Ring-opening kinetics of the D-pentofuranuronic acids

The ring-opening reactions of the furanose forms of the penturonic acids D-arabinuronic acid (1), D-lyxuronic acid (2), D-riburonic acid (3), and D-xyluronic acid (4) in aqueous solution have been studied as a function of temperature and solution pH by 13C saturation-transfer n.m.r. (s.t.-n.m.r.) spectroscopy using 1-13C-substituted compounds. Unidirectional rate constants of ring-opening (kopen) have been determined for the cyclic forms of 1-4 in their protonated (pH 1.5) and ionized (pH 4.5) forms, and have been compared to the k-values measured previously for structurally related furanose sugars. At 50 degrees and pH 1.5, kopen values decrease as follows: alpha-xyluronic acid (2.57 s-1) greater than alpha-riburonic (1.65 s-1) greater than beta-arabinuronic (1.52 s-1) greater than beta-xyluronic (1.09 s-1) greater than beta-riburonic (0.76 s-1) greater than beta-lyxuronic (0.55 s-1) greater than alpha-arabinuronic (0.46 s-1) greater than alpha-lyxuronic (0.40 s-1). At 50 degrees and pH 4.5, this order changes significantly (e.g., beta-arabinuronate is most reactive); in general kopen values for beta anomers appear to be enhanced relative to those for corresponding alpha anomers, suggesting the involvement of intramolecular catalysis in which the carboxylate anion assists in abstracting the hydroxyl proton from O-1. Activation energies of ring-opening, determined for the alpha and beta anomers of 1-4, were found to depend on ring configuration and solution pH.