Structural Biochemistry/Carbohydrates/Monosaccharides - Wikibooks, open books for an open world
His successful negotiation of the stereochemical maze presented by the . and its exchange relationship to D-(+)-glucose was demonstrated by oxidation to a . the same reaction with D-galactose, shown in the blue-shaded box, produces a. How do the 3 sugars differ in terms of their stereochemical properties? relationship between the hydroxyl groups at C-2 and C-3 of each sugar? In order to produce the pyranose form (6-membered ring, OH-5 in each case The models below show the consequences of this for D-Glucose, D-Mannose, and D- Galactose. Dissociation ions specific to stereochemical differences at C2 and C4 in thus allowing unambiguous differentiation of glucose, galactose, mannose, and talose .
The analytical tools that were available to him were combustion analysis, melting point, optical rotation and reason. In the late 's, Heinrich Kiliani developed the chain extension method using hydrogen cyanide to form aldonic glyconic acids containing an additional carbon atom. The reduction of these polyhydroxylated carboxylic acids with red phosphorus and HI afforded alkanoic acids. Using these techniques arabinose was shown to be aldopentose in that it produced n-hexanoic acid.
Glucose was shown to be an aldohexose n-heptanoic acid while fructose, through the formation of 2-methylhexanoic acid, was formulated as a 2-ketohexose.
Early in Fischer's careerat the age 23, he published a paper on the formation of phenylhydrazine by the reduction of a phenyldiazonium salt. Ironically, this substance was to prove critical in the elucidation of the structure of glucose and its isomers. This was true of the respective aldonic acids, gluconic and mannonic acids wherein the aldehyde group had been converted into carboxylic acid groups.
D and L Sugars — Master Organic Chemistry
In fact, upon heating either of these acids in the presence of the base quinoline, a mixture of gluconic acid and mannonic acid was produced. Furthermore, the two monosaccharides formed different phenylhydrazone derivatives but they formed the same osazone, which were identical to the osazone formed from - -fructose.
The process of osazone formation is an oxidation that destroys the stereochemistry at C2. In addition, reduction of - -fructose with sodium amalgam afforded both glucitol sorbitol and mannitol.
Thus, the absolute configuration at C3, C4, and C5 must be the same in all three monosaccharides. Thus, 1 is the enantiomer of 9, 2 the enantiomer 10, Both glucaric acid and glucitol are optically active.
Thus structure 1 is the same as 9 and 7 is the same as They are meso compounds and optically inactive. Scheme 2 Now, mannaric acid, as well as mannitol, is optically active.
But structure 7 is optically inactive. For the same reason, Structures 2 and 10 in Scheme 2 cannot represent glucaric acid or glucitol because structure 1 is optically inactive and it cannot represent mannaric acid or mannitol.
Fischer was half way home; he was now left with the options presented by Scheme 3. Scheme 3 Chain extension through the Kiliani procedure was now going to play a key role.
D and L Sugars
From these new enantiomers were prepared both - -glucose and - -mannose. The implications were clear.
Here was a seemingly simpler problem; determine the structure of arabinose. The pentose structures are situated between the two hexoses they form on chain extension. Note that the newly introduced hydroxyl groups are designated in red, and A and A' are enantiomers as are B and B'.
Scheme 4 Fischer again called upon symmetrization. These apparent conflicting observations were too important to ignore. Realizing that borax complexes of optically active polyols give enhanced rotations, Fischer applied this technique to arabitol; optical rotation was observed. The example on the right shows D-Glucose with priorities of each substituent numbered. When rotated to view down the C-H bond, the priorities decrease in a clockwise fashion, hence that stereocenter is designated R.
However, the enantiomer of D-glucose, the priorities decrease in a counterclockwise fashtion indicating that the stereocenter is designated S. Fischer Stereochemistry Proof[ edit ] Herman Emil Fischer presented the stereochemical configuration relationship in sugar through a series of experiments with ribose.
At the time when this experiment was conducted, all they had was optical rotation to determine stereochemistry. Fischer was able to manipulate a series of reactions to assign stereochemistry among sugars.
At first he just assumed the penultimate position of the experimental arabinose was in R-configuration. Luckily, the arabinose was later proved to be in D-conformation. Under the Kiliani-Fischer synthesis condition, arabinose will produce two epimeric sugars, mannose and glucose. Although it remained unknown which one was glucose and which one was mannose.
Out of the four possible aldaric acid derivatives from a set R penultimate configuration, two were eliminated because they were not optically active. Possible aldaric acid Next, mannose and glucose were oxidized by HNO3. Mannaric acid and glucaric acid were also optically active. With only one unknown stereocenter, there are two possible forms of aldaric acid for each sugar.
Out of the four total predictions of glucose and mannose, one of the aldaric acids is meson and therefore cannot be either Mannaric or glucaric acid. Mannaric acid and glucaric acid should have the same stereocenters except for the inverted C2 stereocenter.