Aldoses aldotetrose

As shown for the aldotetroses an aldose belongs to the d or the l series accord mg to the configuration of the chirality center farthest removed from the aldehyde func tion Individual names such as erythrose and threose specify the particular arrangement of chirality centers within the molecule relative to each other Optical activities cannot be determined directly from the d and l prefixes As if furns ouf bofh d eryfhrose and D fhreose are levorofafory buf d glyceraldehyde is dexfrorofafory... 

Transaldolase, which catalyzes reactions with d-erythrose 4-phosphate and D-fructose 6-phosphate as substrates. As in the case of fructose-1,6-bisphosphate aldolase, this enzyme uses a e-amino side-chain to form a Schiff base intermediate. In this case, however, the triose phosphate moiety is not released but is transferred to the other aldose (in this case, the aldotetrose). 

The two aldotetroses, erythrose and threose, differ from the other aldoses in their behavior.23 Ring formation, to give furanoses, can occur only through the primary hydroxyl group, and is therefore less favored than with the higher sugars. Consequently, considerable proportions of the aldehydo and aldehydrol forms are found in solution. Like all a- and /J-hydroxyaldehydes, the aldehydo form of the aldotetroses readily forms dimers in concentrated solutions of the tetroses, the signals of the dimers are readily visible in their n.m.r. spectra. In the syrupy state, the tetroses consist mainly of dimers, rather than of furanoses they have never been crystallized. 

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. 

The final reaction to be covered in this section is known as the Kiliani-Fischer synthesis. It is a method that converts an aldose to two diastereomeric aldoses that contain one more carbon than the original sugar. The Kiliani-Fischer synthesis is illustrated in the following reaction sequence, which shows the formation of the aldopentoses D-ribose and D-arabinose from the aldotetrose D-erythrose ... 

The names of all the aldotetroses, -pentoses, and -hexoses end in -ose. Glyceraldehyde is the parent aldose thus, its name, glycerose, is valid. 

Whereas glyceraldehyde has one chiral center, aldotetroses, -pentoses, and -hexoses have two, three, and four, respectively. Each chiral center gives rise to optical activity. The net optical activity of an aldose will depend on contributions from each chiral center and will be (+) or (-). 

Sugars are further classified by the number of carbons. Thus, aldoses include aldotrioses (C3 D-glyceraldehyde, HO—CH2—GHO), aldotetroses (Gy D-erythrose) aldopentoses (C-, D-ribose, D-arabinose, D-xylose) and aldohexoses (Ce D-glucose, D-mannose, D-gulose, D-galactose). Ketoses include ketotrioses (Gy dihydroxyacetonephosphate, HO-CH2-CO— CH2OH), ketotetroses (C D-erythrulose), ketopentoses (C3 D-ribulose, D-xylulose) and ketohexoses (C6 D-fructose). 

Fischer projections of the four-, five-, and six-carbon d aldoses are shown in Figure 25.3. Starting with n-glyccraldehyde, we can imagine constructing the Iw o n aldotetroses by inserting a new chirality center just below the aldehyde carbon. Fach of the two d aldotetroses then leads to two o aldopento.ses (four total), and... 

To continue forming the family of D-aldoses, we must add another carbon atom (bonded to H and OH) just below the carbonyl of either tetrose. Because there are two D-aldotetroses to begin with, and there are two ways to place the new OH (right or left), there are now four D-aldopentoses D-ribose, D-arabinose, D-xylose, and D-lyxose. Each aldopentose now has three stereogenic centers, so there are 2 = 8 possible stereoisomers, or four pairs of enantiomers. The D-enantiomer of each pair is shown in Figure 27.4. 

The derivation of the aldopentoses from the aldotetroses is shown in Figure 1.5. Aldoses are numbered from the carbonyl group - thus ribose differs from arabinose in the configuration at C2 and xylose in the configuration at C3. Carbons and attached hydrogens and oxygens are referred to as Cl, C2 C6, HI, H3, H4, 02, 04 etc. 

Aldoses with four, five, six, and seven carbon atoms are called aldotetroses, aldopentoses, aldohexoses, and aldoheptoses, respectively. The corresponding ketoses are ketotetroses, ketopentoses, ketohexoses, and ketoheptoses. Six-carbon sugars are the most abundant in nature, but two five-carbon sugars, libose and deoxyribose, occur in the structures of RNA and DNA, respectively. Four-carbon and seven-carbon sugars play roles in photosynthesis and other metabolic pathways. 

The simplest carbohydrate is the aldotriose, glyceraldehyde (Fig. 3-1). The 4-carbon aldoses are related to D- and L-glyceraldehyde and can be viewed as having been stracturaUy derived from them by the introduction of a hydroxylated chiral carbon atom between C-1 and C-2. Thus there are four aldotetroses. Two simple aldopentoses can be derived stmcturally from each of the fom aldotetroses described, making a total of eight aldopentoses. 

What is an aldose, an aldotetrose, a ketose, a ketohexose Give an example of each. 

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