Towards the biotechnological isomerization of branched sugars: d-tagatose-3-epimerase equilibrates both enantiomers of 4-C-methyl-ribulose with both enantiomers of 4-C-methyl-xylulose

Synthesis of Branched-Chain Sugars with a DHAP-Dependent Aldolase: Ketones are Electrophile Substrates of Rhamnulose-1-phosphate Aldolases

Dihydroxyacetone phosphate (DHAP)-dependent rhamnulose aldolases display an unprecedented versatility for ketones as electrophile substrates. We selected and characterized a rhamnulose aldolase from Bacteroides thetaiotaomicron (RhuABthet) to provide a proof of concept. DHAP was added as a nucleophile to several α-hydroxylated ketones used as electrophiles. This aldol addition was stereoselective and produced branched-chain monosaccharide adducts with a tertiary alcohol moiety. Several aldols were readily obtained in good to excellent yields (from 76 to 95 %). These results contradict the general view that aldehydes are the only electrophile substrates for DHAP-dependent aldolases and provide a new C-C bond-forming enzyme for stereoselective synthesis of tertiary alcohols.

Advances in the enzymatic production of L-hexoses

Rare sugars have recently drawn attention because of their potential applications and huge market demands in the food and pharmaceutical industries. All L-hexoses are considered rare sugars, as they rarely occur in nature and are thus very expensive. L-Hexoses are important components of biologically relevant compounds as well as being used as precursors for certain pharmaceutical drugs and thus play an important role in the pharmaceutical industry. Many general strategies have been established for the synthesis of L-hexoses; however, the only one used in the biotechnology industry is the Izumoring strategy. In hexose Izumoring, four entrances link the D- to L-enantiomers, ketose 3-epimerases catalyze the C-3 epimerization of L-ketohexoses, and aldose isomerases catalyze the specific bioconversion of L-ketohexoses and the corresponding L-aldohexoses. In this article, recent studies on the enzymatic production of various L-hexoses are reviewed based on the Izumoring strategy.

An approach to 8 stereoisomers of homonojirimycin from (D)-glucose via kinetic & thermodynamic azido-γ-lactones

Crystal structures were obtained for the two C2 epimeric azido-γ-lactones 2-azido-2-deoxy-3,5:6,7-di-O-isopropylidene-d-glycero-d-ido-heptono-1,4-lactone and 2-azido-2-deoxy-3,5:6,7-di-O-isopropylidene-d-glycero-d-gulo-heptono-1,4-lactone prepared from kinetic and thermodynamic azide displacements of a triflate derived from d-glucoheptonolactone. Azido-γ-lactones are very useful intermediates in the synthesis of iminosugars and polyhydroxylated amino acids. In this study two epimeric azido-heptitols allow biotechnological transformations via Izumoring techniques to 8 of the 16 possible homonojirimycin analogues, 5 of which were isolated pure because of the lack of stereoselectivity of the final reductive amination. A side-by-side glycosidase inhibition profile of 11 of the possible 16 HNJ stereoisomers derived from d-glucose and d-mannose is presented.

Enzymes for the biocatalytic production of rare sugars

Carbohydrates are much more than just a source of energy as they also mediate a variety of recognition processes that are central to human health. As such, saccharides can be applied in the food and pharmaceutical industries to stimulate our immune system (e.g., prebiotics), to control diabetes (e.g., low-calorie sweeteners), or as building blocks for anticancer and antiviral drugs (e.g., l-nucleosides). Unfortunately, only a small number of all possible monosaccharides are found in nature in sufficient amounts to allow their commercial exploitation. Consequently, so-called rare sugars have to be produced by (bio)chemical processes starting from cheap and widely available substrates. Three enzyme classes that can be used for rare sugar production are keto–aldol isomerases, epimerases, and oxidoreductases. In this review, the recent developments in rare sugar production with these biocatalysts are discussed.

6-De-oxy-3,4-O-isopropyl-idene-2-C-methyl-l-galactono-1,5-lactone

X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title mol­ecule, C₁₀H₁₆O₅, in which the 1,5-lactone ring exists in a boat conformation. The absolute stereochemistry was determined by the use of d-ribose in the synthesis. The crystal exists as O—H⋯O hydrogen bonded chains of mol­ecules running parallel to the a axis with each mol­ecule acting as a donor and acceptor for one hydrogen bond.

4-C-Me-DAB and 4-C-Me-LAB - enantiomeric alkyl-branched pyrrolidine iminosugars - are specific and potent α-glucosidase inhibitors; acetone as the sole protecting group

The syntheses of 4-C-Me-DAB [1,4-dideoxy-1,4-imino-4-C-methyl-d-arabinitol] from l-erythronolactone and of 4-C-Me-LAB [from d-erythronolactone] require only a single acetonide protecting group. The effect of pH on the NMR spectra of 4-C-Me-DAB [pK(a) of the salt around 8.4] is discussed and illustrates the need for care in analysis of both coupling constants and chemical shift. 4-C-Me-DAB (for rat intestinal sucrase K(i) 0.89 μM, IC(50) 0.41 μM) is a competitive - whereas 4-C-Me-LAB (for rat intestinal sucrase K(i) 0.95 μM, IC(50) 0.66 μM) is a non-competitive - specific and potent α-glucosidase inhibitor. A rationale for the α-glucosidase inhibition by DAB, LAB, 4-C-Me-DAB, 4-C-Me-LAB, and isoDAB - but not isoLAB - is provided. Both are inhibitors of endoplasmic reticulum (ER) resident α-glucosidase I and II.

2-Azido-2-de-oxy-3,4-O-isopropyl-idene-2-C-methyl-d-talono-1,5-lactone

The relative stereochemistry of the title compound, C(10)H(15)N(3)O(5), was confirmed by the crystal structure determin-ation. The absolute configuration was determined from the use of d-lyxonolactone as the starting material. The six-membered ring adopts a boat conformation with the larger azide group, rather than the methyl group, in the bowsprit position. In the crystal structure, a bifurcated inter-molecular O-H⋯O/O-H⋯N hydrogen bond links mol-ecules into chains running parallel to the b axis.

2-Azido-3,4;6,7-di-O-isopropyl-idene-α-d-glycero-d-talo-heptopyran-ose

In the title compound, C(13)H(21)N(3)O(6), the six-membered ring adopts a twist-boat conformation with the azide group in the bowsprit position. The azide group is disordered over two sets of sites in a 0.642 (10):0.358 (10) ratio. The crystal structure consists of O-H⋯O hydrogen-bonded trimer units. The absolute configuration was determined from the use of d-mannose as the starting material.

3,4-O-Isopropyl-idene-2-C-methyl-d-galactonolactone

X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title mol­ecule, C₁₀H₁₆O₆, in which the 1,5-lactone ring exists in a boat conformation. The use of d-galactose in the synthesis determined the absolute stereochemistry. The crystal exists as O—H⋯O hydrogen-bonded layers in the ab plane, with each mol­ecule acting as a donor and acceptor for two hydrogen bonds.

2,6-Dide-oxy-2,6-imino-l-glycero-d-ido-heptitol

The title mol­ecule, C₇H₁₅NO₅, the major product from selective enzymatic oxidation followed by hydrogeno­lysis of the corresponding azido­heptitol, was found by X-ray crystallography to exisit in a chair conformation with three axial hydroxyl groups. One of the hydroxymethyl groups is disordered over two sets of sites in a 0.590 (3):0.410 (3) ratio. In the crystal, O—H⋯O, O—H⋯(O,O), O—H⋯N and N—H⋯O hydrogen bonding occurs.

tert-Butyl 2-deoxy-4,5-O-isopropylidene-D-gluconate

The relative configuration of tert-butyl 2-de­oxy-4,5-O-iso­propyl­idene-d-gluconate, C₁₃H₂₄O₆, an inter­mediate in the synthesis of 2-de­oxy sugars, was determined by X-ray crystallography, and the crystal structure consists of chains of O—H⋯O hydrogen-bonded mol­ecules running parallel to the a axis. There are two mol­ecules in the asymmetric unit. The absolute configuration was inferred from the use of d-erythrono­lactone as the starting material.

3-O-Benzhydryl-2,5-dide-oxy-2,5-imino-2-C-methyl-l-lyxono-1,4-lactone

The title bicyclic lactone, C₁₉H₁₉NO₃, is an inter­mediate in the synthesis of chiral α-methyl­prolines and branched C-methyl pyrrolidines; the absolute configuration was determined by the use of d-erythronolactone as the starting material. It exhibits no unusual crystal packing features, and each mol­ecule acts as a donor and acceptor for one C—H⋯O hydrogen bond.