Aldehydes, condensation with sugars

Numerous publications have been dedicated to the genesis of alkylthiazoles. A model reaction based on a mixture of cysteine/xylose/tributyrin was studied by Ledl and Severin (1973), who proposed the decarboxylation of cysteine into cysteamine (2-aminoethanethiol), followed by a condensation with sugar degradation products and a subsequent oxidation. Mulders (1973c) also proposed a model based on a system of cysteine/cystine/carbohydrates, with the formation of thiazolidines subsequently oxidized into thiazoles. The same pathway has been proposed by Flament (Firmenich, 1973), thiazolidines easily being formed by the reaction of cysteamine with aldehydes. Kato et al. (1973a) also found thiazoles in the volatile compounds produced by the reaction of L-cystine with carbonyl compounds. Similarly,... 

Aldehydes, e.g. benzaldehyde , may be condensed with sugars to form six-membered rings as in methyl 4,6-0-benzylidene-j8-D-glucop3u anoside... 

The third hypothesis (C) is that reaction takes place through an aldol condensation between the ethyl acetoacetate and the carbonyl form of the sugar. There is some precedent indicating that it is possible for /3-ketonic esters to undergo aldol condensation. These compounds react mole to mole with the aliphatic aldehydes and afford unsaturated substances, which could be formed via an aldol condensation with subsequent dehydration. 

Henry reactions of nitro sugars can be promoted by catalytic amounts of mild bases and it has widely extended the use of triethylamine. A recent example involves the condensation of sugar derived a,p-unsaturated aldehyde 15 with nitromethane in the presence of triethylamine, to give... 

Only aldehydes with an a-methylene group (i, e., R—CH2CHO) condense with 3,5-diaminobenzoic acid to form quinaldines.92 Consequently, in the carbohydrate series only 2-desoxysugars will react. The usual procedure adopted is to add to the unknown sugar solution an equal volume of a 1.3 percent solution of 3,5-diaminobenzoic acid hydrochloride in 50 percent aqueous perchloric acid. If the carbohydrate solution contains a 2-desoxysugar it will assume a yellow color with a green fluorescence.93 Use of an ester instead of a salt of 3,5-diaminobenzoic acid reduces the sensitivity of the test. 

B. Grzcszczyk, K. Dziewiszek, S. Jarosz, and A. Zamojski, Asymmetric induction in acid-catalysed condensations of sugar aldehydes with furan and 2-methylfuran, Carbohydr. Res., 145 (1985) 145-151. 

The three-component reaction of indole (2) with sugar hydroxyaldehyde 281 and Meldrum s acid 282, with a catalytic amount of D,L-proline, afforded the 3-substitution product 283 as a single isomer [203]. The substituent possesses the czs-fused furo [ 3,2- b ] pyranonc skeleton. The proline catalyzes the Knoevenagel condensation of the sugar aldehyde 281 and Meldrum s acid 282 to provide the alkylidene derivative 284 of Meldrum s acid. Then a diastereo-selective Michael addition of indole and an intramolecular cyclization of this adduct 285 with evolution of carbon dioxide and elimination of acetone furnish the furopyranone in one-pot (Scheme 62). 

Sucrose, or cane sugar, is a disaccharide in which the anomeric hydroxyl of a-D-glucose is condensed with the anomeric hydroxyl of /3-D-fructose (Example 2.22). It is therefore both an a-glucoside and a /3-fructoside. Neither unit possesses an anomeric hydroxyl and neither ring can open to give an aldehyde. 

The furanic aldehydes 5-(hydroxy-methyl)furfural and 2-furaldehyde, systematically present in the toasted wood, can be formed by the thermal degradation of 3-deoxyosone during sugar pyrolysis or Maillard reactions (27). They could also be formed from glyceraldehyde, coming from degradation of DDMP, by condensation with subsequent elimination of water or formaldehyde (24). 

An alternative to noncovalent approaches is covalent attachment of ligands to the surface. Methods for covalent attachment can be broadly categorized as follows 1) condensation with the aldehyde of a reducing sugar, 2) nucleophilic addition to or... 

Isomerism at the acetal carbon atom is theoretically possible in five-membered acetal rings formed when pyranose or furanose sugars are condensed with aldehydes. For furanose derivatives, the isomerism is exo-endo, and the superior stability of exo isomers ensures that one isomer only will be obtainable, and this of the exo configuration. " 1,2-0-Benzylidene-a-D-glucofuranose " therefore has the complete configuration XLV. If the... 

The nitronate anion derived from nitro sugar 82, available from glucopyranoside 1 using standard methods, condenses with aldehyde 33 to afford adduct 83. Reduction with diimide of the ii-olefin, produced in low yield by radical elimination of the nitroaldol, and removal of the... 

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). 

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