Synthesis of free and Nα-Fmoc-/Nγ-Boc- protected (2S,4S)- and (2S,4R)-4-aminopipecolic acids

Article abstract of DOI:10.1002/ejoc.200400045

The preparation of (2S,4S)- and (2S,4R)-4-aminopipecolic acid, a conformationally constrained basic amino acid bearing orthogonal N ^α/N^γ-protection suitable for solid-phase peptide synthesis, is reported. These amino acids were synthesised from enantiopure ethyl (2S)-4-oxo-1-(1-phenylethyl)-piperidine-2-carboxylate (1) with introduction of the side-chain amino group by reductive amination, followed by protection/deprotection of the functional groups. A mixture of Fmoc-Sly(Boc)-OH 6 and 7 was used to establish their compatibility in solid-phase peptide synthesis. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400045

FULL PAPER
Synthesis of Free and Nα-Fmoc-/Nγ-Boc-Protected (2S,4S)- and (2S,4R)-
4-Aminopipecolic Acids
Fabrizio Machetti,*[a] Franca M. Cordero,[a] Francesco De Sarlo,[a] Anna M. Papini,[a]
Maria C. Alcaro,[a] and Alberto Brandi[a]
Keywords: Constrained amino acids / Natural products / Reductive amination
The preparation of (2S,4S)- and (2S,4R)-4-aminopipecolic
acid, a conformationally constrained basic amino acid bear-
ing orthogonal Nα/Nγ-protection suitable for solid-phase pep-
tide synthesis, is reported. These amino acids were syn-
thesised from enantiopure ethyl (2S)-4-oxo-1-(1-phenylethyl)-
piperidine-2-carboxylate (1) with introduction of the side-
chain amino group by reductive amination, followed by pro-
tection/deprotection of the functional groups. A mixture of
Fmoc-Sly(Boc)-OH 6 and 7 was used to establish their com-
patibility in solid-phase peptide synthesis.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
Over recent years considerable efforts have been made in
the synthesis of non-natural amino acids employable in the
assembly of peptides, with two main goals: i) the synthesis
of biologically active peptides capable of surviving proteol-
ysis, and ii) the generation of conformationally constrained
peptides able to reproduce the biological activity of larger
proteins.[1Ϫ3]
As part of a program aimed towards the synthesis of new
peptides as pharmacological tools, we required rigid mimics
of basic amino acid residues (lysine and ornithine) in both
enantiomeric forms, and we focused on a target containing
the pipecolic (piperidine-2-carboxylic) acid nucleus, with an
additional amino functionality at the C-4.
For both of the natural amino acids cited, an important
design consideration may include the restriction of the side
chain χ1, χ2 and χ3 (only for lysine) torsion angles,[4] and
also of the ϕ torsion angle between the Cα and Nα positions.
Different conformationally constrained lysine and ornithine
Pipecolic acid substituted with an amino group at C-4, with
its constrained ϕ, χ1 and χ2 angles, thus represents an endo-
cyclic-Nα/exocyclic-Nγ basic constrained amino acid. Our
Sly[14] imino acids can also be seen as lysine or ornithine
residues bearing shortened side chains. They also mimic the
hydrophobicity of lysine, as the two methylene groups for-
mally absent with respect to the lysine side chain are still
contained in the pipecolic ring. Furthermore, pipecolic acid
derivatives bearing amino functionalities at their C-4 posi-
tions are included among the natural occurring non-pro-
teinogenic α-amino acids found in some plants. 4-Aminopi-
pecolic acid has been found in the leaves of Strophantus
scandens (Apocinaceae),[15Ϫ17] its stereochemistry not being
assigned with certainty, and (2S,4S)-4-acetylamino pipec-
olic acid has been isolated from the leaves of Calliandra
hæmatocephala (Leguminosæ).[18]
analogues have been reported. These include pyrrolidine-
[5Ϫ7]
and piperidine-based endocyclic-Nα/exocyclic-Nε
or
[8Ϫ10]
Nδ
analogues, pyrrolidine-containing side chain ex-
ocyclic-Nα/endocyclic-Nε analogues,[11] azabicyclohexane
constrained side chain exocyclic-Nα/endocyclic-Nδ ana-
logues[12] and anthracene-based exocyclic-Nα/exocyclic-Nε
analogues.[13]
[a]
`
Dipartimento di Chimica Organica ‘‘U. Schiff’’ Universita di
This amino acid moiety has been incorporated into dif-
ferent synthetic biologically active compounds such as cyc-
lic peptides,[19Ϫ21] oligomers[22] and small molecules.[23Ϫ25]
In addition, some biologically active 4-aminopipecolic acid
derivatives have also been reported.[26Ϫ28] The synthesis of
Firenze and ICCOM Ϫ CNR,
Via della Lastruccia 13, 50019 Sesto Fiorentino (Firenze), Italy
Fax: (internat.) ϩ 39-055-4573531
E-mail: fabrizio.machetti@unifi.it
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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DOI: 10.1002/ejoc.200400045
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Synthesis of Free and Nα-Fmoc-/Nγ-Boc-Protected Aminopipecolic Acids
FULL PAPER
racemic 4-aminopipecolic acid through the reduction of 4-
Compound 8 could also be transformable into the 4-am-
aminopicolinic acid with sodium-alcohol has been re- ino derivative through an aza-Mitsunobu reaction.[34] In-
ported.[15,16,29] Some syntheses of 4-aminopipecolic acids deed, treatment of 8 with PPh3, diethyl azodicarboxylate
derivatives are known,[28,30,31] but none seems suitable for (DEAD) and diphenylphosphoryl azide (DPPA)[35] did give
the synthesis of orthogonally protected 4-aminopipecolic the N-Boc-4-trans-azidopipecolic acid methyl ester (10) in
acids 6 and 7.
low yield.[36] A possible nitrogen donor such as phthalimide
In this paper we describe the use of esters 1 and 3 of 4- was not suitable for that substrate, as the Mitsunobu reac-
oxo- and 4-hydroxypipecolic acids to synthesise enantiom- tion[37] gave a poor yield (22%) of the corresponding
erically pure cis- and trans-(2S)-4-aminopipecolic acids, phthalimido ester together with larger amounts (36%) of
either unprotected (4 and 5) or suitably protected for solid- elimination products.[36]
phase peptide synthesis (SPPS) by the Fmoc/tBu strategy
Hydrogenation of the azide 10 in EtOH in the presence
of 20% of palladium/C gave the amino compound 11, which
was directly protected by treatment with benzyl chloro-
formate to give the final compound 12 in good yield.
Hydrolysis of the methyl ester 11 with lithium hydroxide
and subsequent deprotection of the corresponding N-Boc
acid with trifluoroacetic acid gave the trans free amino acid
5 in 88% combined yield.
The racemic N-Boc- and N-Bn-oxopipecolic esters 13 and
14 were then both considered as starting materials. Direct
reductive amination[38] with NaBH3CN/ammonium acetate
in methanol afforded good yields of equimolecular mixtures
of the diastereomeric amines 19 and 11 from the ketone 13
and of the amines 15 and 16 from the ketone 14. The crude
mixture of the amines 15 and 16 was treated with BOC2O
and DIPEA to provide the final compounds 17 and 18.
Similarly, the products 20 and 12 were obtained by treat-
ment of the mixture of 19 and 11 with CbzCl and TEA.
The reductive amination of both 4-oxo substrates 13 and
14 was greatly improved by the use of molecular sieves in
the reaction mixture (Scheme 2). The synthesis of the above
differently protected 4-aminopipecolic acids was ac-
complished in order to find which pair of diastereoisomers
would be most amenable to separation. Attempts to separ-
ate the diastereomers by fractional crystallisation failed. Of
the candidates, only the couple 15Ϫ16 proved separable by
RP-HPLC. Taking advantage of the experimental con-
ditions found for this pair of compounds, we considered the
(Fmoc-cisSly-(Boc)-OH 6 and Fmoc-transSly-(Boc)-OH 7).
Results and Discussion
We have already described a large-scale method for the
asymmetric synthesis of both enantiomers of the protected
4-oxopipecolic acids 1 and 2, with the 1,3-dipolar cycload-
dition between C-ethoxycarbonyl-N-phenylethylnitrone and
but-3-en-1-ol as the key step, followed by the elaboration of
the adducts.[32] We have also previously prepared the pro-
tected cis-4-hydroxypipecolic acid 3 by stereoselective -Sel-
ectride reduction of the corresponding 4-oxo com-
pound.[33]
The functionality present in these protected 4-oxo- and
4-hydroxypipecolic acids should make them appealing start-
ing material for the synthesis either of 4-aminopipecolic ac-
ids 4 and 5, or of the orthogonally protected amino acids 6
and 7.
We first investigated which starting material, out of the
racemic hydroxy derivative 8 and the oxo derivative 13,
would be preferable for the introduction of an amino func-
tionality at C-4.
In order to synthesise the trans-4-aminopipecolic acid
isomer we started with an SN2 reaction on the cis alcohol
8. This compound was treated with methanesulfonyl chlo-
ride in pyridine to afford compound 9 in 90% yield and,
without further purification, this mesylate was treated with
sodium azide in DMF to give the azido compound 10 in
80% yield with inversion of configuration at C-4.
Scheme 1. Reagents and conditions: (i) MsCl, Py, DMAP, 3 h, (ii)
NaN3, DMF, room temp., 18 h, (iii) H2, 80 psi, room temp., 20%
Pd/C, (iv) CbzCl, TEA, 0 °C, (v) a) LiOH, MeOH, room temp.,
16 h, 88%, b) TFA, CH2Cl2, 100%
˚
Scheme 2. Reagents and conditions: (i) NaBH3CN, AcONH4, 3-A
MS, (ii) CbzCl, TEA, 0 °C, (iii) Boc2O, DIPEA, room temp., 3 days
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F. Machetti, F. M. Cordero, F. De Sarlo, A. M. Papini, M. C. Alcaro, A. Brandi
FULL PAPER
use of the reductive amination method for the synthesis of tuted piperidine was achieved by a complete study of the
the enantiopure cis- and trans-4-aminopipecolic acids. NMR spectroscopic data of the fully deprotected com-
Thus, a chromatographic separation followed by suitable pounds 4 and 5. The proton assignments were ac-
deprotection steps might give direct access to both the final complished with the aid of a COSY spectrum, starting from
compounds 28 and 29.
the H-2 protons. The proton H-2 (δ ϭ 3.99 ppm in 4) dis-
Our synthetic route carried out from the enantiopure plays coupling constants of 12.8 and 3.2 Hz with H-3ax and
(1ЈR,2S) ketone 1 is shown in Scheme 3. The reductive ami- H-3eq, respectively, requiring H-2 to be axial. The orien-
nation of ketone 1 with ammonium acetate and NaBH3CN tation of proton H-4 could not be assigned in the same way,
provided an almost equimolecular mixture of the epimeric because the coupling constants with protons H-3 and H-5
amines 21 and 22, in 76% combined yield. These were sepa- could not be determined.
rated by semipreparative reversed-phase high pressure
The stereochemistry of proton H-4 was determined from
liquid chromatography (RP-HPLC), and were then in turn 1H-NOESY cross-peaks observed between H-4/H-2 and H-
protected as Nγ-Boc urethanes by standard procedures in 4/H-6ax that indicated their axial orientations (Figure 1). If
good yields.
a chairlike conformation is assumed for the piperidine ring,
these NMR considerations are consistent with the cis rela-
tive configuration for 4 and consequently for 6. The assign-
ment of the trans configuration to the diastereomeric com-
pound 5 is consistent with the observation that its 1H NMR
spectrum is identical to that obtained, by deprotection by
standard procedures, of the trans racemic compound 11 and
it is also consistent with NMR spectroscopic data. The res-
onance frequencies of protons H-4 and H-6 are too close
for the different signals to be distinguished. Addition of tri-
fluoroacetic acid to the NMR tube (pH ϭ 1) shifted the
H-4 signal (δ ϭ 3.48 ppm) downfield, after which it then
appeared as a triplet of triplets with a large coupling con-
stant (14.0 Hz), indicating that this proton was in an axial
orientation. The methyne proton H-2 (δ ϭ 4.43 ppm, trip-
let) shows a low value of the two JH-2,H-3 coupling constants
(Ͻ 5 Hz), indicating an axial orientation of the carboxylic
group (Figure 1). If a chairlike conformation is assumed for
the piperidine ring, these NMR considerations are consist-
ent with a trans relative configuration for 5 and conse-
quently for 7.
Scheme 3. Reagents and conditions: (i) a) NaBH3CN, AcONH4,
76%, b) RP-HPLC, (ii) Boc2O, DIPEA, (iii) H2, Pd/C(20%), EtOH,
(iv) LiOH, MeOH, vi) Fmoc-OSu, 10% Na2CO3/H2O, acetone,
room temp., (v) TFA/H2O
Figure 1. Determination of the relative configuration of com-
pounds 4 and 5 by means of coupling constants and NOE
The assignment of the structure of 5 allowed the relative
The catalytic hydrogenation of the epimers 23 and 24 was configurations of 4-N-substituted natural pipecolic acids to
carried out in ethanol in the presence of Pearlman’s catalyst. be elucidated. Marlier et al. reported the isolation of a 4-
The reaction had gone to completion after one day, to give acetylaminopipecolic acid from the leaves of Calliandra
deprotected Nα-amino esters 25 and 26 in high yields. Hy- haematocephala.[18] They obtained the parent 4-aminopipe-
drolysis of ethyl esters 25 and 26 with LiOH gave the acids colic acid by acid hydrolysis of the corresponding natural
27 and 28, respectively, and these were in turn protected by amide and ascribed the 2S configuration and the trans
a (fluorenylmethoxy)carbonyl (Fmoc) group to afford the stereochemistry to these amino acids. The trans relative
1
final orthogonally protected compounds 6 and 7 in good configuration was assigned by H NMR considerations but
yields.
the authors wrongly reported the stereoisomer as (2S,4R)
Stereochemical assignment of compound 6 as the cis-2,4- instead of (2S,4S). Consequently this compound is reported
1
disubstituted piperidine and of 7 as the trans-2,4-disubsti- in Chemical Abstract as (2S) cis.[39] Comparison of our H
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Synthesis of Free and Nα-Fmoc-/Nγ-Boc-Protected Aminopipecolic Acids
FULL PAPER
NMR spectra with that reported by Marlier now allows the carboxylate (1), the side-chain amino group being intro-
correct configuration of the natural compound to be unam- duced by reductive amination. Amino acids 6 and 7 are
biguously established as (2S,4S).
suitably protected for solid-phase organic synthesis. The in-
In the earlier literature relating to the natural 4-aminopi- corporation of these constrained amino acids into peptides
pecolic acid the stereochemistry was not indicated.
is under investigation.
Finally we evaluated the coupling of the constrained Sly
amino acids to solid supports, anchoring them to two dif-
ferent resins commonly used for Fmoc/tBu SPPS: Rink re-
sin and trityl chloride resin (Scheme 4). While the Rink re-
sin was chosen for demonstrating the possible preparation
of peptide amides containing the Sly amino acids at the C-
terminal position, the trityl resin was selected to further
potential peptide coupling. Coupling of the Sly amino acid
to the Rink resin by amide bond formation was performed
by use of DIPCDI (1,3-diisopropylcarbodiimide) and
HOBt (1-hydroxybenzotriazole) for 16 h at room tempera-
ture. The loading determined by the Fmoc release, moni-
tored by UV absorption at 301 nm, showed quantitative
anchoring of the amino acid (98% yield). The trityl resin
proved preferable because it allows the modulation of the
final loading with low amounts of the first amino acid (stoi-
chiometric) for its functionalization, in comparison to the
Wang resin, which required 10 equivalents of the first
amino acid. Moreover, the steric hindrance of the trityl
resin reduces or totally blocks the formation of diketopiper-
azines at the dipeptide level during the deprotection step,
when low resin loadings are used.[35] The use of a such resin
is therefore particularly important when the peptide se-
quence includes elements favouring diketopiperazine forma-
tion, such as imino acids stabilizing a cis amide bond (as in
our Sly amino acid) or a -/-amino acid combination. In
order to obtain a low resin loading, coupling to the trityl
resin (0.080 mmol) was performed with 0.080 mmol of the
Sly amino acid, and DIPEA for 2 h at room temperature.
The loading (0.27 mmol/g) was determined from the Fmoc
release, monitored by UV absorption at 301 nm (34% yield).
Experimental Section
General Remarks: Melting points are uncorrected and were meas-
ured with a RCH Kofler microscope apparatus. Chromatographic
separations were performed under pressure by FCC (flash column
chromatography; silica gel unless otherwise stated); Rf values refer
to TLC, carried out on 0.25 mm silica gel plates (Merck F254) with
the same eluent as indicated for the column chromatography. IR
spectra (CDCl3 solution unless otherwise stated) were recorded on
a PerkinϪElmer 881 spectrophotometer, and NMR spectra (CDCl3
solution unless otherwise stated) on Varian Gemini 200 and Varian
Mercury 400 (1H, 200, 400 MHz; 13C 50, 100 MHz respectively)
spectrometers: the chemical shifts are given in ppm from TMS.
3
Coupling constants J (in Hz) refer to JH,H. EI mass spectra were
carried out at 70 eV ionizing voltage by direct introduction into a
QMD 1000 CarloϪErba instrument. The electrospray mass spectra
were performed on a ThermoFinnigan LCQ Advantage Ion Trap
apparatus. Microanalyses were carried out with a PerkinϪElmer
240C elemental analyser. Optical rotation measurements were car-
ried out with a JASCO DIP-360 digital polarimeter. All reactions
requiring anhydrous conditions were performed in oven-dried
glassware. Analytical reversed-phase HPLC was carried out on a
Beckman System Gold instrument fitted with a diode array detec-
tor, on a 5 µm (250 ϫ 4.6 mm) C18 Vydac column. Semipreparative
purifications were performed with a 10 ϫ 250 mm C18 Vydac col-
umn. The flow rates were 1 mL/min for analytical HPLC and 4 mL/
min for semipreparative HPLC.
1-tert-Butyl 2-Methyl cis-4-Hydroxypiperidine-1,2-dicarboxylate
(8): A solution of -Selectride in THF (1.0 , 1.14 mL) was added
dropwise at Ϫ78 °C to a previously prepared solution of methyl
1-tert-butoxycarbonyl-4-oxopiperidine-2-carboxylate (208 mg, 0.80
mmol) in dry THF (16 mL). The reaction mixture was stirred at
Ϫ78 °C for 2 h, and the temperature was then slowly increased to
0 °C. After the mixture had been stirred for 30 min, satd. aq.
NH4Cl was added (16 mL). The aqueous layer was extracted with
EtOAc (3 ϫ 16 mL). The combined organic layer was washed with
water (3 ϫ 20 mL) and brine (3 ϫ 20 mL), dried (Na2SO4), filtered
and concentrated under reduced pressure. The residue oil was puri-
fied by FCC (CH2Cl2/MeOH, 15:1, Rf ϭ 0.34) to afford a colour-
less oil corresponding to compound 8 (120 mg, 58%). 1H NMR:
δ ؍ 1.41 [s, 9 H, C(CH3)3], 1.50Ϫ1.75 (m, 2 H, H-3ax and H-5ax),
1.89 (ddd, J ϭ 2.9, 6.8, 15.5 Hz, 1 H, H-5eq), 2.40 (dm, J ϭ 2.7 Hz,
1 H, H-3eq), 3.20Ϫ3.42 (m, 1 H, H-6ax), 3.71 (s, 3 H, OCH3),
3.60Ϫ3.90 (m, 1 H, H-6eq), 4.08Ϫ4.18 (m, 1 H, H-4), 4.65 and
4.80 (rotamers, br. s, 1 H, H-2) ppm. 13C NMR: δ ؍ 27.8 [q, 3 C,
C(CH3)3], 30.6 (t, C-5), 32.9 (t, C-3), 34.6 and 35.7 (rotamers, t, C-
6), 49.9 and 51.0 (rotamers d, C-2), 51.5 (q, OCH3), 62.0 (d, C-4),
79.6 (d, CMe3), 155.3 (s, CO2tBu), 172.6 (s, CO2Me) ppm. MS (EI):
m/z (%) ϭ 200 (6) [M Ϫ CO2Me]ϩ, 144 (60), 100 (50), 82 (21), 57
(100). IR (cmϪ1): ν˜ ϭ 2979, 2955, 2904, 1725, 1686. C12H21NO5
(259.30): calcd. C 55.58, H 8.16, N 5.40; found C 55.30, H 8.35,
N 5.10.
Scheme 4. Reagents and conditions: (i) DIPEA/DCM, (ii) a) piper-
idine/DMF (1:5), b)DIPCDI/HOBt/DMF
Conclusions
In conclusion, two novel basic constrained amino acids 6
and 7, each incorporating a rigid pipecolic skeleton, were
synthesised. These amino acids were synthesised from en-
1-tert-Butyl 2-Methyl cis-4-[(Methylsulfonyl)oxy]piperidine-1,2-di-
carboxylate (9): MsCl (0.036 mL, 0.46 mmol) was added dropwise
antiopure ethyl (2S)-4-oxo-1-(1-phenylethyl)piperidine-2- at 0 °C to a solution of the alcohol 8 (116 mg, 0.45 mmol) and
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F. Machetti, F. M. Cordero, F. De Sarlo, A. M. Papini, M. C. Alcaro, A. Brandi
FULL PAPER
DMAP (4 mg) in pyridine (1 mL). The mixture was stirred at room
11 (40 mg, 0.15 mmol) and TEA (0.030 mL, 0.20 mmol) in CH2Cl2
temperature for 3 h and was then quenched with satd. aq. NaCl (1 mL). The mixture was stirred at room temperature for 12 h. The
(1 mL) and extracted with CH2Cl2 (3 ϫ 2 mL). The organic phases
were dried with Na2SO4, filtered and concentrated under reduced
pressure to give 9 (135 mg, 90%) as a syrup, which was employed
in the next step without further purification. An analytical sample
solvent was evaporated, and the residue was purified by flash chro-
matography (petroleum ether/ethyl acetate, 3:1; Rf ϭ 0.20) to afford
1
a clear oil corresponding to compound 12 (58 mg, 95%). H NMR
(mixture of rotamers): δ ϭ 1.41 and 1.44 [s, 9 H, C(CH3)3],
was obtained by FCC (CH2Cl2/MeOH, 20:1, Rf ϭ 0.34). 1H NMR: 1.60Ϫ1.70 (m, 1 H, H-5ax), 1.95Ϫ2.10 (m, 1 H, H-3ax), 2.50 (dm,
δ ϭ 1.44 [s, 9 H, C(CH3)3], 1.60Ϫ1.82 (m, 2 H, H-3ax and H-5ax),
1.98 (ddd, J ϭ 15.0, 7.5, 2.6 Hz, 1 H, H-5eq), 2.60Ϫ2.75 (dm, J ϭ (m, 1 H, H-6ax), 3.73 (s, 3 H, OCH3), 3.90Ϫ4.05 (m, 2 H, H-6eq,
16 Hz, 1 H, H-3eq), 2.95 (s, 3 H, SO2CH3), 3.20Ϫ3.42 (m, 1 H, H- and H-4), 4.54Ϫ4.68 (m, 1 H, NHCO), 4.80 and 5.00 (br. s, 1 H,
6ax), 3.71 (s, 3 H, OCH3), 3.80Ϫ4.12 (br. m, 1 H, H-6eq), 4.82 and H-2), 5.07 (s, 2 H, OCH2Ph), 7.32 (s, 5 H, Ar-H) ppm. 13C NMR
4.92 (rotamers, br. s, 1 H, H-2), 5.01Ϫ5.08 (m, 1 H, H-4) ppm. 13
(mixture of rotamers): δ ϭ 28.3 [q, 3 C, (CH3)3C], 31.9 (t, C-5),
NMR: δ ϭ 28.3 [q, 3 C, C(CH3)3], 29.8 (t, C-5), 31.5 (t, C-3), 36.2 33.0 (t, C-3), 40.1 and 40.8 (t, C-6), 45.9 (d, C-4), 52.4 (q, OCH3),
J ϭ 14 Hz, 1 H, H-5eq), 2.95Ϫ3.20 (m, 1 H, H-3eq), 3.40Ϫ3.65
C
(q, SO2CH3), 38.5 (t, C-6), 51.2 and 50.1 (rotamers, d, C-2), 51.4
(q, OCH3), 74.6 (d, C-4), 80.6 (s, CMe3), 151.3 (s, CO2tBu), 171.2
(s, CO2Me) ppm. IR (cmϪ1): ν˜ ϭ 1720, 1670, 1354. MS (EI): m/z
53.5 and 54.6 (d, C-2), 66.8 (t, OCH2Ph), 80.5 (s, CMe3), 128.1 (d,
Ar-H), 128.2 (d, Ar-H), 128.6 (d, Ar-H), 136.3 (s, Ar-H), 155.4 and
155.5 (s, CO2tBu), 171.4 (s, CO2Me) ppm. IR (cmϪ1): ν˜ ϭ 3439,
(%) ϭ 278 (6) [M Ϫ CO2Me], 222 (16), 182 (15), 154 (15), 126 (71), 2977, 2953, 1760Ϫ1660. MS (EI): m/z (%) ϭ 336 (4) [M Ϫ
82 (100), 57 (74). C13H23NO7S (337.39): calcd. C 46.28, H 6.87, N
4.15; found C 46.46, H 6.97, N 4.08.
Me2CH2]ϩ, 291 (4), 277 (9) [M Ϫ CO2Me], 213 (11), 169 (20), 140
(49), 125 (31), 91 (92), 82 (73), 79 (12), 57 (100). C20H28N2O6
(392.44): calcd. C 61.21, H 7.19, N 7.14; found C 61.56, H 7.52,
N 6.89.
1-tert-Butyl 2-Methyl trans-4-Azidopiperidine-1,2-dicarboxylate
(10): NaN3 (44 mg, 0.68 mmol) was added to a solution of com-
pound 9 (60 mg, 0.18 mmol) in dry DMF (0.5 mL) under nitrogen.
The mixture was heated at 60 °C for 18 h, H2O (0.1 mL) was then
added, and the solvent was evaporated under reduced pressure. The
crude residue was purified by FCC (CH2Cl2; Rf ϭ 0.30) to afford
a clear oil corresponding to compound 10 (41 mg, 80%). 1H NMR:
δ ϭ (rotamers) 1.44 [s, 9 H, C(CH3)3], 1.40Ϫ1.75 (m, 2 H, H-3ax
and H-5ax), 1.80Ϫ2.04 (m, 1 H, H-5eq), 2.38Ϫ2.58 (m, 1 H, H-
3eq), 2.81Ϫ3.12 (m, 1 H, H-4), 3.20Ϫ3.42 (m, 1 H, H-6ax), 3.72
(s, 3 H, OCH3), 4.00Ϫ4.20 (m, 1 H, H-6eq), 4.82 and 5.02 (br. s, 1
H, H-2) ppm. 13C NMR (rotamers): δ ϭ 28.3 [q, 3 C, C(CH3)3],
30.4 (t, C-5), 31.9 (t, C-3), 39.7 and 40.5 (t, C-6), 53.3 and 54.3 (d,
C-2), 55.4 (d, C-4), 80.7 (s, CMe3), 151.2 (s, CO2tBu), 173.5 (s,
CO2Me) ppm. MS (EI): m/z (%) ϭ 225 (8) [M Ϫ CO2Me]ϩ, 197
(8), 169 (41), 141 (11), 125 (31), 97 (11), 57 (100). IR (cmϪ1): ν˜ ϭ
2095, 1724, 1672. C12H20N4O4 (284.14): calcd. C 50.69, H 7.09, N
19.71; found C 50.44, H 7.19, N 19.91.
Methyl cis- and trans-4-Amino-1-benzylpiperidine-2-carboxylate (15
and 16): NH4AcO (2.58 g, 33.5 mmol) and NaBH3CN (190 mg,
3.03 mmol) were added to a solution of ketone 16 (0.790 g,
3.03 mmol) in dry MeOH (9.5 mL) in the presence of molecular
˚
sieves (3A). The resulting mixture was stirred at room tempera-
ture for 48 h. The mixture was filtered and concentrated under re-
duced pressure. The resulting residue was purified by chromatogra-
phy on basic alumina (CH2Cl2, Rf ϭ 0.63) to give an equimolecular
mixture of the amines 15 and 16 (0.620 g, 82% combined yield).
Yellow oil. 1H NMR: δ ϭ 1.60Ϫ1.95 (m, 2 H, H-3 and H-5),
2.18Ϫ2.40 (m, 1 H, H-5), 2.35Ϫ2.64 (m, 1 H, H-3), 2.74Ϫ3.22 (m,
2 H, CH2Ph), 3.38Ϫ3.62 (m, 1 H, H-4), 3.66Ϫ3.86 (m, Ar-H) ppm.
MS (EI): m/z (%) ϭ 248 (1) [Mϩ], 189 (4) [Mϩ Ϫ CO2Me], 174
(39), 172 (22), 91 (100), 86 (22), 84 (35). IR (cmϪ1): ν˜ ϭ 3360Ϫ3280
(br), 3179, 3085, 3063, 3029, 2980, 2963, 2929, 2855, 1724.
C14H20N2O2 (248.32): calcd. C 67.71, H 8.12, N 11.28; found C
67.56, H 8.02, N 11.48.
1-tert-Butyl 2-Methyl trans-4-Aminopiperidine-1,2-dicarboxylate
(11): The azide 10 (80 mg, 0.28 mmol) was dissolved in MeOH
(0.56 mL), Pd/C (20%, 80 mg) was added, and the apparatus was
flushed three times with H2. The reaction mixture was hydrogen-
ated at a pressure of 80 psi at room temperature for 16 h, filtered
through a short pad of Celite (3 ϫ 1 mL rinse) and then concen-
trated under reduced pressure to give the amine 11 (64 mg, 90%),
sufficiently pure to be used in the next step without further purifi-
cation. An analytical sample was obtained by chromatography on
Methyl cis- and trans-1-Benzyl-4-[(tert-butoxycarbonyl)amino]piper-
idine-2-carboxylate (17 and 19): Boc2O (416 mg, 1.91 mmol) was
added portionwise to a solution of the amines 15 and 16 (500 mg,
1.91 mmol) and DIPEA (0.35 mL, 1.91 mmol) in CH2Cl2/EtOH
(4:1, 4 mL). The mixture was stirred at room temperature for 3 days
and was then concentrated under reduced pressure. The residue was
partitioned between Et2O/CH2Cl2 (2:1) and NaHSO4 (10%). The
aqueous layer was extracted with Et2O/CH2Cl2 (2:1). The solvent
was removed from the organic combined solutions and the residue
was purified by FCC (CH2Cl2/MeOH, 9:1; Rf ϭ 0.51) to give an
equimolecular mixture of 17 and 19 (594 mg, 1.64 mmol, 86%
yield). Clear oil. 1H NMR (mixture of rotamers): δ ϭ 1.50Ϫ2.09
(m, 2 H, H-3 and H-5), 1.41 and 1.44 [s, 9 H, C(CH3)3], 2.20Ϫ2.40
(m, 1 H, H-3 and H-5), 2.45Ϫ2.68 (m, 1 H, H-6), 2.80Ϫ3.20 (m, 2
H, H-6 and H-4), 3.32Ϫ3.72 (m, 2 H, CH2Ph), 3.77 and 3.78 (s, 3
H, OCH3), 4.05 Ϫ4.40 (m, 2 H, H-2 and BocNH), 4.70Ϫ4.85 (m,
1 H), 7.12Ϫ7.40 (m, 5 H, Ar-H) ppm. 13C NMR: δ ϭ 31.7 (t), 31.9
(t), 33.7 (t), 35.9 (t), 37.1 (t), 45.9 (t), 47.2 (t), 49.5 (d), 59.9 (d),
60.3 (t), 60.8 (t), 63.0 (d), 64.8 (d), 66.9 (d), 81.1 (s), 127.0 (d),
127.2 (d), 128.1 (d), 128.7 (d), 129.1 (d), 129.3 (d), 137.4 (s), 138.6
(s), 155.0 (s), 172.8 (s), 173.1 (s) ppm. MS (EI): m/z (%) ϭ 363 (2)
[Mϩ], 289 (10), 190 (11), 172 (87), 91 (100), 65 (12), 57 (40), 55
(10). C19H28N2O4 (348.44): calcd. C 65.49, H 8.10, N 8.04; found
C 65.56, H 8.02, N 7.88.
1
basic alumina (CH2Cl2, Rf ϭ 0.43). Clear oil. H NMR (mixture
of rotamers): δ ϭ 1.42 and 1.45 [s, 9 H, C(CH3)3], 1.70Ϫ1.90 (m,
3 H, H-5eq, H-5ax, H-3ax), 2.20Ϫ2.38 (m, 1 H, H-3eq), 2.58Ϫ2.80
(m, 1 H, H-4), 2.80Ϫ3.10 (m, 1 H, H-6ax), 3.70 (s, 3 H, OCH3),
3.88Ϫ4.13 (m, 1 H, H-6eq), 4.80 and 4.96 (br. s, 1 H, H-2) ppm.
13C NMR (mixture of rotamers): δ ϭ 28.2 [q, 3 C, (CH3)3C], 34.6
(t, C-5), 36.1 (t, C-3), 40.2 and 41.0 (t, C-6), 45.7 and 45.8 (d, C-
4), 52.1 (q, OCH3), 53.5 and 54.6 (d, C-2), 80.2 (s, CMe3), 155.2
and 155.6 (s, CO2tBu), 171.9 (s, CO2Me) ppm. MS (EI): m/z (%) ϭ
258 (3) [Mϩ], 201 (7), 199 (8), 157 (25), 143 (28), 141 (30), 99 (87),
86 (52), 84 (80), 57 (100). IR (cmϪ1): ν˜ ϭ 3356, 3300 (br), 2926,
2851, 1739, 1692. C12H22N2O4 (258.32): calcd. C 55.80, H 8.58, N
10.84; found C 55.56, H 8.54, N 10.59.
1-tert-Butyl
2-Methyl
trans-4-{[(Benzyloxy)carbonyl]amino}-
piperidine-1,2-dicarboxylate (12): Benzyl chloroformate (0.030 mL,
0.20 mmol) was added dropwise at 0 °C to a solution of the amine
2932
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 2928Ϫ2935

Synthesis of Free and Nα-Fmoc-/Nγ-Boc-Protected Aminopipecolic Acids
FULL PAPER
1-tert-Butyl 2-Methyl cis- and trans-4-Aminopiperidine-1,2-dicar-
boxylate (19 and 11): The procedure described above for the syn-
thesis of the amines 15 and 16 was applied to ketone 13 (790 mg,
3.03 mmol), to give a 3:2 mixture of the amines 19 and 11 (618 mg,
Ethyl
(2S,4R)-4-[(tert-Butoxycarbonyl)amino]-1-[(1R)-1-phenyl-
ethyl]piperidine-2-carboxylate (23): The trifluoroacetate salt 21
(340 mg, 0.87 mmol) was dissolved in CH2Cl2 (5 mL), anhydrous
EtOH (1 mL) and DIPEA (0.32 mL, 1.83 mmol), and Boc2O
79% combined yield). Purification: FCC on neutral alumina (209 mg, 0.96 mmol) was then added portionwise. The resulting
(CH2Cl2/MeOH, 25:1; Rf ϭ 0.41). Yellow oil. 1H NMR (amide
mixture was stirred for 5 h at room temperature and was then con-
rotamers): δ ϭ 1.42 and 1.44 [s, 9 H, C(CH3)3], 1.70Ϫ1.90 (m, 3 centrated under reduced pressure. The residue was purified by FCC
H), 2.10Ϫ2.40 (m, 1 H), 2.50Ϫ2.75 (m, 1 H), 2.80Ϫ3.10 (m, 1 H),
3.20Ϫ3.40 (m, 1 H, 11), 3.70 (s, 3 H), 3.80Ϫ4.18 (m, 1 H, 19),
with hexane/EtOAc (6:1) as eluent to give 23 (Rf ϭ 0.57, 232 mg,
1
71%). Clear oil. [α]2D3 ϭ ϩ26.2 (c ϭ 1.04, CHCl3). H NMR: δ ؍
4.42Ϫ4.65 (m, 1 H, 11: H-2), 4.96 and 4.80 (br. s, 1 H, 19: H-2) 1.25 (d, J ϭ 6.8 Hz, 3 H, NCHCH3), 1.26 (t, J ϭ 7.2 Hz, 3 H,
ppm. 13C NMR (signal of cis isomer 19, amide rotamers): δ ؍ 29.5,
29.6, 31.5, 33.4, 37.3, 38.0, 44.2, 44.9, 45.8, 47.6, 50.4, 51.3, 51.9,
79.8 155.2, 155.6, 173.2 ppm. MS (EI): m/z (%) ϭ 258 (11) [Mϩ],
203 (18), 159 (20), 99 (56), 57 (100). C12H22N2O4 (258.31): calcd.
C 55.80, H 8.58, N 10.84; found C 55.72, H 8.40, N 10.76.
OCH2CH3), 1.39 [s, 9 H, (CH3)3C], 1.63Ϫ1.85 (m, 2 H, H-3ax and
H-5ax), 2.05Ϫ2.32 (m, 2 H, H-3eq and H-5eq), 2.62 (ddd, J ϭ
12.0, J ϭ 5.8, J ϭ 3.6 Hz, 1 H, H-6ax), 3.40Ϫ3.68 (m, 3 H, H-2,
H-6 and H-4), 3.93 (q, J ϭ 6.8 Hz, 1 H, NCHCH3), 4.17 (q, J ϭ
7.2 Hz, 2 H, OCH2CH3), 4.62 (d, J ϭ 9.0 Hz, 1 H, BocNH),
7.18Ϫ7.43 (m, 5 H, Ar-H) ppm. 13C NMR: δ ؍ 12.7 (q,
OCH2CH3), 14.2 (q, NCHCH3), 28.3 [q, 3C, (CH3)3C], 31.6 (t),
35.6 (t), 41.9 (t, C-6), 46.3 (d), 58.5 (d), 60.3 (d, C-2), 60.6 (t,
OCH2CH3), 79.2 (s, CMe3), 126.6 (d, Ar-H), 127.5 (d, 2 C, Ar-H),
127.9 (d, 2 C, Ar-H), 143.9 (s, Ar-H), 154.9 (s, NCO2tBu), 173.7
(s, CO2Et) ppm. MS (EI): m/z (%) ϭ 303 (19) [M Ϫ CO2Et]ϩ, 262
(27), 203 (26), 186 (89), 105 (100), 91 (68), 82 (25), 56 (31). IR
(cmϪ1): ν˜ ϭ 3440, 2981, 1710, 1506. C21H32N2O4 (376.49): calcd.
C 66.99, H 8.57, N 7.44; found C 70.04, H 8.44, N 7.78.
1-tert-Butyl 2-Methyl cis- and trans-4-{[(Benzyloxy)carbonyl]-
amino}piperidine-1,2-dicarboxylate (20 and 12): The procedure de-
scribed above for the synthesis of 12 was applied to a mixture of
amines 21 and 13 (80 mg, 0.32 mmol), to give a mixture of pro-
tected amines 20 and 12 (141 mg) in 89% yield. Purification: FCC
1
(petroleum ether/ethyl acetate, 3:1; Rf ϭ 0.20). Clear oil. H NMR
(mixture of rotamers): δ ϭ 1.41 and 1.44 (s, 9 H), 1.70Ϫ2.10 (m, 2
H), 1.95Ϫ2.10 (m, 1 H), 2.40Ϫ2.50 (m, 1 H), 2.95Ϫ3.20 (m, 1 H),
3.40Ϫ3.68 (m, 1 H), 3.73 and 3.71 (s, 3 H), 3.90Ϫ4.10 (m, 2 H),
4.50Ϫ4.60 (m, 1 H), 4.80Ϫ5.05 (br. m, 1 H), 4.99 and 5.07 (s, 2 H),
7.32 and 7.38 (s, 5 H) ppm. MS (EI): m/z (%) ϭ 277 (9), 213 (11),
169 (19), 140 (49), 125 (31), 108 (11), 107 (10), 91 (92), 82 (73), 79
(12), 57 (100). C20H28N2O6 (392.44): calcd. C 61.21, H 7.19, N
7.14; found C 61.26, H 7.42, N 6.94.
Ethyl(2S,4S)-4-[(tert-Butoxycarbonyl)amino]-1-[(1R)-1-phenylethyl]-
piperidine-2-carboxylate (24): The procedure described above for
the synthesis of N-Boc amine 23 was applied to salt 22 (228 mg,
0.58 mmol), to give (14 h room temp.) the N-Boc amine 24 (148
mg, 68%) as a white solid (FCC, hexane/EtOAc, 6:1); Rf ϭ 0.57.
[α]2D3 ϭ ϩ34.2 (c ϭ 0.5; CHCl3). M. p. 155Ϫ156 °C. 1H NMR: δ ϭ
1.25 (t, J ϭ 6.8 Hz, 3 H, NCHCH3), 1.31 (d, J ϭ 7.2 Hz, 3 H,
(2S,4R)-2-(Ethoxycarbonyl)-1-[(1R)-1-phenylethyl]piperidin-4-
aminium Trifluoroacetate (21) and (2S,4S)-2-(Ethoxycarbonyl)-1-
[(1R)-1-phenylethyl]piperidin-4-aminium
Trifluoroacetate
(22): OCH2CH3), 1.41 [s, 9 H, (CH3)3C], 1.42Ϫ1.82 (m, H-3 and H-5),
NH4OAc (7.26 g, 94.2 mmol) and NaBH3CN (0.418 g, 6.80 mmol) 2.32 (dm, 12.0 Hz, 1 H, H-5), 2.47 (dm, 12.0 Hz, 1 H, H-3), 2.86
were added to a solution of ketone 1 (1.73 g, 6.28 mmol) in dry
EtOH (8 mL) in the presence of molecular sieves (3 A). The re-
sulting mixture was stirred at room temperature for 48 h. The mix-
ture was filtered and concentrated under reduced pressure. The re-
sulting residue was purified by chromatography on basic alumina
(td, J ϭ 12.0, 3.0 Hz, 1 H, H-6), 3.58 (m, 1 H, H-4), 3.91 (q, J ϭ
6.8 Hz, 1 H, NCHCH3), 3.80Ϫ3.98 (m, 1 H, H-6), 4.04Ϫ4.18 (m,
3 H, H-2 and OCH2CH3), 7.15Ϫ7.35 (m, 5 H, Ar-H) ppm. 13C
NMR: δ ϭ 14.5 (q, OCH2CH3), 21.4 (q, NCHCH3), 28.4 [q, 3C,
(CH3)3C], 32.6 (t), 35.1 (t), 36.5 (t, C-6), 44.5 (d), 56.9 (d), 60.3 (t,
˚
(CH2Cl2/MeOH, 20:1; Rf ϭ 0.26) to give the amines 21 and 22 OCH2CH3), 61.3 (d, C-2), 79.6 (s, CMe3), 126.7 (d, Ar-C), 126.9
(1.32 g, 76% combined yield). These amines were separated by pre- (d, 2 C, Ar-C), 128.3 (d, 2 C, Ar-C), 140.9 (s, Ar-C), 146.3 (s,
parative RP-HPLC with isocratic 15% H2O/CH3CN containing NCO2tBu), 173.0 (s, CO2Et) ppm. MS (EI): m/z (%) ϭ 303 (17) [M
0.1% TFA as eluent: 0.67 g of 21 (Rt ϭ 13.1 min) and 0.53 g of 22
(Rt ϭ 16.3 min) were obtained.
Ϫ CO2Et]ϩ, 271 (5), 247 (8), 203 (7), 186 (42), 154 (9), 105 (100),
82 (47). IR (cmϪ1): ν˜ ϭ 3445, 2978, 1706, 1492. C21H32N2O4
(376.49): calcd. C 66.99, H 8.57, N 7.44; found C 66.64, H 8.72,
N 7.24.
Compound 21: Colourless oil. [α]2D2 ϭ Ϫ5.2 (c ϭ 0.2, H2O). 1H
NMR: δ ϭ 1.26 (t, J ϭ 6.8 Hz, 3 H, OCH2CH3), 1.58 (d, J ϭ
6 Hz, 3 H, NCHCH3), 1.95Ϫ2.60 (m, 4 H, 2 ϫ H-5 and 2 ϫ H-3),
2.94Ϫ3.10 (m, 1 H, H-6ax), 3.10Ϫ3.32 (m, 1 H), 3.32Ϫ4.00 (m, 2
H), 4.00Ϫ4.23 (q, J ϭ 6.8 Hz, 2 H, OCH2CH3), 4.52Ϫ4.71 (m, 1
H, H-2), 7.25Ϫ7.38 (m, 5 H, Ar-H) ppm. MS (EI): m/z (%) ϭ 276
(Ͻ 1) [Mϩ], 203 (26) [M Ϫ CO2Et], 186 (4), 171 (2), 160 (7), 105
(100). C18H25F3N2O4 (390.40): calcd. C 55.38, H 6.45, N 7.18;
found C 55.68, H 6.65, N 7.18.
Ethyl
(2S,4R)-4-[(tert-Butoxycarbonyl)amino]piperidine-2-car-
boxylate (25): Compound 23 (105 mg, 0.28 mmol) was dissolved
in EtOH (3.5 mL), Pd(OH)2/C (20%, 115 mg) was added, and the
apparatus was flushed three times with H2. The reaction mixture
was hydrogenated under atmospheric pressure at room tempera-
ture for 24 h, filtered through a pad of Celite (3 ϫ 2 mL of EtOH
rinse) and then concentrated under reduced pressure to afford the
amine 25 as an oil (73 mg, 95%) sufficiently pure to be used in the
Compound 22: Colourless oil. [α]2D2 ϭ ϩ7.8 (c ϭ 0.125, H2O). 1H
NMR: δ ؍ 1.28 (t, J ϭ 7.1 Hz, 3 H, OCH2CH3), 1.46 (d, J ϭ next step without purification. Yellow oil. [α]2D3 ϭ ϩ38.2 (c ϭ 0.5,
7.0 Hz, 3 H, NCHCH3), 1.85Ϫ2.21 (m, 2 H, H-3 and H-5), CHCl3). 1H NMR (CDCl3): δ ؍ 1.27 (t, J ϭ 7.2 Hz, 3 H,
2.38Ϫ2.65 (m, 2 H, H-3 and H-5), 2.88Ϫ3.10 (m, 1 H, H-6ax),
3.25Ϫ4.08 (m, 3 H, H-4, H-6eq and NCHCH3), 4.16Ϫ4.34 (m, 2 and H-3ax), 2.49 (dm, J ϭ 12.4 Hz, 1 H, H-3eq), 2.94 (td, J ϭ
H, OCH2CH3), 4.38Ϫ4.52 (m, 1 H, H-2), 7.36Ϫ7.45 (m, 5 H, Ar- 12.0, J ϭ 3.2 Hz, 1 H, H-6ax), 3.60Ϫ3.74 (m, 3 H, H-4, H-6ax and
H) ppm. MS (EI): m/z (%) ϭ 276 (Ͻ 1) [Mϩ], 203 (45) [M Ϫ H-2), 4.23 (q, J ϭ 7.2 Hz, 2 H, OCH2CH3), 4.74 (d, J ϭ 8.4 Hz, 1
CO2Et]ϩ, 186 (4), 171 (6), 160 (12), 105 (100). C18H25F3N2O4 H, BocNH) ppm. 13C NMR (CDCl3): δ ؍ 14.1 (q, OCH2CH3),
(390.40): calcd. C 55.38, H 6.45, N 7.18; found C 55.49, H 6.62, 28.4 [q, 3 C, C(CH3)3], 29.1 (t), 33.0 (t), 43.2 (t, C-6), 46.4 (d,
OCH2CH3), 1.41 [s, 9 H, (CH3)3C], 1.60Ϫ2.12 (m, 3 H, 2 ϫ H-5
N 6.88.
C-4), 56.7 (d, C-2), 62.5 (t, OCH2CH3), 79.9 (s, CMe3), 154.9 (s,
Eur. J. Org. Chem. 2004, 2928Ϫ2935
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2933

F. Machetti, F. M. Cordero, F. De Sarlo, A. M. Papini, M. C. Alcaro, A. Brandi
NCO2tBu), 168.6 (s, CO2Et) ppm. MS (EI): m/z (%) ϭ 227 (2) [M NCO2CH2CH), 4.30 Ϫ4.47 (m, 2 H, NCO2CH2), 4.70 and 4.90 (d,
FULL PAPER
Ϫ OEt]ϩ, 199 (78) [M Ϫ CO2Et]ϩ, 143 (86), 81 (100), 56 (40). IR J ϭ 4.4 Hz and J ϭ 5.2 Hz, 1 H, H-2), 7.26Ϫ7.38 (m, 4 H, Ar-H),
(cmϪ1): ν˜ ϭ 3440, 2981, 1710, 1506. C13H24N2O4 (272.34): calcd.
C 57.33, H 8.88, N 10.29; found C 57.64, H 9.06, N 10.04.
7.42Ϫ7.62 (m, 2 H, Ar-H), 7.69Ϫ7.76 (m, 2 H, Ar-H) ppm. 13C
NMR (D2O): δ ؍ 28.2 (t), 28.3 [q, 3 C, C(CH3)3], 30.9 (t), 36.1 (t,
C-6), 44.1 (d, NCO2CH2CH), 47.3 (d, H-4), 51.4 (d, H-2), 67.7 (t,
NCO2CH2), 81.5 [s, C(CH3)3], 119.9 (d, Ar-C), 125.1 (d, Ar-C),
127.1 (d, Ar-C), 127.6 (d, Ar-C), 141.3 (s, Ar-C), 143.9 (s, Ar-C),
156.2 (s), 157.8 (s), 175.2 (s, CO2H) ppm. MS (ESI): m/z (%) ϭ
467 [M ϩ H]ϩ. IR (cmϪ1): ν˜ ϭ 3306, 2981, 2934, 1720, 1691, 1649.
C26H30N2O6 (466.53): calcd. C 66.94, H 6.48, N 6.00; found C
66.64, H 6.58, N 5.80.
Ethyl
(2S,4S)-4-[(tert-Butoxycarbonyl)amino]piperidine-2-car-
boxylate (26): The procedure described above for the synthesis of
amino ester 25 was applied to compound 24 (55 mg, 0.15 mmol),
to give amino ester 26 (36 mg, 90%) as a yellow oil. [α]2D3 ϭ ϩ13.2
(c ϭ 0.5, CHCl3). 1H NMR: δ ϭ 1.25 (t, J ϭ 7 Hz, 3 H,
OCH2CH3), 1.39 [s, 9 H, (CH3)3C], 1.58 Ϫ1.78 (m, 1 H, H-5),
1.85Ϫ2.29 (m, 3 H, 2 ϫ H-3 and H-5), 3.00Ϫ3.40 (m, 2 H, 2 ϫ H-
6), 3.63Ϫ3.80 (m, 1 H, H-4), 4.00Ϫ4.06 (m, 1 H, H-2), 4.20 (q, J ϭ (2S,4S)-4-[(tert-Butoxycarbonyl)amino]-1-[(9H-fluoren-9-yloxy)-
7.0 Hz, 2 H, OCH2CH3), 5.05 (br., 1 H, BocNH) ppm. 13C NMR: carbonyl]piperidine-2-carboxylic Acid (7): Fmoc-OSu (25 mg,
δ ϭ 14.1 (q, OCH2CH3), 28.4 [q, 3 C, C(CH3)3], 29.3 (t), 31.8 (t),
40.6 (t, C-6), 47.4 (d, C-4), 53.9, C-3), 62.0 (t, OCH2CH3), 79.6 (s, 0.041 mmol) in Na2CO3 (10%, 0.07 mL) and acetone (0.7 mL).
CMe3), 155.0 (s, NCO2tBu), 170.2 (s, CO2Et) ppm. MS (EI): m/z After stirring at room temperature for 6 h, the mixture was concen-
(%) ϭ 215 (5), 199 (7) [M Ϫ CO2Et]ϩ, 143 (18), 126 (12), 82 (100), trated under reduced pressure and the residue was purified by FCC
0.074 mmol) was added at 0 °C to a suspension of 28 (10 mg
57 (28). C13H24N2O4 (272.34): calcd. C 57.33, H 8.88, N 10.29;
found C 57.59, H 8.60, N 10.48.
(CH2Cl2/MeOH, 15:1; Rf ϭ 0.25) to give 7 (15 mg, 78%) as a clear
oil. [α]2D5 ϭ Ϫ1.8 (c ϭ 0.41, CHCl3). 1H NMR (amide rotamers):
δ ϭ 1.47 [s, 9 H, (CH3)3C], 1.60Ϫ2.10 (m, 2 H), 2.43Ϫ2.63 (m, 2
H), 3.05Ϫ3.80 (m, 3 H), 4.02 Ϫ4.30 (m, 2 H), 4.30Ϫ4.55 (m, 2 H),
4.40 (dd, J ϭ 14.4, J ϭ 7.0 Hz, 1 H), 4.85Ϫ5.10 (m, 1 H),
7.24Ϫ7.41 (m, 4 H, Ar-H), 7.49Ϫ7.54 (m, 2, Ar-H), 7.76 (d, J ϭ
7 Hz, 2 H, Ar-H) ppm. 13C NMR (D2O, amide rotamers): δ ؍
28.4, 29.7, 32.0, 33.1, 40.8, 47.2, 53.9, 68.0, 80.5, 120.0, 125.1,
127.1, 127.7, 141.3, 144.0, 156.3, 160.3, 167.8 ppm. MS (ESI): m/z
(%) ϭ 467 [M ϩ H]ϩ. IR (cmϪ1): ν˜ ϭ 3399, 2977, 1673, 1592.
C26H30N2O6 (466.53): calcd. C 66.94, H 6.48, N 6.00; found C
66.74, H 6.78, N 5.70.
(2S,4R)-4-[(tert-Butoxycarbonyl)amino]piperidine-2-carboxylic Acid
(27): A mixture of the ester 25 (64 mg; 0.235 mmol) and LiOH
(10.7 mg; 0.45 mmol) in MeOH (1.5 mL) was stirred at room tem-
perature for 24 h. After evaporation of the solvent, the residue was
purified by FCC (CH2Cl2/MeOH, 4:1; Rf ϭ 0.35) to give pure 27
(43 mg, 75%) as a white solid. M.p. 240 °C (dec.). [α]2D2 ϭ ϩ17.6
(c ϭ 0.2, H2O). 1H NMR (D2O): δ ؍ 1.43 [s, 9 H, (CH3)3C],
1.46Ϫ1.69 (m, 2 H, H-3ax and H-5ax), 2.13 (dm, J ϭ 12.8 Hz, 1
H, H-5eq), 2.47 (dm, J ϭ 12.4 Hz, 1 H, H-3eq), 3.07 (td, J ϭ 13.2,
3.0 Hz, 1 H, H-6ax), 3.49 (ddd, J ϭ 13.2, 4.4, 3.0 Hz, 1 H, H-6eq),
3.69 (dd, J ϭ 12.8, 3.4 Hz, 1 H, H-2), 3.62Ϫ3.80 (m, 1 H, H-4) (2S,4R)-2-Carboxypiperidin-4-aminium Trifluoroacetate (4): Boc-
ppm. 13C NMR (D2O): δ ؍ 27.2 [q, 3 C, C(CH3)3], 27.6 (t), 32.2 amine 27 (20 mg, 0.082 mmol) was dissolved in a cooled (0 °C)
(t), 41.7 (t, C-6), 45.6 (d, C-4), 57.9 (d, C-2), 80.9 [s, C(CH3)3], mixture of TFA/H2O 95:5 (1 mL) and the system was stirred at
157.7 (s, NCO2tBu), 172.9 (s, CO2H) ppm. MS (EI): m/z (%) ϭ
room temp. After 2 h the solution was concentrated under reduced
pressure and the resulting residue was washed with Et2O to give
199 (Ͻ 1) [M Ϫ CO2H]ϩ, 142 (16), 126 (29), 82 (100), 57 (70), 56
(60). IR (cmϪ1): ν˜ ϭ 1620. C11H20N2O4 (244.29): calcd. C 54.08, compound 4 (21 mg, 100%) as a colourless solid. M.p. 314 °C (dec.)
H 8.39, N 19.43; found C 53.84, H 8.16, N 19.70.
[α]2D5 ϭ Ϫ5.1 (c ϭ 0.75, H2O). 1H NMR (400 MHz, D2O): δ ؍
1.70Ϫ1.98 (m, 2 H, H-3ax and H-5ax), 2.30 (dm, J ϭ 12.0 Hz, 1
H, H-5eq), 2.68 (dm, J ϭ 13.0 Hz, 1 H, H-3eq), 3.14 (td, J ϭ 13.4,
3.2 Hz, 1 H, H-6ax), 3.52Ϫ3.71 (m, 2 H, H-4 and H-6eq), 3.99 (dd,
J ϭ 12.8, 3.2 Hz, 1 H, H-2) ppm. 13C NMR (100 MHz, D2O): δ ؍
25.7 (t, C-5), 29.7 (t, C-3), 41.6 (t, C-6), 45.9 (d, C-4), 56.0 (d, C-
2), 116.8 (q, JC,F ϭ 293 Hz, CF3CO2H), 163.2 (q, JC,F ϭ 35.2 Hz,
CF3CO2H), 170.3 (s, CO2H) ppm. MS (ESI): m/z (%) ϭ 145 [M ϩ
H]ϩ. IR (cmϪ1): ν˜ ϭ 1610. C8H13F3N2O4 (258.20): calcd. C 37.21,
H 5.07, N 10.85; found C 37.12, H 5.14, N 11.04.
(2S,4S)-4-[(tert-Butoxycarbonyl)amino]piperidine-2-carboxylic Acid
(28): The procedure described above for the synthesis of acid 27
was applied to ester 26 (33 mg; 0.12 mmol), to give acid 28 (23 mg,
77%) as a white solid (FCC, CH2Cl2/MeOH, 4:1; Rf ϭ 0.35). M.p.
Ͼ 345 °C. [α]2D5 ϭ ϩ1.7 (c ϭ 1. 02, H2O). 1H NMR (D2O): δ ϭ
1.41 [s, 9 H, (CH3)3C], 1.70Ϫ2.01 (m, 2 H, H-3ax and H-5ax),
2.01Ϫ2.18 (m, 2 H, H-3eq and H-5eq), 3.10Ϫ3.52 (m, 2 H, H-6ax
and H-6eq), 3.72Ϫ3.84 (m, 2 H, H-2 and H-4) ppm. 13C NMR
(D2O): δ ϭ 27.2 (t), 28.3 [q, 3 C, C(CH3)3], 31.3 (t), 40.1 (t, C-4),
43.8 (d, C-4), 55.4 (d, C-2), 82.0 [s, C(CH3)3], 157.9 (s, NCO2tBu), (2S,4S)-2-Carboxypiperidin-4-aminium Trifluoroacetate (5): The
173.9 (s, CO2H) ppm. MS (EI): m/z (%) ϭ 199 (Ͻ 1) [M Ϫ
procedure described above for the synthesis of salt 4 was applied
CO2H]ϩ, 187 (4), 142 (12), 127 (7), 126 (19), 82 (100), 57 (68), 56 to Boc-amine 28 (10 mg, 0.041 mmol), to give salt 5 (11 mg, 100%)
(80). IR (cmϪ1): ν˜ ϭ 1620. C11H20N2O4 (244.29): calcd. C 54.08, as a colourless solid. M.p. 290 °C (dec.). [α]2D5 ϭ Ϫ10.4 (c ϭ 0.64,
H 8.39, N 19.43; found C 54.04, H 8.49, N 19.33.
H2O). 1H NMR (400 MHz, D2O ϩ TFA): δ ؍ 1.78Ϫ1.85 (m, 1 H,
H-5ax), 2.04Ϫ2.10 (m, 1 H, H-3 ax), 2.17 (dm, J ϭ 13.6 Hz, 1 H,
H-5eq), 2.57 (dm, J ϭ 14.0 Hz, 1 H, H-3eq), 3.28Ϫ3.44 (m, 2 H,
H-6eq and H-6ax), 3.48 (tt, J ϭ 14.0, 3.6 Hz, 1 H, H-4), 4.43 (t,
J ϭ 4.8 Hz, 1 H, H-2) ppm. 13C NMR (100 MHz, D2O): δ ؍ 25.3
(t), 28.1 (t), 39.1 (t, C-6), 43.4 (d, C-4), 54.0 (d, C-2), 115.9 (q,
JC,F ϭ 293 Hz, CF3CO2H), 162.5 (q, JC,F ϭ 35.0 Hz, CF3CO2H),
170.5 (s, CO2H) ppm. MS (ESI): m/z (%) ϭ 145 [M ϩ H]ϩ. IR
(cmϪ1): ν˜ ϭ 1610. C8H13F3N2O4 (258.20): calcd. C 37.21, H 5.07,
N 10.85; found C 37.44, H 4.80, N 11.14.
(2S,4R)-4-[(tert-Butoxycarbonyl)amino]-1-[(9H-fluoren-9-yloxy)-
carbonyl]piperidine-2-carboxylic Acid (6): Fmoc-OSu (80 mg,
0.24 mmol) was added at 0 °C to a suspension of 27 (20 mg
0.082 mmol) in Na2CO3 (10%, 0.3 mL) and acetone (2.5 mL). After
stirring at room temperature for 6 h, the mixture was concentrated
under reduced pressure and the residue was purified by FCC
(CH2Cl2/MeOH, 20:1, Rf ϭ 0.25) to give 6 (32 mg, 84%) as a white
solid. [α]2D5 ϭ Ϫ20.0 (c ϭ 0.34, CHCl3). M. p. 104 °C. 1H NMR
(amide rotamers): δ ϭ 1.47 [s, 9 H, (CH3)3C], 1.60Ϫ1.85 (m, 3 H,
2 ϫ H-5 and H-3), 2.63Ϫ2.85 (m, 1 H, H-3), 3.48Ϫ3.80 (m, 2
H, H-6 and H-4), 3.92Ϫ4.12 (m, 1 H, H-6), 4.18Ϫ4.30 (m, 1 H,
Anchoring of Fmoc-Sly(Boc)-OH on Trityl Chloride Resin and on
Rink Resin
2934
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 2928Ϫ2935

Synthesis of Free and Nα-Fmoc-/Nγ-Boc-Protected Aminopipecolic Acids
FULL PAPER
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0.080 mmol) and DIPEA (56 µL, 0.32 mmol) in dry DCM (1 mL)
was added to the resin under N2 and the reaction was allowed to
proceed at 25 °C for 2 h with continuous mechanical shaking. The
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ϫ 5 min), DCM (3 ϫ 2 mL), DMF (2 ϫ 2 mL) and again with
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[16]
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Fmoc-Rink Resin: The Fmoc-Rink resin (0.44 mmol/g, 100 mg) was
swollen for 1 h in DMF. The resin was deprotected with 20% piperi-
dine in DMF (2 mL, 2 ϫ 15 min) and was then washed with DMF
(5 ϫ 2 mL). DIPCDI (20 µL, 0.22 mmol) was added to a solution
of Fmoc-Sly(Boc)-OH (107 mg, 0.22 mmol) and HOBt (30 mg,
0.22 mmol) in DMF (1 mL). After 10 min this solution was added
to the resin under N2 and the system was mechanically stirred for
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The authors thank the MIUR (Rome, Italy) for financial support
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thesis. Financial support by the Ente Cassa di Risparmio di Firenze
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Received January 13, 2004
Eur. J. Org. Chem. 2004, 2928Ϫ2935
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2935