Saccharides optical activity

An optical rotation-based method of conformational analysis has been applied to methyl P-xylobioside and a semi-empirical model of saccharide optical activity has been used to calculate the NaD rotation of this same compound. Calculated values of optical rotation based on the crystal structures of a series of 1,6-anhydro-p-D-pyranoses of galactose, talose and mannose suggest that solution conformations are very similar to those found in the solid state. However, significant differences were detected for methyl 3,6-anhydro-a- and p-pyranosides of glucose, galactose and mannose. 

We have thus established that thiocarbonyl dienophiles on chiral saccharide carriers can be used for the synthesis of optically active thiopyran derivatives including 5-thiopyranoses. 

Cyclodextrins exhibit chiral recognition characteristics, because the cavities inside are formed from optically active sugars. Cyclodextrins have been introduced as ligands in NPLC as early as 1989 for the separation of sugar alcohols and various saccharides [35]. Risley and Strege [36] used such a stationary phase for the separation of polar chiral compounds, which could not be resolved under nonaqueous NP conditions. 

The term carbohydrate literally means a compound that is a hydrate of carbon. The names sugars and saccharides are synonyms and apply to simple carbohydrates, tjrpically consisting of units containing two to seven carbon atoms (usually five or six). Sugars are typically more or less sweet, water soluble, odorless, optically active, colorless substances. 

Optically active polymers Poij saccharides Fibrous proteincs Polypcplitlcs Poly-P-hydroxyb tylatc Polypropylene oxide Internal rotation of dipoies Optical activity Unisxially oriented state d < 4... 

Saccharide alcohols closely resemble the original sugars in solubility, sweetness, optical activity, and isomerism. Once formed, they are very resistant to further reduction, and require drastic reagents, such as boiling HI, to reduce them to the parent hydrocarbon. 

This optical activity can be observed with many RR-related compounds. Thus, ->amino acids (with the exeption of glycine) can exist in two optically active isomers. Many hydroxy acids (- lactic acid, - tartaric acid, malic acid) are also active, and optical isomerism is frequently observed in saccharide chemistry. 

Danishefsky et al. were probably the first to observe that lanthanide complexes can catalyze the cycloaddition reaction of aldehydes with activated dienes [24]. The reaction of benzaldehyde la with activated conjugated dienes such as 2d was found to be catalyzed by Eu(hfc)3 16 giving up to 58% ee (Scheme 4.16). The ee of the cycloaddition products for other substrates was in the range 20-40% with 1 mol% loading of 16. Catalyst 16 has also been used for diastereoselective cycloaddition reactions using chiral 0-menthoxy-activated dienes derived from (-)-menthol, giving up to 84% de [24b,c] it has also been used for the synthesis of optically pure saccharides. 

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