Sugars Their Structures and Stereochemistry

Aldoses with four, five, six, and seven carbon atoms are called aldotetroses, aldopentoses, aldohexoses, and aldoheptoses, respectively. The corresponding ketoses are ketotetroses, ketopentoses, ketohexoses, and ketoheptoses. Six-carbon sugars are the most abundant in nature, but two five-carbon sugars, libose and deoxyribose, occur in the structures of RNA and DNA, respectively. Four-carbon and seven-carbon sugars play roles in photosynthesis and other metabolic pathways. 

We have already seen (in Section 3.1) that some molecules are not super-imposable on their mirror images and that these mirror images are optical isomers (stereoisomers) of each other. A chiral (asymmetric) carbon atom is the usual source of optical isomerism, as was the case with amino acids. The simplest carbohydrate that contains a chiral carbon is glyceraldehyde, which 

Like bread and pasta, fruits and vegetables are sources of carbohydrates, which provide energy. 

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D A comparison of glyceraldehyde (an aldotriose) and dihydrox) cetone (a ketotriose). 

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The structures and stereochemistry of citreodiol (99)(sugar numbering) and epicitreodiol, metabolites of Penicillium citreo-viride. have been determined by synthesis of their antipodes from L-rhamnose two routes led to the epimeric branched-chain glycenonic esters (100), which were converted to (99) and its epimer by a conventional Wittig sequence.An intramolecular Diels-Alder... 

The use of simultaneous 3 - and 5 -protection by the tetra-isopro-pyl-disiloxan-diyl groups (93) proved to be most profitable in these preparations (91,94). In addition to the core oligonucleotides, also the respective 5 -(tri-)phosphorylated structures have been studied (89,95). Different modifications of the sugar (96-700), and base (101) moieties have been introduced in this way. Several recent papers deal with the chemical (102-105), but also enzymatic (706,707) synthesis of phosphorothioate analogs of 2, 5 - oligoadenylate of controlled stereochemistry and their efficiency in binding and activating RNase L. 

As revealed by the data available, the type of compound closest to the ideal for structural analysis of monosaccharides is the class of dialkyl dithioacetals or their acetates their mass spectra contain a considerable peak due to molecular ion, and their fragmentation patterns are simple enough (due to the absence of a sugar ring) and specific enough to permit determination of the position of substituents on the basis of the position of peaks. Thus, elimination is characteristic of the C-2-substituents, whereas substituents at C-3 tend to be retained, producing a peculiar difference between the mass spectra. However, the mass spectra of dialkyl dithioacetals provide almost no information regarding the stereochemistry of the monosaccharide molecule. 

At the turn of the twentieth century, Emil Fischer was studying the stereochemistry of sugars (Chapter 23), which contain as many as seven asymmetric carbon atoms. To draw these structures in perspective would have been difficult, and to pick out minor stereochemical differences in the drawings would have been nearly impossible. Fischer developed a symbolic way of drawing asymmetric carbon atoms, allowing them to be drawn rapidly. The Fischer projection also facilitates comparison of stereoisomers, holding them in their most symmetric conformation and emphasizing any differences in stereochemistry. 

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