Introduction and General Aspects

The renaissance being enjoyed by carbohydrate chemistry is reflected in the world-wide strength of the 1990 literature of the subject, biology and improved methodologies stretching research further and faster.

While the evolved format of these Reports largely allows for the handling of new material, some adaptation is desirable and (after some debate) treatment of chain-extended sugar derivatives is now included in Chapter 2. Carbohydrates as chiral auxiliaries is another aspect that the normal format does not accommodate too readily; it is treated in Chapter 24, and Chapter 4 now includes brief reference to chemical aspects of the cyclodextrins.

A monograph on carbohydrate chemistry has appeared in Topics in Current Chemistry, the history of the subject from its origins has been surveyed in a Chinese language publication and a data bank of the structures of all complex carbohydrates larger than disaccharides has been set up. Reports of papers given at an American Chemical Society Symposium on computer modelling of carbohydrate compounds have appeared in a collected volume.

The nomenclature committee of the International Union of Biochemistry has recommended that the early method used to number the atoms of myo-inositol be relaxed. According to the proposal substituents need not necessarily be numbered so that the smallest possible locant is used; authors may use alternative designations to bring out structural relationships.

An extensive review has appeared on the anomeric and exo-anomeric effects in carbohydrate chemistry. A further review of the anomeric effect asserts that n[right arrow]σ* contributions to the former effect are small relative to those of n[right arrow]n** destabilising ccomponents. Further consideration has been given to the effect following analysis of 529 crystal structures of carbohydrates, in particular the C-O bond lengths and C-O-C and O-C-O bond angles as they depend on dihedral angles in the sequence C-O-C-O-C. The work is an extension of an earlier study (Vol. 18, p.2, ref. 4).

Reviews have also been published on the following chemical aspects of carbohydrates: thermal decomposition, DMSO-dependent oxidation of hydroxyl groups, and reactions in liquid hydrofluoric acid.

Thermodynamic data on aqueous solutions of mono- and oligo-saccharides have been surveyed; new hypotheses on the state of water in the hydration shells of sugars were considered in the light of interactions with third species.

Free Sugars

Reviews have been published on the chemistry of sucrose and its derivatives (90 refs.), on the synthesis of stable isotope enriched D-glucose (61 refs.), and on the interaction between saccharides, metal ions and polyamines (44 refs.).

1 Theoretical Aspects

Earlier reports of unusually large parity-violating energy differences between enantiomers of sugar precursors (see Vol. 20, p. 2, ref. 4 and Vol. 22, p. 4, ref. 8) have now been shown to be wrong.

Correlated variations of bond lengths in pseudorotating furanose rings have been estimated by a theoretical method, and the quantisation of hydrogen bond lengths in carbohydrate crystals has been investigated by use of a 1-dimensional anharmonic oscillator model.

The kinetic effects of various aldohexoses, ketohexoses, and aldopentoses as solutes on the hydrolysis of 1-benzoyl-3-phenyl-1,2,4-triazole, a reaction which is catalysed by water, have been studied. All the sugars tested caused retardation of the hydrolysis. The results were evaluated in terms of stereochemical features and hydration of the monosaccharides.

2 Synthesis

A summary has been presented on the 2-(trimethysilyl)thiazole method of ascent in the aldose series (see Vol. 23, p. 4, ref. 15 and p. 11, ref. 48). A review on the synthesis of stable isotope enriched D-glucose is referred to above (ref. 2).

The autocatalytic synthesis of carbohydrates from formaldehyde was found to require the presence of trace amounts of acetaldehyde. It was assumed that the initial condensation of formaldehyde with acetaldehyde, followed by retroaldol splitting of the condensation products, furnishes glycolaldehyde and low molecular carbohydrates which further condense with formaldehyde. The self-condensation of glycolaldehyde phosphate in aqueous NaOH is covered in Chapter 7.

2.1 Pentoses and Hexnses. - The transketolase-catalysed synthesis of D-threo-pentulose from L-serine reported in Vol. 21, p. 6, ref. 31, has now been carried out with doubly 13C-labelled starting material to give D[1,2-13C2] threo-pentulose. In a preliminary publication, the cloning and overproduction of bacterial fuculose 1-phosphate aldolase (EC 4.1.2.17) has been described. This enzyme catalyses the aldol reaction between dihydroxyacetone phosphate and various aldehydes to give, as shown in Scheme 1, products with 3R, 4R stereochemistry. The commonly used rabbit muscle fructose 1,6-diphosphate aldolase (RAMA) furnishes 3R, 4S-configurated products. The synthetic utility of a bacterial 2-deoxyribose 5-phosphate aldolase (DERA, EC 4.1.2.4) has been assessed. DERA catalyses the reversible aldol condensation between acetaldehyde and D-glyceraldehyde 3-phosphate, Scheme 2, i.e., between two aldehydes, which is unusual. Many alternative aldehydes, including sugars and their phosphates are accepted as substrates.

High yielding and highly stereoselective aldol condensations involving simple or α,β-unsaturated aldehydes (1), the silylenol ether (2), and the chiral catalyst (3), have been employed to prepare free sugars via aldonolactones. As illustrations, the syntheses of D-ribose and 6-deoxy-L-talose are given in Scheme 3.

The reaction of 2,3-O-cyclohexylidene-D-glyceraldehyde with the α-hydroxyacetyl anion equivalent (4), prepared from benzyl chloromethyl ether, 2,6-xylyl isocyanide and SmI2, proceeded with excellent stereoselectivity to furnish, after acetylation and imine hydrolysis, the D-erythro-pentulose derivative (5). Deprotection gave the free sugar (6) as shown in Scheme 4.

Full experimental details have been published...