Introduction and General Aspects

The 1989 literature of mono- and oligo-saccharide chemistry illustrates continuing general advances on all fronts with strong emphasis on the development of synthetic methods and their application to problems with origins in biology. Many examples of this are clearly given in the Sections on oligosaccharides, nucleosides and antibiotics, while the role of carbohydrate chemistry in general organic chemistry is well illustrated by the many complex conversions of sugar derivatives into enantiomerically pure natural substances of a non-carbohydrate nature. Once again, however, biological issues – in this case medicinal – are commonly the driving forces behind the work.

Relevant reviews have appeared on recent developments in modern aspects of synthetic carbohydrate chemistry, stereoselective chemical syntheses of sugar derivatives, syntheses of unusual sugars by combinations of chemical and enzymic methods, and purely enzymic syntheses. Baer has surveyed recent synthetic studies of nitrogen-containing, deoxygenated sugars and related compounds, and more specifically, the use of allylboronates in the synthesis of carbohydrates, and the conversion of 7-oxanorbornenes to sugars and their derivatives have been reviewed. The strategies for bonding sugars to proteins have been covered in a survey of neoglycoproteins, and two Chinese reports have dealt with the applications of carbohydrates in the synthesis of other natural products, and the effects of ultrasound on the reactions of derivatives of β-D-ribofuranose.

Free Sugars

1 Theoretical Aspects

The historic development of the understanding of carbohydrate stereochemistry has been briefly reviewed, and a lecture with four references on the energetics and geometry of furanoid systems has been published. Two complementary descriptions of the conformational behaviour of furanose rings have been presented: (i) by quantum-mechanical energy calculations; and (ii) by a geometrical model of pseudorotation in five-membered rings.

Molecular dynamic simulations of β-D-ribofuranose and 2-deoxy-β-D-erythro-pentofuranose in solution showed that their hydroxy groups are better hydrogen-bond donors, but worse acceptors, than water and that the ring oxygen atoms accept even less hydrogen-bonding. In a similar study with α-D-glucopyranose, solvation was found to have little effect on the preferred conformation of the sugar molecule. According to molecular dynamics simulation experiments both glucitol and mannitol have bent energy minima in vacuo: in aqueous solution mannitol is also bent, but glucitol prefers a planar zig-zag conformation.

A computer program for molecular modelling which is part of the 3D-Molmaster system has been used to carry out computations of the energetics of monosaccharide, polysaccharide and glycoside conformations. By use of the MM2 method a conformational energy map of β-laminarabiose has been constructed which makes allowance for the presence of several inter-converting conformers ("structural relaxation"). The finding that this map differs from one based on a fixed structure indicates the importance of "structural relaxation" within the glucose residue. Newly developed computational methods for describing and understanding molecular motion and flexibility have been employed to describe the "relaxed" potential energy surface of maltose, which was chosen as a prototypical carbohydrate system. Calculations to give solvent-specific "relaxed" energy surfaces have been applied to carbohydrate molecules, in particular to a mannobiose, with the aim of providing new insights into the conformational properties of sugars in solution.

By an examination of optical rotation and n.m.r. measurements, a picture has been developed of the potential energy surfaces of cellobiose and maltose in aqueous solution.

2 Synthesis

The enzyme-catalysed synthesis of mono-, oligo- and poly-saccharides has been covered in a major review with 310 references, and a review has appeared on the synthesis of saccharides uniformly labelled with 14C, starting from D-[U-14C]glucose.

The distribution of products of the formose reaction carried out in aqueous DMF has been studied. Considerable control was possible by adjustment of the water content. When, for example, formaldehyde was heated at 75°C for 1 hour with triethylamine and thiamine hydrochloride in 8:1 DMF-H2O, DL-2-C-hydroxymethyl-3-pentulose, characterised as the tetraacetate (1), was produced in 28% yield. In the absence of water, the major products are dihydroxyacetone and DL-glycero-tetrulose (cf Shigemasa et al., Carbohydr. Res., 1987, 162, C1). In the radiation-initiated formose synthesis in aqueous solution, pentaerythritol and ethylene glycol were the main products, their ratio depending on the initial formaldehyde concentration.

An efficient new procedure for the general ascent of the aldose series, which lends itself to repetitive application and gives an all-anti configurated chain, is shown in Scheme 1. The usefulness of this approach is further extended by the epimerisation referred to in Section 4, Scheme 11, of this Chapter. A new multigram method for trapping aldoses in their aldehydo-form is exemplified in Scheme 2. It exploits the discovery that oxime ethers are cleaved by ozone to give the parent aldehydes in high yields.

In a study of the hydrolysis of cellulose to D-glucose by dilute sulphuric acid, the reversion products which represent 10% of the total yield have been analysed. The main components (>50%) were 1,6-anhydro-β-D-gluco-pyranose and -furanose (7:3). Isomaltose and gentiobiose were the major disaccharides, and (1 [right arrow] 2)- and (1 [right arrow] 3)- linked α-disaccharides predominated over their β-anomers.

2.1 Tetroses.- The D- and L-erythrose derivatives (3) and (4), respectively, are available from chlorobenzene by the enantiodivergent route shown in Scheme 3, via the optically pure microbial oxidation product (2). Small but significant asymmetric induction was observed in the aldolisation of the glycolaldehyde derivative (5) bearing an enantiomerically pure acetal-type protecting group...