Introduction and General Aspects

This year saw the publication of a triple issue (numbers 7–9) of Glycoconjugate Journal highlighting 'Glycobiology at the Millennium, a look back and a glance ahead'. Topics covered range from an appreciation of A. Kabat (Feizi and Lloyd) and affinity enhancement of lectin–carbohydrate interactions (Lee and Lee) to carbohydrates as future anti-adhesion drugs for bacterial disease (Sharon and Ofek). A symposium issue of Journal of Carbohydrate Chemistry features a collection of articles from the First Euroconference on Carbohydrates in Drug Research. A special issue of Chemical Reviews has been published entitled 'Frontiers in Carbohydrate Research', and sub-titled by the guest editor (J.K. Bashkin) 'Carbohydrates – A Hostile Scientific Frontier Becomes Friendlier'. Amongst other topics, many of which are referred to at the beginning of relevant chapters, this issue covers solid-phase oligosaccharide synthesis and combinatorial carbohydrate libraries (Seeberger and Haase), intramolecular O-glycoside formation (Schmidt and co-workers), enzyme-based and programmable one-pot strategies for synthesis of complex carbohydrates and glycoconjugates (Koeller and Wong, who have written a further review on this topic) and theoretical approaches and experimental validation of studies on the structure, conformation and dynamics of bioactive oligosaccharides (Imberty and Perez).

A review from Zechel and Withers, entitled 'Glycosidase Mechanisms: Anatomy of a Finely-tuned Catalyst', addresses issues of transition state structure, substrate distortion, acid–base catalysis and trapping of covalent intermediates. Winchester and Fleet have reviewed modification of glycosylation of glycoconjugates as a therapeutic strategy; the use of glycosphingolipid synthesis inhibitors as therapy for glycolipid storage disorders has been also reviewed. The glycan repertoire of genetically modified mice has been analysed by nano-NMR spectroscopy – a key step on the way to understanding the role of glycosylation in vivo. At the whole-cell level, synthetic A-glycolylmannosamine pentaacetate has been used to prime A-glycolylneuraminic acid formation in neural cell cultures and hence alter cell phenotype. Synthetic heparin–diazeniumdiolate conjugates have been shown to act as inhibitors of thrombin-induced blood coagulation by virtue of their ability to generate nitric oxide. A comprehensive, and comprehensible, survey of topical issues in glycobiology appears in Essentials of Glycobiology.

The synthesis and biological activity of glycolipids, with a focus on gangliosides and sulfatide, has been reviewed. Extensive studies at the forefront of chemical synthesis, detailing the successful total synthesis of the oligosaccharide antibiotic everninomycin 1, have been reported by Nicolaou and co-workers. The use of olefin metathesis in carbohydrate chemistry has been reviewed.

Combinatorial methods have been used to generate penta- and hexa-peptides with monosaccharide-recognition ability. A number of review articles have appeared concerning solution and solid-phase approaches to the generation of carbohydrate-based combinatorial libraries. The use of anomeric radicals in the synthesis of O- and C-glycosides has been reviewed, as have the conformations of the radicals and their reactions under reductive conditions. An extensive review of iodine and iodine-based reagents in carbohydrate chemisry has appeared.

Last year saw the death of Professor Guy Dutton, well known for his leading work on polysaccharide structural analysis; his obituary has appeared.

Free Sugars

1.1 Tetroses and Pentoses. – A new synthesis of 2-O-benzyl-3,4-O-isopropylidene -D-erythrose (2) from 2,3-O-isopropylidene-D-glyceraldehyde involved chain-extension by use of methyl tolyl sulfoxide, followed by benzylation of the new hydroxyl group to give 1. Quantitative transformation of the sulfoxide to a formyl group (1 ->2) was achieved by exposure to lutidine-trifluoroacetic anhydride, then aq. sodium hydrogen carbonate.

Compound 3 was prepared from commercially available (R)-(+)-5 -hydroxymethyl-5H-furan-2-one by O-benzylation and subsequent conjugate addition of (PhMe2Si)2 Cu(CN)Li2, and converted to 2-deoxy-L-ribose (5) via the 2-deoxy-L-ribonolactone derivative 4. 2'-Deoxy-D-ribose 5-phosphates 13C-labelled at C-3 and C-4, and/or at C-5, were prepared in a chemoenzymatic approach by cyclizing appropriately labelled dihydroxyacetone monophosphates with unlabelled acetaldehyde. By use of [13C2]-, [1-13C]- or [2-13C]-acetaldehyde, labels were also introduced at C-1 and/or C-2.

1.2 Hexoses. – The de novo syntheses of enantiopure D- as well as L-hexoses from vinylfuran (see Vol. 33, Chapter 2, Ref. 10) were greatly improved by use of optimal conditions for the required Sharpless catalysed asymmetric dihydroxylation.

Hetero Diels–Alder cycloaddition of α,β-unsaturated carbonyl compounds and dioxygenated alkenes in the presence of a chiral bisoxazoline-Cu(OTf)2 complex as Lewis acid catalyst furnished hexopyranose precursors in good yields and high enantomeric excess. In the synthesis of the precursor 6 of ethyl tetra -O-acetyl-β-D-mannopyranose outlined in Scheme 1, for example, a 69% overall yield and 99% ee were achieved. A new route to hex-2-uloses involving boron-or, preferably, lithium-enolates is exemplified in Scheme 2. Only 3,4-trans- products were formed when the chiral lithium amide 7 was used to generate the enolate, with the D-tagatose derivative 8 as the major (69%) and its D-psicose isomer 9 as the minor (8%) products.

A new approach to multiply-protected aminodeoxyhexoses using an Sn(OTf)2-catalysed cross-aldol condensation between lactaldehyde and a tricarbonyliron/α-aminoheptadiene complex is referred to in Chapter 9.

The synthesis of L-sugars has received considerable attention: the furfural-derived, optically active, bicyclic enone 10 served as common...