Aldotetrose Aldotriose

Glyceraldehyde can be considered to be the simplest chiral carbohydrate It is an aldotriose and because it contains one chirality center exists in two stereoisomeric forms the D and l enantiomers Moving up the scale m complexity next come the aldotetroses Examining their structures illustrates the application of the Fischer system to compounds that contain more than one chirality center... 

II. Transformation of Monomeric Saccharides 1. Aldopentoses, Aldotetroses, and Aldotrioses... 

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). 

Problem 349 (a) Using cross formulas to show configuration, outline all steps in a Kiliani-Fischer synthesis, starting with the aldotriose R-( f>glyceraldehyde, CH2OHCHOHCHO. How many aldotetroses would be expected (b) Give configura-... 

There are one, two, three, and four chiral carbons, respectively, in aldotrioses, aldotetroses, aldopentoses, and aldohexoses. There are zero, one, two, and three chiral carbons, respectively, in ketotrioses, ketotetroses, ketopentoses, and ketohexoses. 

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. 

We can illustrate the Kiliani-Fischer synthesis with the synthesis of D-threose and D-erythrose (aldotetroses) from D-glyceraldehyde (an aldotriose) in Fig. 22.6. 

Glyceraldehyde (aldotriose) Threose (aldotetrose) Ribose (aldopentose) Fructose (ketohexose)... 

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