13C-Substituted pentos-2-uloses: synthesis and analysis by 1H- and 13C-n.m.r. spectroscopy

Abstract

d- erythro-Pentos-2-ulose and d- threo-pentos-2-ulose and their 1- 13C- and 2- 13C-substituted derivatives have been prepared by oxidizing the corresponding natural and 13C-substituted d-aldopentoses ( d-arabinose, d-xylose) with cupric acetate, and purifying the products by chromatography on a cation-exchange resin in the calcium or barium form. The equilibrium compositions of the pentos-2-uloses in 2H 2O were determined by 13C-n.m.r. spectroscopy (75 MHz) at 25° and 80°. Among the eighteen possible monomeric acyclic, cyclic, and bicyclic forms, the anomeric pairs of the unhydrated aldopyranoses, aldopyranose endocyclic hydrates, aldofuranose endocyclic hydrates, and ketofuranose exocyclic hydrates were identified on the basis of 13C chemical shifts and 13C- 1H and 13C- 13C spin-coupling constants. 1H-N.m.r. (300, 500, and 620 MHz) and 13C-n.m.r. (75 MHz) spectroscopic data in one and two dimensions (DQF-COSY, homonuclear 2D- J) were used to evaluate the conformational properties of the cyclic structures. The unhydrated pyranoses are highly conformationally homogeneous; the erythro and threo isomers prefer 1 C 4 and 4 C 1 conformations, respectively. d- threo-Pentos-2-ulopyranose hydrate prefers the 4 C 1 conformation whereas the erythro isomer exists in both the 4 C 1 and 1 C 4 conformations. The furanoid forms favor structures having quasi-axial anomeric hydroxyl groups and quasi-equatorial exocyclic hydroxymethyl or dihydroxymethyl groups.