Synthesis of Fmoc- and Boc-Protected (2 S,5 S)- and (2 R,5 R)-5-Aminomethylprolines

Article abstract of DOI:10.1055/s-0037-1610304

The proline derivatives (2 S,5 S)dmamPro, (2 S,5 S) N -Boc-amPro and (2 R,5 R)dmamPro are useful building blocks for peptides since they allow conformational fixation of peptidyl-CO-N-prolyl bonds in the unusual cis conformation. The stereoselective synth

Full text of DOI:10.1055/s-0037-1610304

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2018, 50, A–I
paper
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en
A. L. Bartuschat et al.
Paper
Synthesis
Synthesis of Fmoc- and Boc-Protected (2S,5S)- and (2R,5R)-5-Ami-
nomethylprolines
Amelie L. Bartuschat
O
O
(R)
(S)
Nina Hegmann
5
5
(R)
2
O
2
(S)
O
O
O
N
N
(S)
Markus R. Heinrich*
0
0
0
0-0
0
0
1-7
1
1
3-
2
0
2
5
(S)
O
O
Department of Chemistry and Pharmacy,
Pharmaceutical Chemistry, Friedrich-
Alexander-Universität Erlangen-Nürnberg,
Nikolaus-Fiebiger-Str. 10, 91058 Erlangen,
Germany
8 steps*
8 steps*
9 steps*
OH
O
(S)
O
(S)
(R)
markus.heinrich@fau.de
BocHN
O
O
NH
N
NH
O
N
(S)
N
(S)
(R)
Fmoc
Fmoc
Fmoc
(2S,5S)dmamPro (A)
23% overall yield
(2S,5S)N-Boc-amPro (B)
33% overall yield
(2R,5R)dmamPro (C)
12% overall yield
* only two purifications by column chromatography required
Received: 11.09.2018
Accepted: 17.09.2018
Published online: 25.10.2018
DOI: 10.1055/s-0037-1610304; Art ID: ss-2018-t0612-op
tertiary amide, which basically suggests that the cis as well
as the trans rotamer of the amide should be observed.5–7
Astonishingly, only very few prolines (ca. 10%) incorporated
in native peptides and proteins do actually prefer the cis
conformation of their peptidyl–CO–N-prolyl bond.7 Syn-
thetic studies on how to influence the stability and the
folding properties of peptides and proteins have so far
mainly been carried out with 3- and 4-substituted proline
derivatives 1–4,8 in which the additional substituents
mainly exert strong effects on the conformation of the pyr-
rolidine ring (Figure 1, grey background). Among the broad
variety of 3- and 4-substituted prolines,9,10 the 4-substitut-
ed derivatives 2–4 represent the more frequent approach
and such compounds have often been employed to tune the
properties of collagen11 and other biomolecules.12
Abstract The proline derivatives (2S,5S)dmamPro, (2S,5S)N-Boc-am-
Pro and (2R,5R)dmamPro are useful building blocks for peptides since
they allow conformational fixation of peptidyl–CO–N-prolyl bonds in
the unusual cis conformation. The stereoselective syntheses of these di-
methylaminomethyl-prolines is achieved from literature-known precur-
sors with overall yields of 23% [over 8 steps for (2S,5S)dmamPro], 33%
[over 9 steps for (2S,5S)N-Boc-amPro] and 12% [over 8 steps for
(2R,5R)dmamPro]. The applicability of (2S,5S)N-Boc-amPro in peptide
synthesis is demonstrated through the preparation of an Fmoc-Val-am-
Pro-OMe dipeptide.
Key words proline, conformation, switchable, amides, peptides
Proline’s unique cyclic structure combined with the re-
sulting conformational restraints enables this amino acid to
play an important role in the folding and stability of many
proteins and peptides.1–4 In addition, when incorporated in
a peptide, proline is the only protein amino acid featuring a
In contrast to the conformational effects on the pyrroli-
dine core observed for prolines 1–4, an additional substitu-
ent(s) at the 5-position of proline, as in compounds 5–7,
can be useful to impact the conformation of the CO–N-pro-
lyl bond.13–16 A cis to trans ratio of 66:34 was observed for
R1
R
R
R
4
3
4
4
R2
CO2H
CO2H
CO2H
CO2H
N
H
N
N
H
N
H
H
4
1
3
2
(3R)- and (3S)-
substituted prolines
R = alkyl, aryl, N3,
NHBoc, OH, OtBu, ...
(4S)-
(4R)-
4,4-
substituted prolines
R = NH2, N3, OH, OR', SR',
F, Cl, ...
substituted prolines
R = N3, OH, OR', SR',
F,..
disubstituted prolines
R1 = Me, F
R2 = Me, F
5
+ H
5
5
5
CO2H
CO2H
N
CO2H
N
CO2H
N
N
N
N
– H
H
R
O
O
R
O
R
O
R
7a
7b
5
6
5-tert-butylproline
5,5-dimethylproline
5-aminomethylproline
Figure 1 Examples of 3-, 4- and 5-substituted proline derivatives
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

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A. L. Bartuschat et al.
Paper
Synthesis
the (5S)-tert-butyl derivative 5,17 whilst 5,5-dimethyl sub-
stitution, as in proline 6, led to an even higher prevalence of
the cis rotamer of 90% in water.18–20 Further studies includ-
ing the incorporation of 5,5-dimethylproline in peptides
demonstrated the high potential of such derivatives in bio-
logical and medicinal applications.21 In a recent study on
the influence of positive charges on the conformation of
amide bonds, it was found that protonated 5-aminomethyl-
prolines 7a represent a valuable alternative to known 5-
substituted proline derivatives, favoring a cis conformation
of the prolyl amide.22,23 Besides a strong conformational fix-
ation observed for 7a in a range of solvents (87% to >95% cis
rotamer), the charge-induced effect is pH-dependent and it
can thus be used to reversibly favor either the cis rotamer
7a or the trans rotamer 7b through the addition of acid or
base.22 Against the background that these initial studies on
the conformational fixation of prolylamide 7a were carried
out with racemic 2,5-trans-configured proline derivatives,
we now describe stereoselective syntheses of trans-config-
ured 5-aminomethylprolines.24 The suitability of the newly
obtained Fmoc-protected acids for incorporation into pep-
tides is further demonstrated with the synthesis of a Fmoc-
L-Val-(2S,5S)amPro-OMe dipeptide.
NH
O
O
DCC, HOBt
DIPEA
5
NaOH
2
O
O
N
N
O
O
N
N
H
(CHCl3)
r.t., 16 h
(S)
(S)
(MeOH/H2O)
45 °C, 4.5 h
O
O
8a (2S,5S)
8b (2R,5R)
9a (84%, 97% brsm)
9b (98%, quant. brsm)
1. Pd/C, H2, TFA
(EtOAc)
50 °C, 2 h
BH3·SMe2
(THF)
50 °C, 6 h
O
O
O
O
N
N
N
(S)
2. BnBr, K2CO3
(MeCN)
r.t., 2 h
then MeOH
reflux, 18 h
O
O
10a (85%)
10b (73%)
11a (91%, 83% from 9a)
11b (90%)
O
O
O
1. Pd/C, H2, TFA, (EtOAc)
50 °C, 2 h
O
HN
N
N
2. NaOH, (MeOH/H2O)
r.t., 2 h
3. Fmoc-Cl, NaHCO3
(THF/H2O)
Fmoc
13a (47%)
13b (39%)
12a (60%)
12b (49%)
0 °C to r.t., 6 h
Scheme 1 Synthesis of Fmoc-protected 5-(dimethylaminomethyl)pro-
lines 13a and 13b; brsm = based on recovered starting material
The chiral, enantiomerically pure pyrrolidines 8a and
8b used as starting materials (Scheme 1) were readily avail-
able from adipic acid dichloride via dibromination, esterifi-
cation and cyclization with (S)-1-phenylethylamine.25 Fol-
lowing a procedure reported by Yamamoto,25b the separa-
tion of the diastereoisomers 8a and 8b was achieved
through a combination of crystallization and column chro-
matography.
the borane–THF adduct.28 The (S)-α-methylbenzyl group of
10a was thus cleaved through hydrogenation with palladi-
um on charcoal (1 bar H2), and alkylation with benzyl bro-
mide gave 11a in high yield without purification of the in-
termediate (Scheme 1).
The selective monohydrolysis of 8a has been reported in
the literature using 1.7 equivalents of sodium hydroxide in
a methanol/water mixture at 20 °C to give 9a in 79% yield
after three days.26 In this work, the single hydrolysis could
be achieved in a much shorter reaction time of 4.5 hours
with 1.5 equivalents of sodium hydroxide at a slightly ele-
vated temperature of 45 °C. Unreacted 8a was separated
from 9a by column chromatography or, more conveniently
for larger scale reactions, via stepwise extraction at accu-
rately defined pH values. N,N′-Dicyclohexylcarbodiimide
(DCC) and 1-hydroxybenzotriazole (HOBt) were chosen for
the synthesis of amide 10a from acid 9a and dimethylamine
due to the later on simple removal of N,N′-dicyclohexylurea
through crystallization (Scheme 1). The purification of am-
ide 10a was possible via column chromatography or ex-
traction, which is again favorable for larger reaction scales.
The subsequent change of the protecting group from
(S)-α-methylbenzyl to benzyl was necessary since the origi-
nally intended reduction of the amide in 10a with borane–
dimethyl sulfide27 led to a number of side reactions includ-
ing demethylation at the amino group. In various attempts,
suitable conditions for selective reduction of 10a could nei-
ther be found with the borane–dimethyl sulfide nor with
Overall, amide 11a could be obtained in three steps
from 9a (83%), requiring no chromatography and only one
crystallization step to remove N,N′-dicyclohexylurea from 10a.
The selective reduction of the amide in 11a was then
achieved with borane–dimethyl sulfide under strictly anhy-
drous conditions.27 Borane species coordinated to the ami-
noalkylproline 12a were removed through heating in re-
fluxing methanol (Scheme 1).27b,c,29 In this particular step,
complete consumption of the starting material 11a was
crucial since the separation of 12a from unreacted starting
material 11a through column chromatography turned out
to be laborious.
Deprotection of 12a and attachment of the 9-fluorenyl-
methoxycarbonyl (Fmoc) group was conducted in three
steps. First, the cleavage of the benzyl protecting group was
achieved via hydrogenation with palladium on charcoal in
the presence of trifluoroacetic acid. Secondly, the ester
moiety was hydrolyzed with 10 equivalents of sodium hy-
droxide in aqueous methanol, and the reaction mixture was
then neutralized and concentrated in vacuo. In the third
step, attachment of the Fmoc protecting group was per-
formed under slightly alkaline conditions using sodium bi-
carbonate in aqueous tetrahydrofuran (Scheme 1). After
quenching with trifluoroacetic acid, the Fmoc-protected
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

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A. L. Bartuschat et al.
Paper
Synthesis
proline 13a could be purified by HPLC. From reactions on
larger scales, 13a was obtained in good purity only by puri-
fication through extraction (see the Supporting Informa-
tion for NMR spectra). After completion of the synthesis of
13a, the synthetic strategy was transferred to the prepara-
tion of 13b from the (2R,5R)-configured diester 8b. In this
sequence, a clean reduction of the amide 11a to 12a again
required a prior change of the protecting group to benzyl.
The yields for the single reaction steps were in most cases
similar to those reached in the preparation of 13a from 8a,
thus indicating the reproducibility of the sequence. The
lower outcome for the final steps from 11b to 13b can be
attributed to the smaller reaction scale which complicates
especially the borane reduction. Determination of the opti-
cal purity of the target compounds 13a and 13b by chiral
HPLC gave ee values of more than 95%.
To enable further synthetic options, and to allow con-
formational fixation as a result of acid-induced deprotec-
tion, a N-Boc protected 5-aminomethylproline derivative
was prepared (Scheme 2). Starting from acid 9a, coupling
with ammonia provided amide 14a. The exchange of the
chiral auxiliary for the benzyl protecting group proceeded
as described for 10a (see Scheme 1). Only the benzylation
step required a longer reaction time to give 15a. Selective
reduction of the amide functionality, removal of complexed
borane, and subsequent Boc protection gave 16a. In the
next steps, the benzyl protecting group was cleaved by hy-
drogenolysis and the ester was hydrolyzed. As the protec-
tion with Fmoc chloride led to an unexpected twofold at-
tachment of the Fmoc group including the formation of a
mixed anhydride from the carboxylic acid, this final trans-
formation was combined with a mild alkaline hydrolysis to
provide 17a. After the final step, the orthogonally protected
proline derivative was purified via extraction to give 17a in
95% purity according to 1H NMR analysis.30
Finally, the applicability of the new building blocks for
peptide synthesis was demonstrated by coupling the N-
Boc-aminomethylproline methyl ester 18 to N-Fmoc-pro-
tected valine. The building block 18 was obtained in quanti-
tative yield via hydrogenation of 16a. Coupling of 18 and
Fmoc-L-Val-OH could be achieved using HATU {1-[bis(di-
methylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridini-
um 3-oxide hexafluorophosphate} and N,N-diisopropyl-
ethylamine (DIPEA) in dry DMF. HPLC monitoring of the re-
action course indicated a conversion of 18 to the dipeptide
of 66% without formation of detectable side products
(Scheme 3). Work-up and purification by column chroma-
tography finally gave the dipeptide 19; during this step
cleavage of the Boc group occurred since a polar solvent
mixture and twofold chromatography on silica had to be
used for purification. The fact that 19 showed only one set
1
of signals when analyzed by H and 13C NMR under acidic
conditions demonstrates that the conformational fixation is
also effective for this particular type of N-prolyl dipeptide.
O
(S)
1. Fmoc-L-Val-OH
DIPEA, HATU, (DMF)
r.t., overnight
O
O
O
(S)
N
(S)
H2N
(S)
O
BocHN
N
R
HN
Fmoc
19 (66%)
2. SiO2 (EtOAc/MeOH)
Pd/C
H2
18: R = H
R = Bn
16a:
Scheme 3 Synthesis of protonated Fmoc-L-Val-(2S,5S)amPro-OMe (19)
NH4Cl
1. Pd/C, H2, TFA
(EtOAc)
50 °C, 2 h
O
O
(S)
(S)
(S)
(S)
DCC, HOBt
DIPEA
O
O
O
H2N
N
N
(S)
(S)
H
2. BnBr, K2CO3
(CHCl3)
O
O
(MeCN), r.t., 16 h
r.t., 4.5 h
9a
14a (88%)
1. BH3·SMe2, (THF)
50 °C, 6 h
O
O
(S)
(S)
then MeOH, 18 h, reflux
O
H2N
O
BocHN
N
N
(S)
(S)
2. Boc2O, K2CO3, (MeCN)
r.t., 6 h
O
15a (92%)
16a (75%)
1. Pd/C, H2, (EtOAc)
45 °C, 2 h
O
(S)
2. NaOH, (MeOH/H2O), r.t., 2 h
O
BocHN
N
(S)
3. Fmoc-Cl, NaHCO3
(THF/H2O), r.t., 1 h
then NaHCO3
Fmoc
17a (56%)
(THF/H2O), r.t., 7 h
Scheme 2 Synthesis of N-Boc-aminomethylproline 17a
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

D
A. L. Bartuschat et al.
Paper
Synthesis
In summary, the synthesis of three new 5-(aminometh-
yl)proline building blocks has been achieved from litera-
ture-known precursors. The eight to nine step sequences
could be performed with good to high yields for all single
steps. Suitability for larger reaction scales is shown since
only two purifications by column chromatography are re-
quired within each synthetic pathway. The starting materi-
als 8a and 8b were available in two steps. All three 5-(ami-
nomethyl)prolines were prepared in Fmoc-protected form
to allow direct use in peptide synthesis, which has been
demonstrated through incorporation of 18 into the Fmoc-
Val-amPro-OMe dipeptide 19.
(2S,5S)-5-(Methoxycarbonyl)-1-[(S)-1-phenylethyl]pyrrolidine-2-
carboxylic Acid (9a)
Sodium hydroxide (2 M, 14.3 mL) was added to 8a (5.53 g, 19.0 mmol)
in water (58 mL) and methanol (143 mL) and was stirred for 4.5 h at
45 °C. Water (35 mL) was added and the mixture was extracted with
ethyl acetate (1 × 200 mL then 1 × 75 mL). The combined organic
phases were dried over sodium sulfate and the solvent was removed
under reduced pressure. Column chromatography (silica gel, hex-
ane/ethyl acetate = 4:1
→
100% ethyl acetate
→
ethyl
acetate/methanol = 9:1) allowed recovery of unreacted 8a (710 mg,
2.45 mmol). The aqueous phase of the previous extraction was ad-
justed to a pH value of 3 with hydrochloric acid (3 M), supersaturated
with sodium chloride and extracted with ethyl acetate (4 × 75 mL).
The combined organic phases were dried over sodium sulfate and the
solvent was removed under reduced pressure to give title compound
9a (4.43 g, 16.0 mmol, 84%, 97% brsm) as a white solid.
Solvents and reagents were obtained from commercial sources and
used as received. Analytical TLC was carried out on Merck silica gel
plates using shortwave (254 nm) UV light, KMnO4 [3 g KMnO4, 20 g
potassium carbonate, 5 mL aqueous sodium hydroxide (5% w/w) in
300 mL H2O] or ninhydrin (200 mg ninhydrin in 100 mL ethanol) to
visualize components. Merck silica gel (40–63 μm) was used for flash
column chromatography. Yields were calculated in percent (%) based
on the employed starting material or in percent based on recovered
starting material (% brsm). Compounds 8a and 8b, which were used as
starting materials, were prepared according to ref. 25b. The analytical
data obtained were in agreement with those reported in the above
reference. Melting points were recorded using a MPM-H2 instrument.
Optical rotations were obtained using a Perkin–Elmer 241 polarime-
ter. IR spectra were recorded as a substance film on a sodium chloride
crystal using a Jasco FT/IR 410 spectrometer. 1H and 13C NMR spectra
were recorded on Bruker Avance 600 (1H: 600 MHz, 13C: 151 MHz),
Bruker Avance 360 (1H: 360 MHz, 13C: 91 MHz) and Bruker Avance
400 (1H: 400 MHz, 13C: 101 MHz) spectrometers. Chemical shifts are
reported in parts per million (ppm) and coupling constants (J) are re-
ported in Hertz (Hz). 1H NMR measurements are referenced to TMS
(0 ppm) or the signals for the residual protons of the used solvent:
CDCl3 (7.26 ppm), CD3CN (1.94 ppm) or D2O (4.79 ppm). For 13C NMR
measurements, CDCl3 and CD3CN were used as solvents using CHCl3
(77.0 ppm) or CD3CN (1.32 ppm) as standards. The following abbrevi-
ations are used for the description of signals: s (singlet), d (doublet), t
(triplet), q (quadruplet), m (multiplet) and br s (broad signal). Mass
spectra were recorded using electron impact (EI) or electrospray ion-
ization (ESI). A JOEL GC mate II GC-MS-System, Bruker Daltonik mic-
roOTOF II, and Bruker Daltonik maXis 4G were used for HRMS mea-
surements. High-performance liquid chromatography (HPLC) was
performed on a Varian 940 C8 column (21.2 × 150 mm × 5 μm) with
PDA (photodiode array) detection (solvent A: 0.1% CF3CO2H in water;
solvent B: acetonitrile; flow rate: 20 mL/min). HPLC-MS was per-
formed using an Agilent 1100 HPLC-MS with a Zorbax XDB-C8 Col-
umn (2.1 × 50 mm × 2.6 μm) with PDA detection at 254 nm and on-
line ESI mass spectrometry detection (solvent A: 0.1% CF3CO2H in wa-
ter; solvent B: acetonitrile containing 0.1% CF3CO2H; flow rate:
0.5 mL/min. Method A: gradient A/B: 90%/10% → 10%/90% over 20
min; method B: gradient A/B: 100%/0% → 10%/90% acetonitrile over
24 min). Chiral HPLC was performed using a Chiralpak IC column
(4.6 × 250 mm × 5 μm) with UV detection at 254 nm (method A: sol-
vent A: hexane; solvent B: ethanol containing 0.1% ethylenediamine;
flow rate: 0.7 mL/min, A/B: 80%/20% isocratic; method B: solvent A:
hexane; solvent B: isopropanol containing 0.1% ethylenediamine;
flow rate: 0.7 mL/min, A/B: 80%/20% isocratic).
Mp 107.4 °C; Rf = 0.7 (ethyl acetate) [UV, KMnO4]; [α]24 –110.7 (c
0.5, CHCl3).
589
IR (NaCl): 2977, 2956, 2881, 1733, 1456, 1374, 1310, 1207, 1166, 767,
704 cm–1
.
1H NMR (360 MHz, CDCl3): δ = 7.36–7.22 (m, 5 H), 4.20 (q, J = 6.7 Hz, 1
H), 3.92 (dd, J = 1.3 Hz, J = 11.0 Hz, 1 H), 3.63 (d, J = 7.3 Hz, 1 H), 3.57
(s, 3 H), 2.66–2.51 (m, 1 H), 2.16–2.02 (m, 2 H), 1.88–1.80 (m, 1 H),
1.38 (d, J = 6.8 Hz, 3 H).
13C NMR (91 MHz, CDCl3): δ = 176.1, 172.8, 142.5, 128.8, 128.0, 127.1,
63.9, 63.4, 60.1, 51.6, 29.8, 28.8, 22.9.
HRMS (ESI): m/z [M + H]+ calcd for C15H20NO4: 278.1387; found:
278.1391.
(2R,5R)-5-(Methoxycarbonyl)-1-[(S)-1-phenylethyl]pyrrolidine-2-
carboxylic Acid (9b)
For the synthesis of the (2R,5R)-isomer 9b, starting material 8b (413
mg, 1.42 mmol) was treated as described above to give the title com-
pound 9b (390 mg, 1.40 mmol, 98%, quant. brsm) as a white solid.
Rf = 0.7 (ethyl acetate) [UV, KMnO4]; [α]24589 +10.8 (c 0.5, CHCl3).
IR (NaCl): 2974, 2952, 1734, 1617, 1456, 1436, 1375, 1306, 1282,
1200, 1166, 1060, 763, 702 cm–1
.
1H NMR (360 MHz, CDCl3): δ = 7.33 (d, J = 7.2 Hz, 2 H), 7.29 (t, J = 7.4
Hz, 2 H), 7.25 (t, J = 7.2 Hz, 1 H), 4.10 (d, J = 5.7 Hz, 2 H), 3.87 (d, J = 9.1
Hz, 1 H), 3.60 (s, 3 H), 2.52–2.45 (m, 1 H), 2.28–2.21 (m, 1 H), 1.93–
1.89 (m, 2 H), 1.46 (d, J = 6.4 Hz, 3 H).
13C NMR (91 MHz, CDCl3): δ = 175.8, 173.6, 141.8, 128.5, 128.4, 128.3,
63.6, 63.4, 60.0, 51.7, 29.4, 29.1, 21.3.
HRMS (ESI): m/z [M + Na]+ calcd for C15H19NNaO4: 300.1206; found:
300.1210.
Methyl (2S,5S)-5-(Dimethylcarbamoyl)-1-[(S)-1-phenylethyl]pyr-
rolidine-2-carboxylate (10a)
Diisopropylethylamine (1.00 mL, 0.75 g, 5.82 mmol), 1-hydroxyben-
zotriazole (0.79 g, 5.82 mmol) and N,N′-dicyclohexylcarbodiimide
(1.64 g, 7.94 mmol) were added to a solution of monoester 9a (1.47 g,
5.29 mmol) in dry chloroform (20.7 mL) under an argon atmosphere.
Dimethylamine (2 M in dry tetrahydrofuran, 5.29 mL, 10.6 mmol)
was added dropwise and the resulting mixture was stirred for 16 h
overnight at room temperature. Subsequently, water (90 mL) and a
saturated aqueous solution of sodium carbonate (10 mL) were added
and the mixture was extracted with chloroform (4 × 40 mL). The com-
bined organic phases were washed with a saturated aqueous solution
of sodium chloride and dried over sodium sulfate. The solvent was re-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

E
A. L. Bartuschat et al.
Paper
Synthesis
moved under reduced pressure. The crude mixture was redissolved in
ethyl acetate and kept at 4 °C overnight. After removal of the white
precipitate by filtration (containing mainly N,N′-dicyclohexylurea)
and washing of the filter cake with small amounts of ethyl acetate,
the filtrate was concentrated under reduced pressure. The crude
product was further purified by column chromatography (silica gel,
hexane/ethyl acetate = 2:1 → 1:1). The title compound 10a (1.37 g,
4.52 mmol, 85%) was obtained as a white solid.
product was purified via column chromatography (silica, hexane/eth-
yl acetate = 1:1) to give the title compound 11a (527 mg, 1.82 mmol,
91%) as a colorless oil.
Rf = 0.3 (hexane/ethyl acetate = 1:2) [UV, KMnO4]; [α]24589 –84.5 (c 0.5,
CHCl3).
IR (NaCl): 2952, 2850, 1733, 1645, 1495, 1454, 1399, 1265, 1200,
1127, 748, 702 cm–1
.
1H NMR (600 MHz, CDCl3): δ = 7.36 (d, J = 7.2 Hz, 2 H), 7.29 (t, J = 7.2
Hz, 2 H), 7.22 (t, J = 7.2 Hz, 1 H), 4.13–3.94 (m, 3 H), 3.78 (d, J = 12.5
Hz, 1 H), 3.67 (s, 3 H), 2.89 (s, 3 H), 2.70 (s, 3 H), 2.42 (qd, J = 9.3 Hz,
J = 12.4 Hz, 1 H), 2.30–2.20 (m, 1 H), 1.99 (t, J = 9.1 Hz, 1 H), 1.78 (tdd,
J = 2.8 Hz, J = 9.4 Hz, J = 12.1 Hz, 1 H).
Mp 69.1 °C; Rf = 0.2 (1:1 hexane/ethyl acetate) [UV, KMnO4]; [α]24
589
–86.9 (c 0.5, CHCl3).
IR (NaCl): 2949, 2359, 2355, 1750, 1730, 1645, 1494, 1455, 1259,
1192, 1152, 1057, 705 cm–1
.
1H NMR (600 MHz, CDCl3): δ = 7.38–7.34 (m, 2 H), 7.29 (t, J = 7.6 Hz, 2
H), 7.21 (t, J = 7.4 Hz, 1 H), 4.07–3.93 (m, 3 H), 3.73 (s, 3 H), 2.81 (s, 3
H), 2.63–2.55 (m, 1 H), 2.35 (s, 3 H), 2.24–2.15 (m, 1 H), 1.87–1.80 (m,
1 H), 1.65 (dd, J = 7.9 Hz, J = 12.1 Hz, 1 H), 1.26 (d, J = 4.5 Hz, 3 H).
13C NMR (151 MHz, CDCl3): δ = 177.7, 173.8, 145.3, 128.2, 127.7,
127.1, 63.3, 60.5, 60.1, 51.7, 26.5, 35.4, 29.9, 28.5, 23.4.
13C NMR (151 MHz, CDCl3): δ = 175.3, 173.3, 139.0, 129.1, 128.1,
127.1, 63.9, 59.9, 53.9, 51.6, 36.8, 35.5, 28.4, 28.1.
HRMS (EI): m/z [M + H]+ calcd for C16H23N2O3: 291.1703; found:
291.1697.
The 1H NMR and 13C NMR data are in agreement with those previous-
ly reported for the racemic compound.22
HRMS (ESI): m/z [M + H]+ calcd for C17H25N2O3: 305.1860; found:
305.1857.
Methyl (2R,5R)-1-Benzyl-5-(dimethylcarbamoyl)pyrrolidine-2-
carboxylate (11b)
Methyl (2R,5R)-5-(Dimethylcarbamoyl)-1-[(S)-1-phenylethyl]pyr-
For the synthesis of the (2R,5R)-isomer 11b, starting material 10b
(243 mg, 0.80 mmol) was treated as described above to give the title
compound 11b (209 mg, 0.72 mmol, 90%) as a colorless oil. The spec-
troscopic data obtained are in agreement with the data described
above.
rolidine-2-carboxylate (10b)
For the synthesis of the (2R,5R)-isomer 10b, starting material 9b (356
g, 1.30 mmol) was treated as described above to give the title com-
pound 10b (287 mg, 0.94 mmol, 73%) as a white solid.
Mp 111.3 °C; Rf = 0.4 (ethyl acetate) [UV, KMnO4]; [α]24589 +42.6 (c 0.5,
[α]24589 +90.0 (c 0.5, CHCl3).
CHCl3).
IR (NaCl): 3559, 3463, 2970, 2949, 1734, 1647, 1494, 1455, 1434,
Methyl (2S,5S)-1-Benzyl-5-[(dimethylamino)methyl]pyrrolidine-
2-carboxylate (12a)
1398, 1366, 1320, 1280, 1195, 1167, 1130, 1090, 1059, 767, 703 cm–1
.
1H NMR (600 MHz, CDCl3): δ = 7.35 (d, J = 6.6 Hz, 2 H), 7.26 (t, J = 7.6
Hz, 2 H), 7.18 (t, J = 7.2 Hz, 1 H), 4.35 (d, J = 5.4 Hz, 1 H), 4.16 (d, J = 5.2
Hz, 1 H), 3.99 (d, J = 8.4 Hz, 1 H), 3.36 (s, 3 H), 2.83 (s, 3 H), 2.79 (s, 3
H), 2.52–2.41 (m, 1 H), 1.80–1.71 (m, 2 H), 1.33 (d, J = 5.7 Hz, 3 H).
13C NMR (91 MHz, CDCl3): δ = 175.6, 173.6, 143.0, 128.0, 127.4, 126.8,
63.7, 59.8, 59.5, 50.7, 36.4, 35.0, 28.9, 28.4, 22.0.
Borane dimethyl sulfide (2 M in dry tetrahydrofuran, 4.00 mL) was
added to a solution of amide 11a (801 mg, 2.76 mmol) in dry tetrahy-
drofuran (40 mL) under an argon atmosphere and the reaction mix-
ture was stirred for 6 h at 50 °C. Subsequently, dry methanol (35 mL)
was added carefully and the mixture was stirred at 75 °C for 18 h. The
reaction course was monitored by TLC. After the reaction was fin-
ished, the solvent was removed under reduced pressure and the crude
product was purified via column chromatography (silica gel, deacti-
vated with triethylamine, hexane/ethyl acetate = 1:1). Title compound
12a (485 mg, 1.66 mmol, 60%) was obtained as a clear viscous oil.
HRMS (EI): m/z [M + Na]+ calcd for C17H24N2NaO3: 327.1680; found:
327.1672.
Methyl (2S,5S)-1-Benzyl-5-(dimethylcarbamoyl)pyrrolidine-2-
carboxylate (11a)
Rf = 0.1 (hexane/ethyl acetate = 1:2) [UV, KMnO4]; [α]24 –164.2 (c
589
0.5, CHCl3).
A mixture of 10a (609 mg, 2.00 mmol), palladium on carbon (10%
w/w, 120 mg, 0.11 mmol) and trifluoroacetic acid (3.90 mL) in dry
ethyl acetate (39 mL) was stirred under a hydrogen atmosphere (1
bar) at 50 °C for 2 h. The reaction course was monitored via TLC.
When the reaction was finished, the mixture was filtered over Celite®
and the filter cake was washed with ethyl acetate. The solvent was re-
moved under reduced pressure and the crude product was dissolved
in acetonitrile (19.5 mL). Potassium carbonate (1.35 g, 9.77 mmol)
and benzyl bromide (0.70 mL, 1.01 g, 5.89 mmol) were added and the
reaction mixture was stirred for 2 h. Subsequently, water (40 mL) was
added and the aqueous layer was extracted with ethyl acetate (4 × 40
mL). The combined organic phases were washed with a saturated
aqueous solution of sodium chloride and dried over sodium sulfate.
The solvent was removed under reduced pressure and the crude
IR (NaCl): 2948, 2817, 2764, 2364, 1732, 1454, 1196, 1159, 1036, 996,
845, 745, 700 cm–1
.
1H NMR (360 MHz, CDCl3, TFA salt): δ = 7.32–7.18 (m, 5 H), 4.07 (d,
J = 13.7 Hz, 1 H), 3.74 (d, J = 13.7 Hz, 1 H), 3.64 (s, 3 H), 3.59 (d, J = 7.9
Hz, 1 H), 3.40–3.33 (m, 1 H), 2.36 (dd, J = 3.8 Hz, J = 12.2 Hz, 1 H),
2.30–2.17 (m, 8 H), 2.12–2.00 (m, 1 H), 1.83–1.73 (m, 2 H).
13C NMR (151 MHz, CDCl3, TFA salt): δ = 174.7, 139.8, 128.5, 128.2,
126.9, 65.2, 63.0, 59.6, 53.3, 51.0, 46.2, 28.9, 27.9.
HRMS (EI): m/z [M + H]+ calcd for C16H25N2O2: 277.1911; found:
277.1909.
The 1H NMR and 13C NMR data are in agreement with those previous-
ly reported for the racemic compound.22
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

F
A. L. Bartuschat et al.
Paper
Synthesis
Methyl (2R,5R)-1-Benzyl-5-[(dimethylamino)methyl]pyrrolidine-
HRMS (ESI): m/z [M + H]+ calcd for C23H27N2O4: 395.1965; found:
2-carboxylate (12b)
395.1961.
HPLC-MS (ESI): Method A, tR = 6.71 min, m/z = 395 [M + H]+; chiral
HPLC: Method A, tR = 14.79 min.
For the synthesis of the (2R,5R)-isomer 12b, starting material 11b
(174.2 mg, 0.60 mmol) was treated as described above to give the title
compound 12b (81.7 mg, 0.29 mmol, 49%) as a colorless oil. The spec-
troscopic data obtained are in agreement with the data described
above.
(2R,5R)-1-{[(9H-Fluoren-9-yl)methoxy]carbonyl}-5-[(dimeth-
ylammonio)methyl]pyrrolidine-2-carboxylate (13b)
[α]24589 +166.0 (c 0.5, CHCl3).
For the synthesis of (2R,5R)-isomer 13b, starting material 12b (50.9
mg, 0.18 mmol) was treated as described above to give the title com-
pound 13b (27.7 mg, 0.07 mmol, 39%) as a white solid. The spectro-
scopic data obtained are in agreement with the data described above.
(2S,5S)-1-{[(9H-Fluoren-9-yl)methoxy]carbonyl}-5-[(dimethylam-
monio)methyl]pyrrolidine-2-carboxylate (13a)
[α]24589 +47.3 (c 0.25, CHCl3).
HPLC-MS (ESI): Method A, tR = 6.89 min, m/z = 395 [M + H]+; chiral
HPLC: Method A, tR = 25.93 min.
A mixture of amine 12a (276 mg, 1.00 mmol), palladium on carbon
(10% w/w, 100 mg, 0.1 mmol) and trifluoroacetic acid (0.46 mL, 0.68
mg, 6.00 mmol) in dry ethyl acetate (9.5 mL) was stirred for 2.5 h at
50 °C under a hydrogen atmosphere (1 bar). The reaction course was
monitored by TLC. When the reaction was finished, the mixture was
filtered over glass wool and the filter bed was washed with ethyl ace-
tate. The solvent was removed under reduced pressure and the ob-
tained residue was dissolved in methanol (36 mL). After addition of
sodium hydroxide (2 M, 5.60 mL), the mixture was stirred for 1 h at
30 °C. The pH value was adjusted to a value of 7 and the solvent was
removed under reduced pressure. The obtained residue was dissolved
in acetonitrile and the remaining solid was filtered off. The filter cake
was washed with acetonitrile and the solvent was removed under re-
duced pressure. The residue was dissolved in tetrahydrofuran (21 mL)
and water (6 mL). Sodium bicarbonate (168 mg, 2.00 mmol) and 9-
fluorenylmethoxycarbonyl chloride (388 mg, 1.50 mmol) were added
and the mixture was stirred for 2.5 h at 30 °C. Trifluoroacetic acid (1
mL) was added and the mixture was extracted with ethyl acetate
(1 × 20 mL). After addition of hexane (30 mL), the organic phase was
washed with water (4 × 30 mL). The combined aqueous phases were
extracted with ethyl acetate (7 × 50 mL) and the completion of the ex-
traction was monitored by TLC. The solvent was concentrated under
reduced pressure and trifluoroacetic acid (1 mL) was added. After
washing with water, the solvent was removed under reduced pressure
to give the title compound 13a (186 mg, 0.47 mmol, 47%) as a white
solid. The compound was further purified via HPLC (gradient A/B:
min 0–5: 90%/10%; min 5–7: 90%/10% → 60%/40%; min 7–21:
60%/40% → 50%/50%; min 21–22: 50%/50% → 10%/90%; min 22–28:
10%/90%; min 28–29: 10%/90% → 90%/10%; min 29–33: 90%/10%;
tR = 16.3 min).
Methyl (2S,5S)-5-Carbamoyl-1-[(S)-1-phenylethyl]pyrrolidine-2-
carboxylate (14a)
Diisopropylethylamine (11.5 mL, 8.71 g, 67.4 mmol), 1-hydroxyben-
zotriazole (2.19 g, 16.2 mmol) and N,N′-dicyclohexylcarbodiimide
(5.57 g, 27.0 mmol) were added to a solution of monoester 9a (3.74 g,
13.5 mmol) in dry chloroform (54 mL) under an argon atmosphere.
Ammonium chloride (2.16 g, 40.4 mmol) was added and the mixture
was stirred for 4.5 h at room temperature. Subsequently, hexane (200
mL) was added and the mixture was extracted with hydrochloric acid
(3 M, 5 × 100 mL). The combined aqueous phases were washed with
hexane (100 mL) and the pH of the aqueous phase was carefully ad-
justed to a value of 9 by addition of potassium carbonate under vigor-
ous stirring. The aqueous phase was extracted with ethyl acetate
(5 × 200 mL), washed with a saturated aqueous solution of sodium
chloride and dried over sodium sulfate. The solvent was removed un-
der reduced pressure and the title compound 14a was obtained as a
white solid which was used in the next step without further purification.
Mp 183.3 °C; Rf = 0.3 (ethyl acetate) [UV, KMnO4]; [α]24 –129.1 (c
0.5, CHCl3).
589
IR (NaCl): 3458, 3235, 3179, 2992, 1730, 1684, 1666, 1454, 1436,
1391, 1376, 1202, 1162, 1140, 991, 766, 702, 612 cm–1
.
1H NMR (360 MHz, CDCl3): δ = 7.34–7.20 (m, 5 H), 7.06 (br s, 1 H),
5.73 (br s, 1 H), 4.09 (q, J = 6.7 Hz, 1 H), 3.78 (dd, J = 1.9 Hz, J = 11.0 Hz,
1 H), 3.56 (d, J = 7.6 Hz, 1 H), 3.54 (s, 3 H), 2.57 (ddt, J = 7.3 Hz, J = 11.1
Hz, J = 13.0 Hz, 1 H), 2.08 (tt, J = 7.7 Hz, J = 12.7 Hz, 1 H), 1.96 (ddd,
J = 2.0 Hz, J = 7.8 Hz, J = 12.8 Hz, 1 H), 1.78 (dd, J = 7.6 Hz, J = 12.8 Hz, 1
H), 1.36 (d, J = 6.7 Hz, 3 H).
Rf = 0.5 (acetonitrile + 1% TFA) [UV, ninhydrin]; [α]24 –43.9 (c 0.5,
589
CHCl3).
IR (NaCl): 3059, 2961, 2891, 2713, 2607, 2501, 1699, 1451, 1415,
13C NMR (91 MHz, CDCl3): δ = 179.9, 173.5, 143.9, 128.6, 127.6, 127.2,
64.5, 64.2, 60.4, 51.2, 30.4, 28.8, 23.5.
1343, 1268, 1198, 1131, 761, 740, 721 cm–1
.
1H NMR (600 MHz, CD3CN, TFA salt): δ = 7.87–7.80 (m, 2.0 H), 7.67–
7.60 (m, 2.0 H), 7.47–7.40 (m, 2.0 H), 7.40–7.32 (m, 2.0 H), 4.85 (dd,
J = 4.9 Hz, J = 11.3 Hz, 0.1 H), 4.69 (dd, J = 4.5 Hz, J = 11.3 Hz, 0.1 H),
4.44 (dd, J = 6.4 Hz, J = 10.6 Hz, 0.9 H), 4.40–4.36 (m, 1.8 H), 4.32 (dd,
J = 1.8 Hz, J = 8.3 Hz, 0.9 H), 4.24 (t, J = 6.6 Hz, 1.0 H), 4.15 (d, J = 8.3
Hz, 0.1 H), 3.91–3.87 (m, 0.1 H), 3.26 (ddd, J = 1.8 Hz, J = 9.8 HZ,
J = 13.2 HZ, 0.9 H), 3.07 (ddd, J = 2.2 Hz, J = 8.5 Hz, J = 13.5 Hz, 0.9 H),
2.96 (d, J = 5.1 Hz, 2.7 H), 2.82 (d, J = 5.1 Hz, 2.7 H), 2.76–2.72 (m, 0.1
H), 2.58–2.54 (m, 0.1 H), 2.52 (d, J = 5.1 Hz, 0.3 H), 2.38 (d, J = 5.1 HZ,
0.3 H), 2.32–2.12 (m, 2.0 H), 2.08–2.00 (m, 1.0 H), 1.70–1.66 (m, 1.0 H).
13C NMR (151 MHz, CDCl3, TFA salt): δ = 173.2, 158.4, 154.9, 144.9,
144.6, 142.3, 142.2, 128.9, 128.8, 128.5, 128.4, 128.3, 128.2, 126.1,
125.9, 125.7, 121.11, 121.08, 69.4, 67.0, 65.0, 61.0, 60.6, 60.4, 55.1,
53.6, 48.3, 47.8, 46.1, 45.1, 43.8, 43.7, 29.2, 28.7, 28.0, 27.6.
HRMS (ESI): m/z [M + Na]+ calcd for C15H20N2NaO3: 299.1366; found:
299.1369.
Methyl (2S,5S)-1-Benzyl-5-carbamoylpyrrolidine-2-carboxylate
(15a)
A mixture of 14a (max. 13.5 mmol), palladium on carbon (10% w/w,
450 mg, 0.42 mmol) and trifluoroacetic acid (4.50 mL) in dry ethyl ac-
etate (45 mL) was stirred for 2 h at 45 °C under a hydrogen atmo-
sphere. The reaction course was monitored by TLC. When the reac-
tion was finished the mixture was filtered over Celite® and washed
with ethyl acetate. The solvent was removed under reduced pressure
and the obtained residue was dissolved in acetonitrile (67.5 mL). Po-
tassium carbonate (14.9 g, 108 mmol) and benzyl bromide (8.02 mL,
11.5 g, 67.5 mmol) were added and the mixture was stirred at 45 °C
for 16 h. Subsequently, hexane (150 mL) was added and the mixture
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

G
A. L. Bartuschat et al.
Paper
Synthesis
was extracted with water (5 × 75 mL). The combined aqueous phases
were washed with hexane (20 mL) and the pH was adjusted to a value
of 11 by addition of potassium carbonate under vigorous stirring. The
aqueous phase was extracted with ethyl acetate (5 × 250 mL), dried
over sodium sulfate and the solvent was removed under reduced
pressure. The title compound 15a (2.59 g, 9.46 mmol, 70% from 9a)
was obtained as a white solid and was used for the next step without
further purification.
(2S,5S)-1-{[(9H-Fluoren-9-yl)methoxy]carbonyl}-5-{[(tert-butoxy-
carbonyl)amino]methyl}pyrrolidine-2-carboxylic Acid (17a)
A suspension of 16a (912 mg, 2.62 mmol) and palladium on carbon
(10% w/w, 524 mg, 0.49 mmol) in dry ethyl acetate (26 mL) was
stirred at 45 °C for 2 h under a hydrogen atmosphere (1 bar). The re-
action course was monitored by TLC. After the reaction was complete,
the reaction mixture was filtered over Celite® and the filter cake was
washed with ethyl acetate. The solvent was removed under reduced
pressure and (2S,5S)-methyl 5-{[(tert-butoxycarbonyl)amino]meth-
yl}pyrrolidine-2-carboxylate (18) (quant.) was used without further
purification. The obtained product (301 mg, 1.17 mmol) was dis-
solved in methanol (12 mL), sodium hydroxide (2 M, 5.83 mL) was
added, and the reaction mixture was stirred for 2 h at room tempera-
ture. The reaction course was monitored via TLC. Subsequently, the
pH was adjusted to a value of 8 by addition of hydrochloric acid (3 M)
and a saturated aqueous solution of potassium carbonate. The solvent
was removed under reduced pressure and the residue was dissolved
in tetrahydrofuran (29 mL) and water (7.3 mL). Sodium bicarbonate
(392 mg, 4.66 mmol) and 9-fluorenylmethoxycarbonyl chloride (453
mg, 1.75 mmol) were added and the reaction mixture was stirred for
1 h at room temperature. The reaction course was monitored by TLC.
After extraction with ethyl acetate (2 × 50 mL), the combined organic
phases were washed with a saturated solution of sodium bicarbonate,
dried over sodium sulfate and the remaining aqueous phases were
kept for further processing (see below). The solvent of the organic
phase was removed under reduced pressure leading to a residue con-
taining (2S,5S)-1-{[(9H-fluoren-9-yl)methoxy]carbonyl}-5-{[(tert-bu-
toxycarbonyl)amino]methyl}pyrrolidine-2-carboxylic [(9H-fluoren-
9-yl)methyl-carbonic] anhydride. The residue containing the anhy-
dride was dissolved in tetrahydrofuran (29 mL) and water (7.3 mL)
and the mixture was stirred for 7 h at room temperature, while the
reaction course was monitored by TLC. The solvent was removed un-
der reduced pressure, the residue was dissolved in water (30 mL) and
washed with methyl-tert-butyl ether (2 × 30 mL). The remaining
aqueous phase was adjusted to a pH value of 2 with hydrochloric acid
(3 M) and was extracted with ethyl acetate (4 × 40 mL). The combined
ethyl acetate phases were dried over sodium sulfate and the solvent
was removed under reduced pressure to give 17a (52%). Subsequently,
the previously kept combined aqueous phases (see above) were ad-
justed to a pH value of 2 with hydrochloric acid (3 M) and extracted
with ethyl acetate (4 × 70 mL). The combined organic phases of this
extraction were dried over sodium sulfate and the solvent was re-
moved under reduced pressure to afford an additional amount of 17a
(27 mg, 0.06 mmol, 4%). The title compound 17a (304 mg, 94% w/w,
0.61 mmol, 52%, in total 56%) was obtained as a white solid.
Mp 136.7 °C; Rf = 0.3 (ethyl acetate) [UV, KMnO4]; [α]24 –121.1 (c
589
0.5, CHCl3).
IR (NaCl): 3437, 3183, 2945, 2883, 2362, 2333, 1733, 1679, 1453,
1359, 1207, 1158, 754 cm–1
.
1H NMR (360 MHz, CDCl3): δ = 7.35–7.27 (m, 3 H), 7.24–7.21 (m, 2 H),
6.90 (br s, 1 H), 5.61 (br s, 1 H), 3.92 (d, J = 13.1 Hz, 1 H), 3.84 (d,
J = 13.1 Hz, 1 H), 3.77 (dd, J = 3.3 Hz, J = 10.9 Hz, 1 H), 3.74 (d, J = 7.7
Hz, 1 H), 3.69 (s, 3 H), 2.54 (dtd, J = 8.1 Hz, J = 10.9 Hz, J = 12.6 Hz, 1
H), 2.16–2.04 (m, 1 H), 2.00–1.85 (m, 2 H).
13C NMR (91 MHz, CDCl3): δ = 177.7, 173.5, 138.0, 128.7, 128.6, 127.6,
65.8, 62.9, 54.2, 51.4, 29.4, 28.6.
HRMS (ESI): m/z [M + Na]+ calcd for C14H18N2NaO3: 285.1209; found:
285.1202.
Methyl (2S,5S)-1-Benzyl-5-{[(tert-butoxycarbonyl)amino]meth-
yl}pyrrolidine-2-carboxylate (16a)
Borane dimethyl sulfide (2 M in dry tetrahydrofuran, 2.73 mL, 5.55
mmol) was added dropwise to a solution of amide 15a (393 mg, 1.50
mmol) in dry tetrahydrofuran (7.6 mL) under an argon atmosphere
and the reaction mixture was stirred for 6 h at 50 °C. Subsequently,
dry methanol (13 mL) was added dropwise and the reaction mixture
was stirred for 18 h at 75 °C. The solvent was removed under reduced
pressure and the obtained residue was dissolved in tetrahydrofuran
(15 mL) and treated with potassium carbonate (212 mg, 2.00 mmol)
and di-tert-butyldicarbonate (437 mg, 2.00 mmol) for 8.5 h. Water
(50 mL) was added and the resulting mixture was extracted with eth-
yl acetate (3 × 50 mL). The combined organic phases were washed
with a saturated aqueous solution of sodium chloride and dried over
sodium sulfate. The crude mixture was purified by column chroma-
tography (silica gel, hexane/ethyl acetate = 9:1 → 4:1 → ethyl acetate)
to give the title compound 16a (392 g, 1.13 mmol, 75%) as a colorless
oil.
Rf = 0.5 (hexane/ethyl acetate = 4:1) [UV, KMnO4]; [α]24 –110.3 (c
589
0.5, CHCl3).
IR (NaCl): 3392, 2977, 2952, 1733, 1714, 1496, 1454, 1366, 1249,
1163, 743, 699 cm–1
.
Rf = 0.3 (CH2Cl2/methanol, 19:1) [UV, ninhydrin].
1H NMR (600 MHz, CDCl3): δ = 7.38–7.36 (m, 1 H), 7.32–7.28 (m, 2 H),
7.26–7.22 (m, 2 H), 4.79 (br s, 1 H), 3.91 (d, J = 13.5 Hz, 1 H), 3.72 (d,
J = 13.5 Hz, 1 H), 3.66 (s, 3 H), 3.63 (d, J = 8.2 Hz, 1 H), 3.45–3.39 (m, 1
H), 3.34 (dd, J = 8.3 Hz, J = 12.2 Hz, 1 H), 3.05 (d, J = 13.1 Hz, 1 H), 2.16
(qd, J = 9.4 Hz, J = 12.6 Hz, 1 H), 2.05–1.98 (m, 1 H), 1.78 (dd, J = 9.8
Hz, J = 11.9 Hz, 1 H), 1.74-1.68 (m, 1 H), 1.45 (s, 9 H).
13C NMR (91 MHz, CDCl3): δ = 174.4, 156.4, 139.2, 128.5, 128.4, 127.1,
78.1, 63.3, 60.9, 52.9, 51.5, 42.1, 28.4, 28.1, 27.0.
HRMS (ESI): m/z [M + Na]+ calcd for C19H28NaN2O4: 371.1941; found:
IR (NaCl): 3354, 2976, 2360, 2341, 1699, 1520, 1450, 1415, 1365,
1340, 1247, 1166, 1128, 1089, 758, 741 cm–1
.
1H NMR (600 MHz, CDCl3): δ (mixture of rotamers, 1:1) = 7.76 (dd,
J = 3.7 Hz, J = 7.4 Hz, 1 H), 7.73 (d, J = 7.5 Hz, 1 H), 7.59–7.52 (m, 2 H),
7.44–7.27 (m, 4 H), 5.10 (t, J = 5.9 Hz, 0.5 H), 4.72–4.65 (m, 1 H), 4.50–
4.43 (m, 1 H), 4.32–4.28 (m, 0.5 H), 4.26–4.21 (m, 1 H), 4.15–4.11 (m,
0.5 H), 4.08–4.01 (m, 1 H), 3.47–3.40 (m, 0.5 H), 3.33–3.26 (m, 0.5 H),
3.26–3.13 (m, 0.5 H), 2.84–2.77 (m, 0.5 H), 2.63–2.57 (m, 0.5 H), 2.35–
1.76 (m, 4 H), 1.44 (s, 4.5 H), 1.42 (s, 4.5 H).
371.1939.
13C NMR (151 MHz, CDCl3): δ (mixture of rotamers, 1:1) = 175.6,
174.7, 156.4, 156.1, 155.4, 144.0, 143.8, 143.7, 143.6, 141.41, 141.36,
141.3, 127.84, 127.79, 127.7, 127.6, 127.4, 127.3, 127.1, 127.0, 124.9,
124.8, 124.6, 119.94, 119.91, 119.85, 79.5, 79.3, 67.3, 66.8, 59.6, 59.3,
58.9, 58.6, 47.3, 47.2, 43.7, 42.4, 28.8, 28.4, 27.4, 26.9, 26.7.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

H
A. L. Bartuschat et al.
Paper
Synthesis
HRMS (ESI): m/z [M + H]+ calcd for C26H31N2O6: 467.2177; found:
467.2175.
Supporting Information
Supporting information for this article is available online at
Chiral HPLC: Method A: tR = 30.49 min; Method B: tR = 22.42 min.
https://doi.org/10.1055/s-0037-1610304.
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Methyl (2S)-1-({[(9H-Fluoren-9-yl)methoxy]carbonyl}-L-valyl)-5-
(aminomethyl)pyrrolidine-2-carboxylate (19)
References
A suspension of 16a (71.6 mg, 0.03 mmol) and palladium on carbon
(10% w/w, 42 mg, 0.05 mmol) in dry ethyl acetate (5 mL) was stirred
at 45 °C under a hydrogen atmosphere (1 bar). The reaction course
was monitored by HPLC-MS. After the reaction was complete, the re-
action mixture was filtered over Celite® and the filter cake was
washed with ethyl acetate. The solvent was removed under reduced
pressure and (2S,5S)-methyl 5-{[(tert-butoxycarbonyl)amino]meth-
yl}pyrrolidine-2-carboxylate (18) (quant.) was used without further
purification.
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For the synthesis of 19, compound 18 (20 mg, 0.08 mmol), Fmoc-L-
Val-OH (102 mg, 0.3 mmol) and N,N-diisopropylethylamine (0.17 mL,
1.00 mmol) were dissolved in dry N,N-dimethylformamide (2 mL) be-
fore 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyri-
dinium 3-oxide hexafluorophosphate (HATU) (0.4 mmol, 152 mg)
was added and the reaction mixture stirred overnight. The reaction
course was monitored by HPLC. After 18 h, water was added and the
reaction mixture was extracted with ethyl acetate (3 × 15 mL). The
combined organic phases were washed with a saturated aqueous
solution of sodium chloride and dried over sodium sulfate. Subse-
quently, the solvent was removed under reduced pressure. Prepurifi-
cation by column chromatography on silica was conducted using hex-
ane/ethyl acetate = 1:1 → ethyl acetate/methanol = 9:1, during which
cleavage of the Boc group occurred on removal of the solvent. A sec-
ond column chromatography (silica, hexane/ethyl acetate = 4:1) pro-
vided 19 (23.1 mg, 0.05 mmol, 66%) as a colorless oil.
Rf = 0.7 (hexane/ethyl acetate = 1:1) [UV, KMnO4].
1H NMR (400 MHz, CDCl3, acidic): δ = 7.79 (d, J = 7.5 Hz, 2 H), 7.63 (dd,
J = 2.4 Hz, J = 7.5 Hz, 2 H), 7.43 (t, J = 7.5 Hz, 2 H), 7.38–7.32 (m, 3 H),
5.38–5.28 (m, 1 H), 4.55–4.40 (m, 2 H), 4.34 (dd, J = 4.9 Hz, J = 9.1 Hz,
1 H), 4.26 (t, J = 7.0 Hz, 1 H), 3.78 (s, 3 H), 2.24–2.15 (m, 1 H), 1.76-
1.52 (m, 3 H), 1.45–1.43 (m, 1 H), 1.29–1.23 (m, 1 H), 1.15–1.10 (m, 3
H), 1.00 (d, J = 6.9 Hz, 3 H), 0.94 (d, J = 6.9 Hz, 3 H).
13C NMR (151 MHz, CDCl3, acidic): δ = 172.6 (Cq), 156.2 (Cq), 143.9
(Cq), 143.8 (2 × Cq), 141.3 (2 × Cq), 127.7 (2 × CH), 127.1 (2 × CH),
125.1 (2 × CH), 120.0 (2 × CH), 67.0 (CH2), 59.5 (CH2), 59.0 (CH), 52.2
(CH3), 47.2 (2 × CH), 38.2 (2 × CH2), 31.3 (CH), 31.2 (CH), 18.9 (CH3),
17.6 (CH3).
HRMS (ESI): m/z [M + H]+ calcd for C27H34N3O5: 480.2493; found:
480.2493.
Funding Information
Deutsche Forschungsgemeinschaft HE5413/6-1
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Acknowledgment
The authors are grateful for the support of this project by Karina
Wicht, Laura Hofmann, Eva Gans, Jonas Ludwig, Jannis Beutel and
Rebecca Hoffmann (FAU Erlangen-Nürnberg).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I

I
A. L. Bartuschat et al.
Paper
Synthesis
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–I