Product Distribution Control for Glucosamine Condensation: Nuclear Magnetic Resonance (NMR) Investigation Substantiated by Density Functional Calculations

This is the authors' abstract. We don't add key points for this paper.

Selective conversion of glucosamine (GlcNH 2 ) to deoxyfructosazine (DOF) and fructosazine (FZ) with additives was investigated. Significantly enhanced yield of DOF can be improved to 40.2% with B(OH) 3 as the additive. Chemical shift titration (via one-dimensional nuclear magnetic resonance (1D 1 H and 13 C NMR)) and two-dimensional nuclear magnetic resonance (2D NMR) including 1 H– 13 C HSQC and 1 H– 1 H COSY are used to investigate intermolecular interactions between B(OH) 3 and GlcNH 2 . Diffusion-ordered NMR spectroscopy (DOSY) was further employed to identify intermediate species. Mechanistic investigation by NMR combined with electron spray ionization–mass spectroscopy (ESI-MS) discloses that a mixed 1:1 boron complex was identified as the major species, shedding light on the promotional effects of B(OH) 3, which is substantiated by density functional theory (DFT). Boron coordination effects make ring-opening and subsequent dehydration reaction thermodynamically and kinetically more favorable. Dehydration of dihydrofructosazine is a key step in controlling overall process (49.7 kcal/mol). Interestingly, chelating effect results in substantial reduction of this free-energy barrier (31.5 kcal/mol). Notably, FZ was gradually becoming the main product (yield up to 25.3%), with H 2 O 2 as the oxidant.