| A relatively new strategy in enantioselective catalysis is the relay of stereochemical information from (a) permanent chiral center(s) to a transient or fluxional chiral center. The transient or fluxional chiral center may serve to amplify the inherent asymmetric induction provided by the permanent chiral center(s) or function as the primary stereocontrol element. This strategy is now commonly referred to as "chiral relay" and has been employed in the development of chiral relay auxiliaries, ligands, and additives. Applications of chiral relay templates are discussed in the context of enantioselective Diels-Alder, nitrile imine, and diazoacetate cycloadditions in the following chapters. An application of chiral relay ligands in diethylzinc additions to aldehydes is also discussed.;Chapter 1. A study on the role of alpha,beta-unsaturated pyrazolidinone templates in enantioselective Diels-Alder reactions is discussed. The key issues addressed are the impact of N-1 and C-5 substitution on enantioselectivity, the influence of metal geometry on the relay of stereochemical information from the chiral ligand to the pyrazolidinone template, and conformational/mechanistic insights.;Chapter 2. Chiral relay ligands based on a novel pyrazole scaffold are developed for application in enantioselective diethylzinc additions to aldehydes. The key issues approached include the effect of fluxional and permanent chirality on enantioselectivity, and the interplay between these two stereocontrol elements.;Chapter 3. The first catalytic, enantioselective methodology for 1,3-dipolar cycloadditions of nitrile imines is developed for the synthesis of chiral N-aryl dihydropyrazoles. The ability to control rotamer geometry in alpha-substituted and alpha,beta-disubstituted alpha,beta-unsaturated pyrazolidinone imides is also demonstrated.;Chapter 4. Enantioselective 1,3-dipolar cycloadditions of diazoacetates to alpha,beta-unsaturated pyrazolidinone imides are developed to synthesize enantioenriched Delta2-pyrazolines. The cycloaddition methodology is successfully applied to an enantioselective total synthesis of (--)-manzacidin A.;Chapter 5. The Sibi group's design and application of achiral auxiliaries are reviewed. The role of achiral auxiliaries in the development of enantioselective radical conjugate addition; amine conjugate addition; Diels-Alder cycloaddition; and 1,3-dipolar cycloaddition methodologies is the focus of the review. |
