I. The Rates of Formation of Some Hexoside 2,3-Epoxides The secondary tosylates of some 4,6-0-ethylidene derivatives of compounds with the D-glucose configuration have been prepared, and their rates of cyclisation to 2,3-epoxides in alkaline solution have been measured. In excess sodium hydroxide solution, the cyclisation of the tosylates was found to exhibit kinetics first-order with respect to the concentration of the tosylate. The relative rates of reaction obtained from these experiments have been correlated with the structural and stereochemical features of the tosylates in terras of recent theories of reaction mechanism and conformational analysis. It is clear from the results that electronic as well as steric effects must be invoked to explain the differential reactivities found in these systems. Evidence relevant to the effect of conformational energy barriers ("passing interactions") on the rates of reaction is discussed.