Amino acid-catalyzed dynamic kinetic asymmetric transformations (DYKAT): one-step de novo synthesis of polyketide sugars from racemic β-hydroxy aldehydes

Construction of three contiguous stereocenters through amine-catalyzed asymmetric aldol reactions

Three contiguous stereocenters were constructed by an amino acid-catalyzed asymmetric aldol reaction of α-siloxyketones with racemizable α-haloaldehydes via dynamic kinetic resolution. One-pot catalytic asymmetric synthesis of the highly functionalized products could also be accomplished by the α-bromination of simple aldehydes and the subsequent asymmetric aldol reaction.

Dynamic resolution of 2-cyclohexylidene acetaldehydes through organocatalytic dienamine [4+2] cycloaddition

Organocatalytic formed dienamines are shown to be involved in dynamic resolution of 2-cyclohexylidene acetaldehydes.

Dynamic kinetic asymmetric transformations of β-stereogenic α-ketoesters by direct aldolization

Dynamic kinetic asymmetric transformations (DyKAT) of racemic β-bromo-α-keto esters by direct aldolization of nitromethane and acetone provide access to fully substituted α-glycolic acid derivatives bearing a β-stereocenter. The aldol adducts are obtained in excellent yield with high relative and absolute stereocontrol under mild reaction conditions. Mechanistic studies determined that the reactions proceed through a facile catalyst-mediated racemization of the β-bromo-α-keto esters under a DyKAT Type I manifold.

A tandem organocatalytic α-chlorination-aldol reaction that proceeds with dynamic kinetic resolution: a powerful tool for carbohydrate synthesis

A tandem, proline-catalyzed α-chlorination/aldol reaction is described that involves a dynamic kinetic resolution of α-chloroaldehyde intermediates. The resulting syn-chlorohydrins are produced with good to excellent diastereoselectivity in high enantiopurity and provide new opportunities for the synthesis of carbohydrates.

Enamine catalysis

The reversible reaction of primary or secondary amines with enolizable aldehydes or ketones affords nucleophilic intermediates, enamines. With chiral amines, catalytic enantioselective reactions via enamine intermediates become possible. In this review, structure-activity relationships and the scope as well as current limitations of enamine catalysis are discussed.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Resolution of 2,2-disubstituted epoxides via biocatalytic azidolysis

A practical procedure for the enzymatic resolution of 2-alkyl-2-aryl-disubstituted epoxides using the Codex HHDH P2E2 enzyme and sodium azide is reported. This method allowed the synthesis of novel regio- and enantioselective 1-azido-2-arylpropan-2-ols in excellent yields. Furthermore, these intermediates were used for the preparation of enantiomerically enriched amino alcohols and aziridines containing a tertiary center.

Dynamic kinetic asymmetric transformation (DYKAT) by combined amine- and transition-metal-catalyzed enantioselective cycloisomerization

The first examples of one-pot highly chemo- and enantioselective dynamic kinetic asymmetric transformations (DYKATs) involving alpha,beta-unsaturated aldehydes and propargylated carbon acids are presented. These DYKATs, which proceed by a combination of catalytic iminium activation, enamine activation, and Pd(0)-catalyzed enyne cycloisomerization, give access to functionalized cyclopentenes with up to 99 % ee and can be used for the generation of all-carbon quaternary stereocenters.

De-racemization of enantiomers versus de-epimerization of diastereomers--classification of dynamic kinetic asymmetric transformations (DYKAT)

The isolation of single stereoisomers from a racemic (or diastereomeric) mixture by enzymatic or chemical resolution techniques goes in hand with the disposal of 50% (racemate) or more (diastereomeric mixtures) of the "undesired" substrate isomer(s). In order to circumvent this drawback, dynamic systems have been developed for the de-racemization of enantiomers and the de-epimerizations of diastereomers. Key strategies within this area are discussed and are classified according to their underlying kinetics, that is, dynamic kinetic resolution (DKR), dynamic kinetic asymmetric transformations (DYKAT), and hybrids between both of them. Finally, two novel types of DYKAT are defined.

Asymmetric organocatalysis: from infancy to adolescence

After an initial period of validating asymmetric organocatalysis by using a wide range of important model reactions that constitute the essential tools of organic synthesis, the time has now been reached when organocatalysis can be used to address specific issues and solve pending problems of stereochemical relevance. This Review deals with selected studies reported in 2006 and the first half of 2007, and is intended to highlight four main aspects that may be taken as testimony of the present status and prospective of organocatalysis: a) chemical efficiency; b) discovery of new substrate combinations to give new asymmetric syntheses; c) development of new catalysts for specific purposes by using mechanistic findings; and d) applications of organocatalytic reactions in the asymmetric total synthesis of target natural products and known compounds of biological and pharmaceutical relevance.

Asymmetric Organocatalytic Domino Reactions

The current status of organic synthesis is hampered by costly protecting-group strategies and lengthy purification procedures after each synthetic step. To circumvent these problems, the synthetic potential of multicomponent domino reactions has been utilized for the efficient and stereoselective construction of complex molecules from simple precursors in a single process. In particular, domino reactions mediated by organocatalysts are in a way biomimetic, as this principle is used very efficiently in the biosynthesis of complex natural products starting from simple precursors. In this Minireview, we discuss the current development of this fast-growing field.

Modern aldol methods for the total synthesis of polyketides

The aldol reaction is one of the most important methods for the stereoselective construction of polyketide natural products, not only for nature but also for synthetic chemistry. The tremendous development in the field of aldol additions during the last 30 years has led to more and more total syntheses of complicated natural products. This Review illustrates by means of selected syntheses of natural products the new variants of the aldol addition. This includes aldol additions with various metal enolates, as well as metal-complex-catalyzed, organocatalytic, and biocatalytic methods.

Sugar-Assisted Kinetic Resolution of Amino Acids and Amplification of Enantiomeric Excess of Organic Molecules

The origins of biological homochirality have intrigued researchers since Pasteur's discovery of the optical activity of biomolecules. Herein, we propose and demonstrate a novel alternative for the evolution of homochirality that is not based on autocatalysis and forges a direct relationship between the chirality of sugars and amino acids. This process provides a mechanism in which a racemic mixture of an amino acid can catalyze the formation of an optically active organic molecule in the presence of a sugar product of low enantiomeric excess.

A concise enantioselective synthesis of iminosugar derivatives

The concise de novo synthesis of amino- and iminosugar derivatives is presented; the four stereocenters of the iminosugar derivatives are created in two-steps with high chemoselectivity and excellent enantioselectivity.