Further work considered the propensity for DKR in the related benzoxazinone scaffold (Figure ), with benzannulation expected to significantly change both the reactive conformation and relative rates of competitive acylation of the ring-opened phenol or the tetra-substituted lactol. Optimisation using the C(2)-phenyl substituted lactol (see SI for full details) showed that chlorinated solvents (CHCl3 and CH2Cl2) led to significant (up to 50%) formation of the unwanted ring-opened ester product. Toluene led to essentially exclusive formation of the benzoxazinone lactol ester, while the use of diphenylacetic anhydride and the addition of base was necessary to promote full conversion, giving 66 in 86% yield and 92:8 er. Reaction monitoring showed that the lactol remained racemic and the enantioselectivity of the lactol ester remained constant with conversion (see SI), consistent with the rate of enantiomerization being faster than acylation in this system. Subsequent variation of the C(2)-substituent showed that benzyl, methyl, vinyl and allyl substituents were tolerated, alongside N(4)-Me substitution, giving the corresponding lactols 67-71 in up to 88% yield and 91:9 er. The effect of substitution of the benzoxazinone skeleton was also probed, with C(6)-and C(8)-substituted derivatives giving 73-75 in good to excellent yield and up to 90:10 er. Notably, the incorporation of a C( )-F substituent gave preferential formation of the ring-opened ester (63% yield) but gave 72 in 23% yield (90:10 er). Density Functional Theory (DFT) was used to identify the factors that determine the preferences behind cyclic vs acyclic products and enantioselectivity of the acylation. Specifically, PBE level of theory with Grimme's empirical dispersion corrections with Becke Johnson dampening parameter (D3BJ) and the 6-31G (d) basis set was used. All computations were performed under SMD solvation with toluene at 298.15 K. The energies were refined with the larger def2-TZVP basis set. Recognised enantiorecognition motifs in isothiourea-catalyzed KRs include aryl, heteroaryl, alkenyl, alkynyl, heteroatom, C=O, CF2, and P=O substituents. With potentially competitive N-C=O and heteroatom O structural motifs adjacent to the carbinol within the morpholine lactol skeleton in this case, the origins of The (2S,3R)-HyperBTM 13 catalyzed acylation of C(2)-Me substituted morpholinone lactol 1 by isobutyric anhydride was investigated (Figure ). As reaction monitoring of this system (Fig ) had indicated that the lactol is racemic throughout, the barrier to enantiomerisation via reversible ring opening and closure was expected to be small compared to alcohol acylation, and this was confirmed by DFT (See Supporting Information). Following the model developed in our previous kinetic resolution work, enantiorecognition between an acylated HyperBTM isothiouronium intermediate with the enantiomers of the morpholinone lactol 1, as well as acylation of the ring-opened alcohol 76 was probed. In all transition structures, two common structural motifs were seen: i. an O•••S chalcogen bonding (nO to s*S-C) interaction was observed between the acyl oxygen and the catalyst sulfur. ii. the isobutyrate deprotonating the alcohol participated in non-classical H-bonding to the acylated catalyst +NC-H substituent (2.1-2.3 Å). The major morpholinone (S)-lactol 1 acylation transition state TS-II leads to the observed major cyclic (R)-ester product 2 with a barrier of 13.7 kcal/mol (note: CIP priority changes). The acylation TS-I gives the minor acyclic ester product 7 with a barrier of 15.6 kcal/mol. This is 1.9 kcal/mol higher than the favoured TS-II leading to the cyclic ester product 2 (96:4). This result is in line with the observed product ratios of 91:9 (Figure ).