Meso-Epoxide ring opening

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. 

Carree, F. Gil, R. Collin, J. (2004) Samarium iodides catalyzed meso-epoxides ring opening by aromatic amines.. Tetrahedron Lett., 45 7749-7751. 

Scheme 2.15 Enantioselective meso-epoxide ring-opening reaction with 4-metbojgr-pbenol witb tbe use of chiral heterobimetallic gallium(m) lithium(i) bis[binaphtholate).

Scheme 13.30 Cataiytic enantioseiective meso-epoxide ring opening with TMSCN.

Highly active oligomeric (salen)Co complexes such as (198) were designed for asymmetric hydrolysis of meso-epoxides and kinetic resolution of terminal epoxides, based on cooperative bimetallic mechanism postulated for epoxide ring-opening reactions (Scheme 16.59) [83, 84]. 

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... 

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... 

Method B was also used in the preparation of occluded (salen)Cr complexes. ligands Ih and li were prepared within the pores of Cr -exchanged EMT and Y zeolites, respectively [25]. These complexes were tested as catalysts in the ring opening of meso-epoxides with trimethylsilyl azide (Scheme 4). The occluded complexes showed a dramatic decrease in catalytic... 

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-... 

To avoid the inherent limitations of a kinetic resolution process, the reaction was extended to desymmetrization of prochiral meso epoxides. A number of cyclic di-methylidene epoxides were synthesized and subjected to treatment with Et2Zn in the presence of Cu(OTf)2 and ligands 42 or 43. As in the case mentioned above, ligand 42 was superior in terms of selectivity. Cydohexane derivative 46 gave the ring-opened product with a 97% ee and in a 90% isolated yield, with a y/a ratio of 98 2 (Scheme 8.28). The other substrates investigated produced sigmficantly lower ees of between 66% and 85%. 

Eppley, A. W. Totah, N. I. (1997) A new chiral titanium species for the ring opening reactions of meso-epoxides., Tetrahedron, 53 16545-16552. 

Schaus, S. E. Larrow, J. F. Jacobsen, E. N. (1997) Practical synthesis of enantiopure cyclic 1,2-amino alcohols via catalytic asymmetric ring opening of meso-epoxides., J. 

Nugent, W. A. (1998) Desymmetrization of meso-epoxides with halides A new catalytic reaction based on mechanistic insight, J. Am. Chem. Soc., 120 7139-7140. Bruns, S. Haufe, G. (1999) Catalytic asymmetric ring opening of epoxides to chlorohydrins with mild chloride donors and enantiopure titanium complexes.. 

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. 

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