13C-Labeled Idohexopyranosyl Rings: Effects of Methyl Glycosidation and C6 Oxidation on Ring Conformational Equilibria

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• 作者：Bidisha Bose-Basu ; Wenhui Zhang ; Jamie L. W. Kennedy ; Matthew J. Hadad ; Ian Carmichael ; Anthony S. Serianni
• 刊名：Journal of Organic Chemistry
• 出版年：2017
• 出版时间：February 3, 2017
• 年：2017
• 卷：82
• 期：3
• 页码：1356-1370
• 全文大小：909K
• ISSN：1520-6904

文摘

An ensemble of J_HH, J_CH, and J_CC values was measured in aqueous solutions of methyl α- and β-d-idohexopyranosides containing selective ¹³C-enrichment at various carbons. By comparing these J-couplings to those reported previously in the α- and β-d-idohexopyranoses, methyl glycosidation was found to affect ring conformational equilibria, with the percentages of ⁴C₁ forms based on ³J_HH analysis as follows: α-d-idopyranose, ∼18%; methyl α-d-idopyranoside, ∼42%; methyl β-d-idopyranoside, ∼74%; β-d-idopyranose, 82%. J_CH and J_CC values were analyzed with assistance from theoretical values obtained from density functional theory (DFT) calculations. Linearized plots of the percentages of ⁴C₁ against limiting J_CH and J_CC values in the chair forms were used to (a) determine the compatibility of the experimental J_CH and J_CC values with ⁴C₁/¹C₄ ratios determined from J_HH analysis and (b) determine the sensitivity of specific J_CH and J_CC values to ring conformation. Ring conformational equilibria for methyl idohexopyranosides differ significantly from those predicted from recent molecular dynamics (MD) simulations, indicating that equilibria determined by MD for ring configurations with energetically flat pseudorotational itineraries may not be quantitative. J-couplings in methyl α-l-[6-¹³C]idopyranosiduronic acid and methyl α-d-[6-¹³C]glucopyranosiduronic acid were measured as a function of solution pH. The ring conformational equilibrium is pH-dependent in the iduronic acid.