Application of the Rule of Six to the Separation of Diastereoisomeric Esters by Gas-liquid Chromatography An Exchange of Comments SIR: A recent paper on the separation of diastereoisomeric esters by gasliquid chromatography attributed the degree of resolution to a possible combination between bulk dissymmetry and the distance between the two optical centers (7). It was concluded that the data obtained were suggestive of certain steric effects, particularly in regard to an energetic preference for a favorable conformational distribution a t the alcoholic asymmetrical carbon atom of a diastereoisomeric ester. The improved resolutions noted with the polar liquid phase [1,2,3-tris-(2-cyanoethoxy)-propane], as contrasted to the nonpolar phase (silicone oil), must be due to a strong secondary bond interaction between the lactic ester carbonyl and the cyanoethoxy moiety. The same suggestion has been invoked for the resolution of some diastereoisomeric esters on an optically active substrate, and a somewhat related gas chromatographic separation of diastereoisomeric alcohols has been e.xplained by an analysis of various conformational influences on intra- and intermolecular hydrogen-bond formation (3, 4). Likewise, the variance in R j values on Sephadex liquid-solid chromatography for several diastereoisomeric dipeptides is attributed to the formation of a hydrogen-bond stabilized ring formation in the DL compounds, while the LL dipeptides reside selectively in the open chain conformation (9).
A generalized concept of the effects observed here in the gas chromatog-
raphy of the diastereoisomeric esters is in the possible operation of the “Rule of Six” (6). Commencing the count with the carbonyl oxygen, then the number of atoms in position six is called the ‘‘six number’’ and varies from 6 in 2-hexyl 0-acetyllactate (Ia) to 3 for the corresponding 2-methyl-1-pentyl (Ib) and 3-methyl-1-pentyl (IC) esters. The essential data are summarized in Figure 1. It is assumed that the normally coiled structure of the atom chain allows the atoms in the six position to approach the oxygen more closely and indirectly block additions t o the carbonyl group from certain directions. A high six number is therefore directly associated with increased steric hindrance to secondary bond formation. As the magnitude of this effect will be dependent on absolute configuration, it will be different for each diastereoisomer and so lead to a corresponding separation of isomers for Ia. Again, a low six number is connected with the formation of a stronger secondary bond with both diastereoisomers, and this condition ultimately furnishes a poorer resolution for Ib and IC. This argument fits the results described earlier for the hexyl esters and has been applied with equal success to the actual gas chromatographic retention patterns given by eight diastereosiomeric dipeptides ( 5 ) . It is evident that an improved resolution is seen directly as an increase in the relative retention times or CY value, which is related to a more negative A (AGO) sum. Another series of secondary esters was
chromatographed in the paper under discussion, but a modified explanation is required for these compounds. Although these derivatives have the same over-all six count, there is considerable variation in the number of six hydrogens and carbons. The necessary information is summarized in Figure 2. It would appear that excessive branching of the alkyl side chain will hinder the development of the secondary bond in one diastereoisomer, but not in the related conformer. A separation would be expected for the t-butyl ester, which possesses three methyl groups, but not for the ethyl ester where there is no alkyl branching. This explanation confirms the resolution patterns drawn for these compounds-poor for IIa, moderate for IId, and good for IIe. The six number does not distinguish between IIa-e, as the count remains constant, and small alkyl eclipsing (skew) factors are dominant here (2, 8). This latter suggestion may be associated with the slow rise in the number of six carbons, due to branching, in this particular series of compounds. The proposal is ventured that the preparation and gas chromatography of 3,3 - dimethyl - 2 - butyl - 0 - acetyllactate (Id) will produce an excellent CY value, as it contains the elements of maximum methyl interaction, yet retains the net six number for this particular group of compounds. It is recognized that 2-methyl-3-pentyl-0-acetyllactate (Ie) has an identical six pattern, but an inferior separation is expected here as severe alkyl eclipsing and inter-
P H o H CH,-C-0 - C - C - 0 4 - R I I I
II
I
CH,
CH3
-
Ester Ester Ia
Ib IC
Id Ie
6No. 6CS 6”s Structure 6 1 5 -CH(CH3)CH2CH,CHeCH3 -CH2CH (CHJ)CH2CH2CH3 3 2 1 -CH2&H2CH(CH3) CH2CH3 3 2 1 6 3 3 -CH(CH,)C(CH3I3 6 3 3 c H (c~HJ CH (cH~),
-
Figure 1.
1238
Six numbering system for Hexyl esters
ANALYTICAL CHEMISTRY
6 No.
5
I
-C3H1 -C,H,
5
I
5
-CH(CHJ2 -c(cg3
6 6
4
I 2 3
-C2Hs
lib I Ic
Ild Figure 2.
6 C’S -
6 6 6
110
118
6 H‘s
3
Diastereoisomeric esters of secondary alcohols
(4) .Gil-Av, E., Feibush, B., CharlesSigler, R., Tetrahedron Letters 1966, 1009. (5) Halpern, B., Westley, J. W., Weinstein, B., Nature 210, 837.(1966). (6) Newman, M. S., “Steric Effects in Organic Chemistry,” M. S. Newman, ed., p. 206-8, Wiley, New York, 1956. ( 7 ) Rose, H. C., Stern, R. L., Karger, B. L., ANAL.C,HEM. 38, 469 (1966). (8) “Sterychemistry of Carbon Compounds, E. L. Eliel, p. 124, McGrawHill. New York. 1962. (9) W’ieland, T., ’Bende, E., Chem. Ber. 98, 504 (1965).
nal strain effects begin to play increased roles a t this point (1). In a reply to this Communication (see following Comment), it has been said that the Rule of Six would favor a separation of 3-heptyl-0-acetyllactate over 2-hexyl-0-acetyllactate; yet, the reverse situation is actually observed on gas chromatography. As stated previously, a clear distinction cannot be made between two compounds when the six count remains constant and a six carbon summation may be the deciding factor. An inspection indicates that the 3-heptyl ester possesses more internal strain than the 2-hexyl ester because of a pronounced 1,&methyl interaction. On this basis, the 2-hexyl-0-acetyllactate should give a better separation than 3-heptyl-0-acetyllactate.
In conclusion, one might seek further quantitative data to support and possibly extend the suggested role of steric factors, computed empirically by the Rule of Six, in the separation of diastereoisomers by gas chromatography. A measurement and discussion of diffusion coefficients for the various compounds in the liquid phases mentioned here would certainly furnish valuable supplemental data.
SIR: In the previous paper Weinstein (4) has invoked Newman’s Rule of Six as a generalized concept of the effects observed in a publication by Rose, Stern, and Karger (3) (R.S.K.) on the gas chromatographic separation of diastereoisomeric esters. The Rule of Six correctly predicts several of the trends observed in the R.S.K. paper and therefore, the rule is to be recommended as a first order approximation of the causes of diastereoisomeric ester separation. We wish, however, to place the Rule of Six in proper perspective. As Newman states in describing his rule (2): “It should be emphasized that the Rule of Six is empirical and is to be used only as a substitute for molecular models. The examination of models is the best way of estimating steric factors.” One must examine structure, configuration, and conformation in order to elucidate the factors responsible for separation, as we have partially done in our original paper (3). Weinstein, himself, agrees that the Rule of Six cannot explain the change in A(AG’) values for compounds IIa-IIc of his paper, in which the six count remains constant. He suggests small alkyl eclipsing, a rather vague term, to ex-
plain this trend. He further predicts that 3,3-dimethyl-2-butyl-O-acetyllactate (Id) should produce a better separation than 2-methyl-3-pentyl-0-acetyllactate (Ie) due to severe alkyl eclipsing and internal strain. We have recently examined compounds Id and Ie, and the Weinstein prediction is correct as shown in Table I. However, a more reasonable way of looking a t this result is in terms of a change in the time average population of preferred conformations at the alcoholic asymmetric center, compound Id being more conformationally rigid than compound Ie (1). A striking example of the failure of the Rule of Six can be seen in a comparison of compounds Ia (2-hexyl-0-acetyllactate) and If (3-heptyl-0-acetyllactate) in Table 11. Applying the ideas of Weinstein, compound If should produce a better separation than I a because, while the six number is the same, the number of alkyl groups in the six position is greater in If. However, compound If is 32 cal./mole smaller in A(AGo) than compound Ia. A detailed interpretation of the mechanism of diastereoisomeric ester separations in terms of preferred conformation a t both the acidic and alcoholic
LITERATURE CITED
(1) Cram, D. J., “Steric Effects in Or-
BORISWEINSTEIN Department of Chemistry Stanford University Stanford, Calif. 94305
(37*dalt,Y., Felkin, H., Bull. SOC.Chim. France 1965, 742.
WORK supported by the National Institutes of Health under Grant, No. GM-12120-01.
ganic Chemistry,” M. S. Newman, ed., 270, Wiley, New York, 1956. (2e.Dauben, W. G., Pitzer, K. S., Ibid.,
Table 1. Separation Data on 20% 1,2,3-Tris(2-Cyanoethoxy) Propane AW Chromosorb P, DMCS, T = 125” C. NAG”),
Compound H 0
LITERATURE CITED
(1) Karger, B. L., Stern, R. L., Rose,
H. C., Keane, W., Sixth International Symposium on Gas Chromatography and Associated Techniques, Sept. 20-23, 1966, Rome, in press. (2) Newman, M. S., “Steric Effects in Organic Chemistry,” M. S. Newman, ed., p. 206 (footnote), Wiley, New York, 1956. (3) Rose, H. C., Stern, R. L., Karger, B. L., ANAL.CHEM.38, 469 (1966). (4) Weinstein, B., Zbid., p. 1238. BARRY L. KARGER ROBERT L. STERN Department of Chemistry Northeastern University Boston, Mass. 02115 WORKsupported by the National Science Foundation under Grant No. GP-5742.
cal./mole
H
CH3
-80 AH*
asymmetric centers will shortly be published (I). This interpretation not only agrees with the trends observed, but also is able to explain the fact that the LDDL racemic ester pair always has a longer retention time than the DD-LL pair. In conclusion, while the Rule of Six may be a useful empirical method for predicting the relative degree of separation, it is not useful if one is concerned with the details of the mechanism of separation. In order to understand the many and subtle effects responsible for separation, a detailed examination of steric and electronic effects is necessary.
Table 11. Separation Data on 20% 1,2,3-Tris-(2-Cyanoethoxy)Propane AW Chromosorb P, DMCS. T = 125” C. 0 H O H
CHa-
e
-0-
Li-C-O-A-CH~-CHZ-CH~-CH, ‘I
I
l
R
CH3 Ester Ia If
R -CHa -CHzCHa
6No 6 6
6C’s 1 2
6”s 5 4
A(AGo)
cal./rnole - 65 -33
VOL. 38, NO. 9, AUGUST 1966
1239
