Conformation proton magnetic resonance spectra

The conformation of the L-iduronic acid in dermatan sulfate has been the subject of controversy the proton magnetic resonance spectrum measured at 220 MBs (7) suggested the 04(1) conformation, but optical rotatory dispersion gave conflicting results (18). Recent X-ray diffraction (19) studies seem to favour the C] (L) form, and further evidence in sv port of this hypothesis was obtained by periodate oxidation, altough the possibility could not be excluded that a small non sulfated L-iduronic acid unit had the 04(1) conformation (20). 

The complex proton magnetic resonance spectrum of 1,3-butadiene has been analyzed, and the calculated spectrum for this AA BB CC system has been determined (24,25). The temperatme-sensitive coupling constant for the protons on carbons 2 and 3 suggests an equilibrium between the predominant s-trans con-former and a skewed conformer having out-of-plane double bonds (25). The comparatively simple spectrum shows little effect of conjugation on the chemical shifts of the carbons (26). 

The proton magnetic resonance spectrum of chorismic acid (81) was fully analysed by Edwards and Jackman and this analysis confirmed the structure and relative stereochemistry assigned to the molecule. Fm hermore the magnitude of the coupling constant between the protons Hj and H4. (11.7 Hz) as well as both the allylic coupling constants (1.9 Hz) suggested that the most extensively populated conformation of the molecule in solution is that in which the two substituents at C-3 and C-4 are quasi-equatorial (93). With this information in mind Edwards and Jackman postulated that the role of the enzyme chorismate mutase, which promotes the transformation to prephenic acid (86), is to invert the... 

At pH 4.4, 5-thio-D-xylopyranose (215) shows a slow mutarotation, namely, [a]p +202 - +178° (half-time, 10 hours). At pH 6.6, however, the half-time is only about 10 minutes. The direction of the rotation shows that 215 crystallizes in the a-D form. The nuclear magnetic resonance spectrum of 215 in deuterium oxide shows the presence of the H-1 proton of both anomers. The diaxial coupling of 8.2 Hz for the H-1 proton at the higher field (t 5.25) corresponds to the )8-D anomer in the CJ(d) conformation of the molecule having a sulfur-containing ring. 

Some interest is attached to the conformation of the metallocycle in solution, as either conformation VII or VIII are possible. In the former (VII), there is no symmetry plane plane containing the MCSiN-four membered ring, the methylene and Me2Si protons are chemically nonequivalent. In the latter case, (VIII), there is such a symmetry plane and the methylene and Me2Si protons are chemically equivalent. The room temperature and -80°C nuclear magnetic resonance spectrum is consistent... 

Some of the important differences in the nuclear magnetic resonance spectra of the /3-n-furanose form in various solvents are shown in Table I. As expected, the chemical shifts are somewhat altered on changing the solvent however, new bands appear with pyridine. In methyl sulfoxide, the mutarotation is very slow, and only one form is seen soon after dissolution. In anhydrous pyridine, the mutarotation is a little faster, so that, in a few minutes, the a- and /S-n-furanose forms are present in equal amounts. With the addition of deuterium oxide, equilibrium between the three forms is rapidly reached. This last spectrum is essentially the same as that of the crude product mentioned earlier. The small values for Jz 4 indicate that all three forms are in fixed conformations and are, therefore, not acyclic. The spectrum of the a-D-pyranose form is closely related to that of (24). The two furanose forms show, overall, a similar spectrum, except that the proton at C-1 of the )3-d anomer shows a relatively large coupling, J = 3.25, of unknown origin. The spectra of many five-membered heterocyclic compounds are anomalous, and not yet fuUy understood. 

---