In the case of asymmetric epoxidation reactions catalysed by Jacobsen's catalyst, the substrate/catalyst ratios used are low. Often additives, such as N-oxides, are employed to enhance the enantioselectivity of the reaction. We have found that for reactions of sterically hindered, less reactive substrates, a non-selective blank reaction competes with the catalysed reaction. When low substrate/catalyst ratios or additives are used, an acceleration of the epoxidation reaction is observed. As a result a larger difference in rate with the blank reaction occurs, resulting in an increase in the enantioselectivity. (C) 1997 Elsevier Science Ltd.