The synthesis of L-(+)-homophenylalanine hydrochloride

Article abstract of DOI:10.1016/S0957-4166(00)00193-2

L-(+)-Homophenylalanine hydrochloride was synthesized from N-phthaloyl-L-(-)-asparitc anhydride 2 in three steps in 55% overall yield with 99% ee. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00193-2

Tetrahedron: Asymmetry 11 (2000) 2309±2314
The synthesis of l-(+)-homophenylalanine hydrochloride
Qianyong Xu,a Guoxin Wang,a Xuechao Wang,a Tongxing Wu,a Xinfu Pan,a,*
Albert S. C. Chanb and Teng-kuei Yangc
aDepartment of Chemistry, National Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000,
P.R. China
bDepartment of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong, P.R. China
cDepartment of Chemistry, National Chung-Hsing University, Taichung, Taiwan
Received 2 March 2000; accepted 12 April 2000
Abstract
l-(+)-Homophenylalanine hydrochloride was synthesized from N-phthaloyl-l-(^)-aspartic anhydride 2 in
three steps in 55% overall yield with 99% ee. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
a-Amino acids take an important role in biology because many of them are the precursors for a
variety of biologically active compounds.1,2 Among them, l-(+)-homophenylalanine [l-(+)-HPA]
has received great attention because of its pharmaceutical interest as a component of ACE-
inhibitors. Although many methods have been reported for the preparation of l-(+)-HPA, most
of them are not economical enough for practical purposes.3 For example, l-(+)-HPA was
synthesized in four steps from a-t-butyl, b-methyl ester of N-benzyloxycarbonyl-(S)-aspartate in
13% overall yield with 80% ee.4 Herein, we describe a method of the synthesis of l-(+)-HPA
hydrochloride using N-phthaloyl-l-(^)-aspartic anhydride as starting material in three steps in
55% overall yield with 99% ee.
2. Results and discussion
The synthetic route for compound 1 is summarized in Scheme 1. N-Phthaloyl-l-(^)-aspartic
anhydride 2 was prepared according to the literature.5 The following Friedel±Crafts acylation of
1
benzene with 2 had been reported by Dornhege et al.5 to give 5 based on the H NMR spectra,
however, we found that 3 is the authentic product in the reaction (Scheme 2).
* Corresponding author. E-mail: panxf@lzu.edu.cn
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00193-2

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Scheme 1. Reagents and conditions: (a) AlCl3, benzene, 50ꢀC, 3 h, 79%; (b) H2, 10% Pd/C, HOAc, 50ꢀC, 3 h, 92%; (c)
HOAc/6N HCl (v:v=5:2), re¯ux, 6±8 h, 75%
Scheme 2.
This result can be explained by the possible mechanism shown in Scheme 3. As can be seen
from the two canonical structures A and B of 2, the lone pair electrons of the center oxygen were
competed for by two carbonyl groups. The resonance form B is more favored because the N-
phthaloylamino group has a stronger electron withdrawing e€ect to the a-carbonyl group.
Therefore, the more favored intermediate 7 leads to 3 as the sole product in the Friedel±Crafts
acylation reaction.
In order to con®rm this conclusion, an X-ray study on the methyl ester of 3 (Scheme 4) was
carried out (Fig. 1). The X-ray structure of 8 clearly indicates that 3 is the structure of the
Friedel±Crafts product. It is understandable that an incorrect structure might be assigned based
on the ambiguity of the NMR spectra between 3 and 5 [1H NMR (CD3COCD3)5 for 5: 3.79 (q,
2H), 4.25 (q, 1H), 5.77 (q, 4H), 7.88 (m, 4H), 7.34±8.16 (m, 5H), 10.26 (s, 1H), which is essentially
the same as that for 3 assigned by this report]. Although Dornhege's conclusion did not e€ect the
synthesis of the optically active 2-amino-tetralin, on revision of the structure it became a practical
methodology for the preparation of l-(+)-HPA.
Hydrogenolysis of 3 gave 4 in 92% yield, which was deprotected with 6N hydrogen chloride in
acetic acid (v:v=2:5) to a€ord 1 in 75% yield, whose spectral data were in agreement with the
literature.3a In order to determine the ee of 1, it was esteri®ed with ethanolic hydrogen chloride as
ethyl l-(+)-HPA and determined by HPLC to be 99% ee, and thus the ee of 1 would be 99%. The
above conclusion on the regioselectivity of Friedel±Crafts acylation of 2 with benzene was further
con®rmed by the completed synthesis of 1.

Q. Xu et al. / Tetrahedron: Asymmetry 11 (2000) 2309±2314
2311
Scheme 3.
Scheme 4.
3. Conclusion
In brief, we have reported a practical method for the synthesis of l-(+)-HPA hydrochloride in
good overall yield (55%) with an excellent enantiomeric excess (99%). This methodology should
be adaptable to the synthesis of a variety of substituted homophenylalanines.
4. Experimental
4.1. General methods
Melting points were taken on a Ko¯er melting point apparatus and are uncorrected. 1H NMR
spectra were recorded on a Bruker AM-80 NMR spectrometer with TMS as an internal reference
for chemical shifts expressed as d-values in ppm and coupling constants in hertz. Mass spectra
were obtained on a Hewlett±Packard HP5988A mass spectrometer using EI ionization. IR spec-
tra were recorded on a Nicolet FT-IR 170X spectrometer. Optical rotations were measured on a
Jasco J-20C automatic polarimeter. Enantiomeric excess (ee) determination was carried out using

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Figure 1. The X-ray structure of 8
hexane:isopropyl alcohol (90:10, 1.0 mL/min) as eluent with a Chiralcel CDMPC (150Â4.6 mm)
column (Daicel Chemical Industries) on a Varian SD-200 HPLC instrument.
4.2. N-Phthaloyl-l-(^)-4-oxo-homophenylalanine 3
Compound 2 (1 mol) was added portionwise to a solution of 2.5 mol anhydrous AlCl3 in 4.8 L
benzene at room temperature. Then the mixture was heated to 50ꢀC gradually and stirred for 3 h
at that temperature. Cooling to rt, 2000 g crushed ice and 230 mL concentrated HCl were added
with stirring. The precipitated product was ®ltered, washed with water, and dried in vacuo to
a€ord 3 as white solid (79% yield). Mp 161±163ꢀC; ‰ꢀŠD20=^15.0 (c, 4.3, ethanol) [lit.5 mp 165±
20
167ꢀC; ‰ꢀŠD =^14.7 (c, 5.0, ethanol)]; H NMR (CD3COCD3): 3.90 (1H, dd, J=6.7, 17.9, CH2);
4.10 (1H, dd, J=6.7, 17.9, CH2); 5.71 (1H, m, CH); 7.45±7.67 (4H, m, Ar-H); 7.90 (5H, s, Ar-H);
m/z (EI): 323 (2); 305 (34); 218 (18); 174 (7); 105 (100).
1
4.3. N-Phthaloyl-l-(^)-homophenylalanine 4
Compound 3 (1 mol) in 30 L acetic acid was hydrogenated over 10% Pd/C (46 g) for 3 h at
normal pressure and 50ꢀC. The catalyst was ®ltered o€, and the ®ltrate was concentrated in

Q. Xu et al. / Tetrahedron: Asymmetry 11 (2000) 2309±2314
2313
reduced pressure to 100 mL. Water was added until precipitate was produced, then the mixture was
stored in a refrigerator overnight. The product was collected by ®ltration, washed with water, and
dried in vacuo to give 4 as white solid (92% yield). Mp 140±142ꢀC; ‰ꢀŠD20=^10.0 (c, 4.0, ethanol);
1H NMR (CDCl3): 2.48±2.92 (4H, m, CH2); 4.86±5.05 (1H, m, CH); 7.17 (5H, s, Ar-H); 7.63±7.93
(4H, m, Ar-H); 11.23 (1H, s, COOH); m/z (EI): 309 (1); 279 (1); 205(8); 187 (6); 148 (13); 91 (100).
4.4. l-(+)-Homophenylalanine hydrochloride 1
To the solution of 1 mol 4 in 7.5 L acetic acid was added 3 L 6N HCl and the mixture was
re¯uxed for 6±8 h. The acetic acid was then removed in reduced pressure and the residue was
dissolved in 3 L 3N HCl with stirring. After that the solution was ®ltered. The ®ltrate was taken
to dryness under reduced pressure, and the solid was collected by ®ltration and dried in vacuo to
give 1 as white solid (75% yield). Mp 250±260ꢀC; ‰ꢀŠ2D0=+40 (c, 0.6, 3N HCl) [lit.6 ‰ꢀŠ2D0=+45.0
^1
.
(c, 1.0, 3N HCl)]; IR (KBr, cm ): 3000 (NH2), 1740 (CO). Anal. calcd for C10H13NO2 HCl: C,
55.69; H, 6.54; N, 6.49. Found: C, 55.60; H, 6.62; N, 6.40. 1H NMR (CF3COOD): 2.63±2.79 (2H,
m, CH2); 3.09±3.18 (2H, t, J=7.0, CH2); 4.55 (1H, m, CH); 7.41 (5H, s, Ar-H); m/z (EI): 179 (2);
162 (10); 134 (24); 91 (100).
4.5. Methyl 2-N-phthaloyl-l-(^)-4-oxo-homophenylalanine 8
Compound 3 (500 mg) was dissolved in 20 mL methanol with 0.2 mL concentrated H2SO4, and
the resulting solution was re¯uxed until TLC indicated the reaction to be complete. Methanol was
evaporated in vacuo, the residue was dissolved in saturated NaHCO3 solution and extracted with
ethyl acetate (15 mLÂ3). The combined organic solution was washed with brine, dried over
anhydrous Na2SO4, ®ltered, and the solvent was removed under reduced pressure. The residue
1
was recrystallized from ethanol to a€ord 8 as a white solid (90% yield). H NMR (CDCl3): 3.77
(3H, s, OCH3); 3.91±4.21 (2H, m, CH2); 5.66±5.83 (1H, dd, J=5.8, 7.6, CH); 7.45 (5H, s, Ar-H);
7.68±8.05 (4H, m, Ar-H); m/z (EI): 337 (0.2), 305 (2), 105 (100).
X-Ray crystal data collection and re®nement for 8: formula, C19H15NO5; Mr, 337.32. Crystal
system, monoclinic; space group, P21/n; a, 8.586(4) A; b, 17.284(2) A; c, 11.449(3) A; V, 1649.0(9)
A3; Z, 4; Dc, 1.359 g cm^3; F(000), 740; m, 0.099 mm^1; T, 20ꢀC; independent re¯ections, 2891.
Observed data with I=3s(I), 1750; radiation (l/A), MoKꢀ; scan type, !/2ꢁ; 2ꢁmax, 50ꢀ; R,
0.0256; Rw, 0.71073. Goodness-of-®t indicator, 1.140.
4.6. Determination of the ee of 1
.
Amino acid 1 (ꢁ100 mg) was re¯uxed in EtOH HCl (5 mL, 1N) until TLC indicated the
starting material had disappeared. Ethanol was evaporated under reduced pressure, and the
residue was puri®ed as above to a€ord ethyl l-(+)-HPA. Chiral HPLC analysis shows ethyl l-(+)-
HPA with 99% ee.
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