Erythrose enantiomers

Relative to each other both hydroxyl groups are on the same side m Fischer pro jections of the erythrose enantiomers The remaining two stereoisomers have hydroxyl groups on opposite sides m their Fischer projections They are diastereomers of d and L erythrose and are called d and l threose The d and l prefixes again specify the con figuration of the highest numbered chirality center d Threose and l threose are enan tiomers of each other... 

Stereoisomers (a) and (b) are nonsuperposable mirror images and are, therefore, a pair of enantiomers. Stereoisomers (c) and (d) are also nonsuperposable mirror images and are a second pair of enantiomers. One way to describe the four stereoisomers of 2,3,4-trihydroxybutanal is to say that they consist of two pairs of enantiomers. Enantiomers (a) and (b) of 2,3,4-trihydroxybutanal are given the names (2R,3R)-erythrose and (2S,3S)-erythrose enantiomers (c) and (d) are given the names (2R,3S)-threose and (2S,3R)-threose. Note that all of the chiral centers in a molecule are reversed in its enantiomer. The molecule with the 2R,3S configuration is the enantiomer of the molecule with 2S,3R, and the molecule with 2S,3S is the enantiomer of the molecule with 2R,3R. Erythrose and threose belong to the class of compounds called carbohydrates, which we discuss in Chapter 25. Erythrose is found in erythrocytes (red blood cells), hence the derivation of its name. 

One of the following molecules (a)-(d) is D-erythrose 4-phosphale, an intermediate in the Calvin photosynthetic cycle by which plants incorporate C02 into carbo- hydrates. If D-erythrose 4-phosphate has R stereochemistry at both chirality centers, which of the structures is it Which of the remaining three structures is the enantiomer of D-erythrose 4-phosphate, and which are diastereomers ... 

Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group. Thus, there are 22 = 4 possible stereoisomeric aldotetroses, or two d,l pairs of enantiomers named erythrose and threose. 

The R- and -enantiomers of Z-3-methoxymethyl-l-methylpropenylstannane have been allowed to react with the protected erythrose- and threose-derived aldehydes 17-A and 17-B. The products are shown below. Indicate the preferred transition structure for each combination. 

If the structures of the two tetroses are written alongside those of D-erythrose and D-threose using Fischer projection formulas, it is seen that two pairs of mirror images are given. That is, the four aldotetroses constitute two pairs of enantiomers ... 

How is each compound related to the simple sugar D-erythrose Is it an enantiomer, diastereomer. or identical ... 

Because each aldotetrose has two stereogenic centers, there are 2 or four possible stereoisomers. D-Erythrose and D-threose are two of them. The other two are their enantiomers, called L-erythrose and L-threose, respectively. The configuration around each stereogenic center is exactly the opposite in its enantiomer. All four stereoisomers of the D-aldotetroses are shown in Figure 27.3. 

Additions of the transient y-OMOM crotyl indium chloride reagents to a-oxyge-nated aldehydes are strongly reagent-controlled. Thus the (R) and (S) reagents add to protected threose and erythrose aldehydes with high diastereoselectivity (Eq. 59) [75]. These additions are complementary to those previously effected with the y-BusSn counterparts (Eq. 45). It is thus possible to prepare precursors to the eight diastereomeric hexoses and their enantiomers from threose- and erythrose-derived aldehydes and their enantiomers plus the a-OMOM crotyl tributylstannane enantiomers. 

Aldotetroses have two asymmetric carbons and therefore four stereoisomers. Two of the stereoisomers are o-sugars and two are L-sugars. The names of the aldotetroses— erythrose and threose—were used to name the erythro and threo pairs of enantiomers described in Section 5.9. 

These two structures are enantiomers, (-) Threose i an isomer of (-) Erythrose with similar chemical properties. Oxidation of (-) Threose by nitric acid gives an optically active compound with the formula CtjHeOe. 

Figure 3-2. Examples of stereoisomers. D- and L-alanine are configurational enantiomers (I), cis- and frfl 5-dibromoethylene (II), threose (III, left), and erythrose (III, right) are configurative diastereoisomers. Both atropoisomers of 2,8-dinitro-6,12-dimethyl biphenyl (IV) and both conformers of 2,3-dibromobutane shown (V) are conformational enantiomers.

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