Generation of 3-deoxypentulose by the isomerization and β-elimination of 4-O-substituted glucose and fructose

Aldose-ketose isomerization is commonly used to prepare rare oligosaccharides such as maltulose (4-O-alpha-D-glucopyranosyl-D-fructose) and lactulose (4-O-beta-D-galactopyranosyl-D-fructose). However, both sugars are degraded under alkaline conditions via beta-elimination, while their subsequent benzylic acid rearrangement leads to the formation of isosaccharinic acids. Here, we investigated the behavior of maltose and maltulose upon heating in phosphate buffer solution at pH 7.5. Maltose was initially isomerized into maltulose. Maltulose was degraded via beta-elimination, followed by keto-enol tautomerization, which led to the formation of a 1,3-dicarbonyl intermediate bearing an aldehyde group at the C-1 position and a ketone group at the C-3 position. Subsequent hydrolysis of this intermediate afforded formic acid and 3-deoxy-D-glycero-pent-2-ulose (1) as the major products based on HPLC and NMR data. In contrast, the formation of isosaccharinic acid via benzylic acid rearrangement, not the 3-deoxypentulose, was reported under the strongly alkaline conditions (Knill and Kennedy, 2003). The heat treatment of 1.4 linked oligo- and polysaccharides possessing glucose or fructose residue at the reducing end under neutral pH conditions could be applied for the practical preparation of a 3-deoxypentulose.

Automated summary

The study investigated the behavior of maltose and maltulose during heating in a phosphate buffer solution at pH 7.5, revealing that maltose was isomerized into maltulose which then underwent degradation and tautomerization to form 3-deoxy-D-glycero-pent-2-ulose. The formation of different products under varying acid-base conditions was also observed.