Mimicking Dihydroxy Acetone Phosphate-Utilizing Aldolases through Organocatalysis: A Facile Route to Carbohydrates and Aminosugars

Total Synthesis of ( + )-Swainsonine, (–)- Swainsonine, ( + )-8--

A tactical combination of either ( S)- or ( R)-proline catalyzed aldol reaction followed by intramolecular reductive amination enabled the synthesis of a chiral pyrrolidine derivative with 3 contiguous stereocenters in only 2 synthetic steps, starting from achiral precursors. This product, obtainable in both enantiomeric forms, was further exploited as a common intermediate in total syntheses of the biologically active iminosugars: ( + )-swainsonine, (–)-swainsonine, ( + )-8- epi-swainsonine, and ( + )-dideoxy-imino-lyxitol.

Organocatalysts based on natural and modified amino acids for asymmetric reactions

Small organic molecules predominantly containing C, H, O, N, S and P element are found promising molecule to accelerate chemical reactions and are named organocatalysis. In addition, these organocatalysts are easy availability, stable in water and air, inexpensive, and low toxicity, which confer a huge direct application in organic synthesis when compared to transition metal catalyzed reactions and becoming powerful tools in the construction of a selective chiral product. Interest on organocatalysis is spectacularly increased since last two decades, due to the novelty of the concept and selectivity. Based on the nature of the organocatalysts used, they are classified in to four major classes, among them one of the types is amino acids derived organocatalysts. Natural amino acids are playing important role in building blocks of protein construction, and also intermediate products of the metabolism. α-Amino acid is a molecule, that contains both amine and carboxyl functional group. Their particular structural characteristic determines their role in protein synthesis, and bifunctional asymmetric catalysts for stereoselective synthesis. Two functional groups present on a single carbon acting as an acid and base, which promote chemical transformations in concert similar to the enzymatic catalysis. The post translational derivatives of natural α-amino acids include 4-hydroxy-L-proline and 4-amino-L-proline scaffolds, and its synthetic variants based organocatalysts, whose catalytic activity is well documented. This chapter discussed past and present development of the organocatalysts derived from natural and modified amino acids for various important organic transformations reviewed.

Lactols in an asymmetric aldol-desymmetrization sequence: access to tetrahydro-4H-furo[2,3-b]pyran-2-one and tetrahydro-4H-furo[2,3-b]furan-2-one derivatives

An asymmetric aldol-desymmetrization sequence was developed which provided highly efficient access to important bicyclic oxygen-containing scaffolds with multiple chiral centers and one is a quaternary stereogenic center containing a free hydroxy group. Moreover, starting from racemic precursors, the final products were obtained as two separable diastereomers by flash chromatography. Several other heterocycles could also be easily generated with this strategy.

Asymmetric organocatalytic methods for the synthesis of tetrahydropyrans and their application in total synthesis

Recent advancement in the area of asymmetric organocatalysis led to the development of new methodologies for the construction of valuable enantiopure molecules, including various heterocycles. As one of the latter class of compounds tetrahydropyrans (THPs) constitute a core structure of a wide array of bioactive natural products. A noticeable growth has been observed in the asymmetric synthesis of THPs using small organic molecules as catalysts. This Tutorial Review describes the organocatalytic methods available to furnish THPs as well as the application of these methodologies in the total synthesis of THP-based natural products.

Synthesis of 2-Keto-d- and l-gluconic Acid via Stereoselective Direct Aldol Reactions

Stereoselective direct aldol reaction between optically pure d- or l-glyceraldehyde and hydroxyacetylfuran is demonstrated as an efficient and straightforward methodology for the synthesis of six-carbon atom d- and l-arabino-hex-2-ulosonic acids. syn-Selective aldol reactions realized by using either tertiary amines or a dizinc aldol catalyst constitute two parallel routes to the de novo synthesis of orthogonally protected biologically relevant 2-keto-d- and l-gluconic acids.

Organocatalytic Synthesis of Higher-Carbon Sugars: Efficient Protocol for the Synthesis of Natural Sedoheptulose and d-Glycero-l-galacto-oct-2-ulose

Herein we report a short and efficient protocol for the synthesis of naturally occurring higher-carbon sugars-sedoheptulose (d-altro-hept-2-ulose) and d-glycero-l-galacto-oct-2-ulose-from readily available sugar aldehydes and dihydroxyacetone (DHA). The key step includes a diastereoselective organocatalytic syn-selective aldol reaction of DHA with d-erythrose and d-xylose, respectively. The methodology presented can be expanded to the synthesis of various higher sugars by means of syn-selective carbon-carbon-bond-forming aldol reactions promoted by primary-based organocatalysts. For example, this methodology provided useful access to d-glycero-d-galacto-oct-2-ulose and 1-deoxy-d-glycero-d-galacto-oct-2-ulose from d-arabinose in high yield (85 and 74 %, respectively) and high stereoselectivity (99:1).

Four-component α-bromo-β-phosphoalkoxylation of aromatic α,β-unsaturated carbonyl compounds

Novel α-bromo-β-phosphoalkoxylated carbonyl compounds were produced in moderate to excellent yields via highly selective four-component reaction involving NBS, a cyclic ether, an organic phosphate and an aromatic α,β-unsaturated carbonyl compound.

Four-component reaction leading to highly functionalized sulfoalkoxy carbonyl compounds

Four components together: a new paradigm of four component reaction brings α-bromo-β-sulfoalkoxyl carbonyl compounds with good stereocontrol in moderate to excellent yields.

Amine-catalyzed direct aldol reactions of hydroxy- and dihydroxyacetone: biomimetic synthesis of carbohydrates

This article presents comprehensive studies on the application of primary, secondary, and tertiary amines as efficient organocatalysts for the de novo synthesis of ketoses and deoxyketoses. Mimicking the actions of aldolase enzymes, the synthesis of selected carbohydrates was accomplished in aqueous media by using proline- and serine-based organocatalysts. The presented methodology also provides direct access to unnatural L-carbohydrates from the (S)-glyceraldehyde precursor. Determination of the absolute configuration of all obtained sugars was feasible using a methodology consisting of concerted ECD and VCD spectroscopy.

Total synthesis of (+)-swainsonine and (+)-8-epi-swainsonine

Enantioselective total synthesis of (+)-swaisonine that hinges on a combination of organocatalyzed aldolization and reductive amination, affords the title compound in 9 steps, with 24% overall yield.

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.

Thermoresponsive Polymer-Supported l-Proline Micelle Catalysts for the Direct Asymmetric Aldol Reaction in Water

l-Proline moieties bound to a thermoresponsive polymer nanoreactor efficiently directed the asymmetric aldol reaction in water with excellent yields and enantioselectivity (ee). The reactions were efficient at higher temperatures in direct contrast to the low yields and ee values found when the reaction was carried out in a DMF/water mixture due to the location of the l-proline moieties within the hydrophobic pocket inside the core of the nanoreactors. This ideal environment formed for catalysis allows control over the water content as well as enhancing interactions between the carboxylic acid of l-proline and the aldehyde substrate. The nanoreactors were disassembled to fully water-soluble polymers by lowering the temperature to below the lower critical solution temperature (LCST) of the polymer, resulting in precipitation of the product in near pure form. The product was isolated by centrifugation and the polymer/water solution reused in additional catalytic cycles by heating the polymer above its LCST and thus reforming the nanoreactors. Although a small decrease in yield after five cycles was observed, the selectivity (anti/syn ratio and ee) remained high.

Aminohydroxyacetone synthons: versatile intermediates for the organocatalytic asymmetric aldol reaction

A practical method for the synthesis of 1,3-aminohydroxyacetone synthons was developed, and their utility in the organocatalytic asymmetric aldol reaction was demonstrated in a short synthesis of aza-sugars.

A highly asymmetric direct aldol reaction catalyzed by chiral proline amide – thiourea bifunctional catalysts

A series of chiral proline amide – thiourea bifunctional catalysts derived from l-proline and chiral diamine were prepared and successfully applied to highly enantioselective direct aldol reactions of cyclohexanone with various aldehydes in excellent yields (85%–97%), diastereoselectivities (anti/syn > 20:1) and enantioselectivities (up to 91% ee).

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.

Reversal of diastereoselectivity in reactions of the trifluoroacetaldehyde ethyl hemiacetal with enamines and imines: metal-free, complementary anti- and syn-selective synthesis of 4,4,4-trifluoro-1-aryl-3-hydroxy-2-methyl-1-butanones

A complete reversal of diastereoselectivity was observed for reactions of the trifluoroacetaldehyde ethyl hemiacetal with enamines and imines, derived from propiophenones, that produce 4,4,4-trifluoro-1-aryl-3-hydroxy-2-methyl-1-butanones. This process serves as the first reliable, metal-free, complementary anti- and syn-selective method to prepare 4,4,4-trifluoro-1-aryl-3-hydroxy-2-methyl-1-butanones.

Organocatalytic asymmetric assembly reactions for the syntheses of carbohydrate derivatives by intermolecular Michael-Henry reactions

Given the significance of carbohydrates in life, medicine, and industry, the development of simple and efficient de novo methods to synthesize carbohydrates are highly desirable. Organocatalytic asymmetric assembly reactions are powerful tools to rapidly construct molecules with stereochemical complexity from simple precursors. Here, we present a simple and robust methodology for the asymmetric synthesis of pyranose derivatives with talo- and manno- configurations from simple achiral precursors through organocatalytic asymmetric intermolecular Michael-Henry reaction sequences. In this process, (tert-butyldimethylsilyloxy)acetaldehyde 1 was successfully utilized in two ways: as a donor in a highly selective anti-Michael reaction and as an acceptor in a consecutive Henry reaction. Varied nitroolefins served as Michael acceptors and varied aldehydes substituted for 1 as Henry acceptors providing for the construction of a wide range of carbohydrates with up to 5 stereocenters. In these reactions, a catalyst-controlled Michael reaction followed by a substrate-controlled Henry reaction provided 3,4-dideoxytalose derivatives 6 in a highly stereoselective manner. The Henry reaction was affected by addition of a simple base such as triethylamine: A complex chiral base was not necessary. 3,4-Dideoxymannose derivatives 7 were produced by simply changing the base from triethylamine to 1,8-diazabicyclo[5.4.0]undec-7-ene. Extension of this methodology to a syn-Michael initiated sequence was also successful. A mechanistic discussion is provided to explain the unusual substrate-induced stereoselectivity of the Henry reaction.

The direct catalytic asymmetric aldol reaction

Asymmetric aldol reactions are a powerful method for the construction of carbon-carbon bonds in an enantioselective fashion. Historically this reaction has been performed in a stoichiometric fashion to control the various aspects of chemo-, diastereo-, regio- and enantioselectivity, however, a more atom economical approach would unite high selectivity with the use of only a catalytic amount of a chiral promoter. This critical review documents the development of direct catalytic asymmetric aldol methodologies, including organocatalytic and metal-based strategies. New methods have improved the reactivity, selectivity and substrate scope of the direct aldol reaction and enabled the synthesis of complex molecular targets (357 references).