A simplified generic mechanistic scheme for this acylative DKR process is shown in Figure . Acylation of (2S,3R)-HyperBTM 3 with isobutyric anhydride generates an intermediate acyl isothiouronium ion pair. This reacts in the enantiodetermining step preferentially with the (R)-enantiomer of the substrate (4 R = Me, 30 R = t-Bu) to generate the enantioenriched phthalide ester (7 R = Me, 14 R = t-Bu) and an isothiouronium carboxylate ion pair. Subsequent reaction with the 2,6-lutidine generates the free HyperBTM catalyst. Reversible ring-opening and closure of the 3hydroxyphthalide to the corresponding keto-acid is postulated to be fast with respect to acylation of either substrate enantiomer, resulting in substrate enantiomerization and facilitating a DKR. Consistent with this model, reaction monitoring of the acylative DKR of substrate (±)-30 (R = t-Bu) showed that the enantioselectivity of product (R)-ester (99:1 er) was consistent throughout the reaction (see SI for further information). The enantiomers of phthalide 30 could not be resolved satisfactorily by HPLC analysis and so its enantioselectivity could not be unambiguously determined. However, modelling using the DYNRES simulator developed by Faber and coworkers allowed an estimate of the relative rates of enantiomerization, (R)substrate reaction, and (S)-substrate reaction as >100:100:1 (see SI).