THE FORMATION OF 2-FURALDEHYDE AND FORMIC-ACID FROM PENTOSES IN SLIGHTLY ACIDIC DEUTERIUM-OXIDE STUDIED BY H-1-NMR SPECTROSCOPY

The title reaction at 96 degrees C and pD 1.5, 3.0, or 4.5 was followed by H-1 NMR spectroscopy. The rate of pentose degradation increased in the order: arabinose approximate to xylose < ribose < 2-pentuloses. At pD 1.5, the rate of 2-furaldehyde formation increased in the same cader. 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-alpha (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-C-13-substituted aldopentoses, the formic acid was derived exclusively from the terminal pentose carbons, C-1 being somewhat more important than C-5.