An efficient access to both enantiomers of pipecolic acid

Article abstract of DOI:10.1016/j.tetasy.2005.01.017

An efficient and convenient synthesis of both enantiomers of pipecolic acid has been developed using the intramolecular cyclization of 2-amino-6- bromohexanoic acid under mild conditions.

Full text of DOI:10.1016/j.tetasy.2005.01.017

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 903–908
An efficient access to both enantiomers of pipecolic acid
Louis A. Watanabe,a Saori Haranaka,b Binoy Jose,c Minoru Yoshida,c,d Tamaki Kato,b
Mitsuaki Moriguchi,e Kenji Sodaf and Norikazu Nishinob,c,
*
aFaculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
bGraduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan
cCREST Research Project, Japan Science and Technology Agency, Saitama 332-001, Japan
dRIKEN, Saitama 351-0198, Japan
eFaculty of Engineering, Oita University, Oita 870-1192, Japan
fFaculty of Engineering, Kansai University, Suita, Osaka-fu 564-8680, Japan
Received 14 December 2004; accepted 5 January 2005
Abstract—An efficient and convenient synthesis of both enantiomers of pipecolic acid has been developed using the intramolecular
cyclization of 2-amino-6-bromohexanoic acid under mild conditions.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
development of synthetic histone deacetylase inhibi-
tors,13 based on natural products such as Cyl-2, we
found that a convenient route to enantiomerically pure
pipecolic acids will be helpful. The great importance of
1 has fostered the development of many synthetic
approaches involving enzymatic reactions,14 alkylation of
chiral glycine enolates,15 derivatization of natural amino
acids,16 enantioselective reactions,17 and resolution.18
However, most of these methods have some limitations,
such as tedious procedures, low yields, unavailability of
starting materials etc. Therefore new and convenient
methods for the preparation of optically active pipecolic
acid are still required. We recently reported the synthesis
of a-amino-x-bromoalkanoic acid for side-chain modifi-
cations.19 During the course of that work we explored a
convenient route to L- and D-pipecolic acids.
Pipecolic acid 1, a widespread natural non-proteino-
genic amino acid, is an attractive synthetic target be-
cause it is a key constituent of many synthetic and
natural bioactive molecules and useful building blocks
for the preparation of peptides and peptide mimetics.1
For example, biologically important natural products
such as immunosuppressant FK506,2 anesthetic bupiva-
caine,3 anticancer agent VX710,4 oxytoxin antagonist L-
365209,5 antifungal antibiotic demethoxyrapamycin,6
antitumour antibiotic sandramycin,7 phytotoxic metab-
olite Cyl-28 and antiprotozoal agent as well as histone
deacetylase inhibitor apicidin,9 trapoxins10 etc. contain
a pipecolic acid moiety. (2R,4R)-Methylpipecolic acid
2 is a key component for the preparation of a highly
selective thrombin inhibitor.11 The replacement of pro-
teinogenic amino acids with cyclic imino acids has been
carried out in structure–activity relationship studies and
in search of new peptidomimetics that have improved
pharmacological profiles.12
N
H
COOH
N
H
COOH
All these factors have contributed to the growing inter-
est in finding a convenient and efficient synthetic route
to pipecolic acid and related imino acids in high enantio-
meric excess. Over the course of a project directed to the
2
1
2. Results and discussion
*
The synthesis of pipecolic acid started with commer-
cially available diethyl acetamidomalonate 3 and
Corresponding author. Tel./fax: +81 93 695 6061; e-mail:
nishino@life.kyutech.ac.jp
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.01.017

904
L. A. Watanabe et al. / Tetrahedron: Asymmetry 16 (2005) 903–908
dibromobutane. An excess of dibromobutane was re-
acted with 3 in the presence of sodium ethoxide in abso-
lute ethanol. One equivalent of 1 M NaOH was added at
0–5 ꢁC to the reaction mixture and we obtained the cor-
responding monoacid monoester as a white solid after
work-up. It was then decarboxylated by refluxing in tolu-
ene. After evaporation of toluene, the resulting acetyl-
DL-2-amino-6-bromohexanoic acid ethyl ester as an oil
was dissolved in ethanol and hydrolyzed using NaOH
at 0–5 ꢁC to yield acetyl-DL-2-amino-6-bromohexanoic
acid 4 as a white powder in a quantitative yield after
work-up. Aspergillus genus aminoacylase (TCI) was
applied to resolve the racemic mixture. During the res-
olution we observed spontaneous intramolecular
cyclization of L-2-amino-6-bromohexanoic acid to L-
pipecolic acid 1a. The reaction mixture was concen-
trated and then acidified with 1 M HCl to pH 3. The
unreacted acetyl-D-2-amino-6-bromohexanoic acid
(Ac-D-Ab6) was then extracted with ethyl acetate. The
aqueous layer was applied to a column of ion-exchange
resin (DOWEX 50W-X4) and L-pipecolic acid was
eluted with 1 M ammonia. After solvent evaporation,
L-pipecolic acid was obtained as a white solid and was
recrystallized from acetone. Recovered Ac-D-Ab6 was
treated with Alcaligenes xylosoxydans D-aminoacylase20
to yield D-pipecolic acid 1b (Scheme 1). In another run,
D-pipecolic acid was obtained by the resolution of 4
using the D-aminoacylase. Yamaguchi and Ueki re-
ported the cyclization of N-protected 2-amino-5-bromo-
pentanoic acid ester to the corresponding proline
derivatives using sodium hydride in THF.21 They de-
rived glutamic acid to N-protected 2-amino-5-bromo-
pentanoic acid ester. However, to the best of our
knowledge, there is no cyclization procedure reported
for the synthesis of pipecolic acid from 2-amino bromo-
hexanoic acid.
Even though the synthetic scheme for the preparation of
pipecolic acid is simple, the extraction process is tedious
and time consuming. For this reason, we have carried
out the reaction with a different route, starting from
diethyl Boc-aminomalonate 5, as shown in Scheme 2.
The initial steps are the same as described earlier. After
refluxing in toluene, the resulting Boc-DL-amino-6-
bromohexanoic acid ethyl ester (Boc-DL-Ab6-OEt) 6
was resolved to Boc-L-amino-6-bromohexanoic acid 8
using subtilisin Carlsberg from Bacillus licheniformis
(Sigma) in a mixture of DMF and water (1/3, v/v) at
pH 8. The pH was maintained at 8 by the addition of
1 M ammonia and the reaction was completed in 3 h.
Then the unreacted Boc-D-Ab6-OEt 7 was removed by
ether extraction. Compound 8 was extracted with ethyl
acetate at pH 3 using citric acid. The Boc group of 8
was removed using 2 M HCl in dioxane to give corre-
sponding amino acid hydrochloride. It was then cyclized
to L-pipecolic acid in methanol by the addition of trieth-
ylamine. 7, which was recovered after resolution, was
then hydrolyzed using dilute NaOH. It was then pro-
cessed as mentioned above to yield D-pipecolic acid.
Thus, both enantiomers of pipecolic acid were prepared
with high enantiomeric purity in high overall yield (27%)
in five steps.
Br
+
1,2,3,4)
COOEt
COOEt
5) or 6)
OH
OH
N
H
N
H
Ac
N
H
COOH
Ac
N
H
O
O
3
4
1a
1b
40%
35%
Scheme 1. Reagents and conditions: (1) (i) EtOH, sodium ethoxide, reflux, 30 min; (ii) 1,4-dibromobutane, reflux, 5 h, 73%; (2) NaOH aq, EtOH,
0 ꢁC, 3 h, 85%; (3) toluene, reflux, 3 h, 97%; (4) NaOH aq, EtOH, 0 ꢁC, 3 h, 81%; (5) L-aminoacylase, CoCl2Æ6H2O, H2O, pH 7, 38 ꢁC, 24 h; (6) D-
aminoacylase, H2O, pH 7, 38 ꢁC, 3 days.
Br
Br
Br
4)
7)
1,2,3)
COOEt
COOEt
+
Boc
N
H
Boc
N
H
COOH
Boc
N
H
COOEt
Boc
N
H
COOEt
5
6
7
8
47%
45%
6)
5)
Br
OH
OH
N
N
H
H
O
Boc
N
H
COOH
O
1b
9
1a
94%
100%
95%
Scheme 2. Reagents and conditions: (1) (i) EtOH, sodium ethoxide, reflux, 30 min; (ii) 1,4-dibromobutane, reflux, 5 h, 92%; (2) NaOH aq, EtOH,
0 ꢁC, 3 h, 80%; (3) toluene, reflux, 3 h, 85%; (4) subtilisin, H2O/DMF = 3/1, pH 8, 38 ꢁC, 3 h; (5) (i) 2 M HCl/dioxane, 0 ꢁC, 2 h; (ii) Et3N, DMF,
pH 8, 0 ꢁC, 5 h; (6) NaOH aq, EtOH, 0 ꢁC, 3 h; (7) (i) 2 M HCl/dioxane, 0 ꢁC, 2 h; (ii) Et3N, DMF, pH 8, 0 ꢁC, 5 h.

L. A. Watanabe et al. / Tetrahedron: Asymmetry 16 (2005) 903–908
905
Br
Br
1,2,3)
4)
5)
COOEt
COOEt
Boc
N
H
H2N
COOH
X
12
Boc
N
H
COOH
Boc HN
COOEt
5
OH
10
11
N
H
O
13
Scheme 3. Reagents and conditions: (1) (i) EtOH, sodium ethoxide, reflux, 30 min; (ii) 1,4-dibromopentane, reflux, 3 h; (2) NaOH aq, EtOH, 0 ꢁC,
3 h, 65%; (3) toluene, reflux, 3 h, 89%; (4) subtilisin, H2O/DMF 3:1, pH 8, 38 ꢁC, 3 h, 40%; (5) (i) 2 M HCl/dioxane, 0 ꢁC, 2 h (ii) Et3N, MeOH, pH 8,
0 ꢁC, 1 h, 44%.
To examine the effect of a substituent at the 6-position
on cyclization reaction, we proposed to attach a methyl
group at the 6-position. We reasoned that the presence
of a bulky methyl group at the 6-position could hinder
the cyclization reaction or it could allow the formation
of one enantiomer of pipecolic acid. We synthesized
Boc-L-2-amino-6-bromoheptanoic acid 11 from 5 and
1,4-dibromopentane as shown in Scheme 3. After resolu-
tion using subtilisin, the unreacted D-compound was re-
moved by extraction with ether. The L-compound was
extracted using ethyl acetate under acidic conditions.
After removal of the solvent, the Boc group was depro-
tected using 2 M HCl in dioxane. Dioxane was evapo-
rated to give a white solid, which was then dissolved
in DMF and two equivalent of triethylamine added.
Formation of a new product was observed, but the reac-
tion did not proceed further after 6 h. Another batch
reaction was carried out in methanol using two equiva-
lents of triethylamine. Similar results were obtained in
6 h and even under refluxing conditions no further
changes were observed. The product was protected with
a Boc group and benzyl group and purified by column
chromatography. 1H, 13C and COSY NMR spectra
showed that the product formed was 2-amino-6-hepte-
noic acid 12. Thus, it is clear that the presence of a bulky
methyl group at the 6-position of 2-amino-6-bromo-
hexanoic acid hinders the formation of methyl pipecolic
acid 13.
eluting with solvents as indicated. All compounds were
routinely checked by thin layer chromatography
(TLC) or high performance liquid chromatography
(HPLC). TLC was performed on aluminum-backed sil-
ica gel plates (Merck DC-Alufolien Kieselgel 60 F254
)
with spots visualized by UV light or by heating the plate.
Analytical HPLC were performed on a Hitachi instru-
ment using: column I; chromolith performance RP-18e
column (4.6 · 50 mm, Merck), the mobile phases used
were A: H2O with 10% CH3CN and 0.1% TFA, B:
CH3CN with 0.1% TFA using a solvent gradient of A
to B over 15 min with a flow rate of 2 mL/min, detection
at 220 nm; column II; MCI GEL CRS10W
(4.6 · 100 mm, Mitsubishi Chemical Corporation), the
mobile phases used were 1.0 mM CuSO4 with a flow rate
of 0.2 mL/min, detection at 254 nm; column III; CHI-
RALCEL OD (4.6 · 250 mm, Daicel Chemical Indus-
tries, Ltd.), the mobile phases used were n-hexane with
5% 2-propanol and 0.1% TFA with a flow rate of
0.5 mL/min, detection at 220 nm. Melting points were
determined with a Yamato melting point apparatus
model MP-21. Electrospray ionization mass spectrome-
try (ESI-MS) were carried on an Applied Biosystems
API 150 EX. FAB-mass spectra and high resolution
mass spectra (HRMS) were measured on a JEOL
JMS-SX 102A instrument. Optical rotations were mea-
sured on Jasco P-1020. NMR spectra were recorded
on a Brucker 500 MHz spectrometer. Unless otherwise
stated, all NMR spectra were measured in CDCl3 solu-
1
tions with reference to TMS. All H shifts are given in
3. Conclusion
parts per millions (s = singlet; d = doublet; t = triplet;
m = multiplet). Assignments of proton resonances were
confirmed, when possible, by selective homonuclear
decoupling experiments or by correlated spectroscopy.
In summary, we have developed a convenient method
for the synthesis of L- and D-pipecolic acid based on
the spontaneous cyclization of L- and D-2-amino-6-
bromoalkanoic acid. The pipecolic acids can be ob-
tained in very high enantiomeric purity in high yields.
This approach is useful for the synthesis of substituted
pipecolic acids, which give diversity in designing bioac-
tive peptides.
4.1. Synthesis of acetyl-DL-2-amino-6-bromohexanoic
acid 4
Metallic sodium (2.19 g, 95 mmol) was added to abso-
lute ethanol (100 mL) with stirring. After complete dis-
solution, diethyl acetamidomalonate
3
(21.08 g,
100 mmol) was added and refluxed for 30 min and then
cooled down. 1,4-Dibromobutane (59 mL, 500 mmol)
was added to the solution and refluxed for 5 h. The reac-
tion was monitored by TLC. After completion of the
reaction, aqueous NaOH (2 M, 50 mL) was added to
the reaction mixture at 0 ꢁC in different portions with
4. Experimental
All reagents were used as purchased from commercial
suppliers without further purification. Flash chromato-
graphy was performed using silica gel 60 (230–400 mesh)

906
L. A. Watanabe et al. / Tetrahedron: Asymmetry 16 (2005) 903–908
25
D
1
stirring. After selective hydrolysis, ethanol evaporated
and the unreacted dibromoalkane was removed by
extraction with diethyl ether under basic condition.
Then the aqueous solution was extracted with ethyl ace-
tate at pH = 3–4, by adding citric acid and washed with
brine, dried and evaporated to obtain a white solid of
monoester monoacid. It was then refluxed in toluene
(100 mL) for 5 h and toluene then evaporated. The
resulting oil (17.0 g, 60.9 mmol) was then dissolved in
ethanol (50 mL) and treated with aqueous NaOH
(2 M, 25 mL) at intervals of 20 min in portions at
0 ꢁC. The stirring was continued for a further 3 h. After
the evaporation of ethanol, the unreacted dibromo-
alkane was removed by extraction with diethyl ether
under basic conditions. Then the aqueous solution was
extracted with ethyl acetate, at pH = 3–4, by adding cit-
ric acid and washed with brine. Evaporation of ethyl
acetate yield 4 as a white solid (12.5 g, 49%). Analytical
RP HPLC: Rt: 3.22 min, >99% (column I); mp: 121–
123 ꢁC; 1H NMR (500 MHz, CD3OD) d = 4.37 (m,
1H), 3.45 (t, 2H, J1 = J2 = 6.5 Hz), 1.99 (s, 3H), 1.91–
1.67 (m, 4H), 1.54 (m, 2H); 13C NMR (CD3OD)
d = 175.4, 173.4, 53.5, 33.9, 33.4, 31.8, 25.5, 22.4; ESI-
MS (m/z): 252 (M+H)+, 274 (M+Na)+.
mp: 271–274 ꢁC; ½aꢀ ¼ þ26:3 (c = 1, water); H NMR
(500 MHz, D2O) d = 3.60 (m, 1H), 3.41 (m, 1H), 3.00
(m, 1H), 2.22 (m, 1H), 1.91–1.57 (m, 5H); 13C NMR
(D2O) d = 175.0, 59.4, 44.0, 26.9, 22.2, 21.9; HRMS
(FAB, m/z): (M+H)+ found: 130.0877 (calcd for
C6H12O2N: 130.0868).
4.4. Synthesis of Boc-D-2-amino-6-bromohexanoic acid
ethyl ester 7 and Boc-L-2-amino-6-bromohexanoic acid 8
Metallic sodium (2.19 g, 95 mmol) was added to abso-
lute ethanol (100 mL) with stirring. After complete dis-
solution, diethyl Boc-aminomalonate
5
(21.08 g,
100 mmol) was added and refluxed for 30 min and then
cooled down. 1,4-Dibromobutane (59 mL, 500 mmol)
was added to the solution and refluxed for 5 h. The reac-
tion was monitored by TLC. After completion of the
reaction, aqueous NaOH (2 M, 50 mL) was added to
the reaction mixture at 0 ꢁC in aliquots (10 mL) with
stirring. After selective hydrolysis, ethanol was evapo-
rated and the unreacted dibromoalkane removed by
extraction with diethyl ether under basic condition.
Then the aqueous solution was extracted with ethyl ace-
tate at pH = 3–4, by adding citric acid and washed with
brine, dried and evaporated to obtain monoester mono-
acid as a white solid. It was then refluxed in toluene
(150 mL) for 5 h and toluene then evaporated. The
resulting oil 6 (19.3 g, 59 mmol) was dissolved in a mix-
ture of water (150 mL) and DMF (50 mL) and the pH of
the solution adjusted to 8 by the addition of 1 M NH3.
Then subtilisin Carlsberg from Bacillus licheniformis was
added (60 mg) and incubated at 38 ꢁC. The pH of the
solution was maintained at 8 by the addition of 1 M
NH3. The reaction was continued for 3 h and then the
mixture concentrated. The unreacted Boc-D-Ab6-OEt
7 was removed by extraction with diethyl ether under
basic conditions. The extraction containing unreacted
7 was washed with brine, dried and evaporated to yield
a colourless oil 7 (9.8 g, 29%). The aqueous solution was
extracted with ethyl acetate at pH = 3–4 by the addition
of citric acid and then washed with brine, dried and
evaporated to yield colourless oil 8 (8.0 g, 28%). Analyti-
4.2. Synthesis of L-pipecolic acid 1a
Compound 4 (10.3 g, 41.0 mmol) was dissolved in water
(200 mL) and CoCl2Æ6H2O (48 mg, 0.2 mmol) was
added. The pH of the solution was adjusted to 7 by
the addition of 1 M NaOH. Then Aspergillus genus L-
aminoacylase was added (2.0 g) and incubated at
38 ꢁC. The pH of the solution was kept at 7 by the addi-
tion of 0.1 M NaOH. After 24 h, the mixture was con-
centrated and acidified with 1 N HCl. The unreacted
Ac-D-Ab6 was extracted with ethyl acetate and evapora-
tion of ethyl acetate yielded Ac-D-Ab6 as a white solid
(4.6 g, 18.2 mmol). The aqueous solution was then ap-
plied to a column of Dowex 50W-X4 and elution with
1 M ammonia yielded L-pipecolic acid 1a (1.7 g, 35%).
TLC: Rf, 0.30 (1-butanol/acetic acid/pyridine/H2O);
>99% ee (determined by column II, Rt: 22.73 min);
25
D
1
mp: 270–273 ꢁC; ½aꢀ ¼ ꢁ26:3 (c 1, water); H NMR
cal RP HPLC: Rt: 6.75 min, 99% (column I); >99% ee
25
D
(500 MHz, D2O) d = 3.58 (m, 1H), 3.41 (m, 1H), 3.01
(m, 1H), 2.22 (m, 1H), 1.91–1.54 (m, 5H); 13C NMR
(D2O) d = 175.9, 60.3, 44.9, 27.8, 23.1, 22.8; HRMS
(FAB, m/z): (M+H)+ found: 130.0842 (calcd for
C6H12O2N: 130.0868).
(determined by column III, Rt: 14.78 min); ½aꢀ ¼ ꢁ2:8
1
(c 0.1, MeOH); H NMR (500 MHz, CDCl3) d = 4.32
(m, 1H), 3.41 (t, 2H, J1 = J2 = 7.0 Hz), 1.93–1.67 (m,
4H), 1.56 (m, 2H), 1.45 (s, 9H); 13C NMR (CDCl3)
d = 176.6, 155.6, 80.3, 53.1, 33.2, 32.1, 31.6, 28.3, 23.9;
ESI-MS (m/z): 310 (M+H)+, 332 (M+Na)+.
4.3. Synthesis of D-pipecolic acid 1b
4.5. Synthesis of L-pipecolic acid 1a
Ac-D-Ab6 (3.5 g, 14 mmol) was dissolved in water
(200 mL) and the pH of the solution adjusted to 8 by
the addition of 1 M NaOH. Then Alcaligenes xylosoxy-
dans D-aminoacylase was added (45 mg/45 mL H2O)
and incubated at 38 ꢁC. The pH of the solution was kept
at 7 by the addition of 0.1 M NaOH. The reaction was
continued for three days and then the mixture concen-
trated. The concentrated aqueous solution was then ap-
plied to a column of Dowex 50W-X4 and elution with
1 M ammonia to yield D-pipecolic acid 1b (1.4 g, 40%).
TLC: Rf, 0.30 (1-butanol/acetic acid/pyridine/H2O);
>99% ee (determined by column II, Rt: 14.96 min);
The Boc group of 8 (1.24 g, 4.0 mmol) was deprotected
using 2 M HCl in dioxane (20 mL) to give the corre-
sponding amino acid hydrochloride. After the evapora-
tion of dioxane, it was dissolved in DMF (10 mL) and
triethylamine (1.1 mL, 8.0 mmol) then added. The
reaction was continued for 5 h and the pH of the solu-
tion was kept at 8 by the addition of triethylamine.
The solution was filtered and then neutralized with ace-
tic acid. Then the solution was concentrated and
dissolved in water. The aqueous solution was then
applied to a column of Dowex 50W-X4 with 1 M

L. A. Watanabe et al. / Tetrahedron: Asymmetry 16 (2005) 903–908
907
ammonia elution. The solution was concentrated and
crystallized from acetone to obtain L-pipecolic acid as
a white solid 1a (494 mg, 95%) TLC: Rf, 0.30 (1-buta-
nol/acetic acid/pyridine/H2O); 98% ee (determined by
tate at pH = 3–4, by the addition of citric acid and
washed with brine, dried and evaporated to obtain the
monoester monoacid as a white solid. It was then re-
fluxed in toluene (150 mL) for 5 h and toluene then
evaporated to give oil 10 (40.0 g, 57%). The resulting
oil 10 (14.7 g, 42 mmol) was dissolved in mixture of
water (110 mL) and DMF (40 mL). The pH of the solu-
tion was adjusted to 8 by the addition of 1 M NH3. Then
subtilisin Carlsberg from Bacillus licheniformis was
added (50 mg) and incubated at 38 ꢁC. The pH of the
solution was maintained at 8 by the addition of 1 M
NH3. The reaction was continued for 3 h and then the
mixture concentrated. The unreacted D-2-Boc-amido-6-
bromo-heptanoic acid ethyl ester was removed by
extraction with diethyl ether under basic condition.
Then the aqueous solution was extracted with ethyl ace-
tate at pH = 3–4, by the addition of citric acid and then
washed with brine, dried and evaporated to obtain 11 as
column II, Rt: 22.73 min); mp: 270–273 ꢁC;
25
½aꢀ ¼ ꢁ26:3 (c 1, water).
D
4.6. Synthesis of Boc-D-2-amino-6-bromohexanoic acid 9
Boc-D-Ab6-OEt 7 (4.63 g, 14 mmol) was dissolved in
ethanol (20 mL) and treated with aqueous NaOH
(2 M, 7 mL) in intervals of 20 min in portions at 0 ꢁC
and then the stirring continued for 3 h. After evapora-
tion of ethanol, the solution was extracted with diethyl
ether under basic conditions. Then the aqueous solution
was extracted with ethyl acetate under acidic condition
and washed with brine. Ethyl acetate evaporation gave
Boc-D-2-amino-6-bromo-hexanoic acid 9 as a colourless
oil (4.33 g, 100%). Analytical RP HPLC: Rt: 6.75 min,
99% (column I); >99% ee (determined by column III,
a colourless oil (5.6 g, 40%) Analytical RP HPLC: Rt:
25
7.35 min, 98% (column I); ½aꢀ ¼ ꢁ2:0 (c 0.5, MeOH),
D
25
1
Rt: 15.39 min); ½aꢀ ¼ þ2:8 (c 0.1, MeOH), H NMR
1H NMR (500 MHz, CD3OD) d = 4.16 (m, 1H), 4.06
(m, 1H), 1.83–1.80 (m, 2H), 1.71–1.59 (m, 4H), 1.50–
1.40 (m, 12H); 13C NMR (CD3OD) d = 176.2, 158.1,
80.5, 54.8, 52.2, 41.8, 32.3, 28.8, 26.9, 25.3; ESI-MS
(m/z): 324 (M+H)+, 346 (M+Na)+.
D
(500 MHz, CDCl3) d = 4.32 (m, 1H), 3.41 (t, 2H,
J1 = J2 = 7.0 Hz), 1.92–1.69 (m, 4H), 1.55 (m, 2H),
1.46 (s, 9H); 13C NMR (CDCl3) d = 177.1, 155.6, 80.4,
53.1, 33.2, 32.1, 31.6, 28.3, 23.9; ESI-MS (m/z): 310
(M+H)+, 332 (M+Na)+.
4.9. Attempted synthesis of methyl pipecolic acid 13
4.7. Synthesis of D-pipecolic acid 1b
The Boc group of 11 (3.7 g, 11.5 mmol) was removed
using 2 M HCl in dioxane (58 mL) to give the corre-
sponding amino acid hydrochloride. After evaporation
of dioxane, it was dissolved in mixture of water
(10 mL) and methanol (10 mL) and then triethylamine
(3.2 mL, 23 mmol) added. The reaction was continued
for 5 h and the pH of the solution maintained at 8 by
the addition of triethylamine. After the evaporation of
water and methanol, it was redissolved in a mixture of
water (10 mL) and dioxane (10 mL) and Boc2O
(3.2 mL, 13.8 mmol) and triethylamine (2.4 mL,
17.3 mmol) were added at under 0 ꢁC and stirred for
6 h. After the evaporation of dioxane, unreacted Boc2O
was removed by extraction with diethyl ether under ba-
sic conditions. Then the aqueous solution was extracted
with ethyl acetate under acidic conditions and washed
with brine. Evaporation of ethyl acetate give a colour-
less oil and the oil dissolved in DMF (10 mL), then ben-
zyl bromide (2.1 mL, 17.3 mmol) and triethylamine
(1.9 mL, 13.8 mmol) were added and stirred at 0 ꢁC
for 6 h. The reaction mixture was concentrated and
the residue purified by silica gel chromatography using
a mixture of hexane/ethyl acetate (8:1) to yield a colour-
less oil (2.7 g, 8.0 mmol). Analysis showed that the com-
pound was Boc-L-2-amino-heptenoic acid benzyl ester
12. Analytical RP HPLC: Rt: 10.34 min, 97% (column
The Boc group of 9 (4.33 g, 14 mmol) was removed
using 2 M HCl in dioxane (70 mL) to give the corre-
sponding amino acid hydrochloride. After evaporation
of dioxane, the solid product was dissolved in DMF
(15 mL) and triethylamine (3.9 mL, 28 mmol) was
added. The reaction was continued for 5 h and the
pH of the solution kept at 8 by the addition of trieth-
ylamine. After filtration, the solution was neutralized
with acetic acid. Then the solution was concentrated
and dissolved in water. The aqueous solution was then
applied to a column of Dowex 50W-X4 with 1 M
ammonia elution. The solution was concentrated and
crystallized from acetone to obtain D-pipecolic acid
as a white solid 1b (1.71 g, 94%) TLC: Rf, 0.30 (1-buta-
nol/acetic acid/pyridine/H2O); >99% ee (determined by
column II, Rt: 14.96 min); mp: 271–274 ꢁC;
25
½aꢀ ¼ þ26:3 (c 1, water).
D
4.8. Synthesis of Boc-L-2-amino-6-bromoheptanoic acid
11
Metallic sodium (4.49 g, 195 mmol) was added to abso-
lute ethanol (200 mL) with stirring. After complete
dissolution, diethyl(Boc-amino)malonate
5
(55.1 g,
200 mmol) was added and refluxed for 30 min and then
cooled down. 1,4-Dibromopentane (100 g, 435 mmol)
was added to the solution and refluxed for 5 h. The reac-
tion was monitored by TLC. After completion of the
reaction, aqueous NaOH (2 M, 100 mL) was added to
the reaction mixture at 0 ꢁC in different portions with
stirring. After selective hydrolysis, ethanol was evapo-
rated and the unreacted dibromoalkane removed by
extraction with diethyl ether under basic conditions.
Then the aqueous solution was extracted with ethyl ace-
25
1
I); ½aꢀ ¼ ꢁ27:7 (c 0.5, MeOH); H NMR (500 MHz,
D
CDCl3) d = 7.31–7.38 (m, 5H), 5.40 (m, 1H), 5.32 (m,
1H), 5.11–5.21 (m, 2H), 5.02 (d, 1H, J = 8.5 Hz), 4.33
(d, 1H, J = 5.0 Hz), 2.01 (m, 2H), 1.70–185 (m, 2H),
1.67 (m, 2H), 1.43 (s, 9H); 13C NMR (CDCl3)
d = 172.7, 155.3, 135.4, 129.3, 128.5, 128.4, 128.3,
128.1, 126.3, 114.9, 79.7, 66.9, 53.2, 33.1, 32.4, 28.3,
24.4; HRMS (FAB, m/z): (M+H)+ found: 334.1997
(calcd for C19H28O4N: 334.2018).

908
L. A. Watanabe et al. / Tetrahedron: Asymmetry 16 (2005) 903–908
A. B.; Sprengeler, P. A.; Barbosa, J.; Gusman, M. C.;
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