An efficient synthesis of chiral homophenylalanine derivatives via enantioselective hydrogenation

Article abstract of DOI:10.1016/S0957-4166(99)00424-3

Chiral homophenylalanine derivatives were synthesized via enantioselective hydrogenation of 5a and 5b catalyzed by rhodium complexes bearing chiral phosphine and phosphinite legands. Enantiomeric excesses up to 96.2% were achieved when S-spiroOP(S-1) was used as a chiral ligand under 500 psi of H₂ pressure in acetone.

Full text of DOI:10.1016/S0957-4166(99)00424-3

Pergamon
Tetrahedron: Asymmetry 10 (1999) 3863–3867
An efficient synthesis of chiral homophenylalanine derivatives via
enantioselective hydrogenation
Xingshu Li,a,b Chi-hung Yeung,a,∗ Albert S. C. Chan,a,∗ Dong-Sheng Lee b and
Teng-Kuei Yang b,∗
aUnion Laboratory of Asymmetric Synthesis and Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hong Kong
bDepartment of Chemistry, National Chung-Hsing University, Taichung, Taiwan
Received 23 August 1999; accepted 20 September 1999
Abstract
Chiral homophenylalanine derivatives were synthesized via enantioselective hydrogenation of 5a and 5b cata-
lyzed by rhodium complexes bearing chiral phosphine and phosphinite legands. Enantiomeric excesses up to 96.2%
were achieved when S-spiroOP(S-1) was used as a chiral ligand under 500 psi of H2 pressure in acetone. © 1999
Elsevier Science Ltd. All rights reserved.
The homogeneous asymmetric hydrogenation has attracted much attention ever since L-DOPA was
prepared in this way.1 In spite of numerous reports about the syntheses of non-racemic chiral amino
acids through homogeneous asymmetric hydrogenation of the corresponding dehydroamino acids, it is
quite rare to see the asymmetric synthesis of homophenylalanine, a highly interesting pharmaceutical
intermediate for the synthesis of ACE-inhibitors, such as benazepril, lisinopril and quinapril,2 and has
served as a constituent of many pharmaceuticals which function as protease inhibitors and neuronal
receptor ligands (Fig. 1).3 In the literature, chiral homophenylalanine has often been prepared by
chemical or biological resolution, or by enzyme-catalyzed reactions.4 Recently, RajanBabu et al. reported
carbohydrate phosphinites as chiral ligands in Rh-catalyzed asymmetric hydrogenation of β-aromatic
substituted dehydroamino acids with excellent enantioselectivity, but low to moderate e.e. values were
obtained with the ester of 2-acetylamino-4-phenyl-2-butenoic acid which is a β-aliphatic substituted
dehydroamino acid ester.5 Herein we report a successful asymmetric synthesis of homophenylalanine
using homogeneous asymmetric hydrogenation.
The synthesis of the dehydroamino acids 5 began with the mono-addition of 2-phenylethyl magnesium
chloride 3 to diethyl oxalate. The resulting ester 4 was then reacted with methyl carbamate to obtain
compound 5 in 60% overall yield (Scheme 1).6 The results of asymmetric hydrogenation of compound
5 catalyzed by rhodium complexes bearing various chiral phosphine and phosphinite ligands (see Fig. 2)
∗
Corresponding authors. E-mail: tkyang@mail.nchu.edu.tw
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00424-3

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X. Li et al. / Tetrahedron: Asymmetry 10 (1999) 3863–3867
Figure 1.
are shown in Table 1.7 Inspection of Table 1 leads to some important features: (a) of the various catalysts
used, [Rh(S-1)(COD)]BF4 in acetone under 500 psi of H2 pressure affords the best enantioselectivity
with 96.2% e.e. in favor of S-form (entries 1, 10, 11, 16, 18, 20, 22, 24, 28); (b) solvent effects on
enantioselectivity are important, and acetone is the best for the hydrogenation as compared with the
alcohols and other solvents used (entries 1–6); (c) the hydrogenation of acid 5a provided better facial
selectivity than the corresponding ester, thus 95% e.e. was obtained for 5a and 92% e.e. for 5b under
350 psi of H2 pressure (entries 8 and 24); (d) there is no significant effect of hydrogen pressure on the
enantioselectivity, and the e.e. values changed from 96.2% to 94% when the pressure increased from 500
psi to 700 psi. On the other hand, lower conversion (84%) with 93.8% e.e. was obtained when only 150
psi of hydrogen pressure was used (entry 1 vs 7 vs 9). These results indicated that the optimal hydrogen
pressure should be around 500 psi. Meanwhile, we also observed that the change of the configuration of
ligand would invert the configuration of product (entries 1 and 10).
Scheme 1. The synthesis of homophenylalanine and its derivatives
The other parameters of this reaction, such as concentration, temperature and ratio of substrate to
catalyst, were also studied (Table 2). The results showed 0.3 M is the best concentration to obtain high
enantioselectivity (entries 1–4); however, the poor solubility of the substrate prevents this reaction being
run at even higher concentrations. Similar to the other related studies, we found the enantioselectivity

X. Li et al. / Tetrahedron: Asymmetry 10 (1999) 3863–3867
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Figure 2.
Table 1
Asymmetric hydrogenation for compounds 5a and 5ba

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X. Li et al. / Tetrahedron: Asymmetry 10 (1999) 3863–3867
Table 2
The effect of concentration, temperature and ratio of substrate to catalysta
increased with higher quantities of catalyst (entries 1 and 5–7), and if a lower reaction temperature was
used (entries 1, 8, 9 and 10–12).
In spite of the higher enantioselectivity observed in the hydrogenations of β-aliphatic substituted N-
carbamoyl dehydroamino acid derivatives compared to the corresponding N-acetyl dehydroamino acid
derivatives, we recognize that the rate of hydrogenation on the former substrates are relatively slower
than the latter compounds.5–7
In summary, we have developed an efficient synthesis of chiral homophenylalanine via asymmetric
hydrogenation of the corresponding dehydroamino acid derivatives, which can be applied to the synthesis
of valuable chiral β-aliphatic substituted amino acids.
Acknowledgements
The authors thank the Hong Kong Research Grants Council (project number ERB003), The Hong
Kong Polytechnic University and Ministry of Economy of Taiwan for financial support of this study.
References
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5. RajanBabu, T. V.; Ayers, T. A.; Halliday, G. A.; You, K. K.; Calabrese, J. C. J. Org. Chem. 1997, 62, 6012.
6. The substrates 5 were synthesized according to the literature: Melillo, D. G.; Larsen, R. D.; Mathre, D. J.; Shukis, W. F.;
Wood, A. W.; Colleluori, J. R. J. Org. Chem. 1987, 52, 5143. Substrates and products were characterized with 1H NMR
and MS, selected data as follows: 5a 1H NMR (CDCl3) δ 7.19–7.38 (m, 5H, H-Ph), 6.95 (t, 1H, J=7.4 Hz, H-3), 6.51 (br,
1H, NH), 3.76 (s, 3H, CO2CH3), 3.59–3.61 (d, J=7.2 Hz, 2H, H-4); m/z: 235 (100%); 2a 1H NMR (CDCl3) 7.12–7.29 (m,
5H, H-Ph), 6.24 (br, d, J=7.9 Hz, 1H, NH), 3.71 (s, 3H, CO2CH3), 2.59 (t, J=7.9 Hz, 2H, H-4), 1.96–2.14 (m, 2H, H-3);
m/z: 237 (83%).
7. The S-spiroOP, R-spiroOP and DIMOP were prepared according to the literature: (a) Chan, A. S. C.; Hu, W.; Pai, C.-C.;
Lau, C.-P.; Jiang, Y.; Mi, A.; Yan, M.; Sun, J.; Lou, R.; Deng, J. J. Am. Chem. Soc. 1997, 119, 9570; (b) Chen, Y.; Li,
X.; Tong, S.-K.; Choi, M. C. K.; Chan, A. S. C. Tetrahedron Lett. 1999, 40, 957. The typical procedures for preparation
of the catalyst and the asymmetric hydrogenation are as follows: (a) in situ catalyst [Rh(S-1)(COD)]BF4: in a dry box,
[Rh(COD)2]BF4 (4.1 mg, 0.01 mmol) was dissolved in acetone (0.5 ml). A solution of S-1 (5.5 mg, 0.0105 mmol) in
acetone (0.5 ml) was added to the above solution under stirring to give an acetone solution of [Rh(S-1)(COD)]BF4; (b) the
asymmetric hydrogenation of dehydroamino acid derivatives: a vessel equipped with a magnetic stirring bar was charged
with 0.5 ml acetone and 50 µl of acetone solution which contained 0.0005 mmol of catalyst in situ prepared as above and
0.05 mmol of substrate under nitrogen atmosphere. The vessel was pressurized with 500 psi of H2 and the hydrogenation
was carried out at room temperature for 7 h. The ethyl ester was directly analyzed via chiral capillary GLC with a 25-meter
Chrompack capillary column (CP-Chirasil-L-Val). The acid was converted to the corresponding methyl ester with methyl
iodine/K2CO3 before GLC analyses.