All calculations were gas-phase (no implicit solvation) performed at the BP86-D3(BJ)-def2-SVP DFT level using the ORCA 4 software package . This was achieved by initial calculation of the relaxed potential energy surfaces (rPES) for rotation of the dihedral angles where the acetate groups are attached directly to the ring carbon atom (Figure ). Due to the additional C-C bond between the acetate and sugar unit, for the 6-OAc, barriers to rotation were obviously lower and peaked at around 7 kcal/mol. Several minima were observed. For the 2-OAc, full rotation was presented with an ~30 kcal/mol barrier. For the 3-OAc, full rotation was not achievable and the maxima, before failure of the calculation, approached 40 kcal/mol. The minima were used for calculation of the respective TS. Relative to the 6-OAc TS (∆∆G TS = 0 kcal/mol), the 2-OAc TS ∆∆G TS = 4.6 kcal/mol and the 3-OAc TS ∆∆G TS = 21.3 kcal/mol. At room temperature, the Boltzmann population of an energy level of 4.6 kcal/mol, relative to 0 kcal/mol, is 0.00042. This clearly indicates that 6-acetylation in the gasphase should be strongly favoured. In all cases, H-bonding plays a part in stabilisation of each TS, as do steric interactions with adjacent atoms and chains (reduced conformational freedom) in increasing ∆∆G TS values for the 2-and 3-OH acetylations. Figure . DFT rPES for acetate rotation and transition-state modelling of the acetylation mechanism, for 2-, 3-and 6-OH acetylation using acetic anhydride.