl carbon (see 76). The shifts of a-tetralone (77) shown in parentheses with its molecular formula serve as an example to illustrate the modified rule. 

Pyridine-induced solvent shifts of the methyl protons of some model cyclohexanones °... 

Thermarol (525), a new -pimarane-type diol, has been isolated from Jungermannia species, together with the previously known ent-pimara-8(14),15-dien-19-ol (524) and -pimara-8(14),15-dien 19-oic acid (526) 249), Diol (525) was acetylated to give a monoacetate, which on dehydration afforded a tricyclic diene identical with -pimara-8(9),15-dien-19-yl acetate prepared by isomerization of the acetate of (524) with formic acid. Position and stereochemistry that the tertiary hydroxyl group of (525) was at C-8 and a was deduced by comparing of the C-NMR spectra of (524—526) and the pyridine-induced solvent shifts of C-18 and C-20 methyl signals in the H-NMR spectra of (525) and (526). 

The ring H- and C-chemical shifts of 8 aldofuranoses have been recorded in DMSO-de and those of 14 aldopyranoses in DMSO-de as well as D2O, to assess the solvent dependencies and the accuracy in calculated predictions of chemical shifts. A new method for determining the absolute configuration of optically active secondary alcohols (menthol, cholesterol, etc.) involves measurement of the differences in the pyridine-induced chemical shifts of their P-d- and P-l-fucofuranosyl derivatives. ... 

The chemist can use these solvent-induced chemical shift changes to clarify complex spectra that feature overlapping multiplets. Often, by adding just a small amount (5-20%) of a benzene-cJg or pyridine-ds to the CDCI3 solution of an unknown, a dramatic effect on the appearance of the spec-tram can often be observed. The chemical shifts of peaks in the proton spectrum can be shifted by as much as 1 ppm, with the result that overlapping multiplets may be separated from one another sufficiently to allow them to be analyzed. The use of this benzene trick is an easy way to simplify a crowded spectrum. 

Pyridine-d5 was found to show the most favorable solvent-induced changes of the chemical shifts for the three compounds in so far as a first-order analysis of the 60-MHz spectra was possible, the observed splittings gave no indication for solvent-induced, conformational changes. 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Pyridine is a useful alternative solvent when the use of benzene is precluded on solubility grounds. In the compounds so far studied, the shifts (A = cDCL <5c5H5n P-pm.) observed for pyridine are in general more negative than the corresponding benzene-induced shifts. The empirical use of pyridine for the simplification of spectra was first reported by Slomp and McKellar in 1960. An example of the simplification achieved is shown in Fig. 7 for 4,4,17a-trimethyl-17 -hydroxy-5-androsten-3-one (68). 

Solvent. For all practical purposes, NMR spectra are recorded in solution, although pure ( neat ) liquids and even gases can, in principle, also be examined. The solvents must meet certain requirements (Sec. 12.1) and a compromise must often be employed between using concentrated solutions (for high sensitivity) and dilute solutions (for measuring chemical shifts uninfluenced by solute-solute interactions). Besides the commonly used carbon tetrachloride, deuterochloroform, and D2O, a range of deuterated solvents (dimethyl sulfoxide, benzene, pyridine, acetone, dioxane) is commercially available. It must be emphasized that direct comparison of chemical shifts obtained in different solvents is invalid, as solvent-induced changes of up to... 

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