Highly Stereoselective Synthesis of Optically Pure C-Aryl Imines from α-L-Amino Acid Methyl Esters

Article abstract of DOI:10.1081/SCC-120026863

The title compounds were easily synthesized starting from readily available α-L-amino acid methyl esters in excellent overall yields. The key feature of the methodology here reported was the TiCl₄ mediated condensation of the 4-methoxybenzaldeh

Full text of DOI:10.1081/SCC-120026863

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Highly Stereoselective Synthesis of Optically Pure C-
Aryl Imines from α-l-Amino Acid Methyl Esters
Antonella Leggio a , Adolfo Le Pera a , Professor Angelo Liguori a , Anna Napoli b , Claudio
Romeo b , Carlo Siciliano a & Giovanni Sindona b
a Dipartimento di Scienze Farmaceutiche , Università degli Studi della , Calabria, Italy
b Dipartimento di Chimica , Università degli Studi della , Calabria, Italy
Published online: 21 Aug 2006.
To cite this article: Antonella Leggio , Adolfo Le Pera , Professor Angelo Liguori , Anna Napoli , Claudio Romeo , Carlo
Siciliano & Giovanni Sindona (2003) Highly Stereoselective Synthesis of Optically Pure C-Aryl Imines from α-l-Amino Acid
Methyl Esters, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
33:24, 4331-4338, DOI: 10.1081/SCC-120026863
To link to this article: http://dx.doi.org/10.1081/SCC-120026863
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 24, pp. 4331–4338, 2003
Highly Stereoselective Synthesis of
Optically Pure C-Aryl Imines from
a-L-Amino Acid Methyl Esters
Antonella Leggio,1 Adolfo Le Pera,1 Angelo Liguori,1,
*
Anna Napoli,2 Claudio Romeo,2 Carlo Siciliano,1
and Giovanni Sindona2
1Dipartimento di Scienze Farmaceutiche,
2Dipartimento di Chimica, Universita degli Studi della
Calabria, Italy
ABSTRACT
The title compounds were easily synthesized starting from readily
available a-L-amino acid methyl esters in excellent overall yields.
The key feature of the methodology here reported was the TiCl4
mediated condensation of the 4-methoxybenzaldehyde with the
free a amino function of the amino acid derivatives. This process
*Correspondence: Professor Angelo Liguori, Dipartimento di Scienze
Farmaceutiche, Universita degli Studi della Calabria, Via P. Bucci, Cubo 15/C,
I-87036 Arcavacata di Rende (CS), Italy; Fax: þ39 0984 492055;
E-mail: A.Liguori@unical.it.
4331
DOI: 10.1081/SCC-120026863
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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4332
Leggio et al.
was clean and highly stereoselective, and the corresponding
enantiomerically pure chiral imines were recovered without need of
chromatographic procedures.
Key Words: Aryl amines; Amino acid methyl esters; Titanium tet-
rachloride; Stereoselectivity.
INTRODUCTION
Formation of the imine functional group represents today an
increasingly fashionable aspect in many organic synthesis purposes.[1–4]
Imines, in fact, are suitable precursors of a very broad family of
compounds, such as a-branched amines,[5] a,a-dibranched amines,[6]
a- and b-amino acids,[7] aziridines,[8] oxaziridines,[9] a- and b-amino
phosphonic acids,[10] g-amino carbonyl compounds,[11] and five- and
six-membered nitrogen containing heterocycles.[12] Literature[13] reports
general preparative methods that are useful to obtain enantiomerically
pure imines, but all of them are restricted to the reaction of the corres-
ponding carbonyl compounds with optically pure or racemic chiral
amines. In some applications, the rate of the condensation can be
accelerated by using TiCl4 as water scavenger.[14] However, in all the
examined cases, reactions afforded imines as mixtures of the expected
E and Z isomers, without stereoselectivity. Furthermore, in the majority
of the experiments done, the chiral imine derivatives have not been
separated and their stereochemistry has not been determinated. It is
relevant to note, as a final point, that unisolated chiral imines, derived
from a-L-valine methyl ester, have been used as important substrates in
one-pot methodologies that can allow for the diastereoselective addition
of carbanions to the C¼N bond.[15]
RESULT AND DISCUSSION
We now report a very short and straightforward procedure in which
the use of TiCl4, in the presence of triethylamine, elicits the smoothly
stereoselective preparation of enantiomerically pure imines, starting from
commercially available methyl esters of the natural a-L-amino acids.
a-L-Valine methyl ester hydrochloride (1a, Sch. 1) was initially
condensed with a stoichiometric amount of 4-methoxybenzaldehyde
(2), in dry ethanol-free dichloromethane at room temperature and in

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Synthesis of Optically Pure C-Aryl Imines
4333
CHO
H
(i)
OCH3
+
OCH3
+
Cl- H3N
N
77%
O
O
H3CO
OCH3
(E:Z = 1:1)
1a
2
3a
(i) CH2Cl2, 4Å molecular sieves, r.t., 24 h.
Scheme 1.
CHO
H
R
R
(i)
OCH3
+
OCH3
+
Cl - H3N
N
91-96%
O
O
H3CO
OCH3
(E isomer)
2
1a-e
3a-e
a: R = CH(CH3)2
b: R = CH3
c: R = CH2Ph
d: R = CH2CH(CH3)2
e: R = CH(CH3)CH2CH3
(i) TiCl4, Et3N, CH2Cl2, anhydrous Na2SO4, r.t., 8 h.
Scheme 2.
the presence of 4A activated molecular sieves. After 24 h, reaction gave
an equimolar mixture of the E and Z isomers of the corresponding
imine 3a, in a good overall yield. GC-MS and 1H and 13C NMR analysis,
performed either on the crude reaction mixture or on the product
isolated after the work-up procedure, confirmed the presence of both
isomers of 3a. Reaction between 1a and 2 was then repeated by setting
the temperature at ꢀ15ꢁC. Under these conditions, after 24 h, imine 3a
was recovered once again as a 1:1 mixture of its E and Z isomers, in 77%
total yield.
When a mixture of 1a and an equivalent amount of 2 in dry ethanol-
free dichloromethane was reacted with TiCl4 and dry triethylamine, at
room temperature and in the presence of Na2SO4 as water scavenger
(Sch. 2), surprisingly the E isomer of 3a was recovered as the only one
product in a remarkable overall yield and in a hitherto obtained high

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4334
Leggio et al.
purity grade, without need of chromatographic purification. No traces of
the Z isomer were detected by GC-MS analysis of the reaction mixture.
The structure of the optically pure chiral imine 3a was unambiguously
1
assigned by H and 13C NMR spectroscopy.
a-L-Amino acid methyl ester hydrochlorides 1b–e were subjected to
the TiCl4 mediated condensation with 4-methoxybenzaldehyde,
under experimental conditions similar to those adopted for the above
discussed preparation of 3a. All the reactions examined ran to completion
in 8 h at room temperature, as stated by TLC analysis of the reaction
mixtures, and all of them still provided only the E isomer of the
corresponding imines 3b–e. GC-MS and NMR analysis, performed on
the final product of each reaction, excluded the formation of imines
having the Z geometry. Furthermore, all products 3b–e were obtained
in excellent total yields and enantiomerically pure. As reported for 3a, no
chromatography was applied to purify imines 3b–e. The structure of the
synthesized imines 3a–e was assigned by 1H NMR analysis on the basis of
the chemical shift of the iminic proton that is less shielded in the E isomer
and according to the data reported in Lit.[16]
A total absence of stereoselectivity was observed for the TiCl4
mediated condensation of glycine methyl ester hydrochloride (4) with
4-methoxybenzaldehyde (Sch. 3).
In fact, treatment provided a 1:1 mixture of the E and Z isomers of
imine 5. Similarly, when the achiral amino acid 4 was subjected to
condensation with 2 without adding the Lewis acid to the reaction
mixture, the process yielded 5 as an equimolar mixture of both isomers.
The results here discussed seem to suggest that the highly stereoselective
formation of the enantiomerically pure imines 3a–e is strictly related to
the presence of the asymmetric carbon atom on the amino acid frame and
to the simultaneous use of Lewis species TiCl4.
CHO
H
H
H
H
(i)
H
OCH3
+
OCH3
+
Cl - H3N
N
90%
O
O
H3CO
OCH3
(E:Z = 1:1)
2
4
5
(i) TiCl4, Et3N, CH2Cl2, anhydrous Na2SO4, r.t., 8 h.
Scheme 3.

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Synthesis of Optically Pure C-Aryl Imines
EXPERIMENTAL
4335
All reagents were purchased from Aldrich Chemical Co. Solvents
were purified according to the literature procedures and distilled
prior to use. 1H NMR spectra of all compounds were recorded at
300 MHz on a Bruker AC 300 spectrometer, and 13C NMR spectra
were recorded at 75.5 MHz on the same instrument. CDCl3 was used as
the solvent with 0.3% v/v TMS as the internal standard. 1H
and 13C NMR chemical shift values are expressed in ppm. All coupling
constants are reported in Hz. Infrared spectra were run on a
Perkin–Elmer FT-IR Paragon 1000 PC spectrophotometer by using
neat compounds. Infrared data are reported in cmꢀ1. Exact masses
were determined by ESI-TOF MS analysis, performed on a PE Sciex
API Q-Star Pulsar-i mass spectrometer. GC-MS analysis of the reaction
mixtures was performed on a Hewlett-Packard HP GC 5890 Series II,
equipped with a quadrupolar mass filter HP 5972A and with a column
HP-5MS (crosslinked 5% PhMe Silicon, ID ¼ 0.25 mm; 0.25 mm, 30 m;
flow rate ¼ 1 mL/min). Optical activities were measured at 28ꢁC by using
a digital Atago Polax-D Polarimeter, and CHCl3 as the solvent in a 1 dm
cell (volume ¼ 2 mL). TLC analysis was performed on Merck F-254 silica
gel precoated aluminum plates, with Et2O/n-hexane 70:30 v/v as the
eluent. Ninhydrin and a 0.5% ethanolic 2,4-dinitrophenylhydrazine solu-
tion were used as the visualizing agents. All the reactions were performed
under an inert atmosphere.
General Procedure for the Synthesis of Imines 3a–e and 5
4-Methoxybenzaldehyde (2; 6 mmol) was added to a suspension of
the appropriate a-L-amino acid methyl ester hydrochloride 1a–e or 4
(6 mmol) and anhydrous Na2SO4 in dry ethanol-free dichloromethane
(5 mL). The resulting mixture was kept under magnetic stirring at room
temperature for 5 min, and after this time a commercially available 1 M
solution of TiCl4 in dry dichloromethane (3 mL) was added by using a
syringe. Dry triethylamine (18 mmol, 2.5 mL) was then added dropwise
and the final mixture was stirred until complete conversion of the starting
materials. After 8 h, the reaction mixture was treated with 1 M aqueous
NaOH (20 mL) and brine (10 mL), then extracted three times with
CHCl3. The organic layers were dried on MgSO4, then evaporated to
dryness under vacuum to afford the corresponding pure imines 3a–e
and 5, each one as a pale yellow oil.

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4336
Leggio et al.
Selected Spectroscopic Data of Imines 3a–e and 5
3a. E isomer. Yield: 96%. 1H NMR: ꢀ ¼ 0.93 [d, J ¼ 6.83, 3H,
CH(CH3)2], 0.95 [d, J ¼ 6.83, 3H, CH(CH3)2], 2.27–2.43 [m, 1H,
CH(CH3)2], 3.74 (s, 3H, CO2CH3), 3.83 (s, 3H, CH3OAr), 6.91
(d, J ¼ 8.79, 2H, ArH), 7.73 (d, J ¼ 8.79, 2H, ArH), and 8.16 (s, 1H,
HC¼N). 13C NMR: ꢀ ¼ 17.12, 27.01, 50.40, 56.02, 72.15, 114.21,
129.62, 130.07, 163.73, 164.32, and 172.12. IR: ꢁ ¼ 1740 (C¼O) and
1610 (C¼N). GC-MS (EI, 70 eV): m/z (%) ¼ 249 (Mþ, 6.1), 234 (4.5),
190 (100), 175 (12), 147 (18.5), 135 (16.7), 105 (9), 91 (5), 77 (7.2), 59 (3.4),
and 51 (3.2). ESI-TOF MS: Calcd. for C14H19NO3 249.136. Found:
249.129. [a]D ¼ ꢀ30.59 (c ¼ 0.1).
3b. E isomer. Yield: 91%. 1H NMR: ꢀ ¼ 1.51 (d, J ¼ 6.83, 3H,
CHCH3), 3.72 (s, 3H, CO2CH3), 3.81 (s, 3H, CH3OAr), 4.11
(q, J ¼ 6.83, 1H, CHCH3), 6.92 (d, J ¼ 8.79, 2H, ArH), 7.71 (d,
J ¼ 8.79, 2H, ArH), and 8.22 (s, 1H, HC¼N). 13C NMR: ꢀ ¼ 16.28,
50.38, 56.09, 61.80, 114.81, 129.48, 130.06, 163.71, 164.28, and 174.09.
IR: ꢁ ¼ 1743 (C¼O) and 1608 (C¼N). GC-MS (EI, 70 eV): m/z (%) ¼ 221
(Mþꢂ, 7.5), 206 (3.5), 162 (100), 147 (16.1), 135 (15), 105 (8), 91 (6), 77
(7.2), 59 (3.1), and 51 (4.3). ESI-TOF MS: Calcd. for C12H15NO3
221.105. Found: 221.099. [a]D ¼ ꢀ47.08 (c ¼ 0.1).
3c. E isomer. Yield: 94%. 1H NMR: ꢀ ¼ 3.11 (dd, J ¼ 8.7 and
J ¼ 13.5, 1H, CH2Ph), 3.36 (dd, J ¼ 5.5 and J ¼ 13.5, 1H, CH2Ph), 3.69
(s, 3H, CO2CH3), 3.79 (s, 3H, CH3OAr), 4.12 (dd, J ¼ 5.5 and J ¼ 8.7,
1H, CHCH2Ph), 6.88 (d, J ¼ 8.79, 2H, ArH), 7.11–7.24 (m, 5H, Ph), 7.61
(d, J ¼ 8.79, 2H, ArH), and 7.82 (s, 1H, HC¼N). 13C NMR: ꢀ ¼ 36.58,
51.39, 55.44, 67.40, 113.81, 114.04, 125.70, 127.91, 128.43, 129.12, 130.83,
140.21, 164.12, 165.12, and 174.38. IR: 1741 (C¼O) and 1606 (C¼N).
GC-MS (EI, 70 eV): m/z (%) ¼ 297 (Mþꢂ, 0.9), 238 (13.5), 206 (100), 174
(10.8), 146 (30.6), 132 (5.4), 91 (13.5), 77 (7.3), 65 (11.8), 59 (4.5), and 51
(1.9). ESI-TOF MS: Calcd. for C18H19NO3 297.136. Found: 297.129.
[a]D ¼ ꢀ55.65 (c ¼ 0.1).
3d. E isomer. Yield: 92%. 1H NMR: ꢀ ¼ 0.95 [d, J ¼ 6.91, 3H,
CH(CH3)2], 0.97 [d, J ¼ 6.91, 3H, CH(CH3)2], 1.51–1.63 [m, 1H,
CH(CH3)2], 1.75–1.91 [m, 2H, CH2CH(CH3)2], 3.75 (s, 3H, CO2CH3),
3.82 (s, 3H, CH3OAr), 4.07 (dd, J ¼ 5.86 and J ¼ 8.3, 1H, CHCO2CH3),
6.94 (d, J ¼ 8.80, 2H, ArH), 7.75 (d, J ¼ 8.80, 2H, ArH), and 8.22 (s, 1H,
HC¼N). 13C NMR: ꢀ ¼ 22.12, 23.38, 40.27, 50.12, 57.03, 62.55, 114.28,
129.69, 130.84, 162.03, 165.15, and 174.02. IR: ꢁ ¼ 1741 (C¼O) and 1607
(C¼N). GC-MS (EI, 70 eV): m/z (%) ¼ 263 (Mþꢂ, 2.2), 248 (3.1), 220
(4.7), 204 (100), 176 (32.9), 161 (59.5), 147 (65.3), 135 (42.3), 121
(46.8), 105 (7.6), 91 (15.3), 77 (14.9), 69 (3.5), 59 (3.6), and 51 (5.8).

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Synthesis of Optically Pure C-Aryl Imines
4337
ESI-TOF MS: Calcd. for C15H21NO3 263.152. Found: 263.145.
[a]D ¼ ꢀ28.34 (c ¼ 0.1).
3e. E isomer. Yield: 93%. 1H NMR: ꢀ ¼ 0.88 (t, J ¼ 7.82, 3H,
CH2CH3), 0.93 (d, J ¼ 6.84, 3H, CHCH3), 1.05–1.18 (m, 1H,
CH2CH3), 1.48-1.61 (m, 1H, CH2CH3), 2.10–2.22 (m, 1H, CHCH3),
3.73 (s, 3H, CO2CH3), 3.82 (s, 3H, CH3OAr), 6.92 (d, J ¼ 8.8, 2H,
ArH), 7.73 (d, J ¼ 8.8, 2H, ArH), and 8.19 (s, 1H, HC¼N). 13C NMR:
ꢀ ¼ 11.82, 14.63, 24.81, 32.78, 51.33, 56.03, 69.61, 113.83, 129.68, 130.33,
164.18, 164.85, and 173.02. IR: ꢁ ¼ 1742 (C¼O) and 1603 (C¼N). GC-
MS (EI, 70 eV): m/z (%) ¼ 263 (Mþꢂ, 4.1), 248 (2.1), 206 (25.3), 204 (100),
174 (13.8), 162 (27.2), 147 (85.1), 136 (26.1), 121 (18), 105 (9.1), 91 (11.3),
77 (10.9), 69 (8.3), 59 (4), and 51 (6.1). ESI-TOF MS: Calcd. for
C15H21NO3 263.152. Found: 263.146. [a]D ¼ ꢀ77.09 (c ¼ 0.1).
1
5. Equimolar mixture of E and Z isomers. Yield: 90%. H NMR:
ꢀ ¼ 3.76 (s, 3H, CO2CH3), 3.79 (s, 3H, CO2CH3), 3.81 (s, 3H,
CH3OAr), 3.83 (s, 3H, CH3OAr), 3.85 (s, 2H, CH2CO2CH3), 3.89
(s, 2H, CH2CO2CH3), 6.93 (d, J ¼ 8.78, 2H, ArH), 6.95 (d, J ¼ 8.79,
2H, ArH), 6.98 (s, 1H, HC¼N), 7.45 (d, J ¼ 8.78, 2H, ArH), 7.52
(d, J ¼ 8.79, 2H, ArH), and 7.98 (s, 1H, HC¼N). 13C NMR: ꢀ ¼ 49.73,
49.85, 56.03, 56.12, 56.45, 61.03, 113.31, 114.28, 127.33, 128.08, 132.34,
133.38, 148.03, 162.71, 164.83, 165.71, 171.02, and 175.33. IR: ꢁ ¼ 1745
(C¼O, broad) and 1609 (C¼N, broad). GC-MS (EI, 70 eV):
m/z (%) ¼ 207 (Mþꢂ, 12.1), 192 (11.1), 148 (86.3), 134 (15.2), 121 (100),
104 (7.2), 91 (12.3), 77 (8), 65 (3.3), 59 (2.3), and 51 (5.1). ESI-TOF MS:
Calcd. for C11H13NO3 207.090. Found: 207.085.
ACKNOWLEDGMENTS
This work was supported by MIUR financial grants.
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Received in the UK June 19, 2003