Sogah, Cram
/ Resolution of Enantiomers of Amino Acid and Ester Salts
(32) L. do Amaral, K. Koehier. D. Bartenbach, T. Pietcher, and E. H. Cordes, J. Am. Chem. SOC.,89,3537 (1967). (33) (a) R. P. Bell and W. C. E. Higginson, Proc. R. SOC.London, Ser. A, 197, 141 (1949): (b) ref 5, p 224; (c) ref 3, p 178. (34) D. D. Perrin, "Dissociation Constants of Inorganic Acids and Bases in Aqueous Solution", Butterworths, London, 1969. (35) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", 3rd ed., interscience, New York, 1973, p 394. (36) T. M. Loehr and R. A. Plane, Inorg. Chem., 7, 1708 (1968). (37) (a) Reference 5 , Chapter 7; (b) H. F. Gilbert and W. P. Jencks, J. Am. Chem. SOC., 99, 7931 (1977). (38) (a) The theoretical curves for the BrOnsted plots in Fi ure 4 are based on
B
the following treatment for proton-transferreactions2 37 between an acid HX and a base Y. Application of the steady-state approximation to eq (a) yields eq (b) for the rate constant for proton transfer in the forward direction. HX
+ Y SXH.. .Y & x . . .HY +X + HY k-a
kf
k-p
(8)
k-b
=
kakpkb (b) kpkb t kbk-a t k-.k-, Assuming that kp >> k-a, k-, N kb, and log Kp = A p K [where A p K = pK(catalyst) pK(intermediate)] gives
-
Setting kf = k--d (in eq 6) and K-, = 0 yields eq 8. The same method was
3035
used for the derivation of eq 9, except that it was assumed that kp giving ka
kf =
1
+ k-. + 10-ApK
k-a,
(d)
kP (b) J. P. Fox and W. P. Jencks, J. Am. Chem. Soc.. 98, 1436 (1974). [a) J. Hine, M. S.Cholod, and R. A. King, J. Am. Chem. Soc., 96,835 (1974); (b) R. D. Gandour, Tetrahedron Lett., 295 (1974): (c) J. Hine, ACC.Chem. Res 11. l(19781 .~ J. E. Reimann and W. P. Jencks, J. Am. Chem. SOC.,88,3973 (1966). S.Rosenberg, S.M. Silver, J. M. Sayer, and W. P. Jencks, J. Am. Chem. SOC..96. 7986 (19741. The increase in amine yield caused by a buffer is calculated from the expression AAIAA, = [buffer]/([buffer] t IC ), where A A = increase in amine yield as compared with yield at zero b z e r concentration; AA, = maximum increase possible. A. I. Vogel, "Macro and Semimicro Qualitative Inorganic Analysis", 4th ed., Longmans, London, 1964, p 242. W. P. Jencks and J. Regenstein in "Handbodc of Biochemistry", H. A. Sober, Ed., Chemical Rubber PublishingCo.. Cleveland, Ohio, 1968, p J-153. B, N. Ames, MethodsEnzymol., 8, 115 (1966). A pK2 value of 13.53 has been reported for hexafiuoroacetone hydrate: J. Hine and N. W. Flachskam, J. Org. Chem., 42, 1979 (1979). A recent calculation gives pK2 = 38 f ' 2 for the ionization of the P-H group in HP0s2-: J. P. Guthrie, Can. J. Chem., 57, 236 (1979).
__.
Host-Guest Complexation. 14. Host Covalently Bound to Polystyrene Resin for Chromatographic Resolution of Enantiomers of Amino Acid and Ester G . Dotsevi Yao Sogah3and Donald J. Cram* Contributionfrom the Department of Chemistry of the Unioersity of California, Los Angeles, Lox Angeles, California 90024. Receiced August 30, 1978
Abstract: A host, CH3OCH2PSCH20ED(CH3)2(0EOEO)rD ( ( R , R ) 12), - was synthesized for preparative or analytical chro-
matographic resolution of racemic amino acids and esters. I n ( R . R ) -12, PS is cross-linked polystyrene, -12% of whose phenyl groups are substituted in the para position with a CH30CH2 group, and 0.8% with a spacer unit ( C H Z O C H ~ C Hwhich ~ ) , in turn is attached to a designed complexing site. This site is a macrocycle composed of two I , I '-dinaphthyl or D units of the same K configuration attached to one another at their 2,2' positions by two O E O E O units (E is CH2CH2). The spacer is attached to the remote 6 position of that D unit which contains two methyl groups substituted in its 3.3' positions. Columns of this material were used in chromatographic resolutions in CHCI3-CH3CN of racemic mixtures of CsHsCH(C02H)NH3C104, p-HOCkH4CH(CO>H)NH3CIO4, ChHsCH2CH(C02H)NH3CIO4. C,yHhNCH2CH(CO2H)NH3ClO4, (CH3)2CHCH-
IO~, CH3CH(C02H)NH3C104, (C02H)NH3C104, C ~ H S ( C H ~ ) C H C H ( C O ~ H ) N H ~ C(CH3)3CCH(C02H)NH3CI04, (C02CH3)N H3C104. p - HOChH4CH (CO2CH 3 ) s H3C104, p-C H30,CCH3SC H 2CH2CH (CO2H)NH3C104. C ~ H J C H ChH4CH(C02CH3)NH3CIO4, p-CIChH4CH(C02CH3)NH3ClO~, C ~ H ~ C H 2 C H ( C O ~ C H 3 ) N H 3 C l O 4and , p-HOC~H4CH2CH(C02CH~)NH3ClO4. Separation factors ranged from 26 to I .4, and resolution factors from 4.5 to 0.21. Host of the R.R configuration bound D guest more firmly than L guest by from 1.8 to 0.18 kcal/mol in all cases. A column packed with 9.5 g of ( R . R ) -12 containing the equivalent of 0.42 g of complexing site gave base-line separation of enantiomers of C~HSCH(CO~H)NH~C in Iruns O ~ that involved as much as 15 nig to as little as 0.013 mg of racemate. A host similar to ( R , R ) -12 in Hhich thc two mcthjl group!, are abscnt, CH3OCH2PSCH20ED(OEOEO)2D ( ( K , R ) - l l ) was , found to give lower separation factors t h a n ( K . R ) -12. The results are discussed in terms of complementary vs. noncornplementary stereoelectronic diastereomeric relationships between host and guest.
Amino ester salt racemates as a family have been resolved preparatively by solid-liquid and liquid-liquid chromatography that involved designed host-guest complexation. Separation factors varied from l .52 to 6.4.4Derivatives of amino acid racemates have been resolved analytically as gases on long capillary columns of very high plate value i n which optically active, derivatized peptides served as liquid phases. Separation factors between enantiomers as high as 1.7 have been observed.s Sephadex, covalently bound to L-arginine as a solid phase, was used to resolve preparatively solutions of 3,4dihydroxyphenylalanine with a separation factor of 1.6. A
complementary relationship between ion-pairing sites of the bound amino acid and one enantiomer of the racemate was envisioned.sc This paper reports the first example of the synthesis of designed solid phase hosts useful for both preparative and analytical chromatographic resolution of amino acid and ester racemates as a family.2 Macromolecules ( R ,R ) - 11 and ( R , R ) -12 (Chart I ) are composed of a macroreticular crosslinked polystyrene p-divinylbenzene resin on which have been grafted the chiral hosts ( R , R ) -1 and ( R , R ) - 2 ,respectively. About 0.8% of the para positions of the CkHs groups available
0 1979 American Chemical Society
3036
Journal of the American Chemical Society
Chart Ia chiral
a (R,R)-l,A = B = X = H; (R,R)-2,A = B = H, X = CH,; (R,R)-3, A = B = Br, X = H; (R,R)-4,A = B = Br, X = CH,; (R,R)-5,A = B = CH2CH20H,X = H; (R,R)-6,A = B = CH2CH20H,X = CH,; (R,R)-7, A = CH,CH,OH, B = X = H; (R,R)-8,A = CH,CH,OH, B = H, X = CH,; (R,R)-9,A = CH,CH,OCH,-PS-CH,C1 (PS = polystyrene), B = X = H; (R,R)-10,A = CH2CH,0CH2-PS-CH2Cl,,B = H, X = CH,;
( R , R ) - l l ,A=CH,CH,OCH,-PS-CH,OCH,, B = X = H; (R,R)12 = CH2CH,0CH,-PS-CH20CH,,B = H, X = CH,.
in the polymer are occupied in ( R , R ) - 1 2 by the ( R , R ) - 2 binding sites to provide -0.056 mmol of host/g, or -I 7 800 mass units for an average site.
Results Syntheses of the Host-Bound Resins. Optically pure ( R ) 2,2’-dihydroxy- 1 ,I’-dinaphthylh‘L and (R)-2,2’-dihydroxy3,3’-dimethyl- I , I’-dinaphthyFb were brominated to give (R)-6,6’-dibromo-2,2’-dihydroxy- I , 1’-dinaphthyl (94%) and (R)-6,6’-dibromo-3,3’-dimethyl-2,2’-dihydroxyI , 1 ’-dinaphthy1 (90%), respectively. These substances, with optically pure (R)-2,2’-bis( 1,4-dioxa-6-tosyloxyhexyl)1, I’-dinaphthyl”%‘and KOH-THF, gave cycles ( R . R ) - 3(74%) and ( R , R ) - 4(69%), respectively. Metallation of ( R , R ) - 3with BuLi and treatment of the product with ethylene oxide gave optically pure parent cycle ( R , R ) - l (30%), diethoxylated cycle ( R , R ) - S(IO%), and monoethoxylated cycle ( R , R ) - 7 ( 5 5 % ) . Similarly, ( R , R ) - 4 gave optically pure ( R , R ) - 2 (25%), ( R , R ) - 6 (6%), and ( R , R ) - 8(60%). Hosts ( R , R ) - 7 and ( R , R ) - 8were attached through their 6-substituted C H l C H 2 0 spacer units to a solid phase by reaction of their sodium alkoxides with -I 5% chloromethylated macroreticular polystyrene-divinylbenzene copolymer to give ( R , R ) - 9and ( R , R ) -10, respectively. The differences between the amounts of ( R , R ) - 7and ( R . R ) - 8used and recovered from the reactions were used to calculate the amounts of cycles covalently bound to the resin. These amounts were consistent within experimental error with the loss in the amounts of chlorine content of the resins during the reactions. To destroy the CH2CI groups on the resin unavailable to the large cyclic alkoxides, (R,R)-9and ( R , R ) - 1 0 were treated with NaOCH3 to give ( R , R ) - l l and ( R , R ) -12, respectively. Grafted polymer ( R . R ) -11 contained -0.048 mmol of cycle/g, 0.17 mequiv of Cl/g, and 0.90 mequiv of CH3O groups/g. Grafted polymer ( R , R ) -1 2 contained -0.056 mmol of cycle/g, 0.1 8 mequiv of Cl/g, and 0.87 mequiv of C H 3 0 groups/g. Chromatographic Columns. The host-bound resins were sieved, suspended in CH3CN-CHCI3, and pumped into jacketed and insulated stainless steel columns, which were conditioned by pumping through their beds, in turn, degassed CH301-1, CHC13, and, finally, the solvent used for the runs. The columns were fitted with injection loops for sample introduction. The bottoms of the columns led to conductivity cells attached to a recorder. The relative conductivity of the cell was found to be proportional to the concentration of the alkylammonium salt in chloroform-host solutions.4cThe dead volume of each column was determined by injecting the nonretained compounds methanol, benzene, hexane, and pentane as samples onto the columns, and determining their retention volumes (see Experimental Section). Column A was 60 by 0.75 (i.d.) cm in dimension and was packed with 9.5 g of 250-325 mesh ( R , R ) -12. Column B was 60 by 0.40 (i.d.) cm and was packed with 4.0 g of 325-400 mesh ( R , R ) - 1 2 . Column C was the same size as B and was packed with 4.0 g of 325-400 mesh ( R , R ) -11.
1 101:ll 1 M a y 23, 1979
Chromatographic Resolutions of Enantiomers. The runs were made at constant temperature maintained by passing waterethylene glycol at constant temperature through the jackets of the columns. During the runs, constant flow rates were maintained between 0.27 and 2.0 mL/min with pressure drops between 350 and 900 psi. Between 0.013 and 84 mg/run of racemic *RNH3CI04 or *RNH3PFh was injected into the loop at the top of the column. Corrections, which were less than one third of the dead volume, were made for the loop, detector, and tubing volumes. The appearance of enantiomers in the column eluate was detected conductometrically. The mobile phases were CHC13 or CHzClz containing 5-25% by volume of CH$N or Et02CCH3 to act as salt carriers. The use of ethers or alcohols as the main solvents gave no enantiomer separation. However, small amounts of C H 3 0 H or (CH3)2CHOH as salt carriers in CHC13 or CHzCll gave moderately good results, but were not generally investigated. Plots of relative conductance ( p n h o ) against volume of column eluate (mL) for each chromatographic run provided the parameters that indicate the effectiveness of the separations.x The peaks were Gaussian and showed little tailing. The enantiomer separation factor ( a )is defined by eq 1, in which V K Ais the retention volume of enantiomer A (more firmly complexed by the stationary phase and appearing last in the column eluate); VRB is the retention volume of enantiomer B (less firmly complexed and appearing first in the eluate); and V M is the dead volume of the column. The enantiomer resolution factor ( R , ) is defined by eq 2, in which W A is the bandwidth (mL) of enantiomer A and W Bis that of enantiomer 9. Under ideal conditions, the differences in free energies of complexed enantiomers A and B are represented by eq 3.8 I n some runs, the conditions were probably not ideal. The results of these runs can be conveniently discussed in terms of the -A(AGo) values even though they are only approximations. The configurational identities and optical purities of the faster (less complexed) and slower (more complexed) moving enantiomers were identified by isolation and characterization of the pure antipodes in runs 9,20, and 26 of Table I, which records the results obtained with amino acid perchlorates as guests. I n runs 6-21 and 27, the identifications of the faster and slower moving enantiomers were made by determinations of the signs of rotation of the eluate fractions. The signs of rotations of authentic L-amino acids salts were taken in the solvents used in these runs, and the signs and configurations correlated. In runs 22-26 and 32-34, each peak was collected, the solvent evaporated, and the residue dissolved in absolute methanol. The signs of rotation were taken. The correlations with configurations reported are based on the signs of rotations of authentic L-amino acid salts taken in absolute methanol. In those runs with base-line separation, the areas under the two bands were essentially equal to one another.
Table I1 reports the results of chromatographic runs made with methyl esters of six different amino acid salts and columns A, 9 , and C . Both perchlorate and hexafluorophosphate salts were examined. Base-line separations were observed for all runs except 12- 15. In those runs with base-line separation, the areas under the bands were essentially equal to one another. The more complexed and slower moving enantiomers were identified by isolation and characterization of the pure antipodes in runs 2,4, and 5 (or ones like them). In the other runs, the more and the less bound enantiomers were identified by their signs of rotation. In runs 1-3 and 7-15. the signs were determined in the column eluate. Configurations wereassigned
Sogah, Cram
/ Resolution of Enantiomers of Amino Acid and Ester Salts
3037
Table I. Resolution of Enantiomers of RCH(C02H)NH3C104 Guests (G) by Solid-Liquid Chromatography with R , R Hosts ( H )
colrun umn no. usedo I 2 3 4 5 6 7 8 9 IO II 12 13 14 15 16 17 18 19 20 21
A A A A A A
22 23 24
A
25 26 27
A
28 29 30 31 32 33 34
B
A A A A A A A
A A A A A A A A
guest structure
wt,
guest enantiomers more bound less bound
-A (AGO), res1
mobile phaseC H/
of R
ChHs
0.013 I 1 000 CHCI:, 0.13 I 100 CHC13 0.32 420 CHC13 0.60 220 CHCI3 I .9 70 CHC13 5.0 27 CHC13 10.1 13 CHC13 15.2 8 CHC13 20.5 6 CHCI, I .5 CHC13 84 5.08 27 CHCli 27 CHCI3 5.04 8 CHCI, 16.1 8 CHC13 16.0 5. I 26 CHC13 15.7 8 CHlCI? 14.5 9 CH2CI2 9 Et20 14.7 6.6 21 CHCI3 4.6 31 CHCI3 5.8 28 CHCI,
A A A A B
B B
C C C
MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN EtOAc EtOAc EtOAc EtOAc EtOAc MeCN MeCN VeCN MeCN MeCN MeCN
IO IO IO IO
IO IO IO IO IO IO 5 5 IO 15 25 5 17 IO
IO 4 IO
5.5 base line 8.9 base line 11.0 base line 11.6 base line 12.2 base line 14.6 base line 24.3 base line 12.2 base line 10.0 mini m u m 10.7 minimum 4.5 base line 10.9 base line 7.4 base line 4.7 minimum 4.3 base line 5.3 base line 3.4 minimum 1.0 none 6.1 base line 2.3 base line 0 1.9 minimum
0. 0 0 0 0 0 0 0 0 0 25 0 0 0 0 0 0 0 0 0
0.9 1.2 1.3 1.3 1.4 1.5 1.7 1.4 1.25 1.2 0.9 1.3
D
0.8 0.8 0.9 0.7 0.0 1.0 0.45 0.35
1.99 2.72 2.89 2.86 1.76 1.13 0.74 0.76 0.54 0.20 1.35 1.92 1.23 0.61 0.85 1.22 0.39 0.00 2.31 0.97 0.42
1.1
D D
D
D D D D
-
D
min s
D
D
D D D D
6.1 base line 1.0 2.3 minimum 0.45 1.9 minimum 0.3
1.61 0.45 0.24
D D D
2.0 I .6 6.6
61 63 20
CHC13 MeCN 5 CHC13 MeCN 4 CHC13 ikfeclc’ 4
0 0 0
1.9 minimum 0.3 1.5 minimum 0.2 1.4 minimum 0.8
0.37 0.21 0.25
D
I .7 2.4 I .o I .4 2.0 I .7 1.1
33 CHC13 25 CHC13 58 CHC13 36 CHCll 23 CHC13 30 CHCI, 48 CHCI3
base line base line minimum minimum
0.89 1.55 0.25 0.52 0.35 0.23 0.21
D
0.1
0.2 minimum 0.5 minimum 0.3 minimum 0.3
-
D
0 0 0
0.8 0.85
-
D
CHC13 MeCN 20 CHC13 MeCN I O CHCll MeCh 5
4.1 4.2 1.2 1.4 2.4 1.8 1.6
-
D D
80 74 53
25 25 MeCN I O 25 MeCN 5 25 MeCN 2.5 0 0 MeCN I O 0 MeCN 5
-
D
2.0 I .6 2.3
MeCN I O MeCN I O
100 1 I6
D
+/
-‘ -/
-’ -‘
D D
-
D
D D
D
-/
D
-I
-’
D
I I9 I I4 89 72 54 51 30 30 69 I34 62 49 45 140 37 36 426 123 89 278 69 59 61 52 86 19 71 14 25 17 32 33
+
+ + + + + +
+ + +
+ +
38 34 32 32 29 27 25 26 25 24 34 34 28 29 28 45 28 36 90 67 59
+/
+‘
66 44 42
+‘ +‘ +
44 43 67
-/
+‘ +/
+‘
12 24 13 21 13 22 24
(’Column A contained 9.5 g of ( H . R ) -12 or 0.53 mmol of host ( H ) sites; column B. 4.0 g of ( R . H ) -12 or 0.22 mniol o f H sites; column C. 4.0 g o f ( R , R ) - l l or 0.19 mniol of H sites. Ratio of moles of H to moles of C . Reagent-grade solvents. CHC13 contained 0.75% EtOH. Equation 2. I Enantiomer A . X 578 and 546 nm: solvent r’ By volume. e Equation 1 . Base line means base line separation. R Equation 3. is column eluant unless otherwise noted. P-indolylmethyl. / MeOH as solvent.
,’
J .
based on the signs of rotation of authentic L-amino ester salts taken in the solvent of the run. In runs 4-6, the two eluant bands were evaporated, and the signs of rotation of their salts were determined in methanol. Runs 4-6 and I I involved esters not previously r e s ~ l v e dWith . ~ column A and larger amounts of the salts than in these runs, p-CH302CC6H4CH(C02CH3)NH3C104 and p-CIC6H4CH(C02CH3)NH3C104 were totally resolved. The C D spectra of all four phenylglycine and two phenylalanine methyl ester perchlorate salts were determined in C H 3 0 H . Each of the six salts gave two Cotton effects, one at 2 15-220 nm ( P T * transitions) whose sign was configuration dependent and the other at 250-260 nm whose sign was configuration independent and negative. The L enantiomers of the four salts of known absolute configuration all gave a positive Cotton effect a t the lower wavelength, and this correlation was used to assign configurations to the two salts of unknown configuration (see Experimental Section).
-
Discussion Utility of Chromatographic Columns for Enantiomer Separations and Analysis. Resin-bound host ( R ,R ) - 12 provides a material for effective solid-liquid chromatographic resolution
of racemates into enantiomers for amino acid and ester perchlorate or hexafluorophosphate salts. Column A, containing 9.5 g of ( R , R ) - 1 2 ,provided separations in either CHC13 or CH2CII of 16 salts of the type RCH(C02R’)NH3C104 in which R represents 12 different groups and R’ was either H or CH3. With the exception of N = I in run 8 using Et20 as a solvent, separation factors (CY) for enantiomers ranged from 26 (run I , Table I f ) to 1.4 (run 27. Table I ) , which represents a spread of -A(AGo) values for the diastereomeric complexes of -1.8-0.1 8 kcal/mol. Resolution factors ( R , ) ranged from 4.5 (runs 2 and 3, Table 11) to 0.21 (run 26, Table I). Phenylglycine perchlorate gave the highest and alanine the lowest. Even with methionine perchlorate (run 27), which gave the lowest separation factor (CY = 1.4), proper cutting of eluate fractions provided substantial amounts of pure enantiomers. The same column (A) was used. both analytically and preparatively. Since the column contained -0.53 mmol of host sites, the ratio of host to guest sites ( H / G ) involved in a run could be calculated. With C C , H $ C H ( C O ~ H ) N H ~asCG, I~~ base-line separation of enantiomers was observed with as little as 0.013 mg of salt ( H / G = 1 1 000, run I, Table I) or as much as 15.2 mg ( H / G = 8, run 8, Table I). Thus, the column performed well when the amount of racemate submitted for res-
Journal of the American Chemical Society
3038
/
101:ll
/ May 23, 1979
Table 11. Resolution of Enantiomers of RCH(C02CH3)NH3X Guests (G) by Solid-Liquid Chromatography on R,K Hosts (H) % CH3CN
in CHC13 run no.
column used"
guest structure R
wt,
X
I 2 3 4
A A A A
C104-
5 6 7 8
A A A A
c104
9 IO II
B B B
C104-
12 13
B
C104-
B
14 15
C
c
c104P Fg -
clodP F6-
C104C104PF6PF6-
clod-
mg H/Gb
as mobile phaseC
guest enantiomers more bound less bound sepn -h(SGo), resln sign sign 7, factor kcal/ factor of VRA, of VRB, 'C ad mol' R,J configng o&dh m L a&dh m L
9.5 9.5 2.1 9.5
16 15 77 18
IO IO 5 IO
26 18.5 18.2 12.6
1.8 1.6 1.6 1.4
3.0 4.5 4.5 2.3
D
9.5 5.0 9.5 9.5
17 37 16 17
IO IO IO IO
8.5 8.1 6.4 4.7
1.2 0.8
2.2 I .2 1.9 1.7
D D D D
2.0 2.5 2.2
29 28 36
IO 5
4.3 4.3 9.0
0.9 0.9 1.3
1.02 0.77 0.84
D D
5
25 25 25
D
28 23 30
2.6 2.7
24 24
5 IO
25 25
3.2 2.2
0.7 0.6
0.58 0.24
D D
15 14
5.4 0.5
1I 116
5 IO
0
4.1 1.7
0.8 0.3
0.48 0.25
D D
20 15
0
1.1 1.0
D D
39
D
76
+ + + +' +' +' + + t
22
30
t
14 13 25
+ +
II I2
+
+ t
12 13
Column A contained 9.5 g of ( R , R ) - 1 2 ,or 0.53 mmol of host ( H ) sites; column B, 4.0 g of (/?,I?)- 12, or 0.22 mmol of H sites; column C, 4.0 g of ( R , R ) -11, or 0.19 mmol of H sites. Ratio of moles of H to moles o f G . ' Reagent-grade solvents. CHCI3 contained 0.75% EtOH, percent by volume. Equation 1. Equation 3. f Equation 2. g Enantiomer A. h 578 and 546 nm; solvent is column eluant, unless otherwise noted. ' CH30H as solvent.
olution varied by a factor of > IO3. Even when badly overloaded with 84 mg of salt (run 10, Table I), over half of each enantiomer was obtained in an optically pure state. Although these chromatographic columns were designed to separate and differentiate enantiomers, they probably could be used to identify different amino acids as well. Solvent composition and pressure were not kept constant for all the different amino acids examined, so general comparisons of changes in their retention volumes with changes in structure cannot be made. Usually, solvent composition and pressure were adjusted to give convenient retention volumes. What comparisons can be made suggest that, had standard conditions been maintained, retention volumes would have varied markedly with changes in the structures of the amino acid. For example, runs 20 and 24-27 were all made on column A with -95% CHCI3-5% CH3CN a t about the same pressure (650-700 psi). As the side chain varied in the order ChH5CH2, CH3SCH2CH2, (CH3)3C, C2HdCH3)CH, and CH3, VRA (mL) changed in the order 123,86,6 I , 59, and 52. Runs 6. 19, 21, and 23 were conducted with 90% CHCI3-IO% CH3CN. As the side chain varied in the order p-HOC6H4, p HOChHdCH2, C6H5, and (CH3)2CH, V R / Z(mL) changed in the order, 426, 89, 72, and 69. Tryptophan had such a large V R Awith this solvent mixture that the CH3CN carrier had to be increased to 20%, which gave a V R / Zof 278. The Question of Whether All Host Sites Bound to Polymer Exhibit the Same Degree of Chiral Recognition. The important question arises as to whether, in a host-polymer such as ( R , R ) -12, each binding site shows the same chiral recognition toward a particular guest. The character of the polymer-host is material to a discussion of this question. The macroreticular resin used possessed a n average pore diameter of 90 A and a surface area of 330 m2/g, and was cross-linked enough not to swell noticeably when wet with the solvents used. In the preparations of the host-polymers used in the separations, the conditions were designed to maximize the number of host sites. In the preparation of ( R . R ) - 1 2 ,-0.8% of the CsH5 groups theoretically available in the original polymer became attached to the macrocycle.
Host 2 has a molecular weight of 740. I t is fairly rigid and possesses molecular dimensions of -I 8 by 1 1 by I O (CPK molecular model examination). Apparently, even with the CH2OCH2CH2 spacer group between the polymer and host, only a relatively small number of sites on the polymer surface were sterically available to this large reactant. For ( R , R ) -12, the average number of mass units per host site is -I 7 800, and -4.4% of it by weight is the macrocyclic binding site. The remainder appears to be support structure. In this sense, ( R , R ) -12 resembles the smaller of the enzyme systems. They possess molecular weights in the I O 000-20 000 range, only a fraction of which involves binding and catalytic sites, the rest being support structure. The molecular weights of the guest cations used in the chromatograms ranged from -I40 to 224 and their molecular dimensions are much less than those of the host sites. Thus it seems reasonable to expect that any place on the polymer sterically available to the relatively large host molecules should also be available to the smaller guest molecules. However, it is possible that the chiral recognition exhibited by host binding sites attached to the more sterically confined aryl groups in the narrower pores of the polymer might be different from the nonconfined sites more surrounded by solvent. The results of r u n s 1-10 of Table I that involve ( R , R ) - 1 2 and C(,H5CH(C02H)NH,CIOd bear on this question. I f all the host sites in ( R , R ) -12 produced the same chiral recognition, the A(AG") values that measure chiral recognition should be independent of H / G values. In runs 1-10, -A(AG") varied from 0.9 kcal/mol a t H / G = 1 1 000 to a maximum of 1.7 at H/G = 13, and decreased to 1.25 kcal/mol at H / G = 1.5. These results are explained by the presence of the following two opposing effects, one operative a t high and the other a t low H / G values. When very small amounts of guest were put on the column (e.g., H / G = 1 1 000), only 1: I complexes were formed a t the sterically most available sites, and these sites exhibited lower chiral recognition than the more confined host sites. This explanation presumes the reasonable assumption that the more exposed complexed sites possess a more negative free energy of binding toward both enantiomers
Sogah, Cram
/ Resolution of Enantiomers of Amino Acid and Ester Salts
3039
than do the more confined complexing sites. As the amounts comparable -A(4Go) magnitudes is visible in runs 6, 19, 22, of guest were increased, the sterically more confined sites which 20,23,26, and 27 of Table I, where complexation occurs a t 0 exhibited higher chiral recognition were engaged in binding, "C a t the polymer-CHC13 interface. and the -A(AG") values increased to 1.7 kcal/mol a t H / G With the amino ester salts, RCH(C02CH3)NH3PF6, in solution in CHC13 at 0 "C, the ( R , R ) - Dcomplex was favored = 13. When H / G values decreased further, complexes formed over the ( R , R ) - Lcomplex, by -A(&") values that decreased a t the top of the column that involved two guests to one host. These complexes were less structured, and lower chiral recin magnitude as the R group was changed as follows: C6H5 p-HOC6H4 > C6H5CH2.lob At the polymer-CHCI3 interface ognition resulted. a t ~0 "C for RCH(C02CH3)NH3C104, the same order applied As a result of this study with C ~ H S C H ( C O ~ H ) N H ~ C I O (see runs 1, 2, and 7 in Table 11). as guest, the H / G ratios employed in runs that involved the Other similarities exist between the results in solution other guests were kept mainly in the 8-80 range. Thus the (one-plate extractions)1° and those a t the polymer-solvent amounts of guest involved were practical from the point of view interface. For both environments. higher chiral recognition was of solubility and identification, and this range gave the most observed with binding sites whose chiral barriers were extended representative chiral recognition. However, the fact that the by two methyl groups (see ( R , R ) - 2and ( R , R ) - 1 2 compared ) free energies of complexation were somewhat dependent on with those without the methyls such as ( R , R ) - Iand ( R , R ) - l l H/G values makes fine distinctions in comparisons between (compare, Table I, run 6 with 32, 19 with 33, and 21 with 34, guests impossible. and appropriate pairs of runs in ref 10b with those in loa). For Racemate Resolution by Rational Design of Chiral Comboth environments, higher chiral recognition was observed at plexing Agent. Unlike conventional resolutions of racemates 0 "C than a t 25 "C (compare, Table I , run 6 with 28, 19 with into their enantiomers, those described here are rational in the 29, 20 with 30, and 25 with 3 I ; Table 11, run 2 with 9 , 4 with sense that in most cases the more bound enantiomer was preI I , 7 with 12, and 8 with 13; and appropriate pairs of runs in dicted i n advance of experiment based on complementary ref IOa,b). For both environments, PF6- or C104- as counstereoelectronic compatibility between host and guest and on terions gave nearly comparable results (compare, Table 11, run results of complexation studies in solution.104,bMolecular 2 with 3, 5 with 6, and 9 with IO, and appropriate runs in ref model (CPK) examination of diastereomeric complexes led lob). For both environments, CHC13 or CHC13-CH3CN to generalized complex ( R , R ) - D - 1 3as being the more stable. mixtures as solvent or mobile phase gave the highest chiral recognition. Substitution of CH2C12 for CHC13 as in runs 16 and I7 of Table 1 resulted in lower chiral recognition and larger retention volumes, particularly for the more firmly bound isomer. Substitution of Et20 for CHC13 eliminated chiral recognition altogether (run 18). Other ethers or alcohols as the main mobile phase also gave no enantiomer separation. Apparently ethers and alcohols as solvents themselves provide ( R,R )-D-13 good enough hydrogen bonding sites to inhibit complexation. Similar responses to these solvent changes have been obTherefore, the D-amino acid or ester salt was expected to be served i n one-plate extraction experiments with the more retained enantiomer on the columns. C(,H5CH(COrCH?)NHjPF6 and ( R , R ) - 2 . 'I Small amounts In molecular models of the host, the four naphthalene rings of C H 3 0 H (3%) or (CH3)2CHOH (10%) can be used as guest occupy planes that are roughly perpendicular to the best plane carriers in CHCI3 or CH2C1,. Although lower chiral recogof the macroring. Two of the naphthalene rings extend above nition was observed, lower volumes of solvent were needed to and tangent and two below and tangent to the macroring. The place the guest on the column. Interestingly, r u n s 1 I - 15 of naphthalene rings provide walls or chiral barriers that divide Table I indicate that CHC13-EtOAc can be substituted for into two cavities on each face of the macroring, the space CHCI?-CH,CN with C h H ~ C H ( C 0 2 H ) Y H 3 C 1 0as~ guest available to the R, C02R', and H groups attached to the with little change in chiral recognition. Unfortunately, no asymmetric center of the potential guests. The methyl groups coniparisons are available in one-plate extraction experiattached to one of the dinaphthyls extend the chiral barrier, men ts. and inhibit folding of the macroring. Aside from the side chain Effect on Chiral Recognition of Para Substituents (X) (spacer) attached a t the remote 6 position, the chiral binding in Guests p-XChH4CH(C02CH3)"3ClOj and p-XChH4site possesses a Cr axis, so that essentially the same complex CH2CH(C02CH3)"3CI04. Substitution of p - H O groups in is formed by the approach of a guest to either face. The visuplace of hydrogen i n the phenyl rings of guests alized complex is held together by three IVH+-O interactions C h HS CH (CO2 H) N H 3 C I 0 4 and C(,H SCH ?C H ( C 0 2 H ) in a tripod arrangement, by three N+-O interactions in a N H ~ C I O Jproduced a marked el'fect on the chiral recognition triangular arrangement, and by w-w C02R' to naphthalene of ( R , R ) -12 for these guests. This substitution in phenylglycine attractions. I n (R,R)-D-13,these latter two groups occupy reduced the chiral recognition from about -A(AG") of 1.5 to parallel planes that contact one another. I n ( R , R ) - D - 1 3 ,the 1 .O kcal/mol (runs 6 and 19. Table I ) , but the direction of the H and CO2R' groups attached to the chiral center occupy the chiral bias was not changed. The same substitution in phelower cavity and the bulky R group occupies the upper cavity nylalanine to give tyrosine changed - A ( J G " ) from 0.45 to on the top face of the host. 0.35 kcal/mol (runs 20 and 21, Table I ) . General model Comparison of Chiral Recognition in Solution and at a Polymer-Solution Interface. The directions of the configura- ( R ,R)-D-13 for the more stable diastereomeric complexes tional bias and degrees of chiral recognition in complexation takes no account of substituent effects in positions in the guest in CHC13 solution with ( R , R ) - 2toward a variety of guestsIO remote from the binding sites. Consequently substituent effects were examined somewhat more thoroughly in the ester seare similar to those observed for ( R , R ) -12 a t the polymerCHC13 interface. Thus, in CHCl3-CHjCN a t 0 "C, ( R , R ) - 2 ries. In runs I , 2, 4, and 5 ofTable I I with ( R , R ) - 1 2 a s host, X complexed D - R C H ( C O ~ H ) N H ~ C better I O ~ than it complexed L-RCH(CO~H)NH~CIO by~ ,-A(ACo) values that de- of p-XChH4CH(CO?CH3)NH3CIO4was varied from H O to H to CH302C to CI. In the four runs carried out under idencreased in magnitude as the R groups were changed as follows: tical conditions, the D enantiomer was always the more comChHs > p-HOChH4 > CxHhNCH2 > ChHsCH2 (CH3)lplexed, but the -A(AG") values changed in the respective C H > CH3SCH2CH2 CH3.ioCsd A comparable order with
-
-
N
3040
Journal o f t h e American Chemical Society
/
101:ll
/ May 23, 1979
1H N M R spectrum gave 6 5.07 (s, O H , 2 H), 6.85 (d. ArH8, J j . 8 = order 1.8, 1.6, 1.4, and 1.2 kcal/mol. This order correlates very 9 Hz, 2 H) 7. I5 (d, ArH3,Jd.j = 9 Hz, 2 H), 7.25 (d o f d , ArH7, J j . 8 roughly with the Hammett CT values for these substituents = 9, J 5 , j = 2 Hz, 2 H), 7.75 (d, ArH4, 5 3 . 4 = 9 Hz, 2 H ) , 7.90 (d, (correlation coefficient of 0.84)12 with p = -0.54. In the X-ray ArHS, J5,7= 2 Hz, 2 H). Anal. (C20Hi202Br2) C , H, Br. structure of the less stable diastereomeric complex between ( R ) - and (S)-6,6'-Dibromo-3,3'-dimethyl-2,2'-dihydroxy-l,l'-di(S,S)-1 and D - C ~ H S C H ( C ~ ~ C H ~ ) N the H ~rather P F ~ ,acidic naphthyl. A procedure essentially identical with that described above hydrogen attached to the asymmetric center of the guest was used except the reaction temperature was -50 "C. Thus, 1 I .63 "hydrogen bonds" one of the oxygens of the macroring of the g (37 mmol) of optically pure (R)-3,3'-dimethyL2,2'-dihydroxyhost.I3 This hydrogen is acidified by the NH3+, C02CH3, and 1 ,I'-dinaphthylbb in 250 mL of CHfJz and 5.0 mL (98 inmol) of BrZ p-XC6H4 groups attached to the asymmetric carbon. A in 50 mL of CH2C12 was converted into 15.7 g (90%) of recrystallized (CHCI3-pentane at -60 "C) R product as a foam at 25 OC, [a]:& plausible explanation for the above order of &AGO) values -68.0" (c 1.37, CH2C12). Anal. (CZlHlhOZBr2) C, H , Br. is the effect of the X substituent on the acidity of this carbon Similarly 3.5 g of optically pure (S)-3,3'-dimethyL2,2'-dihyacid, which directly affects the stability of the less stable didroxy- I , I'-dinaphthyPb was dibrominated to give 4.6 g (88%) of deastereomeric complex. Thus the more acidifying the X subsired (S)-dibromide as a foam: [ a ] &+68.2" (c 1.0, CH2C12); ' H stituent, the more stable the ( R , R ) -12-(S)-guest complexes N M R 6 2.43 (s, CH3, 6 H), 5.03 (s, O H , 2 H), 6.85 (d, ArHg,J7.8 = should become. The thermodynamically favored ( R , R ) - 9 Hz. 1 H), 7.27 (d of d, ArH7, J5.7 = 2 , 5 7 3 = 9 Hz, 1 H ) , 7.62 (br 12-(R)-guest complexes should be relatively insensitive to this s. ArH4, I H). 7.91 (d. ArH?, J5.7 = 2 Hz, 1 H). Anal. (Cz2Hi602Br?) particular remote substituent effect, as is suggested by the C, H.I5 general model, ( R , R ) - D - ~ ~ . (R,Ri-2,3,4,5-Di[ 1,2-(6-bromo)naphtho]-13,14,15,16-di( 1,2-naphIn runs 7 and 8 of Table 11 with ( R , R ) -12 as host, the betho)-1,6,9,12,17,20-hexaoxacyclodocosa-2,4,13,15-tetraene (( R,R-3). havior of ~ - X C ~ H J C H ~ C H ( C O ~ C H ~ ) Nguests H ~ Cwas I O ~ In 400 m L of dry T H F , 10.3 g (23.2) mmol) of optically pure ( R ) 6,6'-dibromo-Z.Z'-dihydroxy- I , 1'-dinaphthyl was dissolved and stirred examined.under standard conditions. With X = H and HO, under dry nitrogen. Potassium hydroxide pellets (2.88 g) were added the D enantiomer was again the more ~ o m p l e x e d , with '~ and the mixture was refluxed under nitrogen for 4 h during which it -A(AGo) values of -1.2 and -0.8 kcal/mol, respectively. I n became homogeneous. Optically pure (R)-2.2'-bis(5-tosthe X-ray structureI3 of (S,S)-~ - D - C ~ H S C H ( C O ~ C H ~yloxy-3-oxa-l-pentyloxy)-l,l'-dinaphthy110a )(18 g or 23.3 mmol) NH3PF6, the C02CH3, and oxynaphthyl groups lie in parallel dissolved in 50 mL of pure T H F was added dropwise in 0.5 h. and the planes, and probably K bind one another. An examination of iiiixture was refluxed under nitrogen for 17 h. The reaction mixture CPK molecular models of ( R , R ) - D - indicates ~~ a geometry % a s cooled and filtered, and the solids were washed with CHCI?. The ideal for a similar electronic effect for this more stable diawashings and filtrate were combined, dried, and evaporated to give ;I viscous oil (21.9 g). This material was chromatographed on 500 g stereomer, as well. This K binding should be subject to remote of ncutral alumina with 2 L of CH2C12 to give 17 g of a white foam. substituent effects in complexes for each diastereomer of the This material was crystallized and recrystallized three times from 2: I ester salts of both phenylglycine and phenylalanine. However, (bq volume) benzene-cyclohexane to give a solvate. nip 136- I37 "C. it is impossible to guess which diastereomer is more subject to Removal of the solvent at 80 "C at 0. I nim of Hg gave a glass, 15.0 this K binding. g ( 7 4 % ) o f ( R , R ) - 3[a]&+I57", CY]$^ +166" (c 1.0,CH?C12).The The overall binding energies in these complexes are small, ' H N M R of the cycle gave 6 3.20 (m,CH20CH2, 8 H), 3.80 (ni, and the -A(AGo) values reflect the net of several different ArOCH2, 8 H), 6.80 (d, BrArHg. 1 7 . 8 = 9 Hz. 2 H). 7.05 (in, ArH, opposing binding effects. In view of the complexities, the sur8 ti), 7.13 (d o f d . BrArH7, J j . x = 9, J5.7 = 2 Hz. 2 H). 7.18 (d, prising feature of this study is that model ( R , R ) - D - quali~~ BrArH3,J3.4 = 9 Hz.2 H).7.75(d,ArH4,Ji,4 = 9 H z , 4 H),7.80(d, tatively correlates all of the results. ArH,,J3,4 = 9 Hz, 2 H). 7.90 (d, BrArH?, J5.7 = 2 HI. 2 H ) . Anal.
Experimental Section General.All chemicals and solvents were reagent grade. Tetrahydrofuran (THF) and I .2-dimethoxyethane were distilled from sodium benz.ophenone ketyl immediately prior to use. Thin-layer chromatography was performed on silica-gel-coated glass plates (0.25-mm thickness) with I % Du Pont Phosphor as UV indicator. Melting points belo& 200 "C \rere measured on a Thomas-Hoover apparatus, those nbove 200 "C were measured on a Met-Temp apparatus, and all are uncorrected. Mass spectra were recorded on an AEI Model MS-9 double-focusing spectrometer. The I H N M R spectra were recorded in CDCI, on a Varian T-60 or HA- 100 N M R spectrometer, and the chcniical shifts are given in 6 (parts per million) with internal Me& as standard. Optical rotations uere measured with a Perkin-Elmer 141 polarimetcr in a I dm thermostatcd cell at 25 "C. The O R D and C D spectra were recorded on :I Cary 60 recording spectrophotometer equipped with a circular dichroism accessory, Cary Model 6002. Gltraviolct spectra \+ere taken on a Car? Model 14. All C D spectral recordings were made in a I-cm jacketed cell at 25.0 f I "C. (RI-6,6'-Dibromo-2,2'-dihydroxy-l,l'-dinaphthyl. In 40 m L of CH?CI:, 2.10 g (7.34 mmol) of optically pure (+)-(R)-Z,Z'-dihydroxy- I ,I'-dinaphthylhc' was dissolved. and the system cooled to -75 "C. Bromine ( 1 mL. 19.6 mmol) was added dropwise over 20-30 min with constant stirring at -75 "C. After stirring an additional 2.5 h while warming to 25 "C, the reaction mixture was stirred further for 0.5 h. and the excess Brz was destroyed b? addition of 50 m L of 10% aqueous solution of sodium bisulfite. The layers were separated, and thc organic layer was washed with saturated NaCl solution and dried. Evaporation of the solution gave 3.6 g of solid, which was recrystallized from benzene-cyclohexane to give 3.20g (99%) of the desired product. When the reaction was repeated using 18.0 g of optically pure ( + ) - ( R ) diol in 400 m L of CHzClz and 9.0 m L (176.4 mmol) of Brz i n 50 m L of CH?CI?. the desired dibromide diol was obtained in 94% yield as. a solvate. which after drying gave [ a ] $ 8- 129" (c 1 .O, CH2C12). Its
( C J X H ~ ~ O ~C,B H, ~ ZBr. )
(R,Ri-2,3,4,5-Di[ 1,2-(6-bromo-3-methyl)naphtho]-l3,14,15,l6di( 1,2-naphtholi-1,6,9,12,17,20-hexaoxacyclodocosa-2,4,13,15-tetraene ((R,R)-4i. By a procedure similar to that described for preparing (K.R)-3, 10.24 g of optically pure (R)-6,6'-dibromo-3,3'-diinethyl2.2'-dihydroxy-l,l'-dinaphthyland 15.1 g of optically pure ( K ) 2.2'-bis(5-tosyloxy-3-oxa- 1 -pentox))- I , I'-dinaphthyl'"" were coupled to give 12.1 g (69%) of (R.R)-4, mp 135-143 "C (benzene-cyclo-
hexane solvate). Before crystallization from benzene- cqclohcxane. thc reaction product uas chromatographcd on 600 g of ncutral aluiiiinn-CHzCl?. After drying at 160 "C at 0.01 niin of Hg. (R.R)-4 gave [ a ] $ X+172" (c 1 . I . CHlC12). Anal. (CiOHJ?OhBr2) C. H. Br. ( R,Ri-2,3-( 1,2-~6-(2-Hydroxyethyli-3-methyI]naphtho~-4.5-[ 1.2(3-methylinaphtho]-l3,14,15,16-di( 1,2-naphtho)-1,6,9,12,17.20hexaoxacyclodoeosa-2,4,13,15-tetraene ((R,R)-8), (R,RI-2,3,4,5Di{1,246-(2-hydroxyethylk3-methyl]naphthot-13,14,15,16-di( 1,2naphtho)-1,6,9,12,17,20-hexaoxacyclodocosa-2,4,13,15-tetraene
i(R,R)-6),and (R,Ri-2,3,4,5-Di[l,2-i3-methylinaphtho~-l3,14,15,16-di(1,2-naphthoi-1,6,9,12,17,20-hexaoxacyclodocosa-2,4,13,15-tetraene((R,R)-ti. Into a dry. three-necked flask. fitted with a jacketed (calibrated) and refrigerated addition funnel was placed a solution of 6.92 g (7.95 mniol) of optically pure dibromidc (H,K)-4 dissolved i n dry. purified 1,2-ditnethoxyethane containing a trace of triphenylmethane indicator. The solution was cooled to -75 "C. and. with constant stirring under dry nitrogen, 8.05 niL (17.7 mmol) 01' butyllithiurn (2.2 M in hexane) was added with a dry hypodermic hlringe through a rubber scptuni. The solution turned pink owing to the triphenylmethane anion. Ethylene oxide gas was dried carefully by passing it through a calcium sulfate tower ( 1 2 by 2 in. i d ) , and it was condensed in the addition funnel ( I .5 nil. or 30 minol) into 6.5 m L of dry 1.2-diinethoxyethane. After the initial reaction mixture had stirred for 2 h at -75 "C, the ethylene oxide solution was added dropwise ( I 5 min) under nitrogen with stirring. The reaction mixture
Sogah, Cram
/ Resolution of Enantiomers of Amino Acid and Ester Salts
then was allowed to warm slowly to 25 "C over a period of 2 h, during which time the pink color disappeared. The mixture was stirred for 30 min at 25 "C, and 200 m L of cold water was added. The mixture w,'IS. .haken with CHzC12, and the organic layer was dried and evaporated to give 6.6 g of a white solid, This material was chromatographed on 300 g of neutral alumina and eluted successively with 1 L of CH2CIz to give 2.00 g (34%) of fully protonated cycle ((R,R)-2); uith I .5 L of 2% CH3OH in CH2Clz to give 3.74 g (60%) of monocthoxylated cycle ( R , R ) - 8 ; and with 750 mL of 3% C H 3 0 H in CHzClz to give 0.40 (6%) of diethoxylated cycle (R,R)-6, which was not characterized. The monoethoxylated cycle was submitted to dry column chromatography on neutral alumina with 25% C H 3 0 H in CHzClz (volume) as solvent. The pure (H,R)-8was isolated as a white glass which was dried at 90 OC at 0.01 m m of Hg to give 3.50 g (56%): inass spectrum (76 eV) M+ at nr/e 783: [cy]::x +164O ( c 1.7, CHzC12). The compound's complex ' H N M R spectrum gave an A?B2 pattern a t d 2.80 with a coupling constant ofJAR = 6 Hz corresponding to the group ArCHZCH20H. Anal. ( C ~ ~ H 4 8 0 C, 7 ) H. (R.R)-2,3-(1,2-(6-Hydroxyethyl)naphtho]-4,5,13,14,15,16-tri( 1,2-
naphtho~-1,6,9,12,17,20-hexaoxacyclodocosa-2,4,13,15-tetraene (R,Ri-2,3,4,5-Di[ 1,2-(6-hydroxyethylinaphtho]I3,14,15,16-di(1,2-naphtho~-1,6,9,12,17,20-hexaoxacyclodocosa2,4,13,15-tetraene (( R,R)-5), and (R,R)-2,3,4,5,13,14,15,16tetra(1,2-naphthoi- 1,6,9,12,17,20-hexaoxacyclodocosa-2,4,13,15tetraene ((R,R)-l),Optically pure dibromocycle ( R , R ) - 3 was treated
( ( R,R)-71,
by the above procedure first with butyllithium. then with ethylene oxide, and finally with uater to givc monoethoxylated cycle ( R , R ) - 7 (55%), diethoxylated cycle ( R . H ) - 5 ( I O % ) , and protonated cycle ( R . H ) - 1 (30%). Desired ( R . R ) - 7 was obtained as a foam: [a]:& 165' (c I ,13. CH2C12); mass spectrum at 70 eV M+ ni/e 756. Anal. (Ccnt13407) H; calcd for C. 79.37; found. 78.90. Diethoxylated cycle ( H . H ) - 5was obtained as a foam: [ c ~ ] 3 +51 ~6 2 O (c 0.7, CH2CIl); mass spectrum (70 eV) M + m/e 800. Anal. (C51H4~08)C. H . The pro+227O (c I , CH2CIl). which intonated cycle ( R . K ) - I gave dicates that no racemization occurred during the above react ions . I 0.1 Chloromethylation of Styrene-Divinylbenzene Macroreticular Resin. Amberlite XAD-2 (porosity 42 voI%, surface area 330 m2/g, average pore diameter 90 k , mesh 20-50,' was ground in a laboratory mill (Wiley Laboratory Mill. Standard Model No. 3) fitted with a 150 mesh sieve. The sieved material ( 1 03.4 g) was mixed with I50 mL of dry ethylene dichloride for 30 min at 25 OC. To this slurry was added dropwise thc stirring over a 15-min period 26.82 g (0.30 mol) of chloromethyl methyl ether. Then 7.49 g (0.05 mol) of anhydrous AIC13 \vas added. The slurry was stirred at 25-30 " C for 4 h and quenched w i t h 300 n i L of CH3OH as the temperature was maintained at 25-30 OC with an icc-water bath. The mixture was stirred for 15 min, the solvent was siphoned off into a suction flask. and the remaining resin was washed four timcs uith 300 n i L of C H 3 0 H , each time removing the methanol N i t h a siphon and suction flask. The resin was drained free of interstitial liquid and dried i n a vacuum oven for 30 h a t 90 OC to give 109.6 g ofchloromcthylated resin. The starting resin. Amberlite XAD-2. gnve thc following elementary analysis. Found: C , 91.77: H. 8.07. Elcnientnl analysis of the chloronicthylated resin follows. Found: C. 86.58: H. 7.98: CI. 3.97. Thus I .I2 mequiv of CHzCl groups/g of resin w;is introduccd. Hence thc equivalent weight of'thc chlorinated polymcr is 893. I f the resin is nssunied to be 20 mol O h divinjlbcnzcne and 80 mol ?hstyrene. then the equivalent weight (per C h H s group) of the original resin is - I 37, or - I 5% of the ChHs groups present in thc resin were chloroniethylated. Grafting of (R,R)-8to Resin to Give (R,R)-10and Its Methoxylation to Give(R,Ri-lt. To 2.63 g (3.35 mmol) of ( R , H ) - 8dissolved in 300 mL of purc. dry T H F was added 2.50 g of NnH (50% dispersion in inineral oil), and the solution was heated at reflux for 30 min under N2, after which 29.0 g of dry (deoxygenated) chloromethylated resin was added undcr Nz. The heating at reflux was continued for 7 days. The reaction mixture was cooled and filtered, and the solid was washed successively with CH,OH (exothermic), water (containing a few drops of 6 N HCI solution), CH2C12. and again with C H 3 0 H . The solid was then dried at 90 OC i n a vacuum oven (0. I mni of Hg) for I2 h to give 30.3 g of cycle grafted to resin, ( H , R ) -IO, which contained, on elemental analysis. 3.71% chlorinc. The filtrate and washings were combined: the organic layer was separated, washed with water, brine and dried (MgSO4). The mixture was filtered and evaporated, and the residual solid was washed with pentane to remove the mineral oil. I t was then dissolved in C H ~ C I Zevaporation , of which left I .8 g of
+
304 1
crude solid. This material was chromatographed on 300 g of alumina and eluted successively with 25% pentane-CHzClz ( I L), pure CHzClz ( I L), and CH2CIl-CH30H (98:2 v:v) to give I .30 g of ( R , R ) - S . Thus, of the 2.63 g of ( R , R ) - 8 introduced into the reaction mixture, 1.33 g disappeared during the reaction and is assumed to have become attached to the resin. This assumption is supported by the fact that the resin gained 1.3 g in weight during the reaction, while, from the chlorine analysis, 0.03 g of chlorine was being lost. Hence 1.33 g of other material. presumably the host, was added to the resin. I f 1.33 g of ( R , R ) - 8became attached to polymer to give 30.3 g of ( R , R ) -10, then 1.70 mmol of host sites was present on 30.3 g of resin. or ( R , H ) -I O contained 0.056 mmol of host/g. To 30.3 g of ( R , R ) -10 mixed with 250 mL of absolute methanol was added 15.4 g (0.29 mol) of NaOCH3. The mixture was refluxed for I5 h, cooled, acidified carefully with aqueous HCI (exothermic) to pH 5, and filtered. The polymer was thoroughly washed with water and then absolute tnethanol and was dried at 90 "C at 0. I m m of Hg for 12 h in a vacuum oven to give 30.0 g of (R,R)-12. Elemental analysis of this material showed 0.65% of chlorine. Thus, by difference. -0.87 niequiv/g of C H 3 0 was,calculated to be present in ( R , R ) -12, along with 0. I8 mequiv/g of CI and 0.056 mmol of host/g. Thus the equivalent weight of host sites i n ( R , R ) -12 is -I7 800. If XAD-2 is assumed to be 20 mol % divinylbenzene and 80 mol %styrene, then -0.8% of the original ChH5 groups of the resin are attached to host sites in ( R , R ) - 1 2 . Grafting of (R,Ri-7 to Resin to Cite (R,R)-9,and Its Methoxylation toGive(R,R)-ll. Bya proceduresimilar to that described above. 2.53 g of ( R , R ) - 7and 29.0 g of chloromethylated resin were converted into 29.5 g of (H,H)-9,containing 3.80% of chlorine by elemental analysis, and 0.048 mmol of host/g, which with NaOCH3 was converted into (R,R)-11 containing 0.60?6 chlorine. Thus (R,H)-11contained 0.17 mequiv of Cl/g. 0.90 mcquiv of C H 3 0 groups/g, and 0.048 mmol of host/g. Preparation of Chromatographic Columns. Host-bound resins ( K , R ) - l l and ( H , R ) - 1 2were sieved to give ( R . R ) - 1 2 of 250-325 mesh used in column A and ( R . H ) -12 and ( R . R ) - 1 1 of 325-400 mesh used in columns B and C. respectively. The resin was suspended in CHCI?-CH,CN ( I : 1 v:v) and transferred into a "cartridge" stainless steel column of about twice the length of, but the same bore as, the final chromatographic column. N o swelling of the resin was noted when mixed uith solvent. The cartridge column \cas connected to the stainless stcel precision bore chromatographic column containing the same sojvent mixture. The slurry in the cartridge was pumped at 3 mL/min at 800-900 psi into the chromatographic column fitted with ;I porous plug at the outlet. A Milton Roj Mini-Pump nith a maxin i u m capacity of I6 mL/min was used in both loadings of the columns. Thc columns were jacketed and insulated for constant temperature control. Pure, dry. degassed solvents Mere used in loading. iiashing, atoring. or running the chromatogrnphic column>.The resin particles \$crc rapidly filtered out of the slurrj onto the porous plug a t the bottoiii of the column, leaving a stablc bed. After loading. the columns werc conditioned by v.ashing uith 1 L of CH3OH. I L ofCHC13. and I L of the desired mobile phase. Thc columns here fitted for samplc introduction w i t h injection loops I'rom ;I Waters Associutc chromatograph. Model 202. The bottoms ofthe columns led to conductivity cells o f 0 . IO-mL capncit\ madc o f 1 \ 4 0 brass plates held apart b> a Tellon gasket. Thccells had a const;int ol'-O.OI 7 cm-I. The cells were electrically attached to a Phillips PR 950 I dircct-reading conductivity bridge attached to a recorder. The dcnd volume of cnch column w a s dctermincd b> injecting the nonretained compounds. niethnnol. bcnicne. hexane. and pentane. LIS snmples onto the columns and detcrmining their retention volumes. Column A was 60 by 0.75 (i.d.) cni in dinicnsion and was loaded v,ith 9.5 g of ( H . R ) -12. With fittings and packed with resin. it as found to possess a nonoccupied volume of 23.76 f 0.04 mL. When corrected for the volumes of the connecting tubes and injection loop. and dead volume (that not occupied by rcsin) of the column itself was 18.36 f 0.04 mL. Columns B and C were 60 by 0.40 (i.d.) cni and were packed with 4.0 g of ( R . R ) -12 and ( R , R ) -11, respectively. With fittings and packed with resin. they were found to possess ;I nonoccupied volume of 9.50 & 0.50 mL. which. when corrcctcd for the volumes of tubings and injection loop, was 7.50 f 0.50 mL. .At thc cnd of the runs, the colunins were washed with nicthanol and thcn uith the solvent used I'or tho next r u n . The columns were stored undcr pure methanol and did not deteriorate with time or use if kept wet and out of contact uith air.
3042
Journal of the American Chemical Society
Perchlorate and Hexafluorophosphate Salts of the Amino Acids. The perchlorate salts were prepared as follows. A weighed amount ol' the racemic amino acid was suspended in absolute methanol. One cquivalcnt of 70% perchloric acid in water was added with constant stirring. Suspended solid dissolved when the acid was added. The solvent was evaporated at reduced pressure to leave a solid wet with untcr. The water was removed as a binary azeotrope with benzene. The final dry solid salt was recrystallized from CH3CN-CHC13. This method worked well for the preparation of the perchlorate salts of phenylglycine (mp 238-239.5 "C) (Anal. (C8HloCINOh) C , H), tyrosine (mp 151-153 "C) (Anal. ( C ~ H I ? C I % OC, ~ )H), tryptophan ( n i p 100-101 "C) (Anal. (CllH13CIN206) C , H), hydroxyphenylgljcine ( m p 217-219 "C) (Anal. ( C ~ H I O C I N OC~, )H). valine (mp 143-145 "C) (Anal. (CsH12CINOh) C , H). phenylalanine (mp 207-209 "C) (Anal. (C9H12CINOh) C , i I ) , isoleucine (mp 150-1 5 2 "C) (Anal. ( C ~ H I J C I N OC~,)H), and tert-leucine (mp 242-244 "C) (Anal. ( C ~ H I J C I N OC~,)H). The perchlorate salts of methionine and alanine were used directly. The perchlorate salts of the amino methyl esters were prepared from their corresponding as follows. A weighed amount ol' the ester hydrochloride salt was suspended in CHCI3-CH3CN ( I : I v / v ) . To this suspension was added I equiv of anhydrous NH4C104 ( LiClOJ could also be used) under nitrogen with constant stirring (10 h). The solution was filtered and the filtrate evaporated under reduced prcssure to produce a white solid, which was recrlstallized from CHC13-CH3CN (95:5 v / v ) . This procedure was applied to the preparation of the methyl ester perchlorate salts of phenylglycine (mp 154-1 56 "C) (Anal. ( C ~ H I ~ C I NC. O ~H), ) p-hydroxyphenylglycine (nip 185-188 "C) (Anal. ( C ~ H I ~ C I N C. O ~H), ) p-chlorophenylgljcine ( m p 173-174 "C) ( A n a l . (CsH~lCl?luOh)C. H),p-carbomethoxyphenylglycine (mp 16 I - I63 "C) (Anal. (Cl H14CINOx)C, H). phenylalanine (mp 157-159 " C ) (Anal. ( C I O H I J C I Y O C, ~ ) H), and tyrosine ( m p 172-174 "C) (Anal. ( C I O H I & I N O ~C), H). The hexafluorophosphate salt of racemic phenylglycine methyl ester \+;is prepared a s follows. In I O niL of CH2CI2 was dissolved I .O g (6.1 iiimol) of methjl phenylglycinate. To this \vas added 1.3 g (6.1 mmol) ol'(C?HS)20.HPFh (Aldrich Chemical Co.). White crystals (needles) scparated from this solution at 0 "C to produce 1.8 g (95%) of salt, nip 87-89 "C. This salt was soluble in CHC13 and hygroscopic. Its ' H h MR spectrum in DzO gave 6 3.64 (s, CH3, 3 H), 4.59 (5, "3.3 H). 5.20 ( 5 , C H . 1 H). 7.40 (s. ArH, 5 H). Anal. (C9Hl?O?NPF6)C , H. .Application of this procedure to the methjl esters of racemic p chlorophenylglycine hqdrochloridc.'"b p-carbomethoxyphenylglycine h>drochloride,Ioband phenylalanine hydrochloride'h'' gave >75% !iclds of the corresponding salts. Optical Resolution of the Methyl Esters of p-Chlorophenylglycine, p-Carbomethoxyphenylglycine, and Phenylglyeine Perchlorates and of Alanine Perchlorate (Free Acid). For all resolutions. column A was fitted H i t h a 6-tnL injection loop. The resolution of p C I C ~ , H J C H ( C O ~ C H ~ ) Y H ~isCillustrated IO~ first. I n three runs, 5X. I , 75.0. and 57.2 mg (190.3 mg) of racemic salt were each dissolved i n 6 imL ofCHCIj-CH3Ch (9O:lOv/v) and injectedonto the column, v hich h a s run at a flow rate of 0.5 mL/min and a pressure of 680 psi. Each run produced a \bell-defined minimum between the two maxima for the enantiomers. Three fractions were cut. enantiomer B (just after thc fir\t maximum). ii middle fraction containing B and A . and enantiomer A (just before the second maximum). The middle fractions from thc threc r u n s uerc combined and reinjected. and fractions were siniilurly cut. ,All B fractions from thc four injections were combined and rcinjectcd to give one peak of V H = 25 mL. wt 6 3 mg. +73.7". [ N ] : : ~f 8 4 . 3 ' (c 0.83. CH3OH). All A fractions from the four injcctions were combined and reinjected to give one peak of V A = 40 mL. \ \ t 7 5 mg [ a ] & -69.5'. -80.3" (c 0.77, CH3OH). Similar runs produced the less-retained enantiomer B of the methyl ester pcrchloratc of p-carbomethoxjphenylglycine. [a]:&C75.9". and the more-retained enantiomer A, [ a ] j &-76.0" (c 0.80, C H j O H ). Two runs of 25 mg each of the methyl ester of phenylglycine perchlor;ite w r c similarly made. The final. more-retained A band ( V R A = 39 niL) gave 20 mg of the D isomer, [a]
