To uncover the structural factors leading to the high enantioselectivity observed in this DKR, the interplay between potential aryl and heteroatom-O enantiorecognition motifs within the antipodes of the 3-hydroxyphthalide derivatives 4 (R 1 = Me) and 30 (R 1 = t-Bu), and their interaction with an N-acylated HyperBTM-derived isothiouronium intermediate, were probed computationally. Computations were carried out at the M06-2XPCM(chloroform)/def2-TZVP//M06-2XPCM(chloroform)/def2-SVP level of theory using Gaussian16. Building upon previous work, an O•••S chalcogen bonding (nO to s*S-C) interaction acts as a conformational lock within the acylated HyperBTM intermediate, while the isobutyrate counterion deprotonates the alcohol and engages in a non-classical H-bond to the acylated catalyst +NC-H substituent. To deliver high enantioselectivity, a donor substrate motif is needed to promote enantiorecognition through interaction with the positively charged acylated isothiouronium intermediate. Competitive transition states arising from stabilisation of the acylated isothiouronium intermediate with either the (R)-lactol utilising the O-heteroatom adjacent to the carbinol, or with the (S)-lactol from the benzannulated aryl substituent, were calculated to understand the high product enantioselectivity observed (Figure ).