Asymmetric ring-opening reaction

Several methods have been developed for the synthesis of the taxol side chain. We present here the asymmetric construction of this molecule via asymmetric epoxidation and asymmetric ring-opening reactions, asymmetric dihydroxylation and asymmetric aminohydroxylation reaction, asymmetric aldol reactions, as well as asymmetric Mannich reactions. 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Asymmetric Ring Opening Reaction of Epoxides with Trimethylsilyl Azide (TMSN3)... 

Table 10. Products Yield and ee s of asymmetric ring opening reaction of ieso-stilbene oxide with aniline using complexes 70-72...

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. 

Dioos B. M. L. Jacobs P. A. (2003) Crlll(salen) impregnated on silica for asymmetric ring opening reactions and its recovery via desorption/re-impregnation.. Tetrahedron Lett, 44 8815-8817. 

Song, C. E. Oh, C. R. Rob, E. J. Choo, D. J. (2000) Cr(salen) catalysed asymmetric ring opening reactions of epoxides in room temperature ionic liquids., Chem. Commun., 1743-1744. 

Ogawa, C. Azoulay, S. Kobayashi, S. (2005) Bismuth triflate-chiral bipyridine complex catalyzed asymmetric ring opening reactions of meso-epoxide in water., Heterocycles, 66 201-206. 

Next to the above presented use of SiCl for the in situ preparation of a Lewis acid catalyst with a Lewis base for the aldol reaction, it is possible to apply this compound as a reagent in the ring opening of epoxides leading to chlorinated alcohols. Denmark [104] reported that the chiral phosphoramide 38 catalyzed the asymmetric ring opening reaction of meso-epoxides in the presence of tetrachlo-rosilane. Similar examples were provided by Hashimoto in 2002 [105], applying the A -oxide 39 as catalyst (Scheme 30). 

Later in 2005, Hashimoto [106] reported the asymmetric ring opening reaction of cyclohexane oxide with catalyst 30 and afforded the corresponding chlorohydrin in high yield and enantioselectivity (Scheme 31). 

Chiral Bi(OTf)3-Catalyzed Asymmetric Ring-Opening Reactions of mesw-Epoxides 5... 

Novel Heterogenized Catalysts for Asymmetric Ring-Opening Reactions of Epoxides... 

Sulfur nucleophiles are also able to afford 2-sulfanyl-l,2-dihydronaphthalen-l-ols such as 276 via rhodium catalyzed asymmetric ring-opening reactions, as shown in Equation (163) <2004JOC2194>. 

Figure 7.10 Dimeric Cr(saien) compiex for asymmetric ring opening reaction of epoxides.

Lautens M, Fagnou K (2001) Effects of halide ligands and protic additives on enantioselectivity and reactivity in rhodium-catalyzed asymmetric ring-opening reactions. J Am Chem Soc 123 7170-7171... 

Moreover, catalytic asymmetric ring-opening reactions of me o-epoxides with indoles, alcohols, and thiols proceeded smoothly in the presence of catalytic amounts of Sc(DS)3 and chiral bipyridine ligand 3 in water to afford (3-amino alcohols in high yield and enantioselectivity (Scheme 15.19 and Scheme 15.20)." Note that an excellent hydrophobic, asymmetric environment has been created in water. 

Catalytic asymmetric ring-opening reactions of mcio-epoxides with aromatic amines also proceeded in the presence of a catalytic amount of bismuth triflate (Bi(OTf)3) 3 and SDBS in pure water to give the corresponding p-amino alcohols in good yield with high enantioselectivity. ... 

Rhodium-catalyzed asymmetric ring-opening reactions of 1,4-epoxy-1,4-... 

Table 5 Asymmetric ring-opening reaction of epoxide with amine NH,...

Besides biomimetic complexes, Jacobsen described particularly efficient bis (chromium-salen) catalyst 9 for the asymmetric ring-opening reaction of epoxides with azide (Scheme 9) [42]. The efficiency of this class of catalysts is attributed to a cooperative mechanism, both substrates being activated toward each other by their respective chromium atom. Of note, a less pronounced cooperative effect was initially demonstrated in an intermolecular manner using monomeric Cr(N3)-salen catalyst [43]. Jacobsen also showed that an analogous cooperative mechanism takes place using polymer-supported chiral Co(salen) complexes for the hydrolytic kinetic resolution of terminal epoxides [44, 45]. 

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