Concise synthesis of enantiopure (S)- and (R)-α-trifluoromethyl aspartic acid and α-trifluoromethyl serine from chiral trifluoromethyl oxazolidines (Fox) via a Strecker-type reaction

Article abstract of DOI:10.1016/j.tetasy.2011.01.007

A concise synthesis of enantiopure (S)- and (R)-α-Tfm-aspartic acid and α-Tfm-serine is reported. The key step involves a Strecker-type reaction on chiral CF₃-oxazolidines (Fox) derived from ethyl 4,4,4-trifluoroacetoacetate (ETFAA) or ethyl tr

Full text of DOI:10.1016/j.tetasy.2011.01.007

Tetrahedron: Asymmetry 22 (2011) 309–314
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Concise synthesis of enantiopure (S)- and (R)-
a-trifluoromethyl aspartic
acid and
a
-trifluoromethyl serine from chiral trifluoromethyl oxazolidines
(Fox) via a Strecker-type reaction
Julien Simon, Thi Thuan Nguyen, Evelyne Chelain, Nathalie Lensen, Julien Pytkowicz, Grégory Chaume,
⇑
Thierry Brigaud
Université de Cergy-Pontoise, Laboratoire SOSCO, 5 Mail Gay-Lussac, Neuville sur Oise, 95000 Cergy-Pontoise cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A concise synthesis of enantiopure (S)- and (R)-a-Tfm-aspartic acid and a-Tfm-serine is reported. The key
step involves a Strecker-type reaction on chiral CF3-oxazolidines (Fox) derived from ethyl 4,4,4-trifluoro-
acetoacetate (ETFAA) or ethyl trifluoropyruvate.
Received 16 December 2010
Accepted 12 January 2011
Available online 4 March 2011
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
only one preparation in the asymmetric series was reported by
Zanda et al.7 The synthesis of both enantiomers of
a-Tfm-serine
a-Trifluoromethyl
a
-amino acids have attracted considerable
has also been reported.8 These two stereoselective methodologies
are based on the use of chiral sulfinimines and sulfoxides. In order
to provide a convenient and straightforward access to both (S)- and
interest as natural -amino acids analogues, owing to the unique
a
properties imparted by the Tfm group, such as high electronegativ-
ity, electron density, steric hindrance, and a hydrophobic charac-
ter.1 Thus, the presence of the Tfm group produces significant
changes in the physical, chemical, and biological properties of
(R)-a-trifluoromethyl aspartic acid and a-trifluoromethyl serine,
we herein report another strategy based on highly functionalized
fluorinated oxazolidines (Fox). This approach involves the
commonly used (R)-phenylglycinol chiral inducer and the com-
mercially available fluorinated building blocks ethyl 4,4,4-trifluo-
roacetoacetate (ETFAA) and ethyl trifluoropyruvate as starting
materials. We have designed a straightforward approach that in-
volves Strecker-type reactions in order to introduce cyano groups
as precursors of carboxylic acid groups to synthesize the target
amino acids (Scheme 1).
the molecules that render a-trifluoromethyl a-amino acids as very
attractive for the design of biologically active molecules, particu-
larly peptides.2 However, despite their promising potency, the
preparation of
a-trifluoromethyl a-amino acids in enantiopure
form involving convenient and scalable experimental procedures
remains a challenge and thus, limits systematic investigation of
their biomedicinal and structural features.3
In previous contributions from our group, we have already re-
ported several approaches for the stereoselective syntheses of var-
ious linear and cyclic
a-trifluoromethyl a-amino acids starting
NH2
NH2
*
from chiral CF3-oxazolidines (Fox) or imines derived from ethyl tri-
fluoropyruvate, trifluoromethyl ketones or fluoral.4 In parallel, we
have also developed new methodologies allowing their incorpora-
tion into peptides.5
As an extension of our investigations, we are now focusing our
attention on the stereoselective synthesis of various side chain
*
F3C
CO2H
F3C
CO2H
OH
(R)- and (S)-α-Tfm-serine
and
CO2H
(R)- and (S)-α-Tfm-
aspartic acid
functionalized
a-trifluoromethyl
a-amino acids such as a-Tfm-
aspartic acid and
there have been very few reported procedures for the stereoselec-
a
-Tfm-serine. Despite their potential interest,
Ph
Ph
Strecker-type
reaction
OH
tive synthesis of these amino acids in enantiopure form. Although
HN
several syntheses of racemic
a-Tfm-aspartic acid and derivatives
HN
O
are reported in the literature,6 to the best of our knowledge the
CN
CO2Et
F3C
F3C
n
n
CO2Et
n= 0,1
⇑
Corresponding author. Tel.: +33(0)1 34 25 70 66; fax: +33(0)1 34 25 70 71.
E-mail address: thierry.brigaud@u-cergy.fr (T. Brigaud).
Scheme 1. The strategy.
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.01.007

310
J. Simon et al. / Tetrahedron: Asymmetry 22 (2011) 309–314
OH
NH
OH
2. Results and discussion
The synthesis of the ethyl trifluoroacetoacetate (ETFAA) -based
oxazolidine 1 (Scheme 2) was not trivial because of the presence of
acidic protons at the a-position of the ester function. The acid cat-
Ph
Ph
F3C
NH
CN
conc. HCl
EtO2C
CN
AcOEt
reflux
F3C
EtO2C
alyzed condensation of ETFAA with (R)-phenylglycinol gave the
corresponding oxazolidines 1 as well as the formation of 10–20%
of an enamine side product.9 This enamine is in thermodynamic
equilibrium with the oxazolidine 1 under acidic conditions. During
the course of our work, Saigo et al.9 reported the selective forma-
tion of oxazolidines 1 versus the enamine using 3 equiv of glacial
acetic acid. Following their procedure, the oxazolidines 1 were ob-
tained in 79% yield as two isolated diastereomers (Scheme 2).10
(S)-2
55 : 45 (R)-2
Ph
O
Ph
HN
HN
F3C
EtO2C
O
O
F3C
EtO2C
O
(R)-3 (27%)
(S)-3 (33%)
Ph
Scheme 4. Syntheses of morpholinones 3.
(R)-phenylglycinol
O
AcOH, CHCl3, reflux
HN
O
CO2Et
CO2Et
F3C
F3C
79%
Starting from the pure isolated morpholinone (S)-3 and (R)-3,
the enantiopure (S)- and (R)-a-trifluoromethyl aspartic acid
1
maj (49%) + 1min (30%)
hydrochlorides (S)-4 and (R)-412 were very conveniently obtained
in 83% and 86% yield, respectively, after acidic treatment
(Scheme 5). The hydrolysis of the two ester functions and the re-
moval of the phenylethanol moiety occurred at the same time by
treatment with concentrated HCl at reflux.
Scheme 2. Synthesis of ethyl trifluoroacetoacetate-based oxazolidine 1.
As we have already mentioned in previous reports,4a,b,5 the
isolation of diastereomerically pure oxazolidines is not necessary
because the introduction of the cyano group through the
Strecker-type reaction involves the same iminium intermediate
from both oxazolidines. Therefore, the Strecker-type reaction was
performed from the crude diastereomeric mixture of oxazolidines
1 obtained from ETFAA and (R)-phenylglycinol. This transforma-
tion was achieved according to our previously described conditions
using TMSCN under BF3ꢀOEt2 activation.4a,b,5 The two aminonitriles
(S)-2 and (R)-2 were obtained on a 5 g scale in good yield (77% from
ETFAA) as an inseparable 55:45 diastereomeric mixture (Scheme 3).
This very low diastereoselectivity can be due to the addition of
TMSCN to a mixture of E- and Z-iminium or to an enamine
intermediate.
Ph
NH3
Cl
HN
F3C
(S)-3
conc. HCl
HO2C
EtO2C
O
CO2H
reflux
83%
F3C
O
Enantiopure (S)-α-Tfm-
aspartic acid : (S)-4
[α]D = + 25.0 (c 0.25, H2O)
[α]D (lit.)7 = + 23.5 (c 0.25, H2O)
Ph
NH3
Cl
HN
conc. HCl
O
F3C
F3C
EtO2C
reflux
86%
CO2H
CO2H
OH
NH
OH
NH
O
1) (R)-phenylglycinol,
AcOH, CHCl3, reflux
O
Ph
EtO2C
Ph
F3C
Enantiopure (R)-α-Tfm-
aspartic acid : (R)-4
(R)-3
F3C
CN
CO2Et
55 : 45 (R)-2
CN
2) TMSCN
F3C
CO2Et
BF3.OEt2, CH2Cl2
[α]D = -24.6 (c 0.3, H2O)
[α]D (lit.)7 = -24.3 (c 0.25, H2O)
77% (2 steps)
(S)-2
Scheme 5. Synthesis of both enantiomers of the a-Tfm-aspartic acid 4.
Scheme 3. Synthesis of amino nitriles 2.
The synthesis of both enantiomers of
a-Tfm-serine was then
anticipated through the same three step strategy involving a Strec-
ker-type reaction on a trifluoropyruvate-based oxazolidine fol-
lowed by a selective ester reduction and the simultaneous acidic
hydrolysis of the nitrile and removal of the phenylethanol side
chain. The BF3ꢀEt2O promoted Strecker-type reaction performed
on the chiral ethyl trifluoropyruvate-based CF3-oxazolidine 5
(7.31 g scale)13 gave the corresponding gem-cyanoester 6 in high
yield (96%) with moderate diastereoselectivity14 (Scheme 6). At
this stage, all attempts at the chemoselective reduction of the ester
group of the gem-cyanoester 6 into a hydroxyl group leading to an
The next steps for the synthesis of enantiopure a-Tfm-aspartic
acid required the efficient separation of the two diastereoisomers
and the hydrolysis of the cyano group into a carboxylic acid group.
We envisioned that these two operations could be achieved
according to a previously described strategy relying on the forma-
tion of intermediate morpholinones.4c,e Several acidic conditions
were tested resulting in the formation of six and seven membered
rings mixtures. The use of concentrated HCl solution in AcOEt11
was the best condition found for the selective formation of the
morpholinones (S)-3 and (R)-3 (Scheme 4). Due to the cyclic struc-
ture of the morpholinones, their separation was possible after silica
gel chromatography to give (S)-3 and (R)-3 in 33% and 27% isolated
yields, respectively (Scheme 4).
a-Tfm-serine precursor were unsuccessful, despite that such
reductions being reported in the literature in the non-fluorinated
series (Scheme 6).15

J. Simon et al. / Tetrahedron: Asymmetry 22 (2011) 309–314
311
Ph
Ph
Ph
NH2
OH
OH
TMSCN,
BF3.OEt2
HN
HN
1) conc. HCl, reflux
2) Dowex
HN
O
F3C
CO2H
OH
Enantiopure (R)-α-Tfm-
serine (R)-9
EtO2C
F3C
F3C
CN
CN
CH2Cl2
96%
F3C
CO2Et
75%
OH
(S)-8
5
6 (60:40 dr)
[α]D = + 10.6 (c 1, H2O)
Selective reduction
of the ester group
[α]D (lit.)8a = + 11.3 (c 0.77, H2O)
X
Scheme 6. Synthesis of
a
-Tfm-gem-cyanoester 6.
Ph
NH2
OH
1) conc. HCl, reflux
2) Dowex
94%
HO
HN
We next decided to reverse the order of the reactions and re-
duce the ester group of the oxazolidine 5 before performing the
Strecker-type reaction. We recently reported that the use of LiAlH4
as a reducing agent to achieve the reduction of the ester group of 5
was accompanied with the ring opening of the oxazolidine.16 Due
to the enhanced electrophilicity of the ester moiety due to the elec-
tron withdrawing effect of the trifluoromethyl group, we hypothe-
sized that the use of a mild reducing reagent should be sufficient
enough to carry out the chemoselective reduction of the ester
group and avoid the side ring opening reaction. Indeed, the ester
group of the oxazolidine 5 (71:29 dr) was chemoselectively re-
duced with NaBH4 to afford the expected 2-hydroxymethyl oxazol-
idine 7 (71:29 dr) in 90% yield (Scheme 7). These oxazolidines
could be separated by silica gel chromatography but as previously
demonstrated this operation is not required for the next step. The
Stecker-type reaction was then performed on the diastereomeric
mixture of 7 under the usual conditions. At this point, the two ami-
no nitriles diastereomers 8 were very conveniently separated by
silica gel chromatography17 to give (S)-8 and (R)-8 in 64% and
33% isolated yield, respectively (Scheme 7).
HO
CO2H
F3C
CN
F3C
(R)-8
Enantiopure (S)-α-Tfm-
serine (S)-9
[α]D = - 10.4 (c 0.85, H2O)
[α]D (lit.)8a = - 9.1 (c 0.59, H2O)
Scheme 8. Synthesis of both enantiomers of a-Tfm-serine 9.
4. Experimental
4.1. General
Unless otherwise mentioned, all the reagents were purchased
from commercial source. All glassware was dried in an oven at
150 °C prior to use. Ether and THF were distilled under nitrogen
from sodium/benzophenone prior to use. CH2Cl2 was distilled un-
der nitrogen from CaH2 prior to use. 1H NMR (400.00 MHz), 13C
NMR (100.50 MHz) and 19F NMR (376.20 MHz) were measured
on a JEOL ECX400 spectrometer. Chemical shifts of 1H NMR are ex-
pressed in parts per million downfield from tetramethylsilane
(d = 0) in CDCl3. Chemical shifts of 13C NMR are expressed in parts
per million downfield from CDCl3 as internal standard (d = 77.0).
Chemical shifts of 19F NMR are expressed in parts per million
downfield from C6F6 as an internal standard (d = ꢁ164.9). Coupling
constants are reported in Hertz. Column chromatography was per-
Ph
Ph
NaBH4
MeOH
HN
O
HN
O
OH
F3C
CO2Et
F3C
97%
7 (71:29 dr)
5 (71:29 dr)
Ph
Ph
OH
formed on SDS 60A, (40–63 lm) silica gel, employing a mixture of
OH
TMSCN,
BF3.OEt2
CH2Cl2
HN
HN
the specified solvent as eluent. Thin-layer chromatography (TLC)
was performed on Merck silica gel (Merck 60 PF254) plates. Silica
TLC plates were visualized under UV light, by a 10% solution of
phosphomolybdic acid in ethanol followed by heating. Gas chro-
matography (CPV) was performed on HP 5890 II (detector with
ionization of flame) and with a polydimethylsiloxane column HP
HO
F3C
CN
CN
F3C
OH
(S)-8 (64%)
(R)-8 (33%)
Scheme 7. Chemoselective reduction of oxazolidine 5 and Strecker-type reaction.
ultra I (25 m ꢂ 3.2 mm ꢂ 0.52
lm thickness of layer). Mass spectra
(MS) were obtained on a GC/MS apparatus HP 5973 MSD with an
HP 6890 Series GC. Ionization was obtained by electronic impact
(EI 70 eV). Infrared spectra (IR) were obtained by Fourier-transfor-
The enantiopure (R)-a-Tfm-serine (R)-9 was obtained in 75%
yield after concentrated HCl treatment of the diastereomerically
pure (S)-8 aminonitrile and Dowex purification (Scheme 8). The
clean removal of the phenylethanol side chain and the hydrolysis
of the nitrile group occurred at the same time. Following the same
mation on BRÜCKER TENSOR 27, wavenumbers are given in cmꢁ1
.
Elemental analyses were performed on Perkin–Elmer CHN 2400.
Optical rotations were determined using a JASCO P1010 polarime-
ter. HRMS analyses were performed on a Jeol JMS-GC Mate II. Melt-
procedure, enantiopure (S)-a-Tfm-serine (S)-9 was obtained in 94%
yield from the diastereomerically pure (R)-8. The specific rotations
of (R)-9 and (S)-9 were consistent with those reported in the
literature.8a
ing points were obtained on
uncorrected.
a Büchi apparatus and are
4.2. (4R)-2-Carboethoxymethyl-2-trifluoromethyl-4-phenyl-1,3-
3. Conclusion
oxazolidines 1
In conclusion we have demonstrated that the Strecker-type
reaction from chiral fluorinated oxazolidines (Fox) derived from
(R)-phenylglycinol provides an expedient and convenient access
To a solution of ethyl 4,4,4-trifluoroacetoacetate (3.63 g,
19.7 mmol, 1 equiv) and glacial acetic acid (3.5 mL, 61.1 mmol,
3.1 equiv) in CHCl3 (60 mL) was added (R)-phenylglycinol (3.14 g,
22.9 mmol, 1.2 equiv) and the mixture was refluxed for 12 h. The
mixture was cooled down to room temperature and carefully
to enantiopure side chain functionalized
no acids such as -Tfm-aspartic acid and
a
-trifluoromethyl
a-ami-
a
a-trifluoromethyl serine.

312
J. Simon et al. / Tetrahedron: Asymmetry 22 (2011) 309–314
neutralized with a saturated solution of NaHCO3 and the reaction
mixture was stirred for 1 h. The mixture was extracted with DCM
(3 ꢂ 20 mL), and the combined organic layers were dried over
anhydrous MgSO4, and evaporated under vacuum. The crude prod-
uct could be used without further purification in the next step or
purified by silica gel chromatography (80:20 cyclohexane/ethyl
acetate) to give 1maj (2.89 g, 49%) and 1min (1.77 g, 30%) as pure
isolated compounds.
37.7, 60.6 (q, J = 30.7 Hz), 61.9, 62.4, 67.1, 114.3, 122.3 (q,
J = 286.7 Hz), 126.9, 127.5, 128.3, 139.9, 167.8. 19F NMR
(376.2 MHz, CDCl3) d ꢁ79.5 (s).
Compound (R)-2: 1H NMR (400 MHz, CDCl3) d 1.13 (t, 3H,
J = 7.3 Hz), 1.98 (t, 1H, J = 5.5 Hz), 2.70 (d, 1H, J = 15.6 Hz), 2.80
(d, 1H, J = 15.6 Hz), 3.38 (br s, 1H), 3.46–3.54 (m, 1H), 3.61–3.68
(m, 1H), 4.08 (dq, 1H, J = 7.3, 2.4 Hz), 4.12–4.20 (m, 1H), 4.24 (dd,
1H, J = 7.8, 7.8 Hz), 7.18–7.35 (m, 5H); 13C NMR (100.5 MHz, CDCl3)
d 13.7, 38.0, 59.4 (q, J = 28.8 Hz), 61.4, 62.2, 67.0, 113.8, 122.9 (q,
J = 286.6 Hz), 127.3, 128.3, 128.7, 138.8, 167.2. 19F NMR
(376.2 MHz, CDCl3) d ꢁ79.4 (s); HRMS (EI) calcd for C15H17F3N2O3
330.1191; found 330.1194.
Major diastereomer 1maj: yellow oil; Rf = 0.53 (80:20 cyclohex-
ane/ethyl acetate); ½a D27
ꢃ
¼ ꢁ20:0 (c 1.4, CH2Cl2); IR (neat): 3338,
2938, 2898, 1727, 1469, 1162 cmꢁ1
;
1H NMR (400 MHz, CDCl3) d
1.30 (t, 3H, 3J = 6.9 Hz); 2.84 (d, 1H, 2J = 15.1 Hz); 3.04 (d, 1H,
2J = 15.1 Hz); 3.69 (t, 1H, J = 7.5 Hz); 3.89 (d, 1H, 3J = 10.5 Hz);
4.22 (dq, 1H, 3J = 6.9 Hz, 2J = 13.9 Hz); 4.25 (dq, 1H, 3J = 6.9 Hz,
2J = 13.9 Hz); 4.32 (t, 1H, J = 7.5 Hz); 4.55 (ddd, 1H, 3J = 10.5 Hz,
3J = 7.5 Hz, 3J = 7.5 Hz); 7.30–7.49 (m, 5H); 13C NMR (100.5 MHz,
4.4. (3S,5R)-3-Carboethoxymethyl-3-trifluoromethyl-5-
phenylmorpholin-2-one (S)-3 and (3R,5R)-3-carboethoxy-
methyl-3-trifluoromethyl-5-phenylmorpholin-2-one (R)-3
2
CDCl3) d 14.0, 35.5, 61.5, 62.7, 74.6, 95.1 (q, JC–F = 30.7 Hz), 124.0
1
(q, JC–F = 287.5 Hz), 127.3, 128.4, 129.0, 137.0, 169.5, 19F NMR
To a solution of a 55:45 diastereomeric mixture of aminonitriles
(S)-2 and (R)-2 (6.46 g, 19.6 mmol, 1 equiv) in AcOEt (200 mL) was
added 22.4 mL of HCl concd (270 mmol, 14 equiv) at room temper-
ature then the mixture was refluxed for 24 h. The reaction was
monitored by TLC (80:20 cyclohexane/ethyl acetate eluent). The
reaction was cooled down to room temperature and carefully neu-
tralized with a saturated solution of K2CO3 and extracted with
AcOEt (3 ꢂ 60 mL), dried over MgSO4 and evaporated under vac-
uum. A fraction of the crude material (1.65 g) was then purified
by silica gel chromatography with an elution gradient (95:5–
90:10 cyclohexane/ethyl acetate) to give (S)-3 (0.55 g, 33%) and
(R)-3 (0.45 g, 27%) as pure isolated compounds.
(376.2 MHz, CDCl3) d ꢁ85.9 (s); HRMS (EI) calcd for C14H16F3NO3
303.1082, found 303.1082.
Minor diastereomer 1min: yellow oil; Rf = 0.26 (80:20 cyclohex-
ane/ethyl acetate); IR (neat): 3338, 2938, 2898, 1727, 1469,
1162 cmꢁ1 1H NMR (400 MHz, CDCl3) d 1.31 (t, 3H, 3J = 7.3 Hz);
;
2.83 (d, 1H, 2J = 14.6 Hz); 2.88 (d, 1H, 2J = 14.6 Hz); 3.67 (br s,
1H); 3.81 (t, 1H, 3J = 8.9 Hz); 4.23 (q, 2H, 3J = 6.9 Hz); 4.36 (t, 1H,
3J = 8.9 Hz); 4.72 (br s, 1H); 7.28–7.44 (m, 5H); 13C NMR
2
(100.5 MHz, CDCl3) d 14.1, 37.6, 61.1, 61.4, 75.2, 93.3 (q, JC–
1
F = 30.7 Hz), 124.3 (q, JC–F = 288.5 Hz), 126.8, 127.8, 128.5, 139.3,
168.8; 19F NMR (376.2 MHz, CDCl3) d ꢁ85.8 (s); HRMS (EI) calcd
for C14H16F3NO3 303.1082; found 303.1082.
Compound (S)-3: white solid; mp 94 °C; ½a D21
¼ þ55:3 (c 1.3,
ꢃ
CH2Cl2); Rf = 0.19 (85:15 cyclohexane/ethyl acetate); IR (neat):
4.3. Ethyl (3S)-3-cyano-4,4,4-trifluoro-3-[(1R)-2-hydroxy-1-
phenylethylamino]butanoate (S)-2 and ethyl (3R)-3-cyano-
4,4,4-trifluoro-3-[(1R)-2-hydroxy-1-phénylethyl-
amino]butanoate (R)-2
3335, 2963, 2951, 2956, 2155, 1741, 1454, 1309, 1212, 1162,
687 cmꢁ1 1H NMR (400 MHz, CDCl3) d 1.26 (t, 3H, 3J = 7.1 Hz),
;
2.33 (br s, 1H), 2.60 (d, 1H, 2J = 16.2 Hz), 3.39 (d, 1H, 2J = 16.2 Hz),
4.14–4.24 (m, 2H), 4.26 (dd, 1H, 3J = 3.0 Hz, 2J = 10.2 Hz), 4.44
2
(dd, 1H,
J = 10.2 Hz 3J = 11.0 Hz), 4.52 (dd, 1H, 3J = 11.0 Hz,
To
a
solution of ethyl 4,4,4-trifluoroacetoacetate (3.63 g,
3J = 3.0 Hz), 7.28–7.32 (m, 5H); 13C NMR (100.5 MHz, CDCl3) d
3
1
19.7 mmol, 1 equiv) and glacial acetic acid (3.5 mL, 61.1 mmol,
3.1 equiv) in CHCl3 (60 mL) was added (R)-phenylglycinol (3.14 g,
22.9 mmol, 1.2 equiv). The mixture was refluxed for 12 h. The mix-
ture was cooled down to room temperature and carefully neutral-
ized with a saturated solution of NaHCO3 and the reaction mixture
was stirred for 1 h. The mixture was then extracted with DCM
(3 ꢂ 20 mL), and the combined organic layers were dried over
anhydrous MgSO4 and evaporated under vacuum. The crude mix-
ture of oxazolidines 1 (6.06 g) was used in the next step without
further purification. To a solution of the crude mixture of oxazoli-
dines 1 (6.06 g) in DCM (60 mL) at room temperature under an ar-
gon atmosphere were added dropwise trimethylsilyl cyanide
(3.7 mL, 29.5 mmol, 1.5 equiv) and BF3ꢀOEt2 (3.75 mL, 29.5 mmol,
1.5 equiv). The mixture was stirred until consumption of the start-
ing material (20 h). The mixture was carefully neutralized with a
saturated solution of K2CO3 and the reaction mixture was stirred
for 1 h, until no gas evolution was observed. Then the mixture
was extracted with DCM (3 ꢂ 20 mL), and the combined organic
layers were dried over anhydrous MgSO4 and evaporated under vac-
uum. The crude product was purified by silica gel chromatography
(90:10 cyclohexane/ethyl acetate) to give a 55:45 diastereomeric
mixture of aminonitriles (S)-2 and (R)-2 (5 g, 15.13 mmol, 77%).
Compound (S)-2: yellow oil; Rf = 0.46 (80:20 cyclohexane/ethyl
14.1, 40.3, 52.9, 61.6, 63.4 (q, JC–F = 25.9 Hz), 74.2, 124.9 (q, JC–
F = 291.4 Hz), 127.2, 128.9, 129.1, 136.6, 163.8, 168.9. 19F NMR
(376.2 MHz, CDCl3) d ꢁ76.9 (s). HRMS (EI) calcd for C15H16F3NO4
331.1031, found 331.1038.
Compound (R)-3: white solid; mp 126 °C; ½a D21
¼ ꢁ24 (c 0.5,
ꢃ
CHCl3); Rf = 0.14 (85:15 cyclohexane/ethyl acetate), IR (neat):
3362, 3000, 2160, 1740, 1727, 1297, 1253, 1204, 1179, 1032,
766, 703, 679 cmꢁ1 1H NMR (400 MHz, CDCl3) d 1.28 (t, 3H,
;
3J = 7.1 Hz), 2.12 (br s, 1H), 2.71 (d, 1H, 2J = 17.2 Hz), 3.55 (d,
2J = 17.2 Hz), 4.15–4.25 (m, 2H); 4.34 (dd, 1H, 3J = 2.8 Hz,
2J = 10.8 Hz), 4.41 (dd, 1H, 2J = 10.8 Hz, 3J = 10.8 Hz), 4.80 (d, 1H,
3J = 10.8 Hz), 7.33–7.49 (m, 5H). 13C NMR (100.5 MHz, CDCl3) d
3
14.0, 39.8, 54.6, 61.7, 63.6 (q, JC–F = 26.8 Hz), 74.0, 124.1 (q,
1JC–F = 284.7 Hz), 127.3, 128.8, 137.3, 164.8, 169.3. 19F NMR
(376.2 MHz, CDCl3) d ꢁ80.6 (s). HRMS (EI) calcd for C15H16F3NO4
331.1031, found 331.1032.
4.5. (S)-a-Trifluoromethylaspartic acid hydrochloride (S)-4 and
(R)- -trifluoromethylaspartic acid hydrochloride (R)-4
a
The morpholinone (S)-3 (0.226 g, 0.68 mmol) was dissolved in
concentrated HCl (10 mL). The heterogeneous solution was re-
fluxed for 12 h. The acidic aqueous phase was washed with Et2O
(2 ꢂ 10 mL) and evaporated under vacuum to give analytically
pure (S)-4 (0.118 g, 83%).
acetate), IR (neat): 3456, 3374, 2938, 2930, 2160, 1712 cmꢁ1 1H
;
NMR (400 MHz, CDCl3) d 1.27 (t, 3H, J = 7.3 Hz), 1.93 (t, 1H,
J = 6.4 Hz), 2.86 (d, 1H, J = 16.5 Hz), 3.03 (d, 1H, J = 16.5 Hz), 3.38
(br s, 1H), 3.55 (ddd, 1H, J = 11.6, 6.4, 6.4 Hz), 3.85 (ddd, 1H,
J = 11.6, 6.4, 4.1 Hz), 4.14–4.21 (m, 2H); 4.24 (dd, 1H, 3J = 6.4,
6.4 Hz), 7.15–7.33 (m, 5H). 13C NMR (100.5 MHz, CDCl3) d 13.9,
Spectroscopic data of (S)-4 are in accordance with the literature7
reported data: white solid; ½a D21
ꢃ
¼ þ25 (c 0.25, H2O); 1H NMR
(400 MHz, D2O) d 3.01 (d, 1H, J = 18.1 Hz), 3.35 (d, 1H, J = 18.1 Hz).
13C NMR (100.3 MHz, D2O) d 34.3, 62.8 (q, J = 28.8 Hz, C–CF3),

J. Simon et al. / Tetrahedron: Asymmetry 22 (2011) 309–314
313
122.4 (q, J = 283.7, CF3), 165.6, 171.5. 13C NMR (100.5 MHz, CDCl3) d
34.3, 62.8 (q, J = 28.8 Hz, C–CF3), 122.4 (q, J = 283.7, CF3), 165.6,
171.5. 19F NMR (376.2 MHz, CDCl3) d ꢁ78.4 (s).
the pure minor diastereomer 7min (2.35 g, 29%) was obtained as a
colorless oil after evaporation of the filtrate.
7maj: white solid; mp 156 °C; Rf = 0.40 (80:20 cyclohexane/ethyl
Following the same procedure, the enantiomer (R)-4 (0.119 g,
86%) was obtained from the morpholinone (R)-3 (0.220 g,
acetate); ½a 2D1
ꢃ
¼ þ0:3 (c 1.0, CH3OH); IR (neat): 3277 cmꢁ1
;
1H
NMR (400 MHz, CDCl3) d 3.79 (dd, J = 7.4, 6.6 Hz, 1H), 3.79 (d,
J = 12.1 Hz, 1H), 4.00 (d, J = 12.1 Hz, 1H), 4.42 (t, J = 7.4 Hz, 1H),
4.61 (dd, J = 7.4, 6.6 Hz, 1H), 7.27–7.38 (m, 5H); 13C NMR
(100.5 MHz, CDCl3) d 60.5, 62.2, 75.0, 96.5 (q, J = 31.3 Hz), 124.0
(q, J = 287.2 Hz), 126.8, 128.5, 129.0, 137.8; 19F NMR (376.2 MHz,
CDCl3) d ꢁ84.0 (s). Anal. Calcd for C11H12F3NO2 (247.21): C,
53.44; H, 4.89; N, 5.67. Found: C, 53.44; H, 5.13; N, 5.62.
0.66 mmol). (R)-4: ½a D25
¼ ꢁ24:6 (c 0.3, H2O). The other spectro-
ꢃ
scopic data of (R)-4 were similar to those of (S)-4.
4.6. Ethyl 2-[(1R)-2-hydroxy-1-phenylethylamino]-2-cyano-2-
trifluoromethylacetate 6
To a solution of oxazolidine 5 (7.313 g, 25.3 mmol, 1.0 equiv) in
dichloromethane (85 mL) at 0 °C under an argon atmosphere were
successively added TMSCN (5.08 mL, 37.95 mmol, 1.5 equiv) and
BF3ꢀEt2O (4.48 mL, 37.95 mmol, 1.5 equiv). After stirring at room
temperature overnight, the mixture was poured into a saturated
NaHCO3 aqueous solution (250 mL) and stirred vigorously for 1 h.
The layers were separated and the aqueous layer was extracted
with dichloromethane (3 ꢂ 20 mL). The combined organic extracts
were washed successively with water (2 ꢂ 20 mL), brine (20 mL),
dried over MgSO4, filtered and concentrated under reduced pres-
sure. The crude reaction mixture was purified by flash chromatog-
raphy (85:15 cyclohexane/ethyl acetate) to afford 7.65 g (96%) of
aminonitrile 6 as a 60:40 mixture of diastereomers. Starting from
the oxazolidine 5 (7.12 g, 24.6 mmol, 1.0 equiv), the reaction was
performed in similar manner and the two diastereomers were sep-
arated by flash chromatography (85:15 cyclohexane/ethyl acetate)
to afford 4.67 g (47%) of the major diastereomer 6maj and 3.11 g
7min: colorless oil; Rf = 0.27 (80:20 cyclohexane/ethyl acetate);
½
a 2D1
ꢃ
¼ ꢁ36 (c 1.0, CH3OH); IR (neat): 3227 cmꢁ1
;
1H NMR
(400 MHz, CDCl3) d 3.74 (d, J = 12.4 Hz, 1H), 3.86 (dd, J = 9.5,
8.0 Hz, 1H), 3.91 (d, J = 12.4 Hz, 1H), 4.44 (dd, J = 8.0, 7.3 Hz, 1H),
4.71 (dd, J = 9.5, 7.3 Hz, 1H), 7.25–7.50 (m, 5H); 13C NMR
(100.5 MHz, CDCl3) d 61.9, 62.0, 76.7, 94.9 (q, J = 29.7 Hz), 124.0
(q, J = 288.5 Hz), 126.9, 128.5, 129.1, 139.0; 19F NMR (376.2 MHz,
CDCl3) d ꢁ83.8 (s). Anal. Calcd for C11H12F3NO2 (247.21): C,
53.44; H, 4.89; N, 5.67. Found: C, 53.50; H, 4.94; N, 5.62.
4.8. 2-Trifluoro-3-hydroxy-2-[(1R)-2-hydroxy-1-phenyl-amino]
propanenitrile 8
To a 71:29 dr mixture of hydroxymethyl oxazolidines 7 (2.0 g,
8.1 mmol, 1.0 equiv) in dichloromethane (25 mL) at 0 °C under an
argon atmosphere were successively added TMSCN (1.1 mL,
8.1 mmol, 1.0 equiv) and BF3ꢀOEt2 (1.0 mL, 8.1 mmol, 1.0 equiv).
The corresponding mixture was stirred at room temperature for
30 min and three other additions of TMSCN (0.5 mL, 4 mmol,
0.5 equiv each) and BF3ꢀOEt2 (0.53 mL, 4 mmol, 0.5 equiv each)
were successively achieved in 30 min intervals. The reaction mix-
ture was stirred at room temperature overnight, poured into a sat-
urated NaHCO3 aqueous solution (100 mL) and stirred vigorously
for 1 h. The layers were separated and the aqueous layer was ex-
tracted with dichloromethane (3 ꢂ 20 mL). The combined organic
extracts were washed with brine (20 mL), dried over MgSO4, fil-
tered, and concentrated under reduced pressure. Purification by
flash chromatography (80:20 cyclohexane/ethyl acetate) gave
1.03 g (46%) of a pure fraction of the (S)-8, 510 mg (23%) of a mix-
ture of both diastereomer and 630 mg (28%) of a pure fraction of
the minor diastereomer (R)-8.
(32%) of the minor diastereomer 6min
.
6maj: yellow solid; mp 77 °C; Rf = 0.37 (80:20 cyclohexane/ethyl
acetate); ½a 2D5
¼ ꢁ72:6 (c 0.98, CHCl3); IR (neat): 3562, 3336, 2359,
ꢃ
1739 cmꢁ1; 1H NMR (400 MHz, CDCl3) d 1.00 (t, J = 7.1 Hz, 3H), 2.30
(s, 1H), 3.11 (d, J = 7.4 Hz, 1H), 3.45–3.95 (m, 4H), 4.13 (dt, J = 7.7,
4.7 Hz, 1H), 7.10–7.40 (m, 5H); 13C NMR (100.5 MHz, CDCl3) d
13.3, 62.0, 64.8, 65.6 (q, J = 31.3 Hz), 66.3, 111.2, 121.2 (q,
J = 287.2 Hz), 127.8, 128.5, 128.6, 137.4, 161.6; 19F NMR
(376.2 MHz, CDCl3) d ꢁ77.6 (s). Anal. Calcd for C14H15F3N2O3
(316.27): C, 53.17; H, 4.78; N, 8.86. Found: C, 53.51; H, 5.00; N, 8.89.
6min: yellow solid; mp 46 °C; Rf = 0.21 (80:20 cyclohexane/ethyl
acetate); ½a 2D6
¼ ꢁ96:1 (c 1.09, CHCl3); IR (neat): 3562, 3336, 2359,
ꢃ
1745 cmꢁ1; 1H NMR (400 MHz, CDCl3) d 1.25 (t, J = 7.1 Hz, 3H), 2.20
(s, 1H), 3.20 (s, 1H), 3.50 (dd, J = 11.3, 8.6 Hz, 1H), 3.60 (dd, J = 11.3,
4.1 Hz, 1H), 4.00 (dd, J = 8.6, 4.1 Hz, 1H), 4.20–4.45 (m, 2H), 7.10–
7.45 (m, 5H); 13C NMR (100.5 MHz, CDCl3) d 13.7, 61.7, 65.2, 65.4
(q, J = 31.3 Hz), 67.0, 111.2, 121.0 (q, J = 287.7 Hz), 127.2, 128.2,
128.6, 138.5, 162.2; 19F NMR (376.2 MHz, CDCl3) d ꢁ77.3 (s). Anal.
Calcd for C14H15F3N2O3 (316.27): C, 53.17; H, 4.78; N, 8.86. Found:
C, 53.08; H, 4.70; N, 8.98.
Compound (S)-8: white solid; mp 59 °C; Rf = 0.41 (60:40 cyclo-
hexane/ethyl acetate); ½a D21
¼ ꢁ77:4 (c 1.0, CHCl3); IR (neat): 3228,
ꢃ
3198, 2350 cmꢁ1; 1H NMR (400 MHz, CDCl3) d 2.60 (br s, 3H), 3.52
(dd, J = 12.2, 8.8 Hz, 1H), 3.75 (m, 3H), 4.15 (dd, J = 8.8, 4.3 Hz, 1H),
7.29–7.39 (m, 5H); 13C NMR (100.5 MHz, CDCl3) d 61.1, 62.3, 63.7
(q, J = 28.7 Hz), 66.9, 114.2, 125.0 (q, J = 285.6 Hz), 127.1, 128.6,
129.1, 139.1; 19F NMR (376.2 MHz, CDCl3) d ꢁ77.63 (s). Anal. Calcd
for C12H13F3N2O2 (274.24): C, 52.56; H, 4.78; N, 10.21. Found: C,
52.38; H, 4.69; N, 10.21.
4.7. (4R)-2-Hydroxymethyl-2-trifluoromethyl-4-phenyl-1,3-
oxazolidine 7
Compound (R)-8: white solid; mp 78 °C; Rf = 0.32 (60:40 cyclo-
To a 71:29 dr mixture of oxazolidine 5 (9.45 g, 32.7 mmol,
1.0 equiv) in methanol (142 mL) under an argon atmosphere at
0 °C was added NaBH4 (1.24 g, 32.7 mmol, 1.0 equiv). The reaction
was stirred for 1 h at room temperature, and then quenched with
saturated NH4Cl aqueous solution (100 mL). Methanol was re-
moved under reduced pressure and the corresponding aqueous
solution was taken up with ethyl acetate (30 mL). The layers were
separated and the aqueous layer was extracted with ethyl acetate
(2 ꢂ 30 mL). The combined organic extracts were washed with
brine (20 mL), dried over MgSO4, filtered and concentrated under
reduced pressure. After filtration through a short pad of silica gel,
the reduced oxazolidine 7 was obtained as a 71:29 diastereoiso-
meric mixture. Precipitation in pentane followed by filtration gave
the pure major diastereomer 7maj (5.50 g, 68%) as a white solid and
hexane/ethyl acetate); ½a D21
¼ ꢁ83:3 (c 1.0, CHCl3); IR (neat): 3345,
ꢃ
3250, 2350 cmꢁ1 1H NMR (400 MHz, CDCl3) d 3.00 (br s, 3H), 3.60
;
(dd, J = 11.2, 8.7 Hz, 1H), 3.86 (dd, J = 11.2, 4.0 Hz, 1H), 3.93 (d,
J = 11.7 Hz, 1H), 4.17 (d, J = 11.7 Hz, 1H), 4.20 (dd, J = 8.7, 4.0 Hz,
1H), 7.27–7.39 (m, 5H); 13C NMR (100.5 MHz, CDCl3) d 61.5, 62.7,
64.7 (q, J = 34.5 Hz), 67.3, 99.9, 122.0 (q, J = 268.4 Hz), 127.0,
128.0, 128.7, 139.5; 19F NMR (376.2 MHz, CDCl3) d ꢁ77.56 (s). Anal.
Calcd for C12H13F3N2O2 (274.24): C, 52.56; H, 4.78; N, 10.21. Found:
C, 52.38; H, 5.04; N, 9.87.
4.9. (R)-a-Trifluoromethylserine (R)-9
A solution of aminonitrile (S)-8 (1.46 g, 5.3 mmol, 1.0 equiv) in
concentrated HCl (38 mL) was heated at reflux overnight, and then

314
J. Simon et al. / Tetrahedron: Asymmetry 22 (2011) 309–314
3. For a review, see: (a) Qiu, X.-L.; Meng, W.-D.; Qing, F.-L. Tetrahedron 2004, 60,
6711–6745. and references cited therein; (b) Brigaud, T.; Chaume, G.;
Pytkowicz, J.; Huguenot, F. Chim. Oggi. Chem. Today 2007, 25, 8–10. and
references cited therein; (c) Smits, R.; Cadicamo, C. D.; Burger, K.; Koksch, B.
Chem. Soc. Rev. 2008, 37, 1727–1739. and references cited therein; For an
enzymatic resolution, see: (d) Koksch, B.; Quaedflieg, P. J. L. M.; Michel, T.;
Burger, K.; Broxterman, Q. B.; Schoemaker, H. E. Tetrahedron: Asymmetry 2004,
15, 1401–1407.
4. (a) Lebouvier, N.; Laroche, C.; Huguenot, F.; Brigaud, T. Tetrahedron Lett. 2002,
43, 2827–2830; (b) Huguenot, F.; Brigaud, T. J. Org. Chem. 2006, 71, 7075–7078;
(c) Chaume, G.; Van Severen, M.-C.; Marinkovic, S.; Brigaud, T. Org. Lett. 2006, 8,
6123–6126; (d) Chaume, G.; Van Severen, M.-C.; Ricard, L.; Brigaud, T. J.
Fluorine Chem. 2008, 129, 1104–1109; (e) Caupène, C.; Chaume, G.; Ricard, L.;
Brigaud, T. Org. Lett. 2009, 11, 209–212.
concentrated under reduced pressure. The crude mixture was
washed with diethyl ether to give, after filtration, the (R)-( )-triflu-
a
oromethylserine hydrochloride as a white solid. mp degradation
over 200 °C; ½a 2D1
ꢃ
¼ ꢁ2:5 (c 1.0, MeOH); 1H NMR (400 MHz,
CD3OD) d 4.04 (d, J = 11.7 Hz, 1H), 4.19 (d, J = 11.7 Hz, 1H); 13C
NMR (100.5 MHz, CD3OD) d 65.7, 78.0 (q, J = 32.6 Hz), 125.5 (q,
J = 283.7 Hz), 164.0; 19F NMR (376.2 MHz, CD3OD) d ꢁ75.0 (s);
HRMS (EI): C4H7ClF3NO3: 209.0067; found: 209.0069.
The crude hydrochloride ammonium salt of (R)-(
ethylserine was then loaded onto DOWEX 50W8-400 resin to af-
ford 636 mg (75%) of the pure (R)- -Tfm-serine (R)-9. White
a)-trifluorom-
a
5. Chaume, G.; Lensen, N.; Caupène, C.; Brigaud, T. Eur. J. Org. Chem. 2009, 5717–
5724.
solid; mp degradation over 200 °C; Rf = 0.70 (50:50 dichlorometh-
ane/methanol); ½a D23
ꢃ
¼ þ10:6 (c 1, H2O); {lit. ½a D20
¼ þ11:3 (c 0.77,
ꢃ
6. (a) Soloshonok, V. A.; Gerus, I. I.; Yagupolskii, Y. L.; Kukhar, V. P. Zh. Org. Khim.
1987, 23, 2308–2313; (b) Kobzev, S. P.; Soloshonok, V. A.; Galushko, S. V.;
Yagupolskii, Y. L.; Kukhar, V. P. Zh. Obshch. Khim. 1989, 59, 909–912; (c) Burger,
K.; Gaa, K.; Geith, K. J. Fluorine Chem. 1988, 41, 429–433; (d) Osipov, S. N.;
Kolomiets, A. F.; Fokin, A. Izv. Akad. Nauk SSSR, Ser. Khim. 1989, 2131–2134; (e)
Sewald, N.; Riede, J.; Bissinger, P.; Burger, K. J. Chem. Soc., Perkin Trans. 1 1992,
267–274; (f) Laurent, P.; Henning, L.; Burger, K.; Hiller, W.; Neumayer, M.
Synthesis 1998, 905–909; (g) Dal Pozzo, A.; Dikovskaya, K.; Moroni, M.; Fagnoni,
M.; Vanini, L.; Bergonzi, R.; De Castiglione, R.; Bravo, P.; Zanda, M. J. Chem. Res.,
Synop. 1999, 468–469.
7. (a) Lazzaro, F.; Crucianelli, M.; De Angelis, F.; Frigerio, M.; Malpezzi, L.;
Volonterio, A.; Zanda, M. Tetrahedron: Asymmetry 2004, 15, 889–893; (b)
Lazzaro, F.; Gissot, A.; Crucianelli, M.; De Angelis, F.; Bruché, L.; Zanda, M. Lett.
Org. Chem. 2005, 2, 235–237.
8. (a) Bravo, P.; Viani, F.; Zanda, M.; Soloshonok, V. A. Gazz. Chim. Ital. 1995, 125,
149–150; (b) Sorochinsky, A. E.; Soloshonok, V. A. J. Fluorine Chem. 2010, 131,
127–139.
H2O)};8a 1H NMR (400 MHz, D2O) d 3.00 (br s, 3H); 13C NMR
(100.5 MHz, D2O)
d 59.7, 66.4 (q, J = 25.9 Hz), 122.8 (q,
J = 283.6 Hz), 165.8; 19F NMR (376.2 MHz, D2O) d ꢁ75.2 (s).
4.10. (S)-a-Trifluoromethylserine (S)-9
A solution of aminonitrile (R)-8 (935 mg, 3.4 mmol, 1.0 equiv) in
concentrated HCl (24 mL) was warmed to reflux overnight, then
concentrated under reduced pressure. The crude hydrochloride
ammonium salt of (a)-trifluoromethylserine was then washed
with diethyl ether and then loaded onto DOWEX 50W8-400 to af-
ford 544 mg (94%) of the pure (S)-
a
-Tfm-serine (S)-9. ½a D28
¼ ꢁ10:4
ꢃ
(c 0.85, H2O); {lit. ½a D20
ꢃ
¼ ꢁ9:1 (c 0.59, H2O)}.3 The other spectro-
9. Ishida, Y.; Iwahashi, N.; Nishizono, N.; Saigo, K. Tetrahedron Lett. 2009, 50,
1889–1892.
10. The configuration of each diastereomer was not determined.
11. Agami, C.; Couty, F.; Mathieu, H. Tetrahedron Lett. 1996, 37, 4001–4002.
12. The polarimetric analyses of (S)-4 and (R)-4 were consistent with the literature
data.7
scopic data of (S)-9 were similar to those of (R)-9.
Acknowledgments
13. The oxazolidine 5 was obtained as a 75:25 diastereomeric mixture according to
We thank Central Glass Co. for financial support and the gift of
ethyl trifluoropyruvate. We also thank the French Fluorine Network.
the reported procedures.4d,e
14. The 60:40 diastereomeric mixture of cyanoester 6 can be separated to give 6maj
(47%) and 6min (32%) as pure isolated compounds.
15. For the chemoselective reduction of an ester: (a) Badorrey, R.; Cativiela, C.;
Diaz-de-Viellegas, M.; Galvez, J.; Gil, A. Tetrahedron: Asymmetry 2003, 14,
2209–2214; (b) Collins, C.; Fisher, G.; Reen, A.; Goraslski, C.; Singaran, B.
Tetrahedron Lett. 1997, 38, 529–532.
16. Pytkowicz, J.; Stephany, O.; Marinkovic, S.; Inagaki, S.; Brigaud, T. Org. Biomol.
Chem. 2010, 8, 4540–4542.
References
1. (a) Soloshonok, V. A., Ed. Fluorine-containing Amino acids and Peptides:
Fluorinated Synthons for Life Sciences. In Fluorine-containing Synthons ACS
Symposium Series 911; American Chemical Society: Washington, 2005.;
(b) Bioorganic and Medicinal Chemistry of Fluorine; Begue, J.-P., Bonnet-
Delpon, D., Eds.; Wiley: New York, 2008; (c) Fluorine in Medicinal Chemistry
and Chemical Biology; Ojima, I., Ed.; Wiley-Blackwell: New York, 2009.
2. Koksch, B.; Sewald, N.; Hofmann, H.-J.; Burger, K.; Jakubke, H.-D. J. Pept. Sci.
1997, 3, 157–167.
17. With cyclohexane/ethyl acetate 60:40 eluent system: Rf for (R)-7 = 0.41 and Rf
for (S)-7 = 0.32.