Total Syntheses of Carbohydrates: Organocatalyzed Aldol Additions of Dihydroxyacetone

Dihydroxyacetone: A User Guide for a Challenging Bio-Based Synthon

1,3-dihydroxyacetone (DHA) is an underrated bio-based synthon, with a broad range of reactivities. It is produced for the revalorization of glycerol, a major side-product of the growing biodiesel industry. The overwhelming majority of DHA produced worldwide is intended for application as a self-tanning agent in cosmetic formulations. This review provides an overview of the discovery, physical and chemical properties of DHA, and of its industrial production routes from glycerol. Microbial fermentation is the only industrial-scaled route but advances in electrooxidation and aerobic oxidation are also reported. This review focuses on the plurality of reactivities of DHA to help chemists interested in bio-based building blocks see the potential of DHA for this application. The handling of DHA is delicate as it can undergo dimerization as well as isomerization reactions in aqueous solutions at room temperature. DHA can also be involved in further side-reactions, yielding original side-products, as well as compounds of interest. If this peculiar reactivity was harnessed, DHA could help address current sustainability challenges encountered in the synthesis of speciality polymers, ranging from biocompatible polymers to innovative polymers with cutting-edge properties and improved biodegradability.

Asymmetric Organocatalysis Enables Rapid Assembly of Portimine Precursor Chains

Sequential organocatalytic additions of 2-furanone and dihydroxyacetone derivatives to a crotonaldehyde lynchpin provide polyhydroxylated chains reminiscent of lactonized deoxo Kdn type sugars. Further homologation via Kulinkovich ring opening of the butyrolactone and acylation of the zinc homoenolate derived from the incipient cyclopropanol allows assembly of functionalized chain precursors to portimine. Our experiments probe the stability and reactivity of monosubstituted methylidene pyrrolines and generate advanced intermediates useful for exploring the biosynthesis and de novo synthesis of portimine.

Organocatalysis applied to carbohydrates: from roots to current developments

Organocatalysis emerged in the last decade as a powerful tool for the synthesis of complex molecules. In the field of carbohydrates, it found widespread use in the synthesis of rare and non-natural carbohydrate derivatives. Additionally, it has also found important application in the stereoselective functionalization of the anomeric carbon in glycosylation reactions. These efforts culminated in the development of different types of catalysts operating through distinct activation modes that allow the selective synthesis of α- or β-glycosides even on daunting substrates. All these advances starting from its first examples in carbohydrate synthesis to the current developments in glycosylation reactions are reviewed.

Cell Factory Design and Optimization for the Stereoselective Synthesis of Polyhydroxylated Compounds

A synthetic cascade for the transformation of primary alcohols into polyhydroxylated compounds in Escherichia coli, through the in situ preparation of cytotoxic aldehyde intermediates and subsequent aldolase-mediated C-C bond formation, has been investigated. An enzymatic toolbox consisting of alcohol dehydrogenase AlkJ from Pseudomonas putida and the dihydroxyacetone-/hydroxyacetone-accepting aldolase variant Fsa1-A129S was applied. Pathway optimization was performed at the genetic and process levels. Three different arrangements of the alkJ and fsa1-A129S genes in operon, monocistronic, and pseudo-operon configuration were tested. The last of these proved to be most beneficial with regard to bacterial growth and protein expression levels. The optimized whole-cell catalyst, combined with a refined solid-phase extraction downstream purification protocol, provides diastereomerically pure carbohydrate derivatives that can be isolated in up to 91 % yield over two reaction steps.

Ultrasound mediated synthesis of dihydropyrano[3,2-d][1,3]dioxin-7-carbonitrile derivatives in H2O/EtOH medium

A one-pot cyclocondensation of 1,3-dioxane-5-one (1) with malononitrile and aromatic aldehydes in aqueous sodium hydroxide under ultrasonic irradiation furnishes a series of pyrano[3,2-d][1,3]dioxin derivatives 3. Reactions are completed after a few minutes and the precipitated products are purified by simple crystallization from ethanol. The reaction with ethyl cyanoacetate instead of malononitrile gives the respective analogous products in high yields.

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).

The long underestimated carbonyl function of carbohydrates – an organocatalyzed shot into carbohydrate chemistry

Several novel and highly stereoselective C–C bond formation processes of unprotected carbohydrates are described.

Hydrolase-catalyzed asymmetric carbon–carbon bond formation in organic synthesis

This article reviews the hydrolase-catalyzed asymmetric carbon–carbon bond-forming reactions for the preparation of enantiomerically enriched compounds in organic synthesis.

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.

Organocatalytic methods for C-C bond formation

Beyond doubt organocatalysis belongs to the most exciting and innovative chapters of organic chemistry today. Organocatalysis has emerged not only as a complement to metal-catalyzed reactions or to biocatalysis over the past decade, but also new asymmetric organocatalyzed reactions have been discovered that could not be accomplished by metal- or biocatalyzed reactions so far. This review gives a brief overview of organocatalyzed asymmetric C-C bond formation processes currently available.

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.

Stereoselective amine-catalyzed carbohydrate chain elongation

Aldol additions of unprotected carbohydrates to 1.3-dicarbonyl compounds have been described. This transformation is based on a dual activation by tertiary amines and 2-hydroxypyridine.

Recent advances in the stereoselective synthesis of carbohydrate 2-C-analogs

C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.