Asymmetric epoxidation using TPPP

TABLE 5.9 Catalytic asymmetric epoxidation of 1-Phenylcyclohexene using TPPP and catalysts (24) and (Jl)"... 

TABLE 5.14 Catalytic asymmetric epoxidation of various alkenes using TPPP and catalyst 36 Ph... 

The principal limitation to this system is the restricted range of temperatures in which the epoxidation can be performed (0°C to room temperature). The upper limit is determined by the oxone, which decomposes relatively quickly in the basic medium at room temperature. The lower limit is determined by the use of the aqueous medium the normal ratio of the water and acetonibile solvents used is 1 1, and this mixture freezes at around -8°C. Therefore the development of the TPPP system (tetraphenylphosphonium monoperoxybisulfate) has allowed, for the first time, oxaziridinium salt mediated epoxidation to occur at sub zero temperatures [90]. This has resulted in the highly enantioselective epoxidation of c -alkenes using catalyst 48 (Scheme 1.44) and the asymmetric total syntheses of levcromakalim 49 [91], (-)-(3 5)-lomatin 50, and (-n)-(3 5, 4 i )-trani-khellactone 51 (Fig. 1.9) [92]. 

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