Furanose forms of carbohydrates

The two stereoisomeric furanose forms of d erythrose are named a d erythro furanose and p d erythrofuranose The prefixes a and p describe fhe relative configu ration of fhe anomeric carbon The configurafion of fhe anomeric carbon is compared wifh fhaf of fhe highesf numbered chiralify cenfer m fhe molecule—fhe one fhaf defer mines whefher fhe carbohydrafe is d or l Chemisfs use a simplified informal version of fhe lUPAC rules for assigning a and p fhaf holds for carbohydrates up fo and mclud mg hexoses... 

Sfrucfurally O glycosides are mixed acefals fhaf involve fhe anomeric posifion of furanose and pyranose forms of carbohydrates Recall fhe sequence of mfermediafes m acefal formalion (Secfion 17 8)... 

Haworth formulas (Section 25.6) Planar representations of furanose and pyranose forms of carbohydrates. 

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. 

A Taste of Honey The fructose in honey is mainly in the jS-D-pyranose form. This is one of the sweetest carbohydrates known, about twice as sweet as glucose. The jS-D-furanose form of fructose is much less sweet. The sweetness of honey gradually decreases at a high temperature. Also, high-fructose corn syrup (a commercial product in which much of the glucose in corn syrup is converted to fructose) is used for sweetening cold but not hot drinks. What chemical property of fructose could account for both these observations ... 

The two stereoisomeric furanose forms of D-erythrose are named a-D-erythro-furanose and 3-D-erythrofuranose. The prefixes a and p describe the relative configuration of the anomeric carbon. The configuration of the anomeric carbon is compared with that of the highest numbered chirality center in the molecule—the one that determines whether the carbohydrate is d or l. Chemists use a simplified, informal version of the IUPAC rules for assigning a and p that holds for carbohydrates up to and including hexoses. 

PROBLEM 25.5 Write Haworth formulas corresponding to the furanose forms of each of the following carbohydrates ... 

It has been suggested that in the presence of borate ions the improved seiectivity of mannitoi is due to the formation of an adduct between the carbohydrate and borate ions [2], It was evidenced by NMR that in the about adduct the beta-furanose form of the carbohydrate prevails [2,3], The above form of the carbohydrate is more favourabie for the formation of D-mannitol than for the formation of D-sorbitol [2],... 

The high D-mannitol selectivity obtained over CPG supported copper catalysts needs further discussion. It is known that the surface of CPG is enriched with B-OH and B-(OH)2 moieties in the SiO matrix [8]. We suggest that the above surface moieties are involved in the overall selectivity control, if in the borate-substrate adduct the beta furanose form of the carbohydrate determines the enantio-differentiation step in the formation of D-mannitol [2], then due to the presence of surface B-(OH)2 moieties the concentration of the beta-furanose form of the carbohydrate should also increase resulting in the high selectivity for the formation of D-mannitol. 

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