Introduction and General Aspects

The long awaited comprehensive IUPAC/IUBMB paper on Nomenclature of Carbohydrates which covers all aspects of the subject has received wide dissemination.' Although the contents have the formal status of 'Recommendations', they are effectively the 'rules' to be followed. The subject has been revisited, particularly from the Hungarian perspective. Hanessian's multi-authored book 'Preparative Carbohydrate Chemistry' deals with a wide range of practical aspects of modern synthesis. Examples, which include experimental details, cover such topics as oligosaccharide synthesis, the conversion of carbohydrates into carbocyclic compounds, free radical reactions, the use of organotin reagents in the field, selective O-substitution and oxidation reactions applied to carbohydrates. A further book on synthesis consists of 21 chapters written by experts on various aspects of the field including methodology and the synthesis of different carbohydrate-based natural products.

A method for the representation of oligosaccharide structures by graphic theoretical methods has been developed for the purpose of storing and searching relevant information.

Attention is drawn to the plethora of reviews that have appeared on specific features of the subject and which are noted in relevant chapters. An extreme example is Chapter 4 which begins with a catalogue of 39 references to review material relevant to oligosaccharide chemistry and biochemistry.

Reviews of wider interest have appeared on the following topics: protecting groups of value in carbohydrate chemistry; protecting groups revealed in the 1997 literature; the use of phase transfer catalyses in anomeric transformations and glycosylations; the use of sugar derivatives as phase transfer catalysts and combinational synthesis in the field.

Biologically inclined treatments have dealt with preparative biotransforma-tions of carbohydrate derivatives (survey of the 1997 literature); structure-biological activity relationships of natural products; the role of 1-deoxy-D-threo-pentulose as a precursor in thiamine biosynthesis; chemo-enzymic synthesis of oligosaccharides of importance in molecular glycobiology. A brief discussion of molecular recognition events involving carbohydrates and their biological consequences has been written for non-specialists.

A very interesting proposal by A.I. Scott regarding the prebiotic biosynthesis of polyketides, fatty acids and polyprenoids uses ribonucleosides 5'-linked to RNA as templates, the C-C bonding reactions involving monomeric units linked to O-3'.

A survey has been presented on the carbohydrate transformations that can be induced by hypervalent iodine reagents. Such topics as the direct oxidation of tri-O-acetyl-D-glucal to the corresponding 1-en-3-one, C-1, C-2 bond oxidation of derivatives protected at all the hydroxyl groups except the anomeric and H-1 abstraction from C-glycosidic compounds were covered.

A significant development has been made in use of sodium periodate for oxidizing carbohydrate α-diols with the reagent adsorbed on silica; oxidations are carried out in dichloromethane and can be followed by TLC; product isolation requires decanting of the solution and removal of the solvent; yields are very high.

Carbohydrate-containing dendrimers are attracting considerable attention (see Chapter 3) and an overview of the topic by Stoddart and colleagues has appeared.

The subject of dihexulose dianhydrides has been the subject of a thorough review.

Free Sugars

1 Theoretical Aspects

An improved method for partitioning the overall molecular charge distribution into atom-centred monopole charges has been applied to D-glucose and d-fructose. The influence of the exo-anomeric effect on the conformations of D-glucose, as well as simpler tetrahydropyran derivatives, in the gas phase and in aqueous solution has been investigated by use of ab initio calculations.

2 Synthesis

The synthesis and reactions of sugars and nucleosides under conditions pertinent to early evolution have been incorporated in a review on 'prebiotic chemistry'.

2.1 Pentoses – C-3- Modified D-erythro- and D-threo-2-deoxypentofuranose derivatives, such as compounds 1 and 2, useful as precursors of nucleoside analogues, have been synthesized from 2,3-O-cyclohexylidene-D-glyceraldehyde as outlined in Scheme 1; a zinc-mediated allylation was used for extending the carbon-chain. 1-O-Acetyl-2,3,5-tri-O -benzyl-β-L-ribofuranose was obtained from 3,5-O -Tips-protected methyl L-arabinofuranoside viaa 2-O-trifluoromethanesulfonyl-intermediate.

2.2 Hexoses – Trapping of dihydroxyacetone phosphate (prepared from α-L-glycerol phosphate by use of co-immobilized α-L-glycerolphosphate oxidase and catalase under anaerobic conditions) by DL-glyceraldehyde in the presence of an aldolase gave, after enzymic dephosphorylation, a mixture of D-fructose and D-sorbose. 3,4,6-Tri-O -acetyl-2-azido-2-deoxy-D-glucopyranose (3) (or its manno-analogue) underwent fragmentation on exposure to (diacetoxyiodo)-benzene and iodine to give nitrile 6, by way of alkoxyl radical 4 and a-azido-carbinyl radical 5, as shown in Scheme 2.

In a novel isomerization reaction ditosylate 7 was converted to methyl α-D-altropyranoside (8) by treatment with sodium acetate in DMF, followed by aqueous-KOH; a manno-configured epoxide-intermediate has been postulated.

2,3,4,6-Tetra-O-llyl- and -benzyl-D-glucose have been obtained in 77% yield by thiolysis (EtSH/BF3OEt2,) of per-O-allyl- and per-O-benzylsucrose, respectively and hydrolysis of sucrose, maltose, cellobiose or starch by use of commercial, acidic zeolites at 80–150 °C in aqueous media gave D-glucose in >90% yield. D-Fructose was available similarly by hydrolysis of inulin.

2.3 Chain-extended Sugars – Fructose 1,6-diphosphate aldolase-catalysed condensation of...