Introduction and General Aspects

The field continues to develop on all fronts, and improvements in synthetic methodology are giving access to substantially more complex products. Nowhere is this move apparent than in oligosaccharide and C-glycoside synthesis and in the use of synthetic procedures which are dependent on carbohydrate-derived carbanions and particularly free radicals. The increasing interest shown by "non-carbohydrate chemists" in carbohydrate chemistry is welcomed; their contributions have enlivened the field appreciably as is best illustrated by their descriptions of novel approaches to glycoside synthesis and, of course, many syntheses of enantiomerically pure non-carbohydrates from sugars.

El Khadem has published a book "Carbohydrate Chemistry: Monosaccharides and their Oligomers", and reviews of general interest have appeared on the chemistry and biochemistry of the sweetness of sugars and on the use of immobilised enzymes in preparative carbohydrate chemistry.

Volume 45 of Advances in Carbohydrate Chemistry and Biochemistry (1987) contains articles on circular dichroism n.m.r. (proton relaxation rate data in structural analysis) and mass spectrometry (FAB methodology). The volume also contains tributes to the lives and work of Professors B. Helferich and F. C. Gonzalez.

The succeeding volume of the series contains reviews on H.p.l.c. of carbohydrates, n.m.r. of fluorinated monosaccharides," the use of photosensitive protecting groups in carbohydrate synthesis'' and high-temperature transformations of monosaccharides in aqueous solution.

Appreciations of the lives and work of Professor K. Onedera and V. Deulofeu are also included in this volume.

Free Sugars

1 Theoretical Aspects

Recent progress in the physical chemistry of small carbohydrate compounds has been reviewed. There is accumulating evidence that solvation plays an important part in determining the solution equilibria and conformational properties of sugars. The standard geometries of both anomers of the eight hexopyranoses in the 4C1 conformation have been calculated and are given in the form of orthogonal co-ordinates. The geometries found for pyranose rings corresponded to the averaged ring structures determined from X-ray diffraction analyses of 161 compounds (see B. Sheldrick and D. Akrigg, Acta Crystallogr., 1980, 836. 1615). A revised CHARMM molecular mechanics potential-energy function has been developed for use in the dynamical simulation of simple carbohydrates in aqueous solution. Application to the molecular dynamics simulation of the motions of α-D-glucopyranose in vacuo in both the 4C1 and 1C4 conformation produced a D-glucose molecule less flexible then had previously been determined. CNDO Force constants for both anomers of D-glucopyranose have been evaluated, and an ab initio investigation of the electronic structure of the α-D-glucose molecule by use of the Hartree-Fock-Roothan method has been carried out. The effective charges agreed with those derived from semi-empirical calculations.

The hydrophobic indices, i.e, the ratios of hydrophobic to hydrophilic surface areas, of seven monosaccharides have been determined. They were found to correlate well with the partition coefficients of the polystyrene-water system for monosaccharides. The concept of hydrophobic indices is important in the consideration of the hydrophobic interactions of flat molecules in aqueous solution. In an effort to establish quantitative structure-activity relationships for carbohydrates, an experimental data-matrix containing the Rf values of sixteen monosaccharides in thirteen solvent systems was subjected to principal component analysis (PCA). Four PC's (t1-t4) were found to explain 97.8% of the variance, and the first of these (t1), which described 90% of the variance, was tentatively interpreted as a hydrophobicity scale. Ab initio Calculations have shown that D-ribose is energetically stabilised relative to its L-enantiomer by parity-violating weak interaction (see Vol. 20, p. 2, ref. 4). The possible significance of these findings with regard to evolution has been pointed out.

2 Synthesis

A review on the synthesis of complex carbohydrates by a combination of chemical and enzymic methods has been published. A new strategy for the predictable creation of new chiral centres and its application to the synthesis of sugars and macrocycles is presented in a review on the use of double asymmetric induction in the aldol condensation, the Diels Alder cycloaddition, epoxidation and hydrogenation. Two approaches to the construction of appropriately functionalised six-carbon chains are outlined in a review on the dg novo synthesis of carbohydrates from achiral precursors: (i), hetero-Diels Alder reaction with inverse electron demand of functionalised 1-oxa-1, 3-dienes with dienophiles such as enol ethers followed by diastereoselective addition to the new double bond and (ii), Sharpless oxidation of racemic or meso-divinyl glycols, with concomitant kinetic resolution.

Sugars with four, six, and eight carbon atoms were formed from glycolaldehyde in an aqueous suspension of sodium montmorilIonite at 40°C, hexoses being the main products (see Vol. 20, p. 3, ref. 8). The conversion efficiency was 90% and an aldol-type mechanism has been suggested for the reaction. Aldoses are smoothly decarbonylated by chlorotris(triphenyl)rhodium in N-methylpyrrolidin-2-one at 130°C to give the next lower alditols. D-Glucose for example gives D-arabinose and 2-deoxv-D-erYthro-pentose gives 1-deoxyerythritol in 90% yield. Ketoses undergo more complex dehydration-decarbonylation reactions; thus furfuraldehyde was formed in 79% yield from fructose. Experiments with [1-13C]-labelled substrate showed that C-1 was lost during the reaction.

2.1 Pentoses. – On reaction with aqueous Pb(OH)2 3,4-Q-isopropylidene-L-arabinose rearranged to 2-deoxy-L-ribono-1, 4-lactone which was reduced to 2-deoxy-L-erythro-pentofuranose by sodium borohydride. 2,4-Q-Benzylidene-L-xylose has been prepared in high yield by periodate oxidation of 2,4-O-benzylidene-D-glucitol. Iron(III)trifluoroacetate proved to be an efficient catalyst in...