Preparation of optically active threo-2-amino-3-hydroxy-3-phenylpropanoic acid (threo-beta-phenylserine) via optical resolution.

Article abstract of DOI:10.1248/cpb.51.1363

To obtain optically active threo-2-amino-3-hydroxy-3-phenylpropanoic acid (1), (2RS,3SR)-2-benzoylamino-3-hydroxy-3-phenylpropanoic acid [(2RS,3SR)-2] was first optically resolved using (1S,2S)- and (1R,2R)-2-amino-1-(4-nitrophenyl)-1,3-propanediol as the resolving agents to afford (2R,3S)- and (2S,3R)-2 in yields of 73% and 66%, based on half of the starting amount of (2RS,3SR)-2. Next, the racemic structures of ammonium and some organic ammonium salts of (2RS,3SR)-2 were examined based on melting point, solubility, and infrared spectrum, with the aim of optical resolution by preferential crystallization. The benzylammonium salt of (2RS,3SR)-2 was suggested to exist as a conglomerate at room temperature, although it forms a racemic compound at the melting point. The optical resolution by preferential crystallization of the racemic salt afforded the (2R,3S)- and (2S,3R)-salts with optical purities of 90-97%. The (2R,3S)- and (2S,3R)-2 obtained from the purified salts were hydrolyzed by reflux in hydrochloric acid to give (2R,3S)- and (2S,3R)-1.

Full text of DOI:10.1248/cpb.51.1363

December 2003
Chem. Pharm. Bull. 51(12) 1363—1367 (2003)
1363
Preparation of Optically Active threo-2-Amino-3-hydroxy-3-
b
phenylpropanoic Acid (threo- -Phenylserine) via Optical Resolution
*
Tadashi SHIRAIWA, Reiichi SAIJOH, Masahiro SUZUKI, Kyosuke YOSHIDA, Satoshi NISHIMURA, and
Hisashi NAGASAWA
Unit of Chemistry, Faculty of Engineering and High Technology Research Center, Kansai University; Yamate-cho, Suita,
Osaka 564–8680, Japan. Received June 16, 2003; accepted August 29, 2003
To obtain optically active threo-2-amino-3-hydroxy-3-phenylpropanoic acid (1), (2RS,3SR)-2-benzoylamino-
3-hydroxy-3-phenylpropanoic acid [(2RS,3SR)-2] was first optically resolved using (1S,2S)- and (1R,2R)-2-amino-
1-(4-nitrophenyl)-1,3-propanediol as the resolving agents to afford (2R,3S)- and (2S,3R)-2 in yields of 73% and
66%, based on half of the starting amount of (2RS,3SR)-2. Next, the racemic structures of ammonium and some
organic ammonium salts of (2RS,3SR)-2 were examined based on melting point, solubility, and infrared spec-
trum, with the aim of optical resolution by preferential crystallization. The benzylammonium salt of (2RS,3SR)-2
was suggested to exist as a conglomerate at room temperature, although it forms a racemic compound at the
melting point. The optical resolution by preferential crystallization of the racemic salt afforded the (2R,3S)- and
(2S,3R)-salts with optical purities of 90—97%. The (2R,3S)- and (2S,3R)-2 obtained from the purified salts were
hydrolyzed by reflux in hydrochloric acid to give (2R,3S)- and (2S,3R)-1.
Key words threo-b-phenylserine; optical resolution; conglomerate; diastereoisomeric separation
2-Amino-3-hydroxy-3-phenylpropanoic acid (1; b-phen- separation of its diastereoisomeric salts with optically active
ylserine) that exists as four stereoisomers is a physiologically organic amines. In addition, we attempted to resolve
important a-amino acid.1—3) L-Threonine aldolase has been (2RS,3SR)-2 optically by preferential crystallization. Optical
reported to catalyze the condensation of glycine (Gly) and resolution by preferential crystallization is a simple and use-
benzaldehyde to afford 1, of which 60% is comprised of the ful method for the large-scale separation of enantiomers from
(2S,3R)-form and 40% of the (2S,3S)-form.3) Condensation racemates and is achieved by providing a small amount of
of Gly and benzaldehyde also occurs nonenzymatically in one enantiomer as seed crystals in a racemic supersaturated
aqueous alkaline media to give predominantly (2RS,3SR)-1, solution.7,8,10) Racemates exist in the forms of racemic com-
DL-threo-b-phenylserine.4,5) Therefore we attempted to obtain pounds, racemic solid solutions, and conglomerates. How-
(2R,3S)- and (2S,3R)-1 by optical resolution (Chart 1).
ever, only conglomerates, which are defined as mechanical
Optical resolutions by preferential crystallization and sep- mixtures of crystals of both enantiomers, can be optically re-
aration of diastereoisomers have been successfully employed solved by preferential crystallization. Therefore we examined
to obtain enantiomers from racemates.6—9) Therefore the op- the racemic structures of (2RS,3SR)-1, (2RS,3SR)-2, and am-
tical resolution of (2RS,3SR)-2-benzoylamino-3-hydroxy-3- monium and some organic ammonium salts of (2RS,3SR)-2
phenylpropanoic acid [(2RS,3SR)-2] was first attempted by to screen for a racemic salt that exists as a conglomerate. We
(a) i) benzaldehyde (2 eq), OHϪ, ii) recrystallization from water; (b) benzoyl chloride, OHϪ; (c) i) optical resolution by separation of diastereoisomeric salts of (2RS,3SR)-2 with
(1S,2S)-2-amino-1-(4-nitrophenyl)-1,3-propanediol [(1S,2S)-3], ii) salt formation of 2, from filtrate, with (1R,2R)-3; (d) benzylamine; (e) optical resolution by preferential crystal-
lization; (f) treatment with hydrochloric acid; (g) i) refluxing in 6 mol/l hydrochloric acid for 30 h, ii) triethylamine (pH 6—7), methanol.
Chart 1. Synthetic Route for Optically Active threo-2-Amino-3-hydroxy-3-phenylpropanoic Acid (1)
To whom correspondence should be addressed. e-mail: shiraiwa@ipcku.kansai-u.ac.jp
© 2003 Pharmaceutical Society of Japan

1364
Vol. 51, No. 12
attempted to resolve the racemic salt that exists as a con-
glomerate optically by preferential crystallization and then to
obtain (2R,3S)- and (2S,3R)-1 from the optically active 2.
Results and Discussion
Reaction of Gly with benzaldehyde under alkaline condi-
tions4,5) afforded 1 of which 80% consisted of the (2RS,3SR)-
form and 20% of the (2RS,3RS)-form. Compound 1 was re-
crystallized from water to give (2RS,3SR)-1 in 100% de in a
yield of 52%.
Compound (2RS,3SR)-2, which was obtained by N-ben-
zoylation of (2RS,3SR)-1, was attempted to be resolved opti-
cally by formation of diastereoisomeric salts with basic re-
solving agents, such as optically active a-phenylethylamine,
brucine, cinchonidine, and (1S,2S)- and (1R,2R)-2-amino-1-
(4-nitrophenyl)-1,3-propanediol [(1S,2S)- and (1R,2R)-3]. Of
these resolving agents, only optically active 3 gave a good re-
sult. A salt of (2S,3R)-2 with (1S,2S)-3 [(2S,3R)-2·(1S,2S)-3
salt] was crystallized as a less-soluble diastereoisomeric salt.
The purified (2S,3R)-2·(1S,2S)-3 salt was treated with hy-
drochloric acid to give (2S,3R)-2 in a yield of 66%, based on
half of the starting amount of (2RS,3SR)-2: (2S,3R)-2, [a]D20
Ϫ58.9° (cϭ1.00, ethanol). On the other hand, after collecting
(2S,3R)-2·(1S,2S)-3 salt by filtration, the filtrate was evapo-
rated to dryness and the salt obtained as the residue was
treated with hydrochloric acid to give the partially resolved
(2R,3S)-2. The (2R,3S)-2 obtained was reacted with (1R,2R)-
3 to yield the less-soluble diastereoisomeric (2R,3S)-
2·(1R,2R)-3 salt. The (2R,3S)-2·(1R,2R)-3 salt was treated
with hydrochloric acid to give (2R,3S)-2 in a yield of 73%:
[a]D20 ϩ58.6° (cϭ1.00, ethanol).
Fig. 1. Melting-Point Binary-Phase Diagram of Benzylammonium Salt of
threo-2-Benzoylamino-3-hydroxy-3-phenylpropanoic Acid (4 Salt)
᭹: Beginning of melting. ᭺: End of melting.
Next, we examined the racemic structures of (2RS,3SR)-1
and -2, with the aim of optical resolution by preferential
crystallization. Compounds (2RS,3SR)-1 and -2 have higher
melting points than their enantiomers and show different IR
spectra from their enantiomers. Since racemic compounds
are known to have the above characteristics,6—8) (2RS,3SR)-1
and -2 are determined to form racemic compounds. There-
fore it was attempted to form ammonium, methylammonium,
ethylammonium, 1-propylammonium, isopropylammonium,
tert-butylammonium, and benzylammonium salts from
(2RS,3SR)- and (2S,3R)-2 to screen for racemic salts existing
as conglomerates. The ammonium and organic ammonium
salts of (2RS,3SR)-2 were obtained as crystalline salts, but
not crystalline salts of (2S,3R)-2, with the exception of the
benzylammonium salt, [(2S,3R)-4 salt], of (2S,3R)-2. There-
fore the all salts except for the (2RS,3SR)-4 salt are estimated
to form racemic compounds because their racemic salts can
be envisaged to have higher melting points than the optically
active salts. On the other hand, the (2RS,3SR)-4 salt had a
lower melting point than the (2S,3R)-4 salt. Although con-
glomerates are known to have such melting point characteris-
tics,6—8) the melting-point binary-phase diagram suggested
that the (2RS,3SR)-4 salt forms a racemic compound, as
shown in Fig. 1. However, the IR spectrum of (2RS,3SR)-4
salt was identical to that of the (2S,3R)-4 salt. In addition, the
(2RS,3SR)-4 salt was more soluble than the (2S,3R)-4 salt:
solubility of the (2RS,3SR)-4 salt at 10 °C was 1.486 g
Fig. 2. Solubility Ternary-Phase Diagram of Benzylammonium Salt of
threo-2-Benzoylamino-3-hydroxy-3-phenylpropanoic Acid (4 Salt)
Conditions: Temperature, 10 °C; solvent, ethanol.
be a conglomerate (Fig. 2). The above results suggest that the
(2RS,3SR)-4 salt exists as a conglomerate at room tempera-
ture and forms a racemic compound at the melting point.
The (2RS,3SR)-4 salt was optically resolved by preferen-
tial crystallization in ethanol at 10 °C. To optimize the condi-
tions, the optical resolution was conducted by stirring solu-
tions of the (2RS,3SR)-4 salt in 50 ml of ethanol at 150—
190% supersaturation for 30 min; the (2S,3R)-4 salt (0.100 g)
was employed as seed crystals. The results are shown in Fig.
3. The yield of enantiomer [YE (g)], degree of resolution [DR
(%)], and the amount of crystallization [AC(2S,3R) and AC(2R,3S)
(g)] were calculated from
YE (g)ϭ[yield (g)ϫOP (%)/100]Ϫ0.100,
DR (%)ϭYE (g)ϫ100/{(1/2)[amount of (2RS,3SR)-4 salt (g)Ϫ0.743]},
AC(2R,3S) (g)ϭ(1/2)[yield (g)ϪYE (g)Ϫ0.100],
AC(2S,3R) (g)ϭyield (g)ϪAC(2R,3S) (g)Ϫ0.100,
(100 ml of ethanol)Ϫ1; and solubility of the (2S,3R)-4 salt at where the optical purity [OP (%)] of the obtained (2S,3R)-4
10 °C was 0.666 g (100 ml of ethanol)Ϫ1. The ternary solubil- salt is calculated on the basis of the specific rotation ([a]D20
ity diagram also showed that the (2S,3R)-4 salt is expected to Ϫ41.1° (cϭ0.500, ethanol)) of the (2S,3R)-4 salt, the solubil-

December 2003
1365
ity of the (2RS,3SR)-4 salt was 0.743 g in 50 ml of ethanol at cessive optical resolution was attempted by stirring the 170%
10 °C, and the yield is the sum of the amounts of the crystal- supersaturated solution, as the initial solution, for 30 min
lized 4 salt and seed crystals. The degree of resolution [DR (Table 1). The degrees of resolution [DR (%)] of the (2S,3R)-
(%)] is defined as the yield (%) of the seeded enantiomer, and (2R,3S)-4 salt obtained were calculated from
based on half of the supersaturating portion of a racemate,
and indicates the efficiency of optical resolution; half of the
[operation amount of (2S,3R)- or (2R,3S)-4 salt (g)Ϫ0.372]
supersaturating portion of a racemate means the theoretical
DR (%)ϭYE (g)ϫ100/
yield (g) of the seeded enantiomer.
where the operation amount is the amount of the (2S,3R)-
When the 150—170% supersaturated solutions were em- and (2R,3S)-4 salts in the solution used in the optical resolu-
ployed, the (2S,3R)-4 salts with optical purities of 97—99% tion and those in runs 2—7 in Table 1 were calculated based
were obtained with 58—83% degrees of resolution. There- on the yields and optical purities of the (2S,3R)- and (2R,3S)-
fore the (2RS,3SR)-4 salt was confirmed to exist as a con- 4 salt obtained in runs 1—6, respectively. Half of the solubil-
glomerate at room temperature. When the solutions with ity of (2RS,3SR)-4 salt is 0.372 g in 50 ml of ethanol at 10 °C.
180% and 190% supersaturation were employed, the optical
The optical resolution afforded the (2S,3R)- and (2R,3S)-4
resolutions gave poor results because of rapid crystallization salts with optical purities of 90—97% at 48—90% degrees
of the unseeded (2R,3S)-4 salt; the (2S,3R)-4 salt obtained of resolution. Although the (2R,3S)-4 salt with 90% optical
showed optical purities of 75% and 51%, respectively. From purity was obtained in run 2, the amount of crystallization
these results, the optical resolution for the 170% supersatu- was less than expected. Therefore the optical resolution in
rated solution was determined at resolution times of 15— run 3 was carried out by again providing the (2R,3S)-4 salt as
60 min (Fig. 4).
seed crystals to afford the (2R,3S)-4 salt of 90% optical pu-
Rapid crystallization of the unseeded (2R,3S)-4 salt was rity with 58% degree of resolution. The (2S,3R)- and
not observed for the first 30 min, but the (2R,3S)-4 salt began (2R,3S)-4 salts obtained were recrystallized from ethanol, as
to crystallize rapidly at 45 min. Based on these results, suc- described in the Experimental section, to obtain the optically
pure (2S,3R)- and (2R,3S)-4 salts. Treatment of the purified
Fig. 3. Relationship between the Amount of Crystallization and Degree of
Supersaturation in the Optical Resolution of Benzylammonium Salt of Fig. 4. Relationship between the Amount of Crystallization and Resolu-
(2RS,3SR)-2-Benzoylamino-3-hydroxy-3-phenylpropanoic Acid [(2RS,3SR)-
4 Salt]
tion Time in the Optical Resolution of Benzylammonium Salt of (2RS,3SR)-
2-Benzoylamino-3-hydroxy-3-phenylpropanoic Acid [(2RS,3SR)-4 Salt]
Conditions: (2RS,3SR)-4 Salt, 1.115—1.412 g (150—190% supersaturation); seed
crystals, 0.100 g of (2S,3R)-4 salt; solvent, 50 ml of ethanol; stirring time, 30 min; tem-
perature, 10 °C. Amount of crystallization: ᭺ (2S,3R)-4 Salt; ᭹ (2R,3S)-4 salt.
Conditions: (2RS,3SR)-4 Salt, 1.263 g (170% supersaturation); seed crystals, 0.100 g
of (2S,3R)-4 salt; solvent, 50 ml of ethanol; stirring time, 15—60 min; temperature,
10 °C. Amount of crystallization: ᭺ (2S,3R)-4 Salt; ᭹ (2R,3S)-4 salt.
Table 1. Successive Optical Resolution by Preferential Crystallization of Benzylammonium Salt of (2RS,3SR)-2-Benzoylamino-3-hydroxy-3-phenyl-
propanoic Acida)
Amount of
(2RS,3SR)-4 salt
added (g)
Operation amount (g)
(2S,3R)-4 Salt (2R,3S)-4 Salt
Optical purity
(%)
Run
Yieldb) (g)
YEc) (g)
DRd) (%)
1
2
3
4
5
6
7
1.263
0.220
0.204
0.190
0.310
0.266
0.270
0.632
0.528
0.615
0.696
0.551
0.667
0.539
0.632
0.737
0.650
0.570
0.716
0.599
0.727
(2S,3R)-(Ϫ) 0.319
(2R,3S)-(ϩ) 0.304
(2R,3S)-(ϩ) 0.289
(2S,3R)-(Ϫ) 0.409
(2R,3S)-(ϩ) 0.366
(2S,3R)-(Ϫ) 0.369
(2R,3S)-(ϩ) 0.411
97
90
90
96
91
96
97
0.209
0.174
0.161
0.291
0.233
0.256
0.297
80
48
58
90
68
87
84
a) Conditions: Seed crystals of the (2S,3R)- or (2R,3S)-4 salt, 0.100 g; solvent, 50 ml of ethanol; temperature, 10 °C; resolution time, 30 min. b) The yield is the sum of the
amounts of crystallized 4 salt and seed crystals. c) YE, yield of enantiomer. d) DR, degree of resolution.

1366
Vol. 51, No. 12
(2S,3R)- and (2R,3S)-4 salts with hydrochloric acid quantita- ethanol). After dissolving the salt in ethanol (12 ml gϪ1) at 60 °C, the solu-
tion was allowed to stand overnight at 5 °C. The precipitated salt was col-
tively gave (2S,3R)- and (2R,3S)-2.
The obtained (2S,3R)- and (2R,3S)-2 were hydrolyzed by
lected by filtration and dried. To the suspension of the purified (2S,3R)-
2·(1S,2S)-3 salt in water (4 ml gϪ1) 5 mol/l of hydrochloric acid (0.8 ml gϪ1
)
refluxing for 30 h in 5 mol/l of hydrochloric acid, followed by
treatment with triethylamine to give (2S,3R)- and (2R,3S)-1:
(2S,3R)-1, [a]D20 Ϫ50.5° (cϭ1.00, 5 mol/l HCl) ([a]D Ϫ50.3°
(5 mol/l HCl))11); (2R,3S)-1, [a]D20 ϩ50.5° (cϭ1.00, 5 mol/l
HCl).
was added in an ice bath. After stirring the mixture for 2 h in an ice bath and
then for 1 h at room temperature, the precipitated (2S,3R)-2 was collected by
filtration and then recrystallized from ethanol (1 ml gϪ1).
(2S,3R)-2·(1S,2S)-3 Salt: Yield, 8.24 g (66.5%); mp 169—171 °C; [a]D20
ϩ32.5° (cϭ0.500, ethanol). 1H-NMR (270 MHz, DMSO-d6, TMS) d:
8.22—7.12 (15H, m, arom. H, –NH–CO–), 5.20 (1H, d, Jϭ3.8 Hz,
ϾCH(OH) (2)), 4.85 (1H, d, Jϭ7.3 Hz, ϾCH(OH) (3)), 4.51 (1H, dd,
Jϭ3.8, 7.6 Hz, ϾCH–NH– (2)), 3.51—3.45 (1H, m, –CHH–OH (3)), 3.28—
Experimental
General Specific rotations were measured at 589 nm and 20 °C with a 3.22 (1H, m, –CHH–OH (3)), 3.16—3.10 (1H, m, ϾCH(NH3ϩ) (3)). 13C-
Horiba Seisakusho SEPA-300 auto polarimeter equipped with a quartz cell NMR (67.5 MHz, DMSO-d6, TMS) dϭ165.3 (C6H5–CO–), 149.7 (arom. C),
with a 5.00-cm path length. IR spectra were obtained in the range of 4000— 146.8 (arom. C), 142.9 (arom. C), 134.5 (arom. C), 131.0 (arom. C), 128.3
400 cmϪ1 with a Perkin-Elmer Model 1600 FT-IR spectrometer using the (arom. C), 128.0 (arom. C), 127.9 (arom. C), 127.4 (arom. C), 127.3 (arom.
KBr disk method. 1H- and 13C-NMR spectra were recorded on a JNM- C), 126.7 (arom. C), 126.6 (arom. C), 126.5 (arom. C), 126.2 (arom. C),
FX270 FT NMR system in deuterium oxide (D2O) or dimethylsulfoxide-d6
126.1 (arom. C), 125.4 (arom. C), 123.2 (arom. C), 123.1 (arom. C), 72.3
(DMSO-d6) with sodium 3-(trimethylsilyl)propane-1-sulfonate (DSS) or (ϾCH(OH) (2)), 69.9 (ϾCH(OH) (3)), 59.2 (ϾCH–NH– (2)), 58.7
tetramethylsilane (TMS) as an internal standard. Chemical shifts are re- (–CH2–OH (3)), 58.0 (ϾCH(NH3ϩ) (3)).
ported in d units downfield from DSS or TMS. Melting points were mea-
(2S,3R)-2: Yield, 4.70 g (65.9%); mp 125—126 °C; [a]D20 Ϫ58.9°
(cϭ1.00, ethanol). IR (KBr) cmϪ1: 3213, 1736, 1641, 1340, 1265, 1219,
sured with a Yanaco MP-500 D micro melting point apparatus.
(2RS,3SR)-2-Amino-3-hydroxy-3-phenylpropanoic Acid [(2RS,3SR)-1] 1059, 696. The 1H- and 13C-NMR spectra were virtually identical to those of
To the solution of Gly (56.3 g, 0.750 mol) in 5 mol/l aqueous sodium hydrox- (2RS,3SR)-2. Anal. Calcd for C16H15NO4: C, 67.36; H, 5.30; N, 4.91. Found:
ide (525 ml) 53.1 g (0.500 mol) each of benzaldehyde was added 3 times at C, 67.32; H, 5.34; N, 5.00.
10-min intervals, keeping the temperature below 10 °C in an ice bath. The
On the other hand, after collecting (2S,3R)-2·(1S,2S)-3 salt by filtration,
mixture was further stirred in an ice bath for 1 h and then at room tempera- the filtrate was evaporated to dryness in vacuo at 50 °C to obtain the crude
ture for about 30 min to give a solid condensation cake. After adding 5 mol/l (2R,3S)-2·(1S,2S)-3 salt (15.4 g) as the residue; [a]D20 ϩ0.1° (cϭ1.00,
of hydrochloric acid (750 ml) to the mixture in an ice bath, the resulting so- ethanol). (2R,3S)-2·(1S,2S)-3 salt was treated with hydrochloric acid in a
lution was evaporated to dryness in vacuo at 60 °C. Methanol (750 ml) was
manner similar to (2S,3R)-2·(1S,2S)-3 salt to give crude (2R,3S)-2; [a]D20
added to the residue and then the mixture was filtered to remove sodium ϩ28.6° (cϭ1.00, ethanol). Compound (1R,2R)-3 (6.51 g, 30.7 mmol) was
chloride. The filtrate was adjusted with triethylamine to pH 6—7 to precipi- added to the suspension of (2R,3S)-2 (8.76 g, 30.7 mmol) in 90 ml of
tate 1. The crude 1 was collected by filtration, washed with methanol, and ethanol. After stirring the mixture for 2 h at room temperature, the precipi-
dried; the yield was 117 g (86.0%). 1H-NMR (270 MHz, D2O, DSS) d: tated (2R,3S)-2·(1R,2R)-3 salt was collected by filtration and then was re-
7.51—7.38 (5H, m, C6H5–), 5.35 (0.19H, d, Jϭ4.3 Hz, –CH(OH)–), 5.30 crystallized from ethanol (12 ml gϪ1). The purified (2R,3S)-2·(1R,2R)-3 salt
(0.81H, d, Jϭ4.3 Hz, –CH(OH)–), 4.08 (0.19H, d, Jϭ4.3 Hz, –CH(NH2)–), was treated with hydrochloric acid in a manner similar to (2S,3R)-2·(1S,2S)-
3.91 (0.81H, d, Jϭ4.3 Hz, –CH(NH2)–).
3 salt to give (2R,3S)-2.
The obtained 1 was dissolved in water (3.75 ml gϪ1) at 70 °C. After stir-
(2R,3S)-2·(1R,2R)-3 Salt: Yield, 9.27 g (74.8%); mp 169—170 °C; [a]D20
ring the solution for 30 min in an ice bath, the precipitated (2RS,3SR)-1 was Ϫ32.7° (cϭ1.00, ethanol). The 1H- and 13C-NMR spectra were virtually
collected by filtration and dried.
identical to those of the (2S,3R)-2·(1S,2S)-3 salt.
(2RS,3SR)-1: Yield, 70.0 g (51.5%); mp 189—190 °C. IR (KBr) cmϪ1
:
(2R,3S)-2: Yield, 5.24 g (73.5%); mp 125—126 °C; [a]D20 ϩ58.6°
3615, 3137, 1644, 1488, 1402, 1352, 1017, 709, 526. H-NMR (270 MHz, (cϭ1.00, ethanol). The IR, H- and 13C-NMR spectra were virtually identi-
D2O, DSS) d: 7.51—7.38 (5H, m, arom. H), 5.30 (1H, d, Jϭ4.3 Hz, cal to those of (2S,3R)-2. Anal. Calcd for C16H15NO4: C, 67.36; H, 5.30; N,
ϾCH(OH)), 3.91 (1H, d, Jϭ4.3 Hz, ϾCH(NH2)). 13C-NMR (67.5 MHz, 4.91. Found: C, 67.25; H, 5.57; N, 4.75.
1
1
D2O, DSS) dϭ174.3 (–COOH), 141.5 (arom. C), 131.4 (arom. C), 131.3
(arom. C), 131.8 (arom. C), 128.3 (arom. C), 128.2 (arom. C), 73.8 propanoic Acid (4 Salt) After adding benzylamine (0.536 g, 5.00 mmol)
(ϾCH(OH)), 63.4 (ϾCH(NH2)).
to a solution of (2RS,3SR)-, (2S,3R)-, or (2R,3S)-2 (1.43 g, 5.00 mmol) in
Benzylammonium Salt of 2-Benzoylamino-3-hydroxy-3-phenyl-
(2RS,3SR)-2-Benzoylamino-3-hydroxy-3-phenylpropanoic Acid 15 ml of ethanol, the mixture was stirred for 1 h in an ice bath. The precipi-
[(2RS,3SR)-2] Benzoyl chloride (84.3 g, 0.600 mol) was added dropwise tated (2RS,3SR)-, (2S,3R)-, or (2R,3S)-4 salt was collected by filtration and
over a period of 1 h with stirring in an ice bath to the solution of (2RS,3SR)-
1 (90.6 g, 0.500 mol) in 5 mol/l of aqueous sodium hydroxide (300 ml). After
dried.
(2RS,3SR)-4 Salt: Yield, 1.75 g (89.3%); mp 155—156 °C. IR (KBr)
stirring for 2 h in an ice bath and then for 1 h at room temperature, the reac- cmϪ1: 3394, 3154, 1633, 1601, 1578, 1515, 1484, 1454, 1438, 1380, 1309,
1
tion mixture was adjusted to pH 1 with 5 mol/l of hydrochloric acid. The 1261, 1181, 1141, 1082, 1063, 926, 756, 735, 725, 708, 687, 593, 521. H-
precipitated (2RS,3SR)-2 was collected by filtration, washed with diethyl NMR (270 MHz, DMSO-d6, TMS) d: 8.19—7.14 (16H, m, –C6H5,
ether (250 mlϫ4), and dried. The obtained (2RS,3SR)-2 was dissolved in –NH–CO–), 5.13 (1H, d, Jϭ3.7 Hz, ϾCH(OH)), 4.73 (1H, d, Jϭ3.8 Hz,
100 ml of methanol and then water (1 l) was added to the solution. After stir- ϾCH–NH–), 3.88 (2H, s, –CH2–C6H5). 13C-NMR (67.5 MHz, DMSO-d6,
ring the mixture for 1 h at 20 °C, the purified (2RS,3SR)-2 was collected by TMS) dϭ172.8 (–COOϪ), 165.4 (C6H5–CO–), 143.1 (C6H5–), 135.1
filtration and dried.
(C6H5–), 134.7 (C6H5–), 131.0 (C6H5–), 128.6 (C6H5–), 128.4 (C6H5–),
128.0 (C6H5–), 127.4 (C6H5–), 126.7 (C6H5–), 126.5 (C6H5–), 126.3
(2RS,3SR)-2: Yield, 101 g (70.6%); mp 146—148 °C. IR (KBr) cmϪ1
:
3244, 1732, 1602, 1548, 1397, 1257, 1215, 1066, 1024, 863, 697, 688, 546. (C6H5–), 72.2 (ϾCH(OH)), 58.5 (ϾCH–NH–), 42.4 (–CH2–NH3ϩ). Anal.
1H-NMR (270 MHz, 0.5 mol/l NaOD, DSS) d: 7.71—7.25 (10H, m, arom.
H), 5.42 (1H, d, Jϭ3.5 Hz, ϾCH(OH)), 4.73 (1H, dd, Jϭ3.5, ϾCH–NH–). N, 7.05.
Calcd for C23H24N2O4: C, 70.39; H, 6.16; N, 7.14. Found: C, 70.33; H, 6.18;
13C-NMR (67.5 MHz, 0.5 mol/l NaOD, DSS) dϭ177.1 (–COOH), 170.3
(2S,3R)-4 Salt: Yield, 1.85 g (94.4%); mp 168—169 °C; [a]D20 Ϫ41.1°
1
(Ph-CO–), 142.5 (arom. C), 133.9 (arom. C), 132.7 (arom. C), 129.4 (arom. (cϭ0.500, ethanol). IR and H- and 13C-NMR spectra were virtually identi-
C), 129.2 (arom. C), 128.8 (arom. C), 128.3 (arom. C), 128.0 (arom. C), cal to those of (2RS,3SR)-BZA salt. Anal. Calcd for C23H24N2O4: C, 70.39;
127.6 (arom. C), 126.6 (arom. C), 72.6 (ϾCH(OH)), 58.9 (ϾCH–NH–). H, 6.16; N, 7.14. Found: C, 70.21; H, 6.24; N, 7.11.
Anal. Calcd for C16H15NO4: C, 67.36; H, 5.30; N, 4.91. Found: C, 67.22; H,
5.34; N, 4.91.
(2R,3S)-4 Salt: Yield, 1.82 g (92.9%); mp 168—169 °C; [a]D20 ϩ41.1°
1
(cϭ0.500, ethanol). IR and H- and 13C-NMR spectra were virtually identi-
Optical Resolution of (2RS,3SR)-2-Benzoylamino-3-hydroxy-3- cal to those of (2RS,3SR)-4 salt. Anal. Calcd for C23H24N2O4: C, 70.39; H,
phenylpropanoic Acid [(2RS,3SR)-2] by Separation of Diastereoisomeric 6.16; N, 7.14. Found: C, 70.31; H, 6.05; N, 7.14.
Salts After adding (1S,2S)-3 (10.6 g, 50.0 mmol) to the suspension of
Optical Resolution by Preferential Crystallization The (2RS,3SR)-4
(2RS,3SR)-2 (14.3 g, 50.0 mmol) in 150 ml of ethanol, the mixture was salt (1.115—1.412 g) was dissolved in 50 ml of ethanol at 40 °C to prepare
stirred for 2 h at room temperature. The precipitated (2S,3R)-2·(1S,2S)-3 150—190% supersaturated solutions at 10 °C. The solutions were cooled to
salt (9.41 g) was collected by filtration and dried; [a]D20 ϩ27.0° (cϭ0.500, 10 °C over a period of 30 min and then seeded with 0.100 g of the (2S,3R)-4

December 2003
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1
spectrum was virtually identical to that of (2S,3R)-1 and H- and 13C-NMR
spectra were identical to those of (2RS,3SR)-1. Anal. Calcd for C9H11NO3:
C, 59.66; H, 6.12; N, 7.73. Found: C, 59.44; H, 6.20; N, 7.74.
salt. After stirring the mixture for 15—60 min at 10 °C, the (2S,3R)-4 salt
was collected by filtration and dried.
(2S,3R)-4 Salt obtained from 150% supersaturated solution at 30 min:
Yield 0.210 g; [a]D20 Ϫ40.8° (cϭ1.00, ethanol). (2S,3R)-4 Salt obtained from
160% supersaturated solution at 30 min: Yield 0.293 g; [a]D20 Ϫ39.9°
(cϭ1.00, ethanol). (2S,3R)-4 Salt obtained from 170% supersaturated solu-
tion at 15 min: Yield 0.252 g; [a]D20 Ϫ39.3° (cϭ1.00, ethanol). (2S,3R)-4 Salt
obtained from 170% supersaturated solution at 30 min: Yield 0.319 g; [a]D20
Ϫ39.8° (cϭ1.00, ethanol). (2S,3R)-4 Salt obtained from 170% supersatu-
rated solution at 45 min: Yield 0.355 g; [a]D20 Ϫ34.1° (cϭ1.00, ethanol).
(2S,3R)-4 Salt obtained from 170% supersaturated solution at 60 min:
Yield 0.451 g; [a]D20 Ϫ23.9° (cϭ1.00, ethanol). (2S,3R)-4 Salt obtained from
180% supersaturated solution at 30 min: Yield 0.379 g; [a]D20 Ϫ31.0°
(cϭ1.00, ethanol). (2S,3R)-4 Salt obtained from 190% supersaturated solu-
tion at 30 min: Yield 0.472 g; [a]D20 Ϫ21.1° (cϭ1.00, ethanol).
Successive Optical Resolution by Preferential Crystallization The
(2RS,3SR)-4 salt (1.263 g) was dissolved in 50 ml of ethanol at 40 °C to pre-
pare a 170% supersaturated solution at 10 °C. The solution was cooled to
10 °C over a period of 30 min and then seeded with 0.100 g of the (2S,3R)-4
salt. After stirring the mixture for 30 min at 10 °C, the (2S,3R)-4 salt
(0.319 g) was collected by filtration and dried (run 1 in Table 1). The
(2RS,3SR)-4 salt (0.220 g) was dissolved in the filtrate at 40 °C and the re-
sulting solution was cooled to 10 °C. After adding the (2R,3S)-4 salt
(0.100 g) as seed crystals to the solution, followed by stirring the mixture for
30 min at 10 °C, the (2R,3S)-4 salt (0.304 g) was collected by filtration and
dried (run 2 in Table 1). Optical resolution was carried out at 10 °C by
adding further the (2RS,3SR)-4 salt to the filtrates in a way similar to that de-
scribed above; the detailed conditions are shown in runs 3—7 in Table 1.
(2S,3R)- and (2R,3S)-2-Amino-3-hydroxy-3-phenylpropanoic Acid
[(2S,3R)- and (2R,3S)-1] The suspension of (2S,3R)- or (2R,3S)-2 (2.85 g,
10.0 mmol) in 6 mol/l of hydrochloric acid (300 ml) was refluxed for 30 h.
The reaction mixture was filtered to remove the precipitated benzoic acid.
After evaporating the filtrate to dryness in vacuo at 60 °C, the crude (2S,3R)-
or (2R,3S)-1 hydrochloride obtained as the residue was washed with 50 ml of
diethyl ether. A solution of (2S,3R)- or (2R,3S)-1 hydrochloride in 100 ml of
methanol was adjusted with triethylamine to pH 6—7. After evaporating the
mixture to dryness in vacuo at 50 °C, chloroform (150 ml) was added to the
residue. The precipitated (2S,3R)- or (2R,3S)-1 was collected by filtration,
washed with a small amount of chloroform, and dried.
Solubility and Phase Diagrams The (2RS,3SR)-4 salt (1.50 g) or
(2S,3R)-4 salt (1.20 g) was dissolved in 50 ml of ethanol at 60 °C. After vig-
orously stirring the solution for 12 h at 10 °C, the precipitated (2RS,3SR)- or
(2S,3R)-4 salt was rapidly collected by filtration and thoroughly dried. The
solubility at 10 °C was calculated on the basis of the weight of the precipi-
tated (2RS,3SR)- or (2S,3R)-4 salt. Solubility of the (2RS,3SR)-4 salt at
10 °C: 1.486 g (100 ml of ethanol)Ϫ1. Solubility of the (2S,3R)-4 salt at
10 °C: 0.666 g (100 ml of ethanol)Ϫ1
.
Preparing a ternary solubility diagram, the solubilities of mixtures of
(2RS,3SR)- and (2S,3R)-4 salts were measured at 10 °C similar to the
method described above. The solid 4 salt was filtered off and thoroughly
dried and the specific rotation was measured. The amounts of (2R,3S)- and
(2S,3R)-4 salts in the solution were calculated based on the solubility of 4
salt and the specific rotation of the solid 4 salt.
In preparation of the binary melting point diagram, the melting points of
the mixtures composed of (2RS,3SR)- and (2S,3R)-4 salts were measured;
after dissolving (2RS,3SR)- and (2S,3R)-4 salts in an appropriate ratio in
methanol, the mixtures were obtained by evaporating the solutions to dry-
ness in vacuo. The melting point binary phase diagram was prepared from
the temperatures at the beginning and end of melting.
Acknowledgments This research was financially supported by a Kansai
University Grant-in-Aid for the Faculty Joint Research Program, 2002.
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(2S,3R)-1: Yield, 0.509 g (28.1%); mp 184—187 °C (decomp); [a]D20
Ϫ34.4° (cϭ0.500, water); [a]D20 Ϫ50.5° (cϭ0.500, 5 mol/l HCl) ([a]D
Ϫ50.3° (5 mol/l HCl)).11) IR (KBr) cmϪ1: 3047, 1666, 1609, 1520, 1398,
1350, 1016, 702, 536. The 1H- and 13C-NMR spectra were virtually identical
to those of (2RS,3SR)-1. Anal. Calcd for C9H11NO3: C, 59.66; H, 6.12; N,
7.73. Found: C, 59.45; H, 6.08; N, 7.71.
(2R,3S)-1: Yield, 0.547 g (30.2%); mp 185—187 °C (decomp); [a]D20
ϩ34.5° (cϭ0.500, water); [a]D20 ϩ50.5° (cϭ0.500, 5 mol/l HCl). The IR