Pyranose anomer, separation

Working at low temperatures (0-4°), h.p.l.c. on a cation-exchange resin in the calcium form will separate the pyranose anomers of most of the aldo-hexoses and -pentoses18 under these conditions, mutarotation is slower than separation. The furanoses are not separated, because they interconvert too rapidly, but, at—2 5 to—45 °, the two furanose forms of D-galac-tose and L-fucose have been separated.19 Attempts to separate the various forms of sugars on a preparative scale [p. 24] have not succeeded so far.20... 

It is convenient to separate all fiuctosamine derivatives into several stractural subgroups which display similarities in their substituent type and reactive behavior. The most commonly considered fiuctosamine stractures can be described by a general tautomerization scheme (Scheme 25), and include the most thermodynamically stable P-pyranose anomer and the most reactive enamine species. 

Gas-liquid chromatography may be the method of choice when it is necessary to identify or to quantitate one or more carbohydrates especially when they are present in small amounts. Although this technique is often used because it is possible to resolve carbohydrates with very similar structures, the fact that a and /3 anomers and pyranose and furanose forms of the same carbohydrate all give separate peaks is sometimes a disadvantage. 

Acetylation of the pyranose 227 gives crystalline tetra-O-acetyl-5-thio-/3-D-ribopyranose and the syrupy a-D anomer. Both compounds lack S-acetyl bands in their infrared spectra. Treatment of 227 with methanolic hydrogen chloride furnishes a mixture of anomeric methyl 5-thio-D-ribopyranosides which cocrystallize. Pure anomers are obtained on separation of the glycosides by use of a basic ion-exchange resin, or by separation of the triacetates on silica gel. The sulfur-containing, six-membered ring was established by periodate assay. 

The degradation of 2 -deoxyuridine was studied in aqueous solution at pH values in the range 1-12 and at different temperatures by use of an h.p.Lc. system capable of separating the a-and p-pyranose and furanose forms. The formation of the anomer and ring isomers fiom deoxyuridine indicates both direct Af-glycosidic bond rupture and the formation of a Schiff base intermediatB which can lecyclize in difhaent ways. ... 

From a non-c ohydrate precursor, Davis and Qi achieved the asymmetric synthesis of 2-deoxy-2-fluoro- iylo-D-pyranose (9) (Figure 7) and 2-deoxy-2-fluoro-lyxo-L-pyranose (13). TTie key reaction was the highly diastereoselective fluorination of a chiral enolate using the electrophilic fluorinating agent, N-fluoro(benzenesulfonimide) (NFSi). In another study by Davis (14), the use of a chiral oxazolidinone adjuvant and fluorination with NFSi led to the chiral fluorohydrin 10 (>97% ee), which was oxidized by the Dess-Martin periodinane procedure to the non-racemic a- fluoroaldehyde 11 (94% ee). Conversion of 11 in four steps provided l,2,3-tri-0-acetyl-4-deoxy-4-fluoro-D-arabinopyranose (12) as a 1 1 mixture of anomers which could not be separated by flash chromatography. 

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