Epoxides enantioselective ring-openings

The enantioselectivity was significantly influenced by the steric factor of the thiols employed. When p-MeC6H4SH and PhSH were used, the optical yields decreased to 69% and 3%, respectively. Shibasaki et al. have reported that gallium-lithium-bi-naphthoxide (GLB) 51 became a good catalyst for the enantioselective ring opening reaction of epoxide for the production of 52 (Eq. 7.39) [46]. 

The enantioselective ring opening of mew-epoxides with thiols can also be facilitated by chiral (salen)Ti(IV) complex.43 As shown in Scheme 4-25, in the presence of salen compound 71 and Ti(OPr )4, ring opening of mew-epoxide proceeds at —25° to —40°C, giving a product with good chemical yield and moderate ee. 

Tiible 11. Enantioselective ring opening of various tneso-epoxides with 4-methoxyphenol (105) promoted by Ga-Ii-linked-BINOL complex (116). 

T. Iida, N. Yamamoto, N. Matsunaga, H.-G. Woo, M. Shibasaki, Enantioselective Ring Opening of Epoxides with 4-Methoxyphenol Catalyzed by Gallium Hetero-bimetallic Complexes An Efficient Method for the Synthesis of Optically Active 1,2-Diol Monoethers, Angew. Chem Int. Ed. EngL 1998,32 2223-2226. 

Figure 14.20. Jacobsen s catalyst in enantioselective ring opening of epoxide...

Denmark, S. E. Barsanti, P. A. Wong, K.-T. Stavenger, R. A. (1998) Enantioselective ring opening of epoxides with silicon tetrachloride in the presence of a chiral lewis base., J. Org. Chem., 63 2428-2429. 

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. 

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. 

Schneider, C. (2006) Synthesis of 1,2-difunctionalized fine chemicals through catalytic, enantioselective ring-opening reactions of epoxides., Synthesis, 3919-3944. 

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.

Epoxides that have meso structures can be converted to optically active chiral compounds via enantioselective ring opening (Scheme 125). 

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... 

Tridentate Schiff base chromium(III) complexes were identified as the optimal catalysts for the enantioselective ring opening of meso-aziridines by TMSN3.51 Indeed, preliminary studies have shown that, although the (salen)chromium complexes catalyzed the reaction to some extent, they consistently led to low enantioselectivities (<14% ee). It was rationalized that the diminished reactivity and selectivity of the salen complexes with aziridines compared to epoxides was a result of the steric hindrance created by the /V-substituent of the coordinated aziridine. As expected, improved results were observed using tridentate ligands on the chromium center because they offer a less-hindered coordination environment (Figure 17.7).51... 

Reetz and coworkers tested catalysts for different reactions such as enantiose-lective acylation of a chiral secondary alcohol by lipases, the enantioselective ring opening of epoxides to non-racemic diols, and metathesis reactions [11, 12]. The two first examples are exothermic reactions and catalyst activity is revealed by hot spots in the IR image. The catalytic performance found by use of time-resolved IR-thermography correlated well with already known activity of the tested catalysts [11]. The metathesis reaction is particularly interesting, because it is the first example of the monitoring of endothermic reactions by means of an IR camera [12]. 

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