PROPERTIES OF (13)C-N.M.R. SPECTRA OF O-(1-CARBOXYETHYLIDENE) DERIVATIVES OF METHYL β-D-GALACTOPYRANOSIDE: MODELS FOR DETERMINATION OF PYRUVIC ACID ACETAL STRUCTURES IN POLYSACCHARIDES

Article abstract of DOI:10.1016/S0008-6215(00)81021-6

(13)C-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 (δ_c 107-109.5) or 6-membered (δ_c 100.5-102.4).The C-3 signals of 3,4-(1-carboxyethylidene) acetals are typical, being at ca. δ_c 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 an in 6 (δ_c 81.1) or 5 (δ_c 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 position of acetal substitution and acetal configuration, to structure 6.Compound 6 (barium salt) is of interest as its (13)C-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.