Enantioselective reagents

There is increased emphasis throughout Part B on the representation of transition structures to clarify stereoselectivity, including representation by computational models. The current practice of organic synthesis requires a thorough knowledge of molecular architecture and an understanding of how the components of a structure can be assembled. Structures of enantioselective reagents and catalysts are provided to help students appreciate the three-dimensional aspects of the interactions that occur in reactions. 

Enantioselective reagents for ammonium ions include, for example, a mixture containing a host chiral crown ether such as 196, possessing four (R) centers and symbolized as M, a host achiral crown ether of similar functionality, symbolized as R, and a salt of a guest chiral amine, symbolized as A, which is analyzed by fast atom bombardment MS (FAB-MS), and the relative peak intensity of the equilibrium complexes 7(MA)//(RA) is measured and correlated with the chirality of the guest molecule. Many host and guest molecules have been investigated405. 

The readily available system Ln(OiPr)3 made the running for the others in carbon-carbon bond generations and functional group modifications. Meanwhile, the steadily growing demand for stereo- and enantioselective reagents has also affected the lanthanide elements and as a result more sophisticated ligands have been introduced (Fig. 35A-G). 

Enantioselective Reduction of Imines and Ketoxime O-Ethers. In addition to the reduction of prochiral ketones, chiral oxazaborolidines have been employed as enantioselective reagents and catalysts for the reduction of imines (eq 11) and ketoxime O-ethers (eq 12) - to give chiral amines. It is interesting to note that the enantioselectivity for the reduction of ketoxime O-ethers is opposite that of ketones and imines. For more information, see 2-Amino-3-methyl-l,l-diphenyl-I-butanol. 

Enantioselective protonation of prochiral enols or enolates, which provides synthetic access to optically active carbonyl compounds, is an elegantly simple test reaction for enantioselective reagents and catalysts, for which a number of examples have been described [80]. The only reaction described with alkyl enol ethers concerns the highly enantioselective protonation of enol ethers such as 55 by catalytic antibody 14D9, an antibody raised against hapten 10 [81]. Antibody 14D9 has a practical turnover of /c-at = 0.4 for substrate 55 and produces... 

Another basic reaction that has been studied extensively with the objective of developing enantioselective reagents is the reduction of ketones. All unsymmetrical ketones possess prochiral faces, and chiral reductants should be enantioselective. One class of such reductants are derivatives of the common reducing agents lithium... 

In this Chspter, we undertake our first discussion of a broad class of organic compounds called organometallic compounds, compounds that contain a carbon-metal bond. In recent years, there has been an enormous explosion in our understanding of their chemistry, particularly as stereospecific (and often enantioselective) reagents for synthetic chemistry. We have already seen one example in the Sharpless enantioselective epoxidation of alkenes (Section 11.8D). 

M. D. Mihovilovic, M. M. Kayser, S. Rodriguez, J. D. Stewart, A novel approach to the synthesis of the taxol side chain adapting S. cerevisiae as enantioselective reagent for the reduction of a-keto-p-lactams, ECHET98 Electronic Conference on Heterocyclic Chemistry, June 29-July 24,1998, Imperial College Press, 1998, pp. 162-170. 

Kubota K, Leighton JS. A highly practical and enantioselective reagent for the allylation of aldehydes. Angew. Chem. Lnt. Ed. 2003 42 946-948. 

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