TEXTBOOK CONFIGURATIONS FOR SUGARS* R. W. FINHOLT Union College, Schenectady, New York
ANmrncrAL report
on carbohydrate nomenclature was in the May 31, 1948, issue of Chemical and Engineering News, in which appeared the accepted conventions for configurations of carbohydrates and their derivatives. These are the conventions of E. Fischer' as modified by M. A. Rosanoff2and translated into ring structures by W. N. Haworth.= These structures have been accepted by workers in the field for many years (for amasterly review of the problem, see the article on fundamental conventions by C. S. Hudson in "Advances in Carbohydrate Chemistry," Vol. 111, 1948, Academic Press, New York 10, New York) and it is surprising that most elementary organic textbooks, and many advanced texts, use the wrong conventions, and projection formulas for certain molecules. The accepted configuration convention for D-glycerose is a basis for configurations for the 16 aldohexoses, the ketohexoses, and the t,artarir acids. This confimratiou is:
3.
TH~OH
Fig-
1.
&,OH
-
+
1. D-glycerose mild oxidrttion + lev0 lactic acid 2, o-glucose (1) 2 Wohl degradat.ion, (2) oxidation meso tartaric acid 3. D-galactose (1) 2 Wohl degradations, (2) oxidation levo tartaric acid. 4. u-glueoe 3 Wohl degmrlntions D-glyoeraldehyde
+
+ +
-
-
With these facts in mind, it can be seen that levo lactic acid should be written as shown in Figure 3 and is art,ually D (leva)-lactic acid.
-Glycerose
Note that the reducing group has the lowest number and that the H and OH are above the plane of the paper, two conventions used in all sugar derivatives. This would make the No. 5 carbon of D-glucose have an -OH group on the right to correspond to the No. 2 carbon of D-glyceraldehyde. This is shown in Figure 2. In naming optical isomers related to D-glucose, the Meso tartaric acidwould be best written as in Figure 4 letter D or L (not d or 1) refers solely to the configuration t,o affirm the relationship t,o D-glucose, although it is of the No. 5 carbon of glucose. A plus (+) or minus quite clear that this acid can be rotated in the plane of (-) is often used to indicate optical rotation. Thus the paper to produce the OH'e on t.he left side. This ordinary fructose is D-(-)-fructose and ordinary glucose is D-(+)-glucose. The preferred system is to use levo or dextro for rotation so as to make fructose D(1evo)-fructose. (The Greek letters a and @ are also used for reference to the anomeric forms, a being the form having the higher positive rotation. Ordinary sugars contain an equilibrium mixture of a and @ anomers and so cannot be called either a or 8.) A few chemical facts serve to clear up the relationship of the tartaric and lactic acids to D-glyceraldehyde and thus establish their configurations. Figure 4. M-0-tartarin Acid * Presented before the Division of Chemical Education a t the 116th meetine of the American Chemiod Sooietv at Atlantic Cit,v - - ~ New Jmsev. - ~., .Sent,emher - ~ ~ 19-23. 1949. FISCHER,E., Bw.,24, 2683 (1891). R o s m o ~ r M. , A,, J. Am. Chem. Soc., 28, 114 (1906). W. N., J . Soc. Chem. Ind., 46, 295 (1927). HAWORTA,
.,.--
-
~~.~~
~~~
~
~~
acid is optically inactive because the two assymetric carbons are nonsuperimposable (think of dissecting a model between carbon 2 and 3 and compare fragments) and hence exert equal and opposit,e rotational effects.
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JOURNAL OF CHEMICAL EDUCATION
Note, however, that if the - 4 H 2 0 H of glucose had radioactive carbon, the molecule could not be so reversed without a change in relationships. Levo tartaric acid (from galactose) would be as shown in Figure 5 and is actually D (leva)-tartaric acid.
amples of ar-D-glucoseare written as a projection formula and as the ring structure (of Haworth). If the projection formula is converted into a model using the convention given on page 551, the molecule turns out to be a mirror image of the ring that B gives as a-D-glucose. (3) Text C. "Organic Chemistry." There is a clear case of confusion over the configuration of "levo" tartaric acid. On page 359 a projection formula has the bottom -OH on the right but on page 380 "2"-tartaric is given with the bottom -OH on the left. No mention is made of what convention is used and a most confusing picture is presented to the student. CHO
The pyranose ring structure of glucose (Figure 6), following these conventions, is that of Haworth4and has been copied correctly in most texts but could not be derived from their conventions.
Now, having seen the accepted representations, let us look a t several popular organic texts: (1) Text A. "Textbook of Organic Chemistry." On page 329, Figure 54, a D-aldotrioseis represented as in Figure 7, whereas this molecule is actually L-glyceraldehvde. If a model is built un using A's molecule and a ring structure is formed such as is shown on page 345, the ring turns out to be L-(1evo)-glucose and not the molecule shown, which is a bona fGde n-glucose. (2) Text B. "Organic Chemistry." Page 551: "d-glyceraldehyde is assigned the configuration [in Figure 81 (Rosanoff, 1906) . . The arrangement H-C-OH when the active group and the CHIOH are a t the top and bottom of the formula and in the plane. of the paper so that the H and OH are back of this plane is the d form, and is indicated as regardless of the actual sign of rotation."
On page 436, D-glyceraldehydeis pictured as shown in Figure 9, with the same implied configuration that Text B uses. This text also gives a ring formula on page 436 which is labeled "pyranose form of glucose," but the Haworth formula shown is actually that of 6-D-(+ )galactose. (4) Text D. "An Introduction to Organic Chemistry." This one-semester text uses the correct convention on page 184 but no attempt is made to explain or state the need for a standard convention, perhaps because of the modest scope of the text. CHO H-
-
.
+
CHO
..u.~F
1. ~ . ~ d o t . i -
This is a clear and definite statement-but exactly opposite to the correct statement. On page 566, ex' HAWORTH, W. N., J . Soe. Chem. I d . , 46,295 (1927).
I I
C-OH
Fi-
0.
D-Qlyoadd.hyd.
(5) Text E. "Fundamentals of Organic Chemistry." This is the only text of those examined that states the correct convention and shows all structures correctly. The correct usage may be due to extensive use of photographs and diagrams of actual models of the sugars. (6) Text F. "ThB Chemistry of Organic Compounds." This text brings in the usual confusion between D and L forms and optical rotation. On page 255, a diagram is labeled "dextrolactic acid" and examination shows the molecule to be D-(-Llactic acid. so apparently the "dextro" is intended to refer to configuration here. On page 258, a diagram of D-(-)-tartaric acid is named dextro tartaric acid, which would again refer to configuration. So far, nothing worse than calling the acids dextro in an opposite sense to the conmon names (which refer to rotation) has been done. However, on page 258, a projection formula is labeled "defiro tartaric and has the bottom -OH on the left, as would be true in ,.-(+)-tartaric acid. ~f the tartaric acid projection formula referred to rotation, then \
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NOVEMBER, 1949
the drawing above it is wrong; if the projection formula refers to configuration, then it is incorrectly drawn. The text leaves this confusing picture and then returns to carbohydrates later and gives a projection formula for D-(+)-glyceric aldehyde (page 282) with the No. 2 carbon having an OH on the right. A student might well ask a t this point just what the OH on the right means in view of the tartaric acids. This text also presents lactose (page 294) as a glucosidyl galactose instead of the accepted galactosidyl glucose.
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It is clear from the preceding discussion of these six popular texts that a discerning student (to say nothing of his instructor!) could hardly fail to be confused. Construction of models of the tinker-toy type, using the proper conventions, is an excellent way to avoid confusion on the instructor's part and is also a great aid to the student. It is hoped, however, that future editions of organic texts will use the accepted orientations and explain the need for a standard convention in discussing carbohydrates and related compounds!
