A related, but as yet unexplored, model for cyclic deracemization passes not through a transient achiral intermediate (the imine in Figure ) but instead through a transient state that consists of a pair of rapidly interconverting enantiomers. A model for such a deracemization is shown in Figure . Oxidation of the chiral atropisomeric diol 1 would give hydroxyaldehyde 2, which is expected to racemise rapidly at room temperature owing to a bonding interaction between the phenolic hydroxyl group and the aldehyde carbonyl (shown in square brackets). Covalent bonding interactions that lower energy barriers to bond rotation provide the basis for Bringmann's 'lactone' method and other more recent strategies for the enantioselective synthesis of atropisomers, and related covalent bonding interactions are exploited in the rotary motor of Leigh and co-workers and other stepwise rotary molecular motors. Transient non-covalent bonding interactions, as proposed for hydroxyaldehyde 2, have been used extensively in the asymmetric synthesis of atropisomers through dynamic kinetic resolution methods using organo-, transition metal and enzymatic catalysis. In analogy to Turner's cyclic deracemization of a point chiral centre (Figure ), deracemization of atropisomeric 1 occurs if the oxidation to hydroxyaldehyde 2 proceeds enantioselectively and is coupled to a non-selective reduction back to the diol 1. Such an atropisomeric deracemization is also accompanied by net directional motion: if the oxidation of 1 to 2 is enantioselective, the rapidly interconverting mixture of enantiomeric conformers of 2 is approached selectively from one direction, and every transformation of (Sa)-1 to (Ra)-1 results from a 180° anticlockwise rotation of the upper ring, as viewed from above, indicated by the curved arrow. Fig. | Cyclic deracemization as a model for unidirectional rotation. a Cyclic deracemization of point-chiral amines by way of an achiral imine, coupling enantioselective biocatalytic oxidation with non-selective reduction using ammonia borane. b Cyclic deracemization of an atropisomeric biphenyl by way of a pair of rapidly interconverting enantiomeric conformers. c Design for a unidirectional rotary molecular motor. d Motor 3a and desymmetrized derivatives (Ra)-5a and (S,Sa)-6a. Stereochemical assignment assumes that R 1 has higher priority than OH and R 2 has lower priority than CH2OH.