Other aspects of the MER process are less obvious. Firstly, in addition to the requirements that the coupled reaction is chemostated to be spontaneous and that the system displays kinetic asymmetry, it is essential that the relative rates of different processes are properly arranged for the system to display both stereoselectivity and high yieldif the formation of 4 from 2 and 3 is highly stereoselective but slow and the hydrolysis reaction is highly stereoselective and fast, although at steady state 4 will be highly enantioenriched, the dominant species will be aldehyde 2. These requirements can be described quantitatively using very simple equations (see ESI).