Monosaccharides open-chain form

Identify the following monosaccharide, write its full name, and draw its open-chain form in Fischer projection. 

Glycosides are named by first citing the alkyl group and then replacing the -ose ending of the sugar with -oside. Like all acetals, glycosides are stable to neutral water. They aren t in equilibrium with an open-chain form, and they don t show mutarotation. They can, however, be converted back to the free monosaccharide by hydrolysis with aqueous acid (Section 19.10). 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

Only those compounds which do not have tautomeric aromatic triazole structures will be considered here, the others having been treated as triazoles. The triazolines are unstable and have been subjected to little study. Compounds which are disubstituted at the C(3) or C(5) atom are more stable than the mono- or unsubstituted analogues. The equilibrium has been observed by NMR spectroscopy between the six-membered tetrazine (75) and the triazolinethione (76) via the open-chain form, thus mirroring monosaccharide equilibria (Scheme 12) <90TL3927>. 

The two most common functional groups found in monosaccharides (in open chain form) are aldehyde and ketone. When a monosaccharide contains an aldehyde, it is known as an aldose, e.g. glucose, and in the case of ketone, it is called a ketose or keto sugar, e.g. fructose. 

For most of the reactions of monosaccharides that involve the aldehyde or ketone functional group, the presence of open chain form is crucial, as only in this form do these functional groups exist. A sugar solution contains two cyclic anomers and the open chain form in an equilibrium. Once the aldehyde or ketone group of the open chain form is used up in a reaction, the cyclic forms open up to produce more open chain form to maintain the equilibrium. 

An anomeric carbon is the hemiacetal or acetal carbon in the cyclic form of a monosaccharide. Said another way, it was the carbonyl carbon in the open-chain form of the monosaccharide. 

Using methods similar to Fischer s, the straight-chain form of any monosaccharide can be worked out. As we have seen, however, monosaccharides exist mostly as cyclic pyra-nose or furanose hemiacetals. These hemiacetals are in equilibrium with the open-chain forms, so sugars can react like hemiacetals or like ketones and aldehydes. How can we freeze this equilibrium and determine the optimum ring size for any given sugar Sir Walter Haworth (inventor of the Haworth projection) used some simple chemistry to determine the pyranose structure of glucose in 1926. 

Treatment of a monosaccharide with NaBH, reduces it to a polyalcohol called an alditol. The redxiction occurs by interception of the open-chain form present in the aldehyde/ketone-hemiacctal equilibrium. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced then more is produced by opening of the pyranose form and that additional amount is reduced and so on, until the entire sample has undergone reaction. 

Compounds such as methyl glucopyranoside differ in reactivity from the parent monosaccharide. For example, unmodified glucose reacts with oxidizing agents such as cupric ion (Cu ) because the open-chain form has a tree aldehyde group that is readily oxidized. 

The carbonyl and alcohol groups within the same monosaccharide may react together if the carbon chain is long enough. The result is a cyclic hemiacetal. A new chiral center is formed at the carbon which was previously the carbonyl. The two optical isomers that can result are called anomers. Five- and six-membered cyclic structures predominate with the alcohol oxygen as the last member of the ring. These are referred to as furanoses and pyranoses, respectively. Cyclic structures exist in equilibrium with the open-chain form. 

MUTAROTATION The a- and /T forms of monosaccharides are readily interconverted when dissolved in water. This spontaneous process, called mutarotation, produces an equilibrium mixture of a- and /1-forms in both furanose and pyranose ring structures. The proportion of each form differs with each sugar type. Glucose, for example, exists primarily as a mixture of a- (38%) and j8- (62%) pyranose forms (Figure 7.11). Fructose is predominantly found in the a-and /J-furanose forms. The open chain formed during mutarotation can participate in oxidation-reduction reactions. 

The first term in the denominator of Eq. (66) represents a non-competitive inhibition (compare to Eq. (31)) of the enzyme by the sum of concentrations of A, B and P. This non-specific inhibition could be correlated with an increased viscosity of the reaction medium in the presence of A, B and P147- mi. As the mutarotation of the carbohydrates is fast compared to the enzymatic reaction there was no need for a discrimination between the a,P-anomers or the open-chain form of the monosaccharides. A complete set of kinetic parameters was determined, summarized in Table 7-4. 

Two different procedures can be used to determine what size ring a monosaccharide forms. In the first procedure, treatment of the monosaccharide with excess methyl iodide and silver oxide converts all the OH groups to OCH3 groups (Section 22.12). Acid-catalyzed hydrolysis of the acetal then forms a hemiacetal, which is in equilibrium with its open-chain form. The size of the ring can be determined from the structure of the open-chain form because the sole OH group is the one that had formed the cyclic hemiacetal. 

Solution The monosaccharide, D-fructose, has the open chain form of ... 

We learned in Section 4.3 that aldehydes and those ketones that contain an adjacent —OH gronp are readily oxidized by an alkaline solntion of Cn (Benedict s reagent). Open-chain forms of monosaccharides exist as aldehydes or hydroxyketones and are readily oxidized. As the oxidation proceeds, the cyclic forms in eqnilibrinm are converted to open-chain forms and also react. 

Benedict s reagent is deep blue in color. As the open-chain form of a monosaccharide is oxidized, is reduced and precipitated as CU2O, a red-orange solid. 

Although carbohydrate cyclic hemiacetals and hemiketals are in equilibrium with open-chain forms of the monosaccharides, the glycosides (acetals and ketals) are much more stable and do not exhibit open-chain forms. Therefore, glycosides of monosaccharides are not reducing sugars. 

The basic monomeric unit of the polysaccharide polymer, the monosaccharide, exists in solution in equilibrium between the open chain and cyclized forms. As the open chain form (Figure 2), monosaccharides are represented by two main classes, the aldoses and ketoses. As the names imply, the aldoses contain a terminal aldehyde functionality while the... 

Dissolved in an aqueous environment, monosaccharides exist primarily in the cyclic or hemiacetal form, which is more stable in solution than the open chain form. If an aldohexose is the precursor of cyclic hemiacetal, it forms a six-membered pyranoside (Figure 3) if a ketohexose or aldopentose is the precursor of the cyclic hemiacetal, a five-membered furanoside is formed. 

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