Glucose cyclic structures

The four kelohexoses are fructose, sorbose, allulose and tagatose. See glucose for example of isomerization between open chain and cyclic structures in a typical hexose molecule. 

There is another aspect of the structure of glucose and fructose. They, like other simple sugars, can exist as a straight chain but this form is in equilibrium with a cyclic structure. In solutions the latter form prevails. Reaction (2) shows both forms of glucose. 

By 1920, it was recognized that a successful chemical synthesis would probably require the use of a D-fructose maintained in the proper cyclic structure by the presence of substituents. The substituent groups would have to be stable enou to survive the conditions required in condensation with a suitable D-glucose derivative, but at the same time be capable of ready removal by agents that failed to affect any sucrose... 

In the case of the oximes of the aldose sugars, the situation is more complicated because of the possibility of both open-chain and cyclic structures. That aldose oximes can react in the open-chain form follows from the formation of the nitriles and from the isolation of acylated open-chain aldose oximes as secondary products in preparation of nitriles. For example, Wolfrom and Thompson, by the action of sodium acetate-acetic anhydride on n-glucose oxime, not only obtained pentaacetyl-D-glucononitrile, in 40% yield, but also isolated a small amount of hexaacetyl-oWeAydo-D-glucose oxime (V) identical with that prepared by mild acetylation of pentaacetyl-aWe%do-D-glucose oxime (IV) whose structure was assured by its formation from pentaacetyl-aldehydo-D-glucoae (III). 

More recently alkylated cyclodextrins have been developed as chiral phases. These phases are based on cyclodextrins, which are cyclic structures formed from 6, 7 or 8 glucose units. Alkylation of the hydroxyl groups in the structure of the cyclodextrins lowers their melting points and makes them suitable as GC phases. The cyclodextrins contain many chiral centres and separate enantiomers of drugs according to how well they fit into the chiral cavities of the cyclodextrin units (see Ch. 12 p. 273). 

As we stated previously, glucose forms some, but not all, of the common carbonyl derivatives. The amount of free aldehyde present in solution is so small that it is not surprising that no hydrogen sulfite derivative forms. With amines, the product is not a Schiff s base but a glucosylamine of cyclic structure analogous to the hemiacetal structure of glucose, Equation 20-3. The Schiff s base is likely to be an intermediate that rapidly cyclizes to the glucosylamine ... 

Another important group of heterocycles present in wine is the Carbohydrates, polysubstituted pyrans and furans, which can exist in both open-chain and cyclic structures. The most important of these sugars are glucose 23 and fructose 24, and they represent the basis for the fermentation process giving the three main products CO2, glycerol, and ethanol ... 

Draw the cyclic structures (Haworth projection and conformational structure) for a-D-and p-D-glucose and the corresponding methyl glycosides. 

D-glucopyranose pentaacetate (LXVII) to carbonium ions, followed by a rapid reaction of these ions with chloride ions derived from the catalyst. Since the ions were essentially in the 1,2-a-cyclic structure (LXYIII), the /S-l,2-frans-halide (LXXIX) was the main product of the initial reaction. On prolonging the reaction, rearrangement to the normal 1,2-cfs-aceto-chloro-D-glucose occurred. 

This chapter begins with a discussion of the structure and stereochemistry of monosaccharides. Then the formation of cyclic structures front monosaccharides is discussed. This is followed by the presentation of a small number of reactions of these compounds. The classic series of experiments that was used to establish the structure of glucose is presented next. Finally, the structures of disaccharides, polysaccharides, and a few other types of carbohydrate-containing compounds are introduced. 

The IR spectrum of D-glucose follows. Explain how this spectrum supports the cyclic structure for D-glucose. 

The building blocks for polysaccharides are monosaccharides (sugars) such as glucose or fructose (QH O ). Two cyclic structures of D-glucose are reproduced below (a third, open-chain structure is not shown). 

The cyclic structure is often drawn initially in the Haworth projection, which depicts the ring as being flat (of course, it is not). The Haworth projection is widely used in biology texts, but most chemists prefer to use the more realistic chair conformation. Figure 23-6 shows the cyclic form of glucose both as a Haworth projection and as a chair conformation. 

The limited flexibility of their molecules is due to the presence of glucose rings and additiortal cyclic structures formed by hydrogen bonds of side groups (Table 1). 

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