D- -threose

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... 

Which aldotetrose is the structure shown Is it D erythrose D threose L erythrose or L threose (Be careful The conformation given is not the same as that used to generate a Fischer projection)... 

As shown for the aldotetroses, an aldose belongs to the d or the l series according to the configuration of the chirality center farthest removed from the aldehyde function. Individual nfflnes, such as erythrose and threose, specify the particular ariangement of chirality centers within the molecule relative to each other. Optical activities cannot be determined directly from the d and l prefixes. As it turns out, both D-erythrose and D-threose are levorotatory, but D-glyceraldehyde is dextrorotatory. 

Problem 25.23 Two of the four d aldopentoses yield D-threose on WohJ degradation. What are their structures ... 

Under similar reaction conditions the D-threose derived nitrone (480) is converted in high regioselectivity to diastereomeric cycloadducts (485-488) in an overall yield of 84% (Scheme 2.238). 

Nitrones derived from cyclic acetals of D-erythrose (479) and (689) and of D-threose (480) and (690), reacted with N -phenylmaleimide D15a to afford the corresponding diastereomeric isoxazolidines (691-706) (Scheme 2.295). The stereoselectivity is dependent on the substituents in the nitrone. In the case of nitrones (479) and (689) the cycloaddition is exo-selective. It was observed that microwave irradiation decreased the reaction times of the cycloadditions dramatically. For example, for nitrone (689) and dipolarophile D15a the reaction time decreased from 11 h to 8 min and for nitrone (690) and D15a it decreased from 3 h to 10 min. Moreover, microwave irradiation reversed the ratio of erythro-frans/erythro-r/.v adducts from 63 37 to 39 55, see compound (689). Cycloadditions of the chiral maleimides D15b and D15c are less stereoselective (817). 

Diastereoisomer Stereoisomers with two or more chiral centers and where the molecules are not mirror images of one another, for example, d-erythrose and D-threose often contracted to diastereomer. 

D-Threose diacetamide. (Aqua ammonia.) Thirty grams of tetraacetyl-o-xylononitrile was mixed with 300 ml. of concentrated aqua ammonia (28-29 %) and warmed on a water bath until the solid was all dissolved, and then allowed to stand for three hours. The solution was concentrated in vacuo to a thick sirup, which was dissolved in aqua ammonia and reconcentrated. The final thick sirup was dissolved in absolute ethanol and ether added to the point of turbidity. After forty-eight hours in a refrigerator the crystals which had formed were collected yield, 16.5 g. (78%). They were recrystallized by dissolution in two volumes of warm 75 % ethanol, filtration through carbon and the addition of an equal volume of absolute ethanol. The compound crystallized slowly, forming clear, sharp needles or prisms in resets, m. p. 165-167. ... 

The rate of hydrolysis of D-threose diaoetamide by 0.1 sulfuric acid has been studied by Hockett by titration with iodine, and the rate was found to correspond approximately to that of a unimolecular reaction. The same results were obtained with D-erjrthrose diacetamide by Hockett and Majmard on following the change of rotatory power during the hydrolysis. 

D-Xylose. Maquenne obtained tetraacetyl-D-xylononitrile in 41% 3deld from the D-xylose employed, by treatment of the oxime with sodium acetate-acetic anhydride. From the nitrile, D-threose diacetamide was obtained in 30 % yield by the action of ammonia and it was hydrolyzed... 

Maquenne s method was improved by Hockett, who obtained the nitrile in 48% yield and the D-threose diacetamide in 78% yield the latter product was hydrolyzed with 0.1 sulfuric acid. Tetraacetyl-D-xylononitrile has been degraded by Deulofeu with sodium methoxide and also by hydrolysis of the acetyl groups with sulfuric acid. 

Elsewhere, the reaction of styrene with nitrones derived from cyclic acetals of D-erythrose (e.g., 126) or D-threose has afforded a mixture of diastereomeric isoxazolidines (Scheme 1.28) (170,171). In all cases, nuclear magnetic resonance (NMR) analysis suggests that the major product contains the C(3)/C(4 ) erythro... 

All of the 4-carbon saccharinic acids possible have been prepared and characterized.132 The 4-carbon metasaccharinic acid has been encountered frequently,1,133-137 and a racemic mixture of the 2-methyl-glyceric acids (2,3-dihydroxy-2-methylpropanoic acids) was obtained from 4-O-methyl-D-threose.136... 

D-Erythrose, D-threose, and some of their derivatives were obtained from the diastereoisomeric products resulting from the reaction of 638... 

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