| Résumé | ¹³C-N.m.r. spectra were recorded of compounds containing O-(1-carboxyethylidene) groups linked to galactopyranose and fucopyranose derivatives. These compounds are useful as aids in determination of the positions and configurations of pyruvic acid acetal substituents in polysaccharides. Chemical shifts of non-protonated acetal carbons depend on whether the acetal ring is 5-membered (δc 107–109.5) or 6-membered (δc 100.5–102.4). The C-3 signals of 3,4-(1-carboxyethylidene) acetals are typical, being at ∼δc 81 and in the case of the barium salt of the methyl β-d-galactopyranoside derivative. The exact value depends on the configuration, whether it is as in 6 (δc 81.1) or 5 (δc 80.4). The CH3 signals of proton-n.m.r. spectra are also diagnostically useful, falling at δ 1.97 and 2.07 respectively. (The foregoing shift-values are pH-dependent). The pyruvylated galactan from the snail, Pomacea lineata, was shown to contain some residues that could be assigned a structure corresponding, in the positions of acetal substitution and acetal configuration, to structure 6. Compound 6 (barium salt) is of interest as its ¹³C-n.m.r. spectrum lacks non-protonated carbonyl and acetal carbon resonances, when obtained by the usual procedures. While this is principally because of long T₁ values, the non-protonated acetal carbon signals are comparatively broad, possibly through slow conformational interchange. In the case of the carbonyl resonance, the lack of sensitivity is because of a low n.O.e. value of 1.4, approximately one half that of other carbon atoms in the molecule. |
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