Ketoses Fischer projection

Thus the trans relationship between the hydroxymethyl group and the C-l hydroxy group in a-D-glucopyranose, and the cis relationship between the methyl group and the C-l hydroxy group in P-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5. 

Review carbohydrate terminology by referring to text Table 25.1. A ketotetrose is a four-carbon ke-tose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic center is present, and thus there are only two ketotetroses. They are enantiomers of each other and are known as d- and L-erythrulose. 

Monosaccharides, or simple sugars, are carbohydrates that cannot be hydrolyzed to simpler compounds. Figure 23-1 shows Fischer projections of the monosaccharides glucose and fructose. Glucose is a polyhydroxyaldehyde, and fructose is a polyhydroxy ketone. Polyhydroxyaldehydes are called aldoses (aid- is for aldehyde and -ose is the suffix for a sugar), and polyhydroxyketones are called ketoses (ket- for ketone, and -ose for sugar). 

FIGURE 2.7 Trivial and systematic names of ketoses drawn as their Fischer projections. 

The Bilik reaction applied to 2-ketoses yield 2-hydroxymethyl aldoses in which the tertiary carbon originates from C2 of the ketose and the C2 hydroxyl is on the opposite side to the C3 hydroxyl of the ketose (in the Fischer projection). Thus, o-fructose yield o-hamamelose. The position of equilibrium, however, lies towards the straight-chain sugar, although it can be pulled over somewhat towards the branched-chain aldose by the addition of borate. The mechanism in Figure 6.9 again explains the main reaction, but not the formation of sorbose as a by-product, which probably arises from a metal ion-promoted hydride shift, as there is no isotope exchange with solvent. The Bilik reaction can be applied to the production of l-deoxy-o-xylulose from 2-C-methyl-D-erythrose the reaction is particularly clean and only the two... 

Terms in bold are defined glycoconjugate 238 monosaccharide 238 oligosaccharide 238 disaccharide 238 polysaccharide 238 aldose 239 ketose 239 Fischer projection formulas 240 epimers 240... 

Fischer projection formulas, 973-974, 1007 furanose forms, 978-981, 1007 glycolysis, 1002-1004, 1015 glycoproteins, 995—996 glycosides, 988-991, 1008 Haworth formulas, 980 ketoses, 973, 986-987 mutarotation in, 985—986, 1008 photosynthesis, 976, 1015 polysaccharides, 993-995, 1008 pyranose forms, 981-984, 1007 reactions of... 

Solutions of aldoses at equilibrium contain mixtures of at least two (a and 3) furanoses and two (a and 3) pyranoses. Typically, the two pyranose forms predominate at equilibrium. The anomeric carbon is chiral (C-1 of aldoses and C-2 of ketoses). In the D-series, if the OH on the anomeric carbon is to the right in a Fischer projection or below the plane of a Haworth perspective formula, the isomer is a and it is 3 if the OH is to the left or above. Conversely, in the L-series, the a anomer has the OH at carbon 1 to the left or... 

Carbohydrates are polyhydroxy aldehydes and ketones that are commonly known as sugars. A monosaccharide is a carbohydrate composed of one sugar unit and a disaccharide is a carbohydrate composed of two monosaccharide units. An oligosaccharide is composed of 2-10 monosaccharide units a polysaccharide is composed of 10 or more monosaccharide units. An aldose is a carbohydrate that contains an aldehyde unit and a ketose is a carbohydrate that contains a ketone unit. Monosaccharides are categorized by the total number of carbons in the structure triose, tetrose, pentose, hexose, etc. The d and 1 configurations of a monosaccharide are based on the Fischer projection of d-glyceraldehyde. A Fischer projection is an older representation of sugars. 

The enantiomers D- and L-tetrulose, by formally inserting additional CHOH-groups between the keto and existing CHOH-groups, form a series of D- and L-2-ketoses. Figure 4.2 gives D-2-ketoses in their simplified Fischer projections. 

D sugar, 869 deoxy sugar, 883 disaccharide, 883 Fischer projection, 864 furanose, 874 glycoside, 877 ketose, 864 L sugar, 869 monosaccharide, 863 mutarotation, 874 polysaccharide, 886 pyranose, 873 reducing sugar, 880 simple sugar, 863 uronic acid, 881... 

Figure 24-1 shows Fischer projections of the series of o-aldoses up to the aldohexoses. To prevent confusion, chemists have adopted a standard way to draw these projections The carbon chain extends vertically and the aldehyde terminus is placed at the top. In this convention, the hydroxy group at the highest-numbered stereocenter (at the bottom) points to the right in all d sugars. Figure 24-2 shows the analogous series of ketoses. 

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