D-Penturonic acids: solution studies of stable-isotopically enriched compounds by 1H- and 13C-n.m.r. spectroscopy.

Methyl D-pentofuranosides were prepared by Fischer glycosidation of the aldopentoses D-arabinose, D-lyxose, D-ribose, D-xylose, and 2-deoxy-D-erythro-pentose, and oxidized with O2 over a platinum oxide catalyst to give the corresponding methyl D-pentofuranosiduronic acids. After purification by anion-exchange chromatography, these glycosides were hydrolyzed to give the corresponding D-penturonic acids [D-arabinuronic acid (1), D-lyxuronic acid (2), D-riburonic acid (3), D-xyluronic acid (4), and 2-deoxy-D-erythro-penturonic acid (5)] in 80% yield based on the starting pentofuranoside. 1-13C-Substituted D-aldopentoses were used to prepare D-(1-13C)penturonic acids. Aqueous solutions of the 1-13C-substituted penturonic acids, studied over a range of pH values by 13C-n.m.r. spectroscopy, were found to contain alpha- and beta-furanoses, acyclic aldehyde and hydrate, and/or hydrated 2,5-lactone. The ratio of D-riburonic acid anomers was most sensitive to solution pH (alpha/beta = 0.49 and 1.2 at pH 1.9 and 4.9, respectively). The values of the 1H and 13C chemical shifts, and 1H-1H. 13C-1H, and 13C-13C spin-coupling constants, were determined by 1H-(300, 500, and 620 MHz) and 13C-(75 MHz) n.m.r. spectroscopy with the aid of 2-D 13C-1H chemical shift correlation maps, 2-D 1H-1H COSY data, and 13C substitution, and were compared to those determined previously for structurally-related furanose rings. Isomerization of the penturonic acids at pH 5.0 and 50 degrees gave the corresponding 4-pentulosonic acids.