Catalytic asymmetric epoxidation of olefins is very useful for the synthesis of enantiomerically enriched epoxides, which are versatile building blocks for the synthesis of natural products and biologically active substances. There are many efficient protocols to mediate the epoxidation of allylic alcohols to provide satisfactory yields and enantioselectivities. 1 However, very limited catalyst systems can be used for the asymmetric epoxidation of homoallylic alcohols and those substrates in which olefin is located more far away from the hydroxy group. Sharpless asymmetric epoxidation, which was efficient for allylic alcohols, could not provide homoallylic alcohols with satisfactory enantioselectivities. 2,3 The protocol reported by our group, which used vanadium and a-amino acid-based hydroxamic acid ligands to perform the asymmetric epoxidation of homoallylic alcohols, was found to be efficient. 4,5 Unfortunately, however, the enantioselectivities of trans and cis-substituted olefins were not satisfactory. Thus, there has been no truly efficient catalytic asymmetric epoxidation of homoallylic alcohols reported. Recently, we reported a vanadium-catalyzed epoxidation of allylic alcohols with newly designed bishydroxamic acid (BHA) ligands (1a and 1b), 6 which has the following features: 1) high enantioselectivity for a wide scope of allylic alcohols, 2) less than 1mol% catalyst loading, 3) mild reaction conditions, and 4) use of aqueous tert-butyl hydroperoxide (TBHP) as an achiral oxidant instead of anhydrous TBHP. 7,8,9 (Scheme 1) Herein, we report a new modified BHA ligand that is suitable for highly enantioselective vanadium-catalyzed epoxidation of homoallylic alcohols.Initial experimental modification showed that cumene hydroperoxide (CHP) was better than TBHP to facilitate and complete the transformation and toluene was used as solvent to inhibit cyclization of the produced epoxide to the corresponding tetrahydrofuran by-product. (Scheme 1) One mol% catalyst loading was enough to perform the reaction at room temperature. Reaction proceeded smoothly and moderate enantioselectivity as well as good yield was achieved on 2 a when ligand 1b was used. (Table 1. entry 1) With this promising result in hand, new ligands based on 1b were synthesized. The enantioselectivity was increased to 90%ee with 1c. Finally, 1d, which was introduced with a more hindered substituted phenyl group, was found to be excellent for the reaction; 96% ee was obtained on 2a while the rate of the reaction was also facilitated. (Table 1.)The scope of the reaction was investigated with 1d under the modified conditions. Gratifyingly, both trans-and cis-substituted epoxides were achieved with virtually complete enantioselectivities and satisfactory yields.With the successful results of the asymmetric epoxidation of homoallylic alcohols, this catalyst system was applied to the kinetic resolution of these alcohols with outstanding selectivities (4a, 4b). (Scheme 2) Both the starting homoallylic alcohols and epoxy alcohols were obtained
