Carbohydrates furanose-pyranose

The Michael-type addition reaction of carbon nucleophiles to vinyl sulfone-modified carbohydrates should be considered as an efficient route for the synthesis of branched-chain sugars because almost all carbohydrates in pyranose and furanose form could be converted to their vinyl sulfone derivatives very easily [5, 12-14]. Moreover, the product of the reaction carrying sulfone functionality has the potential to undergo a wide variety of transformations [15]. For a review on desulfonylation reaction, see Ref. [16]. 

Grubbs catalyst second-generation XXVin was used in 2009 by Botta and coworkers in a multicomponent procedure for the preparation of 2,3-dihydropyrans (Section 5.3.2.2, Scheme 5.57). As an application of this MCR, furanose-pyranose C—C-linked disaccharide 106 was obtained as a mixture of four diastereoisomers, which after equihbration with ZnClj and hydrogenation afford 107a,b (Scheme 5.80). This reaction constitutes a versatile and effective approach for the preparation of biologically interesting building blocks in carbohydrate chemistry [80]. 

Carbohydrate Temp. Pyranose Furanose Open-chain... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

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)... 

Derivatives of aldoses in which the terminal aldehyde function is oxidized to a car boxylic acid are called aldonic acids Aldonic acids are named by replacing the ose ending of the aldose by omc acid Oxidation of aldoses with bromine is the most com monly used method for the preparation of aldonic acids and involves the furanose or pyranose form of the carbohydrate... 

Most carbohydrates exist as cyclic hemiacetals Those with five membered rings are called furanose forms those with six membered rings are called pyranose forms... 

Cyclic herniacetal formation is most common when the ring that results is five- or six-rnernbered. Five-rnernbered cyclic herniacetals of carbohydrates are called furanose forms six-rnernbered ones are called pyranose forms. The ring carbon that is derived... 

A particular- carbohydrate can interconvert between furanose and pyranose for-rns and between the a and p configuration of each for-m. The change from one for-m to an equilibrium mixture of all the possible hemiacetals causes a change in optical rotation called mutarotation. 

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

The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

It is evident from the above discussion that more-detailed studies of the P.M.R. spectra of carbohydrate derivatives will reveal an ever-increasing number of long-range couplings. Studies in this laboratory are directed towards the investigation of the occurrence of these couplings in the spectra of both furanose and pyranose derivatives. 

Ionophoresis of Carbohydrates. Part II. Some Pyranose and Furanose Derivatives of D-Glucose, ... 

This is the form employed by the carbohydrate databank CarbBank, and is preferred for most purposes. Each symbol for a monosaccharide unit is preceded by the anomeric descriptor and the configuration symbol. The ring size is indicated by an italic / for furanose or p for pyranose, etc. The locants of the linkage are given in parentheses between the symbols a double-headed arrow indicates a linkage between two anomeric positions. In CarbBank, omission of a/p, D/L, or ftp means that this structural detail is not known. 

Actually, on oxidation of the carbohydrate with sodium periodate, three moles of periodate are consumed and one mole of formic acid is formed. These data agree with the assumption that the disaccharide contains a pyranose and a furanose ring. The possibility that the disaccharide is made up of glucofuranose and sorbopyranose can also be eliminated on the basis of the periodate oxidation data. Glucofuranose would contain two pairs of adjacent hydroxyls, on carbon atoms 2 and 3 and on 5 and 6, and the sorbopyranose would have three adjacent hydroxyls, on carbon atoms 3, 4 and 5. In oxidizing such a disaccharide, a total of four moles of periodate would thus be used, giving rise to one mole of formic acid. This is inconsistent with the experimental data. 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

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. 

Monomeric carbohydrates in their cyclic form (furanoses and pyranoses) are hemiacetals, which, to become acetals, form 0-glycosyl conjugates. The C-atom C(l) that bears two O-atoms is the reactive, electrophilic center targeted by glycosidases. Nonenzymatic hydrolysis is also possible, although, as a rule, under physiological conditions of pH and temperature, the reaction is of limited significance. 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

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