Free sugars, condensation

The second- and oldest-method for the preparation of thionocarbamates on carbohydrate scaffolds consists of the condensation of thiocyanic acid, generated in situ from potassium thiocyanate and a protic acid, with an anomeric-free sugar to form the thermodynamically more stable five-membered ring OZT as the major compound (Scheme 2).27,28... 

Microorganisms have also been developed to produce alternative products, such as lactic acid [65], propane-1,3-diol [67], 3-hydroxypropionic acid [68], butane-2,3-diol [69] and numerous other intermediates. For instance, bacteria such as the Clostridium acetobutylicum ferment free sugars to C4 oxygenates such as butyric acid or butanol. They form the C4 oxygenates by Aldol condensation of the acetaldehyde intermediates. The Weizmann process exploits this property to ferment starch feedstock anaerobically at 37 °C to produce a mixture of w-butanol, acetone and ethanol in a volume ratio of 70 25 5 [3],... 

In the synthesis of the glycolipid asialo GMI described by Ogawa and co-workers [107] the trisaccharide derivative (215) (used in the synthesis of asialo GM2) was converted into the 4,6-O-benzylidene derivative (233) and this was condensed with aceto-bromogalactose in the presence of mercury(II) cyanide and molecular sieves to give the P-linked acetylated galactose derivative in 97 % yield. The product was deprotected and acetylated to give the peracetyl derivative (234) of the tetrasaccharide which was converted into the free sugar (235) with hydrazine hydrate. Compound (235) was converted into asialo GMI via the imidate as described above for the synthesis of asialo GM2 from the imidate (219). 

A modification of the Kiliani-Fischer method for amino functionality introduction involves the condensation of free sugars with arylamines to give imines, which are treated directly with hydrogen cyanide to produce aminonitriles. Hydrogenolysis and hydrolysis of the resulting aminonitriles gives 2-amino-2-deoxyaldoses with one carbon extended [103],... 

Condensation of methoxyamino sugars with free sugars resulted in the interesting oxyamino linked sugars 66 as mimetics of (1 6)-linked sugars [126]. While these di- and trisaccharide mimetics [127] are isosteric to the natural saccharides the conformational behavior was however found to be different [128]. 

Condensation of Simple Alcohols with Free Sugars. 170... 

Although aldol condensation is another type of ionic reaction, it deserves special consideration. It can lead to two classes of sugars. Condensation of an aldehyde in an a-position of a free carbonyl group under mildly akaline conditions still seems possible. This very simple reaction can also be quite efficient. Thus diacetal 7.59, quantitatively prepared by treatment of mannose with acetone in an acidic medium, is at equilibrium with the carbonyl tautomer. Condensation with formaldehyde, used in aqueous solution in the presence of potassium carbonate, gives the hydroxymethylated sugar 7.60 in 86% yield (Ho 1979). 

Natural sialic acids (Schauer 1982 1991) are derivatives of 5-amino-3,5-dideoxy- D-glycero-D-galacto-nonu osonic acid 12.1. This awkward name has been replaced by neuraminic acid . The most common derivative is N-acetyl-neuraminic acid 12.2 whose configuration is easy to memorize because, in the Fischer representation, 12.3, it is presented as an aldolic condensation product of N-acetylmannosamine (2-acetamido-2-deoxy-D-mannose) and pyruvic acid. When the expression sialic acid is used without any other precision, it is in reference to derivative 12.2. It exists in the free state or glycosidated in the d-conformation, which allows an equatorial disposition of the three-carbon side chain. Structure 12.2 represents the stable /3-anomer of the free sugar with an axial anomeric hydroxyl group and all-equatorial non-anomeric substituents. An X-ray spectrum of this crystallized /3- anomer confirms this conformation and reveals, moreover, that the side chain has the zig-zag conformation with two... 

The reaction of amino acids with sugars to form Schiff s base-type compounds has been often suggested to be a feasible process in the formation of coloured condensed material in seawater, particular in the presence of clay minerals (Hedges, 1978). It is also worthy to note that natural free sugars and amino acids when heated in seawater form fluorescent compounds whose excitation and emission maxima correspond with those formed on reaction of amines with aldehydes (Honda et al., 1974). These experiments were performed at high temperatures (G. Liebezeit, R. Dawson and K. Mopper, unpublished results) and there is some suggestion that the apparent disappearance of amino acids on standing in seawater at room temperature, may be connected in part to this abiotic process. 

N-Glycosyl derivatives of daunorubicin have been prepared by direct condensation of the free sugar with the antibiotic these... 

Condensation of simple alcohols with free sugars (Fischer s method of glycosidation)... 

The -(l-azido-napthalene-S-sulphonyl)-hydrazone derivative of lactose was Mcpared for photoaffinity labelling of a lectin from the conger eel. Aldose N-methyI-A -(4-phcnyI- and -p-substituted phenyl-thiazol-2-yl)hydrazones have been prepared and sqipear to have acyclic sugar residues. Tri- and penta-cyclic derivatives such as 92 and 93 were obtained by condensation of free sugars with A -(4-methylquinolin-2-yl)hydrazine or a tetracyclic heteroaromatic hydrazine, respectively, followed by cyclization (catalyzed by FeQa in EtOH). ... 

Lubineau. His first idea was to reinvestigate the Knoevenagel condensation with a free sugar as linear aldehyde/ ... 

For over 40 years synthetic action of /3-galactosidases has also been known. Bourquelot and associates showed that 3-galactosidase from various sources could synthesize such compounds as ethyl galactoside from the free sugar and ethanol when the substrates were present in high concentrations (I). The equilibrium of the reaction is reached in spite of technical difficulties, such as inactivation of the enzyme and formation of condensation products of galactose with itself. 

Improved yields of 3-aminoheptoses were obtained from 2-acetamido-2-deoxy-hexoses by the nitromethane route when the aldehydo-sugar (10) was used instead of the free-sugar for the condensation with nitromethane. The same authors have developed a multi-step procedure for the sequential build-up to A-acetylneuraminic acid from 2-acetamido-2-deoxy-D-mannose which can be applied to the preparation of specifically labelled compounds and stereoisomers (Scheme 8). ... 

Since acetamide, which results from ammonolysis of the ester functions, does not itself condense with free sugars 189) or acetylated aldehydo-sugars 190) the mechanism of formation of the diacetamide derivatives has received considerable study. A possible mechanism 191) is based on intramolecular migration of acetyl groups from oxygen to nitrogen, according to the scheme ... 

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