Meso epoxide opening, enantioselective

Scheme 12.22. Concept for enantioselective meso epoxide opening by electron transfer.

One of the earliest useful methods for asymmetric opening of meso-epoxides with sulfur-centered nucleophiles was reported by Yamashita and Mukaiyama, who employed a heterogeneous zinc tartrate catalyst (Scheme 7.10) [20]. Epoxides other than cydohexene oxide were not investigated, and the enantioselectivity depended strongly on the identity of the thiol. 

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 first example of asymmetric catalytic ring-opening of epoxides with sp2-hybridized carbon-centered nucleophiles was reported by Oguni, who demonstrated that phenyllithium and a chiral Schiff base ligand undergo reaction to form a stable system that can be used to catalyze the enantioselective addition of phenyllithium to meso-epoxides (Scheme 7.24) [48]. Oguni proposed that phenyllithium... 

Complexes of other metals such as gallium, indium, lead, and antimony have also been used as Lewis acids. Catalytic enantioselective meso-epoxide ring-opening reactions using a chiral gallium(III) catalyst (Ga-Li-linked-BINOL) have been reported (Scheme 84).348 The chemical yields are much improved by linking two BINOL units. 

Using the catalytic system described above, the enantioselective opening of meso epoxides could also be pursued. Although many excellent examples of ring-opening of meso epoxides by Sn2 reactions have recently been reported, the reaction planned here is conceptually different [40]. In the SN2 reaction, the path of the incoming nucleophile has to be controlled. In the titanocene-catalyzed reaction, the intermediate radical has to be formed selectively. If an intermediate similar to that invoked in the Bartmann ring-open-... 

Our hypothesis of steric factors dominating the stability of the emerging radical centers in the transition states readily explains the enantioselective epoxide opening of meso-epoxide 35 to 36 that is shown in Fig. 3 [59,60]. In the case of a reversible epoxide opening, a stability difference of at least 3 kcalmol 1 between the two radicals 37 and 38 is necessary to explain the observed selectivity. According to the calculations this seems highly unlikely. A thermodynamically controlled epoxide opening can therefore be ruled out. 

Pakulskia, Z. Pietrusiewicz, K. M. (2004) Enantioselective desymmetrization of phospholene meso-epoxide by nucleophilic opening of the epoxide.. Tetrahedron ... 

Reymond, S. Brunei, J. M. Buono, G. (2000) New development in the enantioselective ring opening of meso-epoxides by various ion silicon sources catalyzed by an o-methoxyaryldiazaphosphonamide Lewis base., Tetrahedron Asymmetry, 11 4441-4445. 

Nakajima, M. Saito, M. Uemura, M. Hashimoto, S. (2002) Enantioselective ring opening of meso-epoxides with tetrachlorosilane catalyzed by chiral bipyridine N,N -dioxide derivatives.. Tetrahedron Lett., 43 8827-8829. 

Sekine, A. Ohshima, T. Shibasaki, M. (2002) An enantioselective formal synthesis of 4-demethoxydaunomycin using the catalytic asymmetric ring opening reaction of meso-epoxide with p-anisidine.. Tetrahedron, 58 75-82. 

Carree, F. Gil, R. Collin, J. (2005) Enantioselective ring opening of meso-epoxides by aromatic amines catalyzed by lanthanide iodo binaphtholates., Org. Lett., 7 1023-1026. 

Kureshy, R. I. Singh, S. Khan, N. H. Abdi, S. H. R. Suresh, E. Jasra. R. V. (2006) Facile enantioselective ring-opening reaction of meso-epoxides with anilines using (5)-(-)-BlNOL-Ti complex as a catalyst., Ewr. J. Org. Chem., 1303-1309. 

The enantioselective ring opening of meso-epoxides such as cyclohexene oxide, which uses benzoic acid as the nucleophile, was also investigated (see Scheme... 

Scheme 2.1.6.6 Enantioselective ring opening of meso-epoxides with benzoic acid.

Jacobsen s cobalt and chromium salen complexes 69 and 70 have proven extremely successful in the enantioselective ring opening of meso-epoxides (and kinetic resolution of racemic epoxides). Recent accounts of these most efficient and practical catalysts can be found elsewhere [71-73]. 

In metal-free catalysis enantioselective ring-opening of epoxides according to Scheme 13.27 path B has been achieved both with chiral pyridine N-oxides and with chiral phosphoric amides. These compounds act as nucleophilic activators for tetrachlorosilane. In the work by Fu et al. the meso epoxides 71 were converted into the silylated chlorohydrins 72 in the presence of 5 mol% of the planar chiral pyridine N-oxides 73 (Scheme 13.36) [74]. As shown in Scheme 13.36, good yields... 

In recent years there has been considerable progress both in the base-catalyzed isomerization of meso-epoxides and in the metal-free catalysis of enantioselective opening of meso epoxides. The former approach has proven its potential in sev-... 

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