J. Org. Chem. 1992,57, 5231-5239
carrsling out the X-ray crystallographicstudy of 8. We also express our appreciation to Dr. K. Inokuchi and the Fujiaawa Pharmaceutical Co., La., Japan, for providing us with a gift of bicyclomycin. Supplementary Material Available: Experimental procedure for the X-ray analysis of 8, ORTEP drawing of 8 with atom labeling scheme (Figure 2), Table 4 listing the final cell constants, as well as other information pertinent to data collection and refinement, and Tables 5-9 giving a complete listing of atomic
5231
coordinates and equivalent isotropic displacement parameters, bond lengths, bond angles, and hydrogen-bonding parameters, select long-range proton-carbon connectivities observed in the proton-detected long-range heteronuclear multiple quantum chemical shift correlation (HMBC) experiments for 8 (Figure 1) and 3 (Figure 3) and ‘H and/or 13C NMR spectra for all new compounds (42pages). This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the joumal, and can be ordered f ” the ACS see any current masthead page for ordering information.
Separation of Remote Diol and Triol Stereoisomers by Enzyme-Catalyzed Esterification in Organic Media or Hydrolysis in Aqueous Media J. Shield Wallace, Bruce W. Baldwin, and Cary J. Morrow* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Received April 30, 1992
The separation of symmetric,remote, secondary diol stereoisomers by stereoselective enzymecatalyzed acetylation with acetic anhydride in anhydrous, low polarity organic solvents or by stereoselective enzyme-catalyzed hydrolysis of the corresponding peracetate in aqueous media is described. Whether or not an alcohol is acetylated or an acetate is hydrolyzed is determined solely by ita own stereochemical arrangement and not by the stereochemistry at any other stereogenic center. Since the enzyme used, Amano P lipoprotein lipase from Pseudomonas species, acetylates secondary alcohol stereogenic centers of the (R)-configuration,an (R,R)-diol is converted to ita diacetate, a meso-diol is converted to the monoacetate at ita (R)-stereogenic center, and an (S,S)-diol is left unchanged. Similarly,when hydrolysis is used,(R)-stereogenic centers are hydrolyzed 80 that the (R,R)-stereoisomer is converted to the corresponding diol while the (S,S)-stereoisomer remains a diacetate. The resulting mixture is separated, and the remaining acetates are removed by hydrolysis to give diols and triols of high stereochemicalpurity. Diols 1, a,a’-dimethyl-l,3successively separated by esterification include a,a’-dimethyl-l,4-benzenedimethanol, 5, and a,d-dimethyl-4,4’-biphenylenedimethanol, benzenedimethanol, 4, a,a’-dimethyl-2,6-pyridinedimethanol, 6. For two cases, a,a’-dimethyl-2,6-pyridinedimethanol, 5, and a,a’,a”-trimethyl-1,3,5-benzenetrimethanol, 7, the separation was achieved using the hydrolysis procedure. The stereochemical purity of each of the separated diol stereoisomers was determined by evaluating the NMFt spectrum of ita bis-MTPA ester. In most cases, it was possible to establish both the stereochemical purity of the fraction and the amount of each contaminating stereoisomerthat was present. The diol products are expected to be of value for preparing optically active polymers and optically active crown ethers.
Diols are valuable intermediates in the preparation of polymers, acetals, and crown ethers, and optically active diols have been widely used for stereochemical control in homochiral syntheses. Unfortunately, the number of optically active diols, other than those associated with carbohydrates, is quite small. Thus, a general source could provide valuable new building blocks for many structures. Most techniques for the preparation of optically active diols focus on the stereospecific synthesis of a single enantiomer.1*2The chemicals for preparing both enantiomers via such a procedure are not always available. In many casea,the stereochemistryat the second stereogenic center is determined by that at the first, limiting the allowable distance between the two. Finally, a completely different approach is generally required for preparing the meso stereoisomer. As a result of our recent activity in the synthesis of optically active [AA-BB], polyesters> the importance of finding an efficient approach to the preparation of all possible stereoisomers of symmetric, secondary diol monomers in a highly purified form became apparent. Having all three isomers allows, for example, the synthesis of an
all (R),an all (S),or the “pseudo-syndiotactic” (R,S)polymer as well as a polymer containing any combination of the above stereochemistries. Moreover, since our interest lay in the use of enzymes to effect polycondensations, preparing diols free of any meso material became particularly important. While such separations can be achieved by WC,4 it seems unlikely they will be useful on a preparative scale. Upon consideration of possible alternative methods for reaching this goal, we concluded that a combination of enzymatic and chemical methods should allow a synthetic mixture of symmetric diol stereoisomers to be separated most easily. The most important feature of such a separation is that it would depend only on the ability of an enzyme to distinguish the chemistry at each stereogenic center in the diol, and would be independent of any interaction between the stereogenic centers. The specificity of hydrolase enzymes for diol stereochemistry has been exploited for some time. However, until recently, their use has been limited to modification of one stereogeniccenter in a mew diol (or diacylated meso diol)6or modification of a specific hydroxyl (or esterified hydroxyl) in a diol bearing a prochiral centerV6 Early
(1) Ito, K.; Harada, T.; Tai, A.; Izumi, Y. Chem. Lett. 1979, 1049. (2) Kitamura, M.; Ohkuma,T.; Inoue, S; Sayo, N.; Kumobayashi, H.; Akutagawa, S.;Ohta, T.; Takaya,H.; Noyori, R.J. Am. Chem. SOC.1988, 110, 629. (3) Wallace,J. S.;Morrow, C. J. J. Polym. Sci. Part A Polym. Chem. 1989,27, 2553.
(4) Koppenhofer, B.;Walser, M.; Bayer, E.; AM&, S.J. Chromutog. 1986, 358, 159. (5) See, for examDle: Hemmerle, H.; Gais, H.-J. Tetrahedron Lett. 1987,28, 3471. (6) See, for example: Ramm Tombo, G. M.;Schar, H.-P.; Fernandez i Busquets, X.; Ghisalba, 0. Tetrahedron Lett. 1986,27,5707.
OO22-3263/92/ 1957-5231$03.00/0
0 1992 American Chemical Society
6232 J. Org. Chem., Vol. 57, No. 19, 1992 HO OH C H I3 C H e A H C H 3
Wallace et al.
1-
Scheme I Ac?O Aman0 P benzene
1
finer OH CH3LH--@dHCH3
I
OH
HO
+
I I
CH3dH*dHCH3
examples of such processes involved stereoselective hydrolysis of one of a pair of esters in aqueous media?+’ The recent that such enzymes also are effective catalysts in low to moderate polarity organic solvents has allowed development of and transe ~ t e r i f i c a t i o nas~viable, ~ ~ ~ ~though ~ ~ ~ ~little ~ ~ ~exploited, ~ alternatives for modifying one of a pair of stereochemically opposite hydroxyls in a diol. Recently, Sih’s group reported a kinetic analysis for the separation of a racemic mixture of (R,R)and (S,S)-diols that is free of the (R,S)-diol, using enzyme-catalyzed acylation.22 Application of the method to the resolution of racemic 2,4-pentanediol was also described. The analysis allows one to determine the composition of the mixture of diol substrates, monoacylated intermediates, and diacylated products present at different extents of conversion of the diol racemate to products. The relative rates of the four forward processes underway are defined in terms of ratios of the experimentally determine rate constants for each of those processes. Unfortunately, the analysis is not readily adapted to the case where the meso compound is also present, for, in the latter case, the number of diol substrates and diester products is increased from two of each to three, the number of monoacylated intermediates is increased from two to four, the number of rate constants required to define the forward reactions in the system is increased from four to eight, and the relative rates of the four possible first steps can no longer (7) Huang, F. C.; Lee, L. F. H.; Mittal, R. S.D.; Ravikumar, P. R.; Chan, J. A,; Sih, C. J.; Caapi, E.; Eck, E. R. J. Am. Chem. SOC. 1975,97, 4144. (8) Ohno, M.; Kobayashi, S.; Limori, T.; Wang, Y.-F.; Izawa, T. J. Am. Chem. SOC. 1981,103, 2405. (9) Chen, C.-S.; Fujimoto, Y.; Sih, C. J. J . Am. Chem. SOC. 1981,103, 3580. (10)Cambou, B.; Klibanov, A. M. J. Am. Chem. SOC. 1984,106,2687. (11) Zaks, A.; Klibanov, A. M. Science 1984,224, 1249. (12) Gatfield, I. L. Ann. N.Y. Acad. Sci. 1984, 568. (13) Zaks, A.; Klibanov, A. M. h o c . Natl. Acad. Sci. U.S.A. 1985,82, 3192. (14) Klibanov, A. M. CHEMTECH 1986, 354. (15) Cambou, B.; Klibanov, A. M. Biotechnol. Bioeng. 1984,26,1449. (16) Kirchner, G.;Scollar, M. P.; Klibanov, A. M. J. Am. Chem. SOC. 1985,107, 7072. (17) Langrand, G.;Secchi, M.; Buono, G.; Baratti, J.; Triantaphylides, C. Tetrahedron Lett. 1985,26, 1857. (18) Langrand, G.; Baratti, J.; Buono, G.;Triantaphylides, C. Tetrahedron Lett. 1986, 27, 29. (19) Gil, G.;Ferre, F.; Meou, A.; Le Petit, J.; Triantaphylides, C. Tetrahedron Lett. 1987,28, 1647. (20) Bianchi, D.; Cesti, P.; Battistel, E. J. Org. Chem. 1988,53, 5531. (21) Francalanci, F.; Cesti, P.; Cabri, W.; Bianchi, D.; Martinengo, T.; FOB,M. J. Org. Chem. 1987,52,5079. (22) Guo, 2.-W.; Wu, S.-H.; Chen, C.-S.; Girdaukas, G.;Sih, C. J. J. Am. Chem. SOC. 1990,112, 4942.
(1) chromatography (2) NaOWCHJOH
OH
HO
+
I I
(1) chromatography (2) NaOHCH30H
OH
CH3&H+!HCH3
be reduced to a single ratio. In a typical organic-phase resolution, the enzyme catalyzes reaction of an activated ester with one enantiomer of a racemic alcohol. When the reaction has reached approximately 50% completion, it is stopped by filtering out the enzyme catalyst. The products of interest are an unchanged, optically active alcohol and an optically active ester. Following separation of the unchanged alcohol enantiomer from the ester, the latter can be hydrolyzed (chemically or enzymatically) to obtain the second enantiomer of the optically active alcohol. In a valuable modification of this procedure, Bianchi, Cesti, and Battistel have shown that acid anhydrides can be used with lipases in place of the activated ester to esterify chiral alcohols stereoselectively.20Because this method exhibits both high reaction rates and selectivities, we decided to adapt it for the separation of diol stereoisomers. Results and Discussion The substrate chosen to test the separation plan was a,d-dimethyl-l,6benzenedimethanol[ 1,1’-(1,6benzene)diethanol], 1. This diol was interesting because its stereogenic centers are well removed from each other so the possibility of one center influencing the stereochemical environment of the other is minimized. The stereochemical mixture of these diols has been used to prepare thermally depolymerizable benzylic polycarbonates.23 It can be easily synthesized in high yield by the sodium borohydride reduction of 1,4-dia~etylbenzene.~~ Acetic anhydride was selected as the esterification agent because of its low cost and high reactivity. Crude Amano P, a lipoprotein lipase from Pseudomonas sp. which had been supported on Celite 577, as described by Bianchi and coworkers,mwas used as the catalyst and anhydrous benzene was chosen as the solvent. These conditions have been shown to acetylate the (R)confiation of 1-phenylethanol selectively.20 As is summarized in Scheme I, using 1 as an example, the following general procedure has been developed. The diol mixture is dissolved or suspended in anhydrous benzene under a dry nitrogen atmosphere. The catalyst is then added, followed immediately by 2 equiv of acetic anhydride. Reaction progress is followed by VPC, and when the reaction is complete, as indicated by a dramatic reduction in the rate, it is stopped by filtering out the enzyme. (We subsequently showed that a useful modification of the Bianchi et al. method is to quench by adding (23) Eichler, E.; Kryczka, B.; Willson, C. G. Makromol. Chem. Rapid Commun. 1986, 7, 121. (24) Smejkal, J.; Kopecky, J. Z. Chem. 1986,26, 397.
Separation of Remote Diol and Triol Stereoisomers
J. Org. Chem., Vol. 57, No.19, 1992 5233
Table I. Specific Rotations of Remote Diol Stereoisomers Separated by Amano P-Catalyzed Stereoselective Acetylation RB RS .c .c ._ -_,diol yield" (%) specific rotationb(deg) yield" (%) specific rotationb(deg) yield" (%) specific rotation* (deg) 1 22.4 +80.5 35.3 -0.35 24.1 -79.9 r-
4 5 6
16.4 14.9 25.0
-9-
+65.9 +44.01 +76.95
41.5' 46.5 32.5
+a77 +10.21d +1.45
16.3 14.1 22.5
-63.6 -42.30 -76.0Y
"The yield of each component is calculated relative to the total amount of the diol mixture used in the reaction. A statistical mixture would contain 25% (R,R), 25% (S,S), and 50% (R,S) isomer. b [ a ] l m(cb = ~ 2, acetone). 'After recycling through the esterification to remove an 11.6% impurity of (R,R)-diol. dNot recycled to remove the (R,R)-diol impurity. "After recycling to remove an impurity of the (R,S)stereoisomer.
excess methanol to the reaction mixture and stirring for -0.5 h before filtering off the enzyme. This destroys the excess acetic anhydride which markedly simplifies the remainder of the work up.) Based on the known selectivity of Aman0 P lipase for catalyzjng esterifkation of secondary alcohol stereogenic centers having the (R)-configuration, it can be concluded that the product mixture includes three components: unreacted diol 1, having the (8,s)-configuration, monoacylated diol 2, having the (R,S')-confiiation, and diacylated diol 3, having the (R,R)-confguration. The mixture is separated by standard techniques such as chromatography. In the case of ar,ar'-dimethyl-1,4benzenedimethanol, and several of the other diols described here, the (S,S)-diol l crystallizes from the mixture upon cooling, and only the (R,S)-monoacetate 2, and (R,R)-diacetate3 need be separated by chromatography. Following separation, the acylated compounds are hydrolyzed to regenerate the @,@- and (R,R)-diols. The results from separating stereoisomers mixtures of 1, 4,5, and 6, using variations of this general method are summarized in Table I. No valid direct measure of the optical purities of the (R,R)- and (S,S)-1 has been reported. The compounds prepared by the method described here have significantly higher values for [aE,, of -79.9O and +80.5O (c = 2, acetone) or +74.8 (c = 2, chloroform)than the value of +60° (c not reported, chloroform)previously r e p o d for the (R,R)-diol formed by enzymatic oxidation of p-diethylbenzene.= An enantiomericexcess of >97% was claimed for that product on the basis of its behavior in the presence of a chiral N M R shift reagent. Our results, therefore, suggest that this method is of questionable validity for determining the stereochemical purity of these diols. From the rotation data alone, it seemed reasonable that the two enantiomers prepared in the present study were optically pure, for the two rotations differed by only 0.75% in magnitude (the (S,S)-l was known to contain minor amounts of impurities) while the rotation of the meso compound (R,S)-lwas essentially zero. Since the separation scheme requires that the enzyme select the (R,R)-stereoisomer twice and the (R,S)-stereoisomeronce but that it always rejects the (S,S)-stereoisomer, it seemed unlikely that partially separated stereoisomershaving this set of specific rotations would occur fortuitously. That this assumption is not always valid will be demonstrated below. To verify the stereochemical purities of the diols, we examined the use of a-methoxy-a-trifluoromethylphenylacetyl (MTPA or "Mosher's") esters prepared by reaction of the diols with (+)-MTPA chloride.2e In contrast with the usual applications of these products, in which only the relative amounta of two enantiomers is sought?' this work required distinguishing the relative (25) Holland, H. L.; Bergen, E. J.; Chenchaiah,C.; Khan, S. H.; Munoz, B.; Ninnies, R. W.; Richards, D. Con. J. Chem. 1987,65, 502. (26) Dale, J. A.; Dull, D. L.; Mosher, H. S. J. Org. Chem. 1969, 34, 2543.
Table 11. Stereochemically Sensitive NMR Regions of the Bin-MTPA Esters from the Stereoisomers of 1 intensities at chemical shifta (6) methoxy" C-methyl" S R SP Ssb R P Rsb diol 6 3.55 3.46 1.63 1.62 1.58 1.57 (RP)-l (RS)-1 (S,S)-l
97
50
50
>97
