Reaction sugar component

The conformation of cyclopentane is important because ribose and deoxyri-bose, the sugar components of RNA and DNA, respectively, assume cyclopentanelike ring conformations. These conformations are crucial to the properties and reactions of RNA and DNA. 

Application of the Wittig reaction in the carbohydrate field is accompanied by certain difficulties. A correct choice of the initial sugar components is the main problem, owing to the basicity of phosphoranes and, especially, to the drastically basic conditions employed with phosphonium ylides (2a). It is not surprising, therefore, that protected (acetalated and aeetylated) aldehydo sugars and resonance-stabilized phosphoranes were used at first,3-5 although partially protected, and even unprotected, aldoses were shown to be amenable to the reaction with various resonance-stabilized phosphoranes, thanks to the presence of the carbonyl form in the mobile equilibrium. The latter reactions, however, are extremely complicated (see Section IV, p. 284). 

The basicity of the medium is considerably less when resonance-stabilized phosphoranes are employed, resulting generally in a normal course for the Wittig reaction. A direct correlation between the basicity of certain phosphoranes and their reactivity towards 2,4 3,5-di-0-benzylidene-aZde/n/do-D-ribose and 2,3,4,5,6-penta-O-acetyl-aldehydo-D-g ucose was not found,21 thus indicating a steric influence prevalent in the sugar components, in comparison with electronic factors in phosphoranes. 

Recently, a systematic study on the Hilbert-Johnson reaction has been undertaken by Sorm et al.2fl Thus the role of solvents, various substituents in position 5 of the pyrimidine component, and various protecting groups in the sugar component have been investigated with a special regard to the total yield of the reaction and to the ratio of the resulting a and /9 anomers (see Section IX, dealing with the stereochemistry of the Hilbert-Johnson reaction). 

Enzymes turned out to be very helpful in the de novo synthesis of certain monosaccharides. Generally, two chiral carbonyl compounds are combined in an aldol-type reaction. In carbohydrate metabolism, aldolases catalyze the condensation of dihydroxyacetone phosphate (DHAP) and aldehydes to higher sugar components. To date, about thirty aldolases have been classified, but only... 

THP-Protected lactamide 459 has been used to synthesize (2R, 3 S)-2,3-(cyclohex-ylidenedioxy)butanal (462), a key intermediate for the synthesis of L-daunosamine, the amino sugar component of natural anthracycline antibiotics [85] (Scheme 66). The crucial reaction in the sequence is the yw-selective hydrosilane/fluoride reduction of ketone 460, which... 

The butyrolactone skeleton of precursors for the sugar component of the glycoside antibiotics L-ristosamine and L-daunosamine can be readily assembled by a nitroaldol reaction of 3-nitropropionate (553) with 831, followed by lactonization [229] (Scheme 111). The initial condensation gives 832 in only moderate yield, probably due to the reversible nature of the nitroaldol reaction. If the crude product is treated with pyridinium tosylate, a mixture of lactones 833 and 834 is produced with concomitant loss of the THP group. The ratio of lactones is dependent on the base used in the nitroaldol condensation. Use of potassium tert-butoxide affords a 2 1 mixture of 833 and 834. The ratio can be increased to 5 1 with KF 2H20/tetrabutylammonium chloride, but the overall yield decreases. 

Maillard reactions between components of the fried food (sugars and amino acids) are responsible for the golden color of fried products. Lipid oxidation products also contribute to the main reactions. Reactive oxidation products include aldehydes, epoxides, hydroxyketones, and dicarbonylic compounds, which react with lysine, proline, and other amino acids. 

This method has been used to synthesize methyl a-pillaroside (10), a derivative of pillarose, a sugar component of an anthracycline antibiotic from S. flavovirens. The starting material was 8, readily available from L-rhamnose. Reaction with 1 led to 9, which was oxidized to 10. Fraser-Reid and Walker have synthesized 10 by a different route. ... 

A peculiar sugar modification occurs in the biosynthesis of the aclacino-mycins. These anthracyclines contain a trisaccharide moiety attached to the aklavinone scaffold at the C-7 position (Scheme 1). The first two carbohydrates in the aclacinomycins are rhodosamine and 2-deoxyfucose, but they differ structurally in their third sugar component, which is rhodinose in AclN, cinerulose A in AclA, L-aculose in AclY and cinerulose B in AclB [157]. The conversion of rhodinose to L-aculose is catalysed in a two-step process by the FAD-dependent enzyme aclacinomycin oxidoreductase [71] (Scheme 5, step 31). The three-dimensional structure of this oxidase revealed that the cofactor FAD is bound via two covalent bonds to the enzyme. Crystal structure and functional data further established a mechanism where the two different reactions are catalysed in the same active site of the enzyme but by different active site residues [71]. 

Acidic hydrolysis of 1-5. Compounds 1-5 (10 mg) were dissolved in 2N HCl (10 ml), respectively. After heating at 80° C in a water bath for 2 hrs, the reaction mixtures were extracted with ethyl acetate (3x10 ml). Then the aqueous phases were concentrated in vacuo and sugar components were identified by TLC chromatography on silica gel with ethyl acetate-methanol-water-acetic acid (65 15 15 20). Glucose was found as the only sugar component of 1-5. 

Interestingly, NeuA was recently discovered to convert various 1,6-linked disaccharides as an aldol acceptor when carrying a o-Man or D-ManNAc residue at the reducing end (Scheme 17.7) [43]. The reactions efficiently yielded 9-glycosylated KDN or NeuSAc products (e.g., 32) that contain a sialic acid in the non-terminal position, which represents rather unusual sugar components. Disaccharides are tolerated as NeuA substrates even when containing a sterically more demanding... 

In most of the other cases where insoluble polymers were employed, monitoring of the solid-phase reactions was carried out by hydrolytic delinking of the sugar components from the polymer and determining their concentration by optical rotation. Unreacted sugar in the reaction mixture can also be determined from optical rotation. 

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