Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with commercially available chiral cyclic secondary amines as catalysts. Structure-based catalyst screening identified L-proline and 5,5-dimethyl thiazolidinium-4-carboxylate (DMTC) as the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding aldol products with high regio-, diastereo-, and enantioselectivities. Reactions employing hydroxyacetone as an aldol donor provide anti-1,2-diols as the major product with ee values up to >99%. The reactions are assumed to proceed via a metal-free Zimmerman-Traxler-type transition state and involve an enamine intermediate. The observed stereochemistry of the products is in accordance with the proposed transition state. Further supporting evidence is provided by the lack of nonlinear effects. The reactions tolerate a small amount of water (<4 vol %), do not require inert reaction conditions and preformed enolate equivalents, and can be conveniently performed at room temperature in various solvents. In addition, reaction conditions that facilitate catalyst recovery as well as immobilization are described. Finally, mechanistically related addition reactions such as ketone additions to imines (Mannich-type reactions) and to nitro-olefins and alpha,beta-unsaturated diesters (Michael-type reactions) have also been developed.

In vitro selection techniques were applied to the development of a DNA enzyme that contains three catalytically essential imidazole groups and catalyzes the cleavage of RNA substrates. Nucleic acid libraries for selection were constructed by polymerase-catalyzed incorporation of C5-imidazole-functionalized deoxyuridine in place of thymidine. Chemical synthesis was used to define a minimized catalytic domain composed of only 12 residues. The catalytic domain forms a compact hairpin structure that displays the three imidazole-containing residues. The enzyme can be made to cleave RNAs of almost any sequence by simple alteration of the two substrate-recognition domains that surround the catalytic domain. The enzyme operates with multiple turnover in the presence of micromolar concentrations of Zn2+, exhibiting saturation kinetics and a catalytic rate of >1 min-1. The imidazole-containing DNA enzyme, one of the smallest known nucleic acid enzymes, combines the substrate-recognition properties of nucleic acid enzymes and the chemical functionality of protein enzymes in a molecule that is small, yet versatile and catalytically efficient.

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