Introduction and General Aspects

The steady advances reported in the 1991 literature show how well the subject connects basic aspects of organic chemistry with medical and biological science. Knowledge of the chemistry of glycosyl cations, anions, free radicals and carbenes has been taken forward on the one hand, and many syntheses have been reported that have been initiated by the needs of biologists, and biological (especially enzymic) methods as tools in synthetic carbohydrate chemistry have become overtly more accessible and useful.

Oligosaccharide syntheses and structural/conformational analyses by spectroscopic and theoretical methods have advanced appreciably and in line with the continuing increase in the recognition of these compounds as key elements of certain biological processes.

An appreciation of the life and work of the great Japanese medicinal chemist Hamao Umezawa has appeared.

Many articles of general interest and reviews on specific topics have been published in 1991. One describes a method for transferring Fischer projection formulae into zig-zag alternatives which should be of interest to those teaching the subject. Of more historical significance is a review of the evolution of the formulae used to depict sucrose, ranging from the earliest to those that are graphic displays which reveal the hydrophobic surfaces. A symposium report covers such topics as configurational and conformational assignments, comparison of solutions and solid state conformations, the orientations of substituent groups and conformational features of oligosaccharides. Related reviews have dealt with the current state of knowledge on the solution properties of low molecular weight polyhydroxy compounds including the influence of solvation on their dynamic characteristics, the stereochemical aspects of hydration of carbohydrates in aqueous solution as derived from density and ultrasound measurements, the role of lone-pair interactions in the chemistry of monosaccharide derivatives including unsaturated compounds, the use of the "prudent ascent" method in the conformational analysis of methyl cellobioside and the sweetener "trichlorogalactosucrose", and molecular dynamics simulation of carbohydrates including conformational energy mapping of mono- and oligo-saccharides.

Special interest will be taken in the topic of association of resorcinol-aldehyde cyclotetramers and sugars in aqueous or non-aqueous media which has been reviewed (in Japanese).

In the area of synthesis, reviews have covered the use of carbohydrate in industrial chemical manufacture, the synthesis of long-chain sugars and branched compounds from furan derivatives, and the preparation of sugars by different methods as synthons for asymmetric synthesis.

A major review covering 203 references on the use of enzyme methods in sugar and oligosaccharide synthesis and acylations and deacylations has appeared. Others have focused on enzymic synthesis of sugars, oligosaccharides and sugar nucleotides' and on enzymic methods for bringing about O-substitution and deprotection.

Vasella has reviewed the chemistry of glycosyl carbanions, carbocations, radicals and carbenes and related work covering his own important contributions and those of others.

Specific areas to have been reviewed are carba-sugars (with ring oxygen atoms replaced by carbon), the enediyne anticancer antibiotics notably calicheamycin and esperamycin (Nicolaou's synthetic work on the disaccharide component especially).

Ally] sulphenates show promise as hydroxyl protecting groups. These are stable under conditions in which many types of groups are cleaved and may themselves be removed by Pd (0)-calalysed transfer (see Chapter 7).

Finally, a paper has been published on the therapeutic applications of various carbohydrate-based compounds.

Free Sugars

1 Theoretical Aspects

An essay on the structural representation of sucrose has been published. Following an historical account of the establishment of the constitutional formula and conformational features of sucrose, the present possibilities for graphics displays of its molecular geometry, contact surfaces, and hydrophobicity potential are given.

Free energy simulations of the equilibrium between the α- and β-anomers of D-glucopyranose in aqueous solution have been described.

2 Synthesis

Two reviews on the use of furan-derived "naked sugars" in the synthesis of unusual carbohydrate derivatives (e.g., L-hexoses, higher carbon sugars) have been published, and recent developments in the synthesis of carbohydrates by means of microbial aldolase catalysis have been reviewed.

In continuation of earlier studies on the formose reaction in aqueous DMF catalysed by triethylamine and thiamine hydrochloride (see Vol.23, Chapter 2, ref. 13), the effects of changes in the concentration of either water, formaldehyde, or triethylamine on the product distribution have been examined.' Improved yields of threo-3-pentulose (19% by g.l.c.) were obtained when the formose reaction was carried out in DMSO in the presence of Pb2O(OH) and thiamine hydrochloride.

Transketolase from spinach leaves has been used in the stereospecific condensation of hydroxypyruvic acid with a variety of aldehydes, including free sugars, which gives ketoses with (5) configuration at the new chiral centre. An example is given in Scheme 1.

1.1 Trioses - Hexoses. - Full details have been published on a high yielding bulk synthesis of O-isopropylidene-D-glyceraldehyde from D-mannitol.

Derivatives (3) of DL-threose and 4-deoxy-DL-threose are available from C-1 oxygenated allylic silanes (1) via α,β-dialkoxyacylsilanes (2) as shown in Scheme 2. The di- and tri-merisation of the acetal protected glycolaldehyde (4) under strongly basic conditions furnished, after deacetalization, DL-threose (ca.10%) and a 2:1 mixture of DL-allose and -mannose (ca.), 20% respectively.

In a preparation of L-lyxose (8) from L-arabinitol (Scheme 3), oxidation of the free primary hydroxyl group of intermediate (5) was achieved by photolysis of the...