Introduction and General Aspects

As always, an extensive range of reviews and symposia proceedings of general interest in the field has appeared. A novel approach to molecular modelling of carbohydrates uses the averages of intra- and inter-molecular bonding energies and potential energy functions, and apparently allows very efficient computation of the energetics and conformational properties of carbohydrates in aqueous solution. A symposium was held on important characteristics of hydrogen bonding in molecular recognition undertaken by sugars, and an analysis has been undertaken of the effects of stereochemistry on carbohydrate hydration.

Two symposia reports have appeared on issues associated with sweetness, the first dealing with natural and artificial sweetners, which included consideration of theoretical, structural and physical chemical correlations, while the second focused on the sweetness of halogenated sugar derivatives. A lecture on the synthesis of glycothanes as receptors in novel compounds derived from α,α-trehalose has been published.

A review on protective group strategies in carbohydrate synthesis has appeared which included consideration of the preparation of phosphate and sulphate esters of heparin oligosaccharides, and a related article covered the detailed consideration of the binding domains associated with the heparin-antithrombin 3 complex.

A survey of the C-homologenation of sugars involving the use of thiazoles in aldol condensations has appeared, and another covers a diverse range of synthetic methods for synthesizing carbohydrate derivatives from acyclic precursors.

Free radical chemistry continues to be of importance in the field and a survey has been published on polar and enthalpic effects in free radical reactions. A further short review dealt with free radicals, carbenes and nitrenes at the anomeric centre of carbohydrates. A useful survey has appeared of the application of carbohydrate based chiral auxilliaries in stereoselective syntheses which covers a range of reactions, for example cycloaddition processes, reductions and Strecker reactions.

Considerable interest continues to be shown in the application of enzymes in synthesis of carbohydrate compounds, and a review has appeared on the use of aldolases, glycosyl transferases and subtilisin. Other reviews have concentrated on aldolases applied in the synthesis of C-C bonds. Another deals with the synthesis of optically active carbohydrates by biooxidation of chlorinated aromatic compounds using Pseudomonas putida. Enzymes may be also used to affect the hydroxyl groups of carbohydrates, and a review on the protection of such groups and deprotection has appeared.

Free Sugars

1 Theoretical Aspects

The chemical reactivity at different sites in α-D-glucopyranose and β-maltose has been examined by use of AMI molecular orbital calculations with a view towards an understanding of the reactivities of glucans. AM1.PM3 molecular orbital calculations on D-xylopyranose and harmonic dynamics calculations on both anomers of glucose in the crystalline state have been reported. Predictions, by means of UNIFAC, of liquid-solid phase diagrams for aqueous sugar systems, (e.g., sucrose/glucose-, sucrose/fructose-, or xylose/mannose-mixtures) compared favourably with experimental data.

The interactions between the surfaces of some heterogeneous catalysts and monosaccharides, e.g., between gallium(I) and glucose in DMSO, have been described.

2 Synthesis

Transketolase (EC 2.2.1.1), which catalyses condensations between aldehydes and hydroxypyruvic acid with loss of CO2 (see Vol. 25, Chapter 2, Ref. 9) has been produced in relatively large quantities from an over-expressed E. coli transformant carrying the transketolase gene. This enzyme was fairly non-specific with respect to the aldehyde component, although best results were achieved with α-hydroxyaldehydes.

2.1 Trioses to Hexoses.- A kinetic analysis of the formose reaction has led to the conclusion that the carbonyl rearrangement and retro-aldol reaction steps play a substantial role in the autocatalytic process. The condensation of D-glyceraldehyde acetonide with racemic alkyl (alkylthiomethyl) sulfoxides proceeded with high selectivity to produce diastereomeric l-alkylsulfinyl-l-alkylthio-3,4-O-isopropylidene-D-tetroses with a predominance of erythro-products. An example is given in Scheme 1. 4-Deoxy-D-L-glycero-tetrulose (4) has been synthesized from butanone (1) via dibromide 2 and diacetate 3. The acetyl groups were hydrolysed by use of a lipase.

The preparation of acyclic 3,4:5,6-di-O-isopropylidene-D-glucose from 1,2:3,4:5,6-tri-O-isopropylidene-D-gluconate is referred to in Chapter 6. L-Glucose derivatives have been synthesized from D-glucose via 2,3,4-tri-(9-benzyl-6-O-t-butyldimethylsilyl-D-glucono-1,5-lactone, following a modification of Shiozaki's method (see Vol. 25, Chapter 2, Ref. 15), and from 2,3,5-tri-O-benzyl-L-arabinofuranose by use of phenyldimethylsilylmagnesium chloride in a highly stereoselective Grignard reaction. Methylenation of protected D-ribono- and D-xylono-1,4-lactone 5 and 8, respectively, with Tebbe's reagent was inefficient, but exposure to Cp2TiMe2 gave alkenes 6 and 9 in 95% and 75% yield, respectively. Dihydroxylation with osmium tetroxide/NMO furnished mainly the β-anomers of D-psicofuranose and D-sorbofuranose derivatives 7 and 10. The fragmentation/rearrangement of carbohydrate anomeric alkoxy radicals formed on treatment of free sugars with iodosylbenzene has been investigated, in particular its application to the synthesis of ketoses from precursors with a free hydroxyl group at C-5 and a hydroxymethyl-branch at C-2. An example is shown in Scheme 2.

A new route to monosaccharides employing catalysed asymmetric aldol condensation has been illustrated by the preparation of L-fucose from crotonaldehyde and ketene silyl acetal 11 (Scheme 3) in 4 steps and 49% overall yield. The preparations of D-tagatose 1,6-diphosphate and of (3S/4R)- configurated ketose 1-phosphates by use of aldolases are covered in...