GUESTAUTHOR J. N. Baxter University of Tasmania Hobart, Austrolio
I I
TextbookErrorr, 53
Systematic Names for the Tartaric Atids
For systematic discussion of stereochemical topics (for instance, the configurations of the amino acids or the sugars) it is desirable to assign to dextrorotatory tartaric mid a systematic name which expresses the fact, discovered relatively recently by Bijvoet and co-workers ( I ) , that it has the absolute configuration (I). Some writers, in attempting to do this, make erroneous use of the small capital letter prefixes D and L of the Rosanoff notation: thus in certain textbooks (I) is named as L(+)-tartaric acid, while in certain other textbooks it is named as D(+)tartaric acid.' This article aims to show that both these names are in error, to explain the nature of the error (equivocal nomenclature), and to show how it can be simply avoided while still satisfying this need of systematic names for (I) and the other stereoisomers. COOH
H-&OH
Incorect Names:
1wtartaric acid >tartaric acid
tartaric x i d l I,(+)-tartaric acid I COOH
HO-c-H I
The systematic names should specify the configurations, but these erroneous names jail in their purpose because of a n ambiguity. Which of the tartaric acids is the n-isomer and which the bisomer? This is a question of nomenclature, and the conventions as they stand today do not establish whether D- is (I) and k is (11) or contrariwise D- is (11) and L- is (I). This will become clear if we examine the conventions which give meaning to these prefixes.
Suggestionsof material suitable far this column and guest columns suitable for publication directly are eagerly solicited. They should he aent with as many details as possible, and particuhrly with refe~encegto modern textbooks, to Karol J. Myaeela, Department of Chemistry, University of Southern California, Los Angeles 7, California. Sinoe the purpose of this column ia to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. In order to be presented an e m must occur in a t least two independent recent standard hobooh. a Incorporating the observed sign of rotation into the name m y show which compound is referred to, but does not specify which caj?gutatia ia meant. Thus, although it is a useful practice for avoiding confusion, it cannot make an incorrect name correct.
(This question of nomenclature should not be confused with the other factual question, solved by Bijvoet, that (I) is the (+)-acid and that (11) is the (-1acid.) Rosanoff's Convention for Carbohydrates
Rnsanoff (2) chose as configurational standard substances the two enantiomorphous glyceraldehydes, (111) and (V). He assigned the sugars to the D-series or ~rserieson the basis of correlation of configuration with D- or bglyceraldehyde, respectively, the correlation being made subject to rules of procedure which make it unambiguous. Neither of the glyceraldehydes had been prepared at the time that Rosanoff put forward his convention, so he defined D-glyceraldehyde in such a way that (+)-glucose, the naturally occurring isomer, was assigned t o the n-series. When D-glyceraldehyde was eventually synthesized it was found to be dextrorotatory. That (111) is the absolute configuration of D(+)-glyceraldehyde was proved by a chain of evidence, the last link in which was supplied by Bijvoet's work. Rosanoff's system has been adopted as official nomenclature by international agreement among carbohydrate chemists (3): Carbohydrates having the same configuration of the highestnumbered asymmetric carbon atom as that of deztm-glyceraldehyde will belong to the D-configurational series; those having the opposite oonfiguration will belong to the >series.
Observe that the convention is framed in such a way as to prevent ambiguity. Of the several asymmetric carbon atoms of the sugar it is the highestnumbered which is correlated with the glyceraldehyde. The correlation is made quite clear then because it is between two tetrahedral arrangements of types Cjklm and Cjklx having three attached groups the same in the two cases. I n this way the aldose (IV) is assigned unambiguously to the D-series. CHO
CHO
H-c-OH
CHO
HO-c-H
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A clear exposition of the Rosanoff system has been given by Hudson (4).
Carbohydrate Convention
COOH
H-c-OH
Convention for Amino Acids
Nomenclature for the a-amino acids (which were not dealt with by the original Rosanoff convention) was codified and made official by international agreement (5)in the following terms: Distinction between the enantiomorphs of the amino acids is made by a prefixed small capital letter D or L to denote the configuration family to which the arcsrbon belongs.
Thus (VI), L(-)-serine, is assigned to the L-series of amino acids in consequence of its configurational relationship to (V). This amino acid convention closely resembles the carbohydrate convention, but there is one very important difference between them that becomes apparent for compounds containing more than one asymmetric center. The amino acid convention is based upon the a-carbon, that is to say the lowest numbered asymmetric center, whereas the carbohydrate convention is based upon the highest-numbered asymmetric center. It is therefore important not to confuse these two systems. Subscript Notation
CHO
. . . . . . . . . . . . . . . HO+H
HO&H
COOH
CH~OH L-glyceraldehyde
L~-tartaricacid
The subscript notation provides unambiguous names for (I): it is D,-tartaric acid, or L,-tartaric acid. Obviously, however, ambiguity results if the small subscript letters are omitted. "~-TartariC acid" would mean (I) if the amino acid convention applied, but would mean (11) if the carbohydrate convention applied, with no way to decide between the two. I t might seem that an additional convention could render the subscript letters unnecessary and give meaning to "D-tartaric acid" by directing which of the two conventions should apply. However the fact is that no such additional convention has been agreed to. And it is fortunate that there is really no need for such an additional convention, for it would be difficult to reach agreement whether carbohydrate or amino acid nomenclature is the more appropriate for tartaric acid. That opinion on this is sharply divided may be seen from the controversy several years ago in the pages of THIS JOURNAL (7-18). One side favors amino acid nomenclature for the historical reason that Freudenberg correlated (+)-tartaric acid experimentally with D-glyceraldehyde (13); but the other side favors carbohydrate nomenclature for the historical reason that Wohl and Momber converted D-glyceraldehyde into (-)-tartaric acid (14). Neither of these arguments is convincing enough to prevail, and it seems pointless to try to reach agreement. Instead, the
Certain compounds bear formal relationships both to amino acids and to sugars. For such compounds Vickery proposed the use of subscripts to indicate whether the carbohydrate convention or the amino acid convention was being used (6). He denoted configurations according to amino acid rules (i.e., lowest numbered asymmetric center) by D, and L, and configurations according to carbohydrate rules (highest numbered asymmetric center) by noand L,. For example, the naturally occurring threonine (VII) may be named systematically either as ~ ~ t h r e o n i n e F~eudmbergCorre2ation (because of configuration a t carbon number 2) or as COOH D,-threonine (because of configuration a t carbon H-LoH number 3). The latter name serves to emphasize its I relationship to the sugar D-threose. HO-&H
-
. . . . . . . . . .",
H-&OH
{
I
The tartaric acids are related to the tetrose sugars but also, being a-hydroxy acids, may be considered related to the a-amino acids. Thus there is no clear indication whether the amino acid convention or the carbohydrate convention should be followed; a priori, either convention might be chosen. Amino-Acid C a v a t i a
CHO
I
CE
CH*
&om1
AooH
half amide of (+):malic acid
CHO
COOH
- H-LOH
sad
I
COOe
I
+
I
H-&OH
cH.NHam I (+)-Isoserine
&HpoH D(- )-GI oerio acid'
Wohl Cowe2atia
CN
CHO
I
H - H
-
D(+)-Glyeerddehyde
COOH
H - L H
(+):Malie
AH20H D(+ )-Glyceraldehyde
Application to the Tartaric Acids
COOH
AooH (+)-Tartaric acid
H-&OH
L.-Threonine D,-Threonine
-
H-A-OH
I
CH, VII
COOH
CN
HO-LHH-LOH H-
H
+
I
H-COH
L o H
&*OH
COOH
COOH
1 I
I
H-c-OH ............... H-c-OH I
HO-C-H COOH D,-tartaric acid
620
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CH~OH D-glyceraldehyde
Journal o f Chemical Education
n( - >Threonie acid
JOOH (-)-Tartaric acid
difficulty is overcome by using the subscript notation; thus either carbohydrate or amino acid notation can be used, ,provided only that it is identified by the approprlate subscript letter. If one does not wish to use the subscript notation, there is an alternative, a t the cost of avoid'mg the name "tartaric." The tartaric acids can be named systematically according to the established carbohydrate non~enclatnre (8). I n this, the dicarboxylic acid obtained by strong oxidation of an aldose is named as the corresponding aldaric acid. In that way (I) may be named unambiguously as L-threaric acid, and (11) as D-threaric acid. No subscript is required because it is quite clear that the carbohydrate convention applies. Racemic tartaric acid may be named as DLthrearic acid, while meso-tartaric acid receives the systematic name erythraric acid (without any prefix onaccount of its symmetry). Another method of nomenclature is provided by the Sequence Rule (15). This names (I) as (RR)-tartaric acid, (11) as (8s)-tartaric acid and (111) as (R)glyceraldehyde. It may be noted that this system does not assign compounds to configurational series, but gives separate specification of each individual asymmetric center. Recommendations
The tartaric acids should not be referred to as "belonging to the D-series" or "belonging to the Lseries." Use of the simple prefixes D- and L- in front of the name "tartaric" is ambiguous and should be avoided. Instead the prefixes D,- or L, and L,- or D,- as appropriate can fulfill the requirements with the minimum necessary change in the textbooks. For instructional
purposes it may be advisable to mention both of the subscript names for each compound. Syslernatie Names COOH D.(+)-Tartaric acid L.(+)-Tartaric acid I,(+)-Threaric acjd (RR)-Tartaric a c ~ d
H-C-OH
1
HO-C-H COOH I COOH
- )-Tartark acid D ~ ( -)-Tartaric aeid D(-)-Threaric acid (8s)-Tartaric acid L,(
HO-C-H d - o H I COOH I1
+
The sigu of observed optical rotation, or -, is not a necessary part of the systematic name. Nevertheless it is wise to include it as a safeguard against confusion. Literature Cited (1) BIJVOET, J. M., PEERDEMAN, A. F.,AND VAN BOMMEL, A. J., Nature (Ladon), 168, 271 (1951). M. A., J. Am. Chem. Soc., 28, 114 (1906). (2) ROSANOPP, (3) Chem. Eng. Nms, 31, 1777 (1953). (4) HUDSON, C. S., Aduan. Carbohydrate Chem., 3, 12 (1948). (5) Chem. Eng. Nms, 30,4522 (1952). (6) VICKERY,H. B., J. Biol. Chem., 169,237 (1947). (7) A B E R N E J. ~ ,L., J. CAEM.EDUC.,33, 88 (1956). G., J. CHEM.EDUC.,33,478 (1956). (8) GORIN, (9) N E N I T ~ E ~C.~ D., U , J. CAEM.EDUC.,34, 147 (1957). J. L.,J. CAEM.EDUC.,34.150 (1957). (10) ARERNETAY, (11) VICKEBY, H. B., J. CAEM.EDUC.,34,339 (1957). J. L.,J. CHEM.EDUC.,34, 566 (1957). (12) ABERNETAY, K., Ber., 47,2027 (1914). (13) FREUDENBERG, Fr., B e . , 50,455 (1917). (14) WOHL,A., AND MOMBER, (15) CAHN,R. S., J. CAEM.EDUC.,41, 116 (1964).
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