ASSIGNMENT OF D AND L PREFIXES TO THE TARTARIC ACIDS
0
An Unsettled Stereochemical Question COSTIN D. NENITZESCU Polytechnic Institute, Bucharest, Rumania
INA receut issue of THIS JOURWAL there appeared an article by John Leo Abernethyl that pointed out some common errors which have resulted from neglecting established rules concerning configurational representations. In this article, the problem of the configuration of tartaric acids is discussed. It is stated, Still unsettled for certain chemists is the matter of assignment of the D and L prefixes to the tartaric acids. Whereas Wohl assigns a n prefix to the levorotatory form, Freudenberg gives this same form s n I. prefix:
COOH
D-glyceraldehyde, as has been shown by Freuden-
I
HI
H--OH
.. .The Wohl system ought to win out because in his procedure the hydroxyl on the bottom asymmetric carbon is to the right for the D-antipode, just as in the case of the D- sugars. Evidence that the matter is still unsettled is displayed in Rodd's" excellent treatise, "Chemistry of Carbon Compounds," Volume I, Part B (Elsevier Publishing Co., Amsterdam, London, New York). On page 786 the (+)-tartaric acid is given the D prefix, whereas the same prefix is assigned to the (-) tartaric acid on page 1249. It is desirable to recall the reasoning related to the experimental facts that caused Freudenberg to place the (+)-tartaric acid in the D- series. Three reasons ought to be taken into consideration: (1) The two asymmetric carbons of the (+)-tartaric acid (as well as the (-)-tartaric acid) have the same configuration. This fact is demonstrated when the projection formulas are used through a translation and rotation in the plane of one of the two asymmetric carbon atoms. H-(!--OH Ho-J-H JOOH
) +
c1 o o H H-c-OH
I
+
CH, I (
COOH
AH%
A
NaOBr
&om1 Amide of (+)-rnalic acid
.
+acid )-Malie
COOH
CHO
H-4-OH
H-&OH
AH~OH
AH~OH
( - )-Glyceric acid
D-Glyceraldehyde
Both of the asymmetric carbons in the (+)-tartaric acid have, therefore, a D-configuration. (3) According to W ~ h l ,a~ correlation between n-glyceraldehyde and (-)-tartaric acid was established, based upon the following well-known reactions: COOH
CN
H O -!(-H
hydrolysis, H - L O H a -
H-(!-OH I
oxidation
HCN C--
H-(!-OH I
600~
meso-Tartaric acid
CN cHo H-c-OH L o H D-Glyceraldehyde
HcN
no-AH H-&-OH
I hyddy:T oxidation
cooH H-&-OH
I
(2) The removal of asymmetry a t one asymmetric carbon of the (+)-tartaric acid, through violent reduction, yields (+)-malic acid. The asymmetric carbon of this comwound has the same confieuration as ABERNETHY, J. L., J. CHEM.EDUC., 33, 88 (1956). VOLUME 34, NO. 3, MARCH, 1957
H-LoH COOH 1
hOOH (+)-Tartaric acid
H-&oH LOOH ~(dextro)-Tartaricaeid (Wohl) ~(dextm)-Tartaricacid (Freudenberg)
COOH
H o - L
COOH
AOOH n(levo)-Tartaric acid (Wohl) ~(levo)-Tartaricacid (Freudenberg)
-
COOH
booq
(- )-Tsrtanc acid
' FREUDENBERG, K. Bw., 47,2027 (1914). FREUDENBERG, K., AND F. BRAUNS, Ber., 55, 1339 (1922). ' W ~ H LA., AND FR.MOMBER, Ber. 50, 455 (1917).
This scheme does not contradict Freudenberg's system, but confirms it. In Freudenberg's scheme the -COOH group originates from the -CHO group of D-glyceraldehyde; in Wohl's scheme the -COOH group (at the bottom of the molecule) proceeds from -CHIOH groups of D-glyceraldehyde. The asymmetric carbon contained in the (-)-tartaric acid has
therefore, the inverse configuration of that in the nglyceraldehyde, since the change of position between two substituents of the asymmetric carbon causes the change of configuration. I t is then more rational to write L(-) and D(+)-tartaric acid. An important advantage of this convention is that it is in accord with the usual convention in the a-amino acid series.
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