A catalytic cycle for the asymmetric synthesis of epoxides using sulfur ylides.

Since the pioneering work1 of Furukawa in the late 1980s, some examples2 of nonracemic epoxidation have been reported using aromatic aldehydes and chiral sulfur ylides. Enantiopure oxiranes are versatile compounds for organic synthesis. They can be transformed3 into 1,2difunctionalized derivatives by nucleophilic ring opening reactions. Our group has recently developed4 an efficient asymmetric conversion of aldehydes into epoxides via a simple C2 symmetric sulfide, (2R,5R)-2,5-dimethylthiolane 1. We reported that trans epoxides could be obtained with high yields (87%-92%) and enantiomeric excesses (86-96%) in a one-pot synthesis employing mild reaction conditions and a stoichiometric amount of sulfide (Scheme 1). In addition, the feasibility of a catalytic process was demonstrated. The use of 0.1 equiv of sulfide 1 led to an excellent yield and ee, but the reaction time at roomtemperature had to be prolonged to one month! We now wish to present5 the results of a new study devoted to (i) reducing the reaction time while maintaining the yields and enantiomeric excesses, (ii) applying these conditions to a variety of aromatic aldehydes, and (iii) demonstrating greater chiral control by using both enantiomers of 1 and a new chiral sulfide, (2R,5R)-2,5-diethylthiolane 2. Results and Discussion