Monosaccharides aldohexoses

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

The approach of Kunieda ami Takizawa is unique, in that elements of the carbon skeleton of the monosaccharide molecule form an acyclic frame up to the very final stage of the synthesis, and yet a high degree of selectivity is achieved, because of the all-tru ns geometry of the starting telomers. On the other hand, this situation limits the range of sugars synthesizable by this method. Only half of the aldohexoses,... 

The monosaccharide D-( + )-glucose, an aldohexose, is formed by plants in photosynthesis and is converted to the polysaccharides cellulose and starch. Simple saccharides are called sugars. Polysaccharides are hydrolyzable to monosaccharides e.g., a mol of trisaccharide gives 3 mol of monosaccharides. 

Depending on the number of carbon atoms present, monosaccharides are classihed as triose, tetrose, pentose or hexose, containing three, four, five or six carbon atoms, respectively. Glucose is a hexose as it contains six carbon atoms. Sometimes, monosaccharides are classified more precisely to denote the functional group as well as the number of carbon atoms. For example, glucose can be classified as an aldohexose, as it contains six carbon atoms as well as an aldehyde group. 

In general, a molecule with n chiral centers can have 2 stereoisomers. Glyceraldehyde has 21 = 2 the aldohexoses, with four chiral centers, have 24 = 16 stereoisomers. The stereoisomers of monosaccharides... 

By means of the cyanohydrin reaction, higher sugars of the heptose. octosc, and nonosc types have been prepared. A monosaccharide such as an aldohexosc may be converted into the next lower monosaccharide, such as an aldopeniosc. by oxidation to the acid, which corresponds to the aldohexose. then treating the calcium salt solution of this acid with a solution or ferrous acetate plus hydrogen peroxide. Carbon dioxide is evolved and aldopentose formed. 

The simplest carbohydrates are the monosaccharides which under specified conditions are structurally characterised as polyhydroxy aldehydes or polyhydroxy ketones these are termed aldoses and ketoses respectively. Aldoses and ketoses are sub-classified, according to the number of carbons present in each molecule, into aldotetroses, aldopentoses, aldohexoses, etc., or ketotetroses, ketopentoses, etc. 

Aldopentose and aldohexose may exist in furanose and pyranose forms. The former favors the furanose form while the latter prefers the pyranose form. Would the energetic differences between the two forms as exemplified by the following two pairs of monosaccharides be sufficient to rationalize the preferences. 

Most molecules tend to favor one conformer over the others based on the stereochemistry of the particular monosaccharide and the steric bulk of the groups that are appended to it. For example, most aldohexoses prefer the chair conformation that places the bulky C5 hydroxymethyl group in the equatorial position. Having said that, the energy barrier between the two possible chair conformations is... 

Glucose is an aldohexose, where aldo- indicates that it is an aldehyde, -hex- designates the number of carbons, and -ose is the suffix used for carbohydrates. Some other common monosaccharides are fructose, a 2-ketohexose that is isomeric with glucose, and ribose, an aldopentose that contains one fewer carbons than glucose. 

Classify the following monosaccharides. (Examples D-aldohexose, L-ketotetrose.)... 

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