DFT calculations for the reaction pathways. (A) o-PDA facilitated isomerization of glucose to fructose via an enediol intermediate. (B) o-PDA facilitated retro-aldol cleavage of fructose to C₃ intermediates. (C) Condensation of PA and o-PDA to produce 1b via sequential dehydrations. Activation and reaction Gibbs free energies (ΔG^≠ and ΔG_r) are given in kJ mol⁻¹. The structural illustrations of transition states are displayed in Figure S20.