The first synthesis of both enantiomers of [α-2H]phenylacetic acid in high enantiomeric excess

Article abstract of DOI:10.1039/b003941l

Arylmalonate decarboxylase (EC. 4. 1. 1. 76) catalysed decarboxylation followed by enantioface-differentiating protonation of [α-²H]- and [α-¹H]phenylmalonic acid in ¹H₂O and ²H₂O respectiv

Full text of DOI:10.1039/b003941l

The first synthesis of both enantiomers of [a-2H]phenylacetic acid in high
enantiomeric excess
Kaori Matoishi,a Satoshi Hanzawa,b Hitoshi Kakidani,a Masumi Suzuki,a Takeshi Sugaia and Hiromichi
Ohta*a
a Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
E-mail: hohta@chem.keio.ac.jp
b Tokyo Research Center, Tosoh Corp., 2743-1 Hayakawa, Ayase 252-1123, Japan.
Received (in Cambridge, UK) 17th May 2000, Accepted 30th June 2000
Published on the Web 21st July 2000
Arylmalonate decarboxylase (EC. 4. 1. 1. 76) catalysed
decarboxylation followed by enantioface-differentiating pro-
1
tonation of [a-2H]- and [a-1H]phenylmalonic acid in H2O
and 2H2O respectively, gave highly enantiomerically en-
riched (R)- and (S)-[a-2H]phenylacetic acid in quantitative
yields.
Regio- and stereospecifically [2H]-labelled compounds are very
useful in studying metabolic pathways and reaction mecha-
nisms. Phenylacetic acid is the intermediary metabolite of
aromatic amino acids, and also the starting material of
Scheme 2
amphetamine. For studies in this field, it will be of great use if
both of the enantiomers of [a-2H]phenylacetic acid 1 are
available. So far, enantiomerically enriched forms of 1 have
been synthesized via deuteriolysis of (R)-(2)-N,N-dimethyl-
phenylglycine1 and oxidative addition of optically active [a-
2H]benzyl halides to palladium complexes, followed by car-
bonyl insertion.2 However, the products of these two methods
suffer from the low enantiomeric excess (551, 76%2) and
contamination of di-deuterated and non-deuterated by-products.
It is practically impossible to raise the ee of the product and to
separate these by-products by ordinary methods, as there are no
chemical differences between the two enantiomers, and labeled
and non-labeled compounds. Accordingly, high selectivity of
the reaction is essential for obtaining the product in high
enantiomeric excess.
Arylmalonate decarboxylase (AMDase, EC. 4. 1. 1. 76),
which was found in our laboratory, catalyses decarboxylation of
arylmalonate derivatives resulting in the formation of enantio-
merically pure (R)-a-substituted phenylacetic acids (Scheme
1).3 It is thought that enantioface-differentiating protonation to
the enolate intermediate in the active site of the enzyme is the
key step of this reaction. Thus, if [a-2H]phenylmalonic acid was
used as the substrate of this enzymatic reaction, it would give
enantiomerically enriched (R)-[a-2H]phenylacetic acid.
In this enzyme-catalysed reaction, it is considered that the
origin of the proton source can be traced back to the solvent
1H2O. Consequently, when the reaction is performed in H2O,
2
then 2H will be incorporated enantioselectively into the product
from the same direction as that of 1H in the above case. As this
enzyme is purified from the culture of E. coli, it is obtained as
1
a solution in H2O. When the replacement of the solvent is
incomplete, it will cause the formation of non-deuterated
1
2
phenylacetic acid. Thus, we tried to replace H2O with H2O.
Fortunately, as this enzyme was not inactivated by repeated
lyophilization, it was possible to replace the 1H2O of the
enzyme solution by 2H2O via successive freeze-drying and
addition of H2O. When substrate 3 was added to this H2O-
exchanged enzyme solution, the reaction proceeded smoothly in
the same manner as for 1H2O, although the rate of reaction was
not measured accurately. As expected, (S)-1§ in over 95% ee
was obtained in a quantitative yield (Scheme 3).
2
2
Scheme 3
The absolute configuration of the product was determined by
comparison of the sign of rotation with the one reported.2 The ee
of 1 was determined by 1H NMR analysis of (R)-a-methoxy-a-
trifluoromethylphenyl acetate (MTPA ester 5¶), which was
prepared by an esterification of the corresponding alcohol
obtained by the reduction of methyl ester 4 (Scheme 4). The 1H
NMR spectra of the benzylic protons are shown in Fig. 1. The
signals of the protons in question were simplified via decou-
pling by irradiation of the adjacent methylene protons.
Scheme 1
[a-2H]Phenylmalonic acid 2† was prepared by deuteration of
the corresponding [a-1H]-diethyl ester followed by hydrolysis
in 2H2O. The amount of non-deuterated diethyl ester was
evaluated from 1H-NMR to be about 3%, which was consistent
with the peak intensity ratio of m/z 136 and 137 (parent ions
corresponding to non- and mono-deuterated phenylacetic acids)
of the enzyme-catalysed reaction product. The asymmetric
decarboxylation of 2 by arylmalonate decarboxylase, which was
obtained by overexpression in E. coli JM 109 and subsequent
purification,4 worked very well. The expected compound (R)-
1‡ of over 95% ee was obtained in a quantitative yield (Scheme
2).
In conclusion, an efficient method for the asymmetric
synthesis of both enantiomers of [a-2H]phenylacetic acid was
established via decarboxylation of phenylmalonate catalysed by
Scheme 4
DOI: 10.1039/b003941l
Chem. Commun., 2000, 1519–1520
This journal is © The Royal Society of Chemistry 2000
1519

pH 8.5, 4 ml). This mixture was incubated at 35 °C for 30 min followed by
acidification with 2 M HCl. The mixture was saturated with NaCl and
extracted with ether. The organic layer was concentrated in vacuo to give
(R)-1 (0.279 g, quant.) as a white solid. This product was revealed to be pure
by 1H NMR. This was recrystallized from hexane to afford (R)-1 (0.250 g,
22
92%) as colorless plates, mp 73.5–74.5 °C; [a]D 21.1° (c 10.0, CHCl3);
n
max/cm21 3033, 1700, 1412, 1263, 1224, 904, 700, 673; (400 MHz,
CDCl3) dH: 3.64 [t, 1H, J = 2.0 Hz, C6H5CDHCO2H], 7.25–7.36 [m, 5H,
C6H5CDHCO2H]; MS (m/z, %) 92 (C7H6D, 100), 136 (1.2), 137 (M+, 34.9),
138 (M + 1, 3.0). HRMS Found: m/z 137.0598. Calcd. for C8H7DO2:
137.0586. Assuming the peak m/z 136 is entirely due to the presence of non-
deuterated phenylacetic acid (C7H8O2), the contamination is calculated to
be about 3%. However, the peak corresponding to M 2 1 (135) was also
observed in the MS of non-deuterated phenylacetic acid. Thus the
contamination of the non-deuterated compound in the enzymatic reaction
product will be less than 3%.
MS of non-deuterated phenylacetic acid (m/z, relative intensity) 91
(C7H7, 100), 135 (1.2), 136 (M+, 72), 137 (M + 1, 7.0), 138 (M + 2, 1.2).
§ (S)-1: The purified arylmalonate decarboxylase (600 units) was dissolved
in a Tris-HCl buffer (2 M, pH 8.5, 5 ml) and lyophilized. The residue was
dissolved in D2O (5 ml) and incubated at 4 °C for 1 h and lyophilized again.
The residue was dissolved in D2O (5 ml) and the disodium salt of 3 (0.550
g, 2.45 mmol) was added. The mixture was incubated at 35 °C for 30 min
and acidified with 2 M HCl. The mixture was saturated with NaCl and
extracted with ether. The organic layer was concentrated in vacuo to give
(S)-1 (0.337 g, quant.) as a white solid. This was recrystallized from hexane
to give (S)-1 (0.300 g, 89%) as colorless plates, mp 74.5–75.5 °C [lit.2 75
°C]; [a]D22 +1.1 (c 9.5, CHCl3), +1.2 (c 25.7, CHCl3) [lit.2 for S enantiomer,
[a]D22 +1.5 ± 0.2 (c 25.7, CHCl3)]; MS (m/z, %) 92 (C7H6D, 100), 136 (1.2),
137 (M+, 34.9), 138 (M + 1, 3.0). HRMS Found: m/z 137.0566. Calcd. for
C8H7DO2: 137.0586. The IR and NMR spectra were identical with those of
(R)-1. The racemic sample of 1 was prepared by the reported procedure.6
¶ 5 from (R)-1: (400 MHz, CDCl3) dH: 3.00 [t, 1H, J = 6.3 Hz,
C6H5CDHCH2OCOC(CF3)(OCH3)C6H5], 3.47 [s, 3H, C6H5CDHCH2O-
COC(CF3)(OCH3)C6H5], 4.53 [d, 2H, J = 6.3 Hz, C6H5CDHCH2O-
COC(CF3)(OCH3)C6H5], 7.17–7.43 [m, 10H, C6H5CDHCH2OCOC(C-
F3)(OCH3)C6H5]. 5 from (S)-1: (400 MHz, CDCl3) dH: 2.97 [t, 1H, J = 7.3
Fig. 1
arylmalonate decarboxylase. As some other analogous com-
pounds having substituents on the aromatic ring also undergo
this enzyme-catalysed reaction,5 this procedure will be applica-
ble to the preparation of the enantiomers of substituted [a-
2H]arylacetic acids.
Hz,
C6H5CDHCH2OCOC(CF3)(OCH3)C6H5],
3.46
[s,
3H,
Notes and references
C6H5CDHCH2OCOC(CF3)(OCH3)C6H5], 4.53 [d, 2H, J = 7.0 Hz, C6H5-
CDHCH2OCOC(CF3)(OCH3)C6H5],
C6H5CDHCH2OCOC(CF3)(OCH3)C6H5].
7.17–7.43
[m,
10H,
† 2: To a dispersion of NaH (12.7 mmol) in dry THF (10 ml) was added a
solution of 6 (4.23 mmol) in dry THF (2 ml). The mixture was refluxed with
stirring for 5 h and cooled to 0 °C. To this mixture was added D2O (3.5 ml)
and DCl (35% in D2O, 1.4 ml). The product was extracted and purified by
column chromatography on silica gel to give diethyl [a-2H]phenylmalonate
7 (0.957 g, 96%). This was contaminated with 6 (3%), as judged from the
signal at dH 4.61 [1H, s, C6H5CH(CO2CH2CH3)2]. The solution of 7 (0.747
g, 3.15 mmol) in CH3OD (5 ml) was treated with NaOD (6.84 mmol) in D2O
(5 ml). The mixture was stirred at rt for 12 h, concentrated in vacuo and
MeOH (15 ml) was added to the residue. The resulting precipitates were
collected by suction filtration, washed with cold MeOH and ether, and then
dried in vacuo to give the disodium salt of 2 (0.640 g, 90%) as a colorless
powder. nmax/cm21 3021, 1591, 1496, 1429, 1324, 1023, 910, 698; (400
MHz, D2O) dH: 7.11–7.21 [m, 5H, C6H5CD(CO2Na)2].
1 H. Dahn and C. O’Murchu, Helv. Chim. Acta, 1970, 53, 1379.
2 K. S. Y. Lau, P. K. Wong and J. K. Stille, J. Am. Chem. Soc., 1976, 98,
5832.
3 H. Ohta, Bull. Chem. Soc. Jpn., 1997, 70, 2895; H. Ohta, Biocatalytic
Asymmetric Decarboxylation, ed. K. Faber, Springer-Verlag, 1999; H.
Ohta and T. Sugai, Stereoselective Biocatalysis, ed. R. N. Patel, Marcel
Dekker Inc., New York, 2000 and references cited therein.
4 Y. Fukuyama, K. Matoishi, M. Iwasaki, E. Takizawa, M. Miyazaki, H.
Ohta, S. Hanzawa, H. Kakidani and T. Sugai, Biosci. Biotechnol.
Biochem., 1999, 63, 1664.
‡ (R)-1: The disodium salt of 2 (0.450 g, 2.00 mmol) was added to a solution
of purified arylmalonate decarboxylase (400 units) in Tris-HCl buffer (2 M,
5 K. Miyamoto and H. Ohta, Biocatalysis, 1991, 5, 49.
6 W. Adam and O. Cueto, J. Org. Chem., 1977, 42, 38.
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Chem. Commun., 2000, 1519–1520