was found between 48 and 66 ppm relative to external 1,4-dioxane.6,7 In natural abundance studies the resonance of the ring oxygen appeared considerably broader than that of the other lines7 as a result of the more restricted rotational freedom of 0-5. From another work the surprisingly low-field oxygen NMR signal at 86.0 ppm of 7-oxanorbornane was also avaialble.8 This prompted us to label the anomeric oxygen atom5b of la (H2170 10% enrichment, in boiling dry 1,4-dioxane) and after acetylation to treat lc with sodium azide. The 48.8-MFIz 170 NMR spectrum of the resulting purified 1,4-anhydro sugar 3b, shown in Figure 2, indicates a very broad signal at 85.9 ppm in perfect agreement with that reported for 7-oxanorbornane8 and strongly deshielded with respect to the ring oxygen atom of monosaccharides.6,7 This result afforded evidence for pathway b in the formation of the 1,4-anhydro sugar 3b.However, neither the 170 chemical shift of the model 2-oxanorbornane nor the signal position of 0-5 of monosaccharides in conformations other than chair was available to us. Thus, it appeared advisable to prove that the nO NMR consistency of 3b with 7-oxanorbornane was not fortuitous.Therefore, labeling of the anomeric oxygen atom of la was also carried out with 50% enriched H2lsO. After acetylation and sodium azide treatment of Id, the resulting 1,4-anhydro sugar 3c was examined by 100.62-MHz 13C NMR spectroscopy. After Gaussian resolution enhancement,9 oxygen-18-induced upfield isotopic shifts5a,5b of 0.025 ppm were detected at the two directly attached carbon sites (99.1 and 81.9 ppm in CDC13), assigned unambiguously by specific proton decoupling1 to C-l and to C-4, respectively. These resonances appeared as a pair of signals corresponding to the 13C-160 and 13C-180 species. This experiment confirmed the C-l oxy anion mechanism of pathway b in the formation of the 1,4-anhydro sugar 3c.
Supplementary Material Available: Figures 1-3, the mass spectra of Os2(CO)6Br4, Os2(CO)6Cl4, and Os3(CO)10Cl2 (3 pages). Ordering information is given on any current masthead page.
The qualitative symmetry arguments and resulting predictions are assumed to be valid despite extensive mixing, which no doubt occurs among metal-ligand and trimetal symmetry orbitals belonging to a given representation.
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