In Figure , we outline how one small change to the biphenyl deracemization substrate 1 reveals a simple design for a unidirectional rotary molecular motor. The same cyclic reaction network is applied to closely analogous molecule 3, which has two hydroxymethyl substituents in the ortho positions of the upper ring (Figure ). Triol 3 is now achiral by virtue of the plane of symmetry that bisects the upper ring. Enantioselective oxidation desymmetrizes triol 3 to give an enantioenriched sample of chiral monoaldehyde (Sa)-4 that itself undergoes rapid racemisation due to the bonding interaction between the phenolic hydroxyl group and the aldehyde carbonyl (shown in square brackets). From monoaldehyde 4, the concurrent nonselective reduction does not return the enantiomer of the starting triol, because this achiral triol is, by definition, superimposable upon its mirror image. Instead, it simply returns the starting The biphenyl rotary motor requires a symmetrically substituted phenyl 'rotor' ring with hydroxymethyl groups in each ortho position, and a phenolic 'stator' ring substituted by R 1 in the final ortho position. The nature of substituent R 1 allows the rotational barriers of the reduced and oxidised states of the motor (triol 3 and monoaldehyde 4) to be adjusted for optimal function. We identified ortho-fluoro triol 3a (R 1 = F), readily prepared through a fivestep synthetic sequence, as a promising motor candidate with a configurationally stable reduced state 3a and a configurationally labile oxidised state 4a (Figure ). Rotational barriers of 3a and 4a were estimated using desymmetrized derivatives. Silylation of 3a with a SiMe2t-Bu (TBS) group and chromatographic resolution gave an enantioenriched sample of desymmetrized triol 5a that showed no detectable racemisation after two days at 100 °C in toluene (ΔG ‡ rot > 136 kJ mol -1 ; Supplementary Information Section 7.3). Acylation of 4a with Mosher's acyl chloride 7 gave ester 6a, whose axial diastereoisomers show resolved signals by H and 19 F NMR spectroscopy. F NMR EXSY analysis of the mixture of diastereoisomers of 6a indicates that the resolved signals undergo chemical exchange and reveals a barrier to rotation ΔG ‡ rot ~ 77 kJ mol -1 in 2:1 DMSO:D2O (Supplementary Information Section 7.3).