The formation of 2-furaldehyde and formic acid from pentoses in slightly acidic deuterium oxide studied by 1H NMR spectroscopy

The title reaction at 96°C and pD 1.5, 3.0, or 4.5 was followed by 1H NMR spectroscopy. The rate of pentose degradation increased in the order: arabinose ≈ xylose < ribose < 2-pentuloses. At pD 1.5, the rate of 2-furaldehyde formation increased in the same order. Increasing pD strongly accelerated the degradation of the aldoses but slightly retarded that of the ketoses. Increasing pD also retarded the formation of 2-furaldehyde, particularly from the ketoses, and increased its deuterium content at H-α (from 8–25 to 50–83 atom %) and H-3 (from 79–100 to 100 atom %). This is explained by assuming that 2-furaldehyde had formed mainly via acyclic intermediates, with reversible formation of a 3-deoxypentosulose. The formation of formic acid was slow and did not proceed via 2-furaldehyde. As evident from experiments with 1- or 5-13C-substituted aldopentoses, the formic acid was derived exclusively from the terminal pentose carbons, C-1 being somewhat more important than C-5.