Asymmetric epoxide opening

In comparison with the asymmetric epoxide-opening reactions described above in Section 9.4, catalytic desymmetrizations of meso-aziridines have relatively limited precedence [151]. Kanai and Shibasaki have disclosed a number of promising results obtained with lanthanide complexes of the chiral phosphinoxide ligand 193 (Scheme 9.24) [152, 153]. These efforts were showcased in the desymmetrization of meso-aziridine 192 with TMSN j to provide azide 194 in 96% yield and 91 % ee. Adduct 194 was subsequently converted into the anti-influenza drug oseltamivir phosphate (Tamiflu, 195) [153]. 

The asymmetric epoxidation of electron-poor cinnamate ester derivatives was highlighted by Jacobsen in the synthesis of the Taxol side-chain. Asymmetric epoxidation of ethyl cinnamate provided the desired epoxide in 96% ee and in 56% yield. Epoxide ring opening with ammonia followed by saponification and protection provided the Taxol side-chain 46 (Scheme 1.4.12). 

Asymmetric ring-opening of saturated epoxides by organoctiprates has been studied, hut only low enantioselectivities f -c 1596 ee) have so far been obtained [49, 50]. Muller et al., for example, have reported that tlie reaction between cyclohexene oxide and MeMgBr, catalyzed by 1096 of a chiral Schiffhase copper complex, gave froiis-2-metliylcyclohexanol in 5096 yield and with 1096 ee [50]. 

Catalytic Asymmetric Epoxide Ring-opening Chemistry... 

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. 

The analogous chromium complex was used in the asymmetric ring opening of meso epoxides with trimethylsilyl azide [15] (Scheme 4). In this case a strong dependence on the anion of the ionic hquid was observed. Anions leading to hydrophobic ionic hquids, such as PFe" and SbFe", led to high... 

Asymmetric Ring Opening of Some Terminal Epoxides Catalyzed by Dimeric Type Novel Chiral Co(Salen) Complexes... 

The asymmetric ring opening (ARO) of racemic terminal epoxides with H2O via hydrolytic kinetic resolution provides an efficient synthetic route to prepare optically pure terminal epoxides. The dimeric type chiral Co(salen)AlX3 complex has great potential to catalyze HKR of terminal epoxides in a highly reactive and enantioselective manner in comparison to their monomeric analogy. 

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