Effect of γ-Substituted Proline Derivatives on the Performance of the Peptidic Catalyst H- d Pro-Pro-Glu-NH2

Article abstract of DOI:10.1055/s-0037-1609547

Substituents at Cγ of proline are valuable probes to tune the trans / cis ratio of Xaa-Pro bonds. We investigated the effect of Cγ-substituents on the reactivity and stereoselectivity of the peptidic catalyst H- d Pro-Pro-Glu-NH ₂. Derivatives that bear electron-withdrawing and -donating substituents (OH, F, N ₃, and SMe) at Cγ of the middle Pro-residue were examined. The results show that substituents at a 4 R -configured Cγ hardly affect the stereoselectivity of the peptidic catalyst whereas substituents at a 4 S -configured Cγ can be used to tune and improve the catalytic performance.

Full text of DOI:10.1055/s-0037-1609547

SYNTHESIS
0
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9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
© Georg Thieme Verlag Stuttgart · New York
2018, 50, A–F
paper
A
en
T. Schnitzer, H. Wennemers
Paper
Synthesis
Effect of γ-Substituted Proline Derivatives on the Performance of
the Peptidic Catalyst H-DPro-Pro-Glu-NH2
Tobias Schnitzer
X
Helma Wennemers*
0
0
0
0
0-
0
0
2
3-
0
7
5
5-
7
4
1
*
H
N
CONH2
CO2H
N
Laboratory of Organic Chemistry, D-CHAB, ETH Zürich,
Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland
Helma.Wennemers@org.chem.ethz.ch
1 mol%
O
R2
O
O
O
NO2
NO2
+
H
R2
NH·TFA
H
R1
Dedicated to Professor Scott E. Denmark
R1
Published as part of the Special Section dedicated to Scott
E. Denmark on the occasion of his 65th birthday
Stereoselectivity?
*
= H, OH, F, N3, or SMe
= (R) or (S)
X
Received: 27.05.2018
Accepted after revision: 18.06.2018
Published online: 02.07.2018
1 mol%
H
N
CONH2
CO2H
N
trans
O
R2
O
DOI: 10.1055/s-0037-1609547; Art ID: ss-2018-c0368-op
O
O
NO2
NO2
+
H
R2
NH·TFA
H
Abstract Substituents at Cγ of proline are valuable probes to tune the
trans/cis ratio of Xaa–Pro bonds. We investigated the effect of Cγ-sub-
stituents on the reactivity and stereoselectivity of the peptidic catalyst
H-DPro-Pro-Glu-NH2. Derivatives that bear electron-withdrawing and
-donating substituents (OH, F, N3, and SMe) at Cγ of the middle Pro-
residue were examined. The results show that substituents at a 4R-con-
figured Cγ hardly affect the stereoselectivity of the peptidic catalyst
whereas substituents at a 4S-configured Cγ can be used to tune and im-
prove the catalytic performance.
R1
R1
CHCl3/i-PrOH (9:1)
2–24 h, r.t.
d.r. 7:1–50:1, 95–98% ee
Scheme 1 Conjugate addition reactions catalyzed by H-DPro-Pro-Glu-
NH₂
Substituents at the pyrrolidine ring of proline also affect
the trans/cis ratio of Xaa–Pro bonds.8–13 Among the most in-
vestigated derivatives are proline residues that bear substit-
uents at Cγ.
Key words proline, peptides, trans/cis isomerization, organocatalysis,
conjugate addition reactions
The substituent influences the pucker of the pyrrolidine
ring and the trans/cis ratio of Xaa–Pro bonds either by steric
effects,8 stereoelectronic gauche effects,9 repulsive interac-
tions, and/or transannular hydrogen bonding.10 For exam-
ple, electron-withdrawing groups such as OH, F, or N3 exert
a gauche effect and favor a Cγ-exo pucker when the substit-
uent is installed at a 4R-configured Cγ and a Cγ-endo pucker
when Cγ is 4S-configured (Figure 1).9
In the case of 4R-configured proline derivatives, the
trans/cis ratio is higher compared to that of unsubstituted
proline but lower in the case of 4S-configured derivatives
(Figure 1c). The trans-conformer is in both diastereoiso-
mers stabilized by an n→π* interaction between adjacent
amide groups (Figure 1a).14 This interaction is weakened in
the case of the 4S-configured derivatives since a repulsion
between the substituent at Cγ and the carbonyl group leads
to an unfavorable Ψ-angle for the n→π* interaction and
hence a higher population of the cis conformer (Figure 1b
and 1c, right). Conversely, the Cγ-exo pucker of 4R-config-
ured derivatives favors the n→π* interaction (Figure 1b and
1c, left). Nature takes advantage of these effects and uses,
for example, (4R)-hydroxyproline (Hyp) to stabilize the col-
Peptides have become valuable as stereoselective cata-
lysts for many different reactions, including acylations, ep-
oxidations, transfer hydrogenations, and C–C bond forma-
tions.1,2 Our group contributed peptidic catalysts of the H-
Pro-Pro-Xaa type (Xaa = any amino acid) for aldol and con-
jugate addition reactions.3–6 These tripeptides are so reac-
tive that catalyst loadings of ≤1 mol% typically suffice to en-
able the C–C bond formation with very good stereoselectiv-
ities and yields. For example, as little as 0.1 mol% of H-DPro-
Pro-Glu-NH2 will suffice to obtain the products of addition
reactions of aldehydes to nitroolefins with excellent enan-
tio- and diastereoselectivity (Scheme 1).4 Recently, we
showed that the trans/cis isomer ratio of the tertiary amide
bond of the DPro-Pro moiety has a significant effect on the
reactivity and stereoselectivity of the peptidic catalyst.7 We
accessed different trans/cis ratios by varying the solvent
and incorporating ring-size analogues of proline in the sec-
ond position. These experiments showed that the higher
the population of the trans-isomer the higher is the reactiv-
ity and stereoselectivity of the peptidic catalyst.7
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–F

B
T. Schnitzer, H. Wennemers
Paper
Synthesis
systems that were assigned by nuclear Overhauser effects
(NOEs) as trans and cis conformers. We determined the
trans/cis ratios by 1H NMR spectroscopy in solutions of
CDCl3/CD3OH (9:1) to be as close as possible to the CDCl3/i-PrOH
(9:1) solvent mixture that is optimal for conjugate addition
reactions of aldehydes to nitroolefins catalyzed by 1.4,7 The
conjugate addition reaction of butanal to nitrostyrene was
then used as model reaction to evaluate the catalytic per-
formance of the peptides. The reaction was performed in
CHCl3/i-PrOH (9:1) with 1 mol% of the peptide TFA-salts
and an equimolar amount of N-methylmorpholine (NMM)
to liberate the secondary amine. All peptides provided the
desired conjugate addition product in a yield of ≥95%, but
their stereoselectivities differed significantly (Tables 1 and
2).
X
X
a)
*
*
O
O
cis
trans
p*
N
N
n
O
O
interaction
b)
c)
Cγ-exo
Cγ-endo
O
X
O
(S)
(R)
H
H
N
O
N
O
X
Ktrans/cis
Ktrans/cis
X = H
4.6
6.1
6.7
6.1
4.1
X = H
4.6
2.4
2.6
2.6
3.1
X
X
(R)
(S)
X = OH
X = F
X = OH
X = F
O
O
N
N
X = N3
X = SMe
X = N3
X = SMe
OMe
OMe
Ac
Ac
First, we focused on the properties of the tripeptides
bearing a 4R-configured proline residue. Peptides 2R–5R
have trans/cis ratios between 30 and 38 (Table 1, entries 2–
5). These values are smaller compared to that of the parent
tripeptide 1 (Ktrans/cis = 46, entry 1). In agreement with our
previous finding that lower trans/cis ratios go hand in hand
with lower stereoselectivity, the dia- and enantioselectivi-
ties of peptides 2R–5R are lower (d.r. ~25:1, 95% ee) com-
pared to those of peptide 1 (d.r. 35:1, 97% ee).
Figure 1 a) trans/cis-Isomers of Pro amide bonds; b) Cγ-exo and Cγ-
endo conformers; c) Ktrans/cis of model compounds Ac-Xaa-OMe in D2O;
data taken from ref. 12, 13
lagen triple helix with all-trans amide bonds.15 Hyp and
other substituted proline derivatives have also been used
by us and others as tools to control the trans/cis ratio of
Xaa–Pro bonds within peptides and proteins.15–17 We there-
fore envisioned that Cγ-substituted proline derivatives
might be valuable to tune the trans/cis ratio in Pro-Pro-Xaa
type catalysts and thereby their stereoselectivity.
Table 1 Conjugate Addition Reaction of Butanal to Nitrostyrene Cata-
lyzed by Peptides 1 and 2R–5R
Herein, we implemented different substituents at Cγ of
the middle Pro residue within H-DPro-Pro-Glu-NH2 (1) and
investigated their effect on the performance of the peptidic
catalysts in conjugate addition reactions. We show that 4R-
configured proline residues decrease the stereoselectivity
of the peptidic catalyst to a small extent whereas 4S-config-
ured proline residues affect the catalytic properties signifi-
cantly. The study resulted in a catalyst with even higher en-
antioselectivity compared to the parent compound.
We started by preparing analogues of 1 bearing differ-
ent substituents at either 4R- or 4S-configured Cγ-carbons
in the middle position. Proline derivatives with hydroxy
(Hyp), fluorine (Flp), azido (Azp), and methyl thioether
(Mtp) moieties were chosen as groups with different steric
and stereoelectronic effects (Figure 2).
Peptide•TFA (1 mol%)
NMM (1 mol%)
O
O
Ph
NO2
NO2
+
H
Ph
H
CHCl3/i-PrOH (9:1)
Et
(1.5 equiv)
Et
6–24 h, r.t.
(1 equiv)
a
Kt/c
Entry
Peptide
X
Yield (%)b d.r.c
ee (%)d
1e
2
1
H
46
>95
>95
>95
>95
>95
35:1
25:1
97
95
95
95
96
2R
3R
4R
5R
OH
F
36
34
30
38
3
26:1
24:1
24:1
4
N3
5
SMe
a Determined by 1H NMR spectroscopic analysis in CDCl3/CD3OH (9:1).
b Yield of the isolated product. Of note, most reactions were complete af-
ter 6–8 h.
c Determined by 1H NMR spectroscopic analysis.
d Determined by chiral stationary phase SFC analysis.
e Data taken from ref. 7.
X
X = H
1
*
H
2R
3R
4R
2S
3S
4S
X = OH
X = F
N
CONH2
N
The smaller trans/cis ratios of 2R–5R compared to 1 are
at first glance surprising as 4R-configured Hyp, Flp, Azp,
and Mtp derivatives have in general higher trans/cis ratios
compared to unsubstituted proline (Figure 1c, left).9,11–13
The reasons become apparent by comparing the conforma-
tion of the 4R-configured proline derivatives with that of
peptide 1. A recent crystal structure of the TFA-salt of pep-
tide 1, which is in agreement with NMR spectroscopic stud-
ies in solution, shows that the middle proline residue
adopts a Cγ-endo pucker (Figure 3).7 In contrast, the pucker
* = (R)
* = (S)
O
X = N3
O
CO2H
NH
5R
5S
X = SMe
Figure 2 Derivatives of the tripeptidic catalyst H-DPro-Pro-Glu-NH₂
Peptides 2R–5R and 2S–5S were readily obtained as
their trifluoroacetic acid (TFA)-salts by standard solid-
phase peptide synthesis following the Fmoc/t-Bu protocol.
As expected, NMR spectra of all peptides show two spin
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–F

C
T. Schnitzer, H. Wennemers
Paper
Synthesis
of 4R-configured proline derivatives with electron-with-
drawing groups is preferentially Cγ-exo. In addition, pep-
tide 1 adopts a type I β-turn in which the adjacent carbonyl
groups of the DPro and Pro residues are not engaged in an
n→π* interaction.7,18 The enhanced n→π* interaction of 4R-
configured proline derivatives is therefore overwritten by
the H-bonded β-turn structure of the peptide and cannot
stabilize the trans conformer of peptide 1.19
Table 2 Conjugate Addition Reaction of Butanal to Nitrostyrene Cata-
lyzed by Peptides 1 and 2S–5S
Peptide•TFA (1 mol%)
NMM (1 mol%)
O
O
Ph
NO2
NO2
+
H
Ph
H
CHCl3/i-PrOH (9:1)
Et
(1.5 equiv)
Et
6–24 h, r.t.
(1 equiv)
a
Kt/c
Entry
Peptide
X
Yield (%)b
d.r.c
ee (%)d
1e
2
1
H
46
>95
>95
>95
>95
>95
35:1
17:1
26:1
29:1
30:1
97
93
98
99
97
2S
3S
4S
5S
OH
F
>50
30
3
4
N3
20
5
SMe
44
a Determined by 1H NMR spectroscopic analysis in CDCl3/CD3OH (9:1).
b Yield of the isolated product. Of note, most reactions were complete after
6–8 h.
c Determined by 1H NMR spectroscopic analysis.
d Determined by chiral stationary phase SFC analysis.
e Data taken from ref. 7
Figure 3 Crystal structure of the TFA salt of H-DPro-Pro-Glu-NH₂ (TFA
anion not shown for clarity). Structure taken from ref. 7.
In conclusion, our study showed that substituents in the
Cγ-position of proline residues are valuable tools to tune
the stereoselectivity of Pro-Pro-Xaa type catalysts. Substit-
uents at a 4S-configured Cγ in the middle position point to-
wards the active site and have a significant effect on the
catalytic performance. Their effect on the catalytic perfor-
mance can even overcome unfavorable trans/cis ratios of
the DPro–Pro bond. The study allowed for the development
of peptide H-DPro-(4S)Azp-Glu-NH2 (4S) as a catalyst with
higher enantioselectivity compared to the parent catalyst
H-DPro-Pro-Glu-NH2 (1).
Thus, the combination of the mismatched pucker and
the lack of an n→π* interaction explain the lower trans/cis
ratio of peptides 2R–5R compared to that of peptide 1.
Noteworthy is also the identical enantioselectivity of pep-
tides 2R–5R. This finding is also easily understandable
when taking the structure of peptide 1 into account: the
substituent at a 4R-configured Cγ points away from the cav-
ity of the peptide and does therefore not interfere with the
C–C bond formation (Figure 3, left).
Next, the catalytic performance of tripeptides 2S–5S
with 4S-configured proline residues was analyzed. In con-
trast to the 4R-configured diastereoisomers, the substituent
points in 2S–5S towards the cavity of the peptidic catalyst
(Figure 3, right). We therefore expected a larger effect of the
substituent on the stereoselectivity than for 2R–5R. Indeed,
their enantioselectivities vary between 93–99% ee and their
trans/cis ratios between 20 and >50 (Table 2 entries 2–5).
The hydroxy functionalized peptide 2S has the highest
Ktrans/cis (>50) but the lowest enantioselectivity (93% ee).
Conversely, the azido-functionalized peptide 4S has the
lowest Ktrans/cis (20) and the highest enantioselectivity (99%
ee). Thus, the trans/cis ratios of the peptidic catalysts 2S–5S
do not correlate with their stereoselectivity. Of note, the en-
antioselectivity of the azido-functionalized peptide 4S is
even higher than that of the parent catalyst 1 (97% ee). This
finding shows that the effect of a substituent that points to-
wards the cavity of the catalyst overwrites that of the
trans/cis ratio. The choice of the substituent therefore al-
lows for fine-tuning of the peptide structure and the cata-
lytic performance.
Reagents and materials were of the highest commercially available
grade and used without further purification. Reactions were moni-
tored by TLC using Merck silica gel 60 F254 aluminum sheets. Visual-
ization of the compounds was achieved by UV or KMnO4. Flash chro-
matography and plug filtrations were performed using Fluka silica gel
60 (particle size 0.040–0.063 mm, 200–400 mesh). Solvents for chro-
matography were of technical quality and distilled before use. 1H and
13C NMR spectra were recorded on a Bruker DRX 400, a Bruker AV III
400 (400 MHz/100 MHz) or a Bruker AV III 600 (600 MHz/150 MHz)
spectrometer. All spectra were recorded at 25 °C, unless stated other-
wise. Chemical shifts (δ) are reported in parts per million (ppm) rela-
tive to the signal of TMS. SFC analyses were performed on an analyti-
cal SFC with a diode array detector ACQUITY-UPLC-PDA from Waters
using the chiral AD column (150 mm × 30 mm) from Daicel under the
reported conditions. A Bruker maXis (UHR-TOF) was used for high-
resolution electrospray ionization (HR-ESI) mass spectrometry.
Peptide Coupling; General Procedure
i-Pr2NEt (6 equiv) was added to a solution of Fmoc-Xaa-OH (3 equiv)
and HATU (3 equiv) in a minimal amount of DMF necessary to obtain
a solution. The solution of the activated amino acid (≈100 mM) was
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–F

D
T. Schnitzer, H. Wennemers
Paper
Synthesis
added to the amino-functionalized resin (pre-swollen in CH2Cl2) and
the mixture was agitated for 1 h before washing with DMF (3 ×) and
CH2Cl2 (3 ×).
H-DPro-(4R)Flp-Glu-NH2·TFA (3R)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.68 (d, J = 7.4 Hz, 1 H),
7.10 (s, 1 H), 6.23 (s, 1 H), 5.35 (dt, J = 51.7, 3.3 Hz, 1 H), 4.74–4.61 (m,
2 H), 4.38 (td, J = 7.7, 3.4 Hz, 1 H), 4.02–3.80 (m, 2 H), 3.56–3.36 (m, 2
H), 2.86–2.68 (m, 1 H), 2.63–1.87 (m, 9 H).
Fmoc-Deprotection; General Procedure
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 178.09, 174.31, 170.33,
169.39, 91.36 (d, JC,F = 179.8 Hz), 59.86, 59.05, 53.77 (d, JC,F = 22.5 Hz),
53.11, 46.84, 35.84 (d, JC,F = 22.0 Hz), 30.44, 29.08, 25.54, 24.72.
A solution of 20% piperidine in DMF was added to the resin pre-swol-
len in CH2Cl2, the reaction mixture was agitated for 10 min, drained,
and the piperidine treatment was repeated for 10 min. Finally, the
resin was washed with DMF (3 ×) and CH2Cl2 (3 ×).
HRMS (ESI): m/z [M + H]+ calcd for C15H24FN4O5: 359.1725; found:
The amino acid coupling steps and the Fmoc-deprotections were
monitored by qualitative Kaiser (primary amines) and chloranil tests
(secondary amines).
359.1719.
H-DPro-(4S)Flp-Glu-NH2·TFA (3S)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.94 (d, J = 6.9 Hz, 1 H),
6.95 (s, 1 H), 6.21 (s, 1 H), 5.36 (dt, J = 51.3, 3.4 Hz, 1 H), 4.78 (dd, J =
8.8, 6.7 Hz, 1 H), 4.63 (d, J = 10.2 Hz, 1 H), 4.43–4.28 (m, 1 H), 4.18
(ddd, J = 22.7, 12.4, 1.9 Hz, 1 H), 3.74 (ddd, J = 36.5, 12.4, 3.5 Hz, 1 H),
3.55 (dt, J = 11.2, 6.7 Hz, 1 H), 2.74–2.59 (m, 1 H), 2.58–2.28 (m, 4 H),
2.26–2.07 (m, 3 H), 2.00–1.90 (m, 2 H).
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 179.09, 174.52, 170.19,
169.58, 91.77 (d, JC,F = 179.1 Hz), 60.52, 59.17, 53.90 (d, JC,F = 24.5 Hz),
53.34, 46.71, 36.03 (d, JC,F = 21.4 Hz), 30.30, 28.68, 24.94, 24.90.
Deprotection of Side Chain Functional Groups and Cleavage of the
Peptide from the Solid Support; General Procedure
A mixture of TFA/TIS/H2O (95:2.5:2.5) was added to the immobilized
peptide and the suspension was agitated for 1 h and a second time for
30 min. Pooling of the filtrates and removal of all volatiles under re-
duced pressure followed by precipitation and thorough washing with
Et2O afforded the peptide as a TFA salt. The peptide was redissolved in
MeCN/H2O (1:1), dried by lyophilization, and used without further
purification.
HRMS (ESI): m/z [M + H]+ calcd for C15H24FN4O5: 359.1725; found:
359.1725.
Analytical Data
The peptides were synthesized following the general procedures and
obtained as white solids. Only the signals of the trans-isomer are re-
ported.
H-DPro-(4R)Azp-Glu-NH2·TFA (4R)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.77 (d, J = 7.3 Hz, 1 H),
7.05 (s, 1 H), 6.21 (s, 1 H), 4.67 (dd, J = 8.8, 6.6 Hz, 1 H), 4.56 (t, J = 7.9
Hz, 1 H), 4.38 (td, J = 7.3, 3.6 Hz, 2 H), 3.91 (dd, J = 11.4, 4.9 Hz, 1 H),
3.57 (ddd, J = 11.3, 3.2, 1.4 Hz, 1 H), 3.49 (dt, J = 11.3, 6.9 Hz, 1 H),
3.45–3.39 (m, 1 H), 2.50 (ddt, J = 13.6, 8.5, 5.6 Hz, 4 H), 2.37 (ddd, J =
13.9, 7.4, 5.3 Hz, 1 H), 2.24–1.93 (m, 5 H).
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 178.35, 174.25, 170.08,
169.32, 59.94, 59.43, 59.07, 53.19, 52.37, 46.75, 34.60, 30.51, 28.98,
25.48, 24.77.
H-DPro-(4R)Hyp-Glu-NH2·TFA (2R)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.75 (d, J = 7.3 Hz, 1 H),
7.14 (d, J = 29.7 Hz, 1 H), 6.21 (s, 1 H), 4.62 (dd, J = 8.9, 6.2 Hz, 1 H),
4.55 (t, J = 8.3 Hz, 1 H), 4.51 (dq, J = 7.0, 4.3, 3.4 Hz, 1 H), 4.37 (td, J =
7.6, 3.2 Hz, 1 H), 3.78 (dd, J = 11.1, 4.0 Hz, 1 H), 3.56 (dt, J = 11.1, 1.9
Hz, 1 H), 3.48 (dt, J = 11.3, 7.0 Hz, 1 H), 3.44–3.37 (m, 1 H), 2.57–2.37
(m, 4 H), 2.22–1.96 (m, 6 H).
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ =178.33, 174.43, 171.29,
169.43, 69.21, 60.35, 59.19, 55.29, 53.15, 46.82, 37.72, 30.43, 28.95,
25.34, 24.74.
HRMS (ESI): m/z [M + H]+ calcd for C15H24N7O5: 382.1833; found:
382.1830.
H-DPro-(4S)Azp-Glu-NH2·TFA (4S)
HRMS (ESI): m/z [M + Na]+ calcd for C15H25N4O6Na: 357.1769; found:
357.1774.
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.97 (s, 1 H), 6.89 (s, 1 H),
6.09 (s, 1 H), 4.65 (dd, J = 8.7, 7.1 Hz, 1 H), 4.49 (dd, J = 9.5, 2.2 Hz, 1
H), 4.34 (tt, J = 5.1, 1.8 Hz, 1 H), 4.29 (td, J = 7.1, 3.0 Hz, 1 H), 3.86–3.77
(m, 1 H), 3.64 (dd, J = 11.2, 5.1 Hz, 1 H), 3.50–3.39 (m, 1 H), 3.32 (pent,
J = 1.7 Hz, 1 H), 2.54–2.30 (m, 4 H), 2.16–1.80 (m, 6 H).
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 179.63, 174.34, 169.82,
169.28, 60.34, 59.56, 59.08, 53.54, 52.49, 46.22, 34.30, 30.63, 28.27,
24.90, 24.65.
H-DPro-(4S)Hyp-Glu-NH2·TFA (2S)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 9.38 (d, J = 5.2 Hz, 1 H),
7.16–6.98 (m, 1 H), 6.46–6.33 (m, 1 H), 4.66–4.60 (m, 1 H), 4.53 (qd,
J = 3.5, 3.0, 1.2 Hz, 1 H), 4.47 (dd, J = 7.0, 5.1 Hz, 1 H), 4.23 (q, J = 4.8
Hz, 1 H), 3.89 (dt, J = 11.0, 1.1 Hz, 1 H), 3.65–3.57 (m, 1 H), 3.47 (ddd,
J = 11.5, 7.5, 6.4 Hz, 1 H), 3.30 (ddd, J = 11.5, 8.4, 7.1 Hz, 1 H), 2.49–
2.25 (m, 5 H), 2.24–2.12 (m, 2 H), 2.08 (ddd, J = 8.5, 5.5, 4.2 Hz, 2 H),
1.89 (dtd, J = 12.9, 8.9, 7.7 Hz, 1 H).
HRMS (ESI): m/z [M + H]+ calcd for C15H24N7O5: 382.1833; found:
382.1831.
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 181.73, 175.33, 171.57,
168.97, 69.66, 60.99, 60.00, 55.64, 54.77, 44.95, 37.29, 32.87, 27.24,
25.35, 24.33.
HRMS (ESI): m/z [M + H]+ calcd for C15H25N4O6: 357.1769; found:
357.1764.
H-DPro-(4R)Mtp-Glu-NH2·TFA (5R)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.84 (d, J = 7.3 Hz, 1 H),
7.09–6.92 (m, 1 H), 6.25 (s, 1 H), 4.70 (dd, J = 8.9, 6.5 Hz, 1 H), 4.58
(dd, J = 8.9, 4.5 Hz, 1 H), 4.39 (td, J = 7.5, 3.1 Hz, 1 H), 4.12 (dd, J = 10.1,
6.0 Hz, 1 H), 3.55–3.43 (m, 3 H), 3.42–3.37 (m, 1 H), 3.43–3.37 (m, 1
H), 2.60–2.42 (m, 4 H), 2.35–2.25 (m, 1 H), 2.25–1.94 (m, 8 H).
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 178.60, 174.30, 170.36,
169.22, 61.10, 59.10, 53.23, 53.18, 46.73, 42.94, 35.38, 30.42, 28.90,
25.33, 24.82, 14.81.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–F

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T. Schnitzer, H. Wennemers
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HRMS (ESI): m/z [M + H]+ calcd for C16H27N4O5S: 387.1697; found:
387.1702.
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H-DPro-(4S)Mtp-Glu-NH2·TFA (5S)
1H NMR (CDCl3/CD3OH 9:1, 600 MHz): δ = 8.80 (d, J = 6.7 Hz, 1 H),
7.07–6.90 (m, 1 H), 6.21 (s, 1 H), 4.72 (dd, J = 8.8, 6.7 Hz, 1 H), 4.52
(dd, J = 9.1, 4.2 Hz, 1 H), 4.33 (ddd, J = 8.1, 6.7, 3.1 Hz, 1 H), 3.90 (dd, J =
11.0, 6.5 Hz, 1 H), 3.74 (dd, J = 11.0, 4.4 Hz, 1 H), 3.59–3.52 (m, 1 H),
3.46 (tt, J = 6.5, 4.5 Hz, 1 H), 3.41–3.38 (m, 1 H), 2.62 (ddd, J = 13.8, 9.2,
6.5 Hz, 1 H), 2.57–2.53 (m, 2 H), 2.53–2.45 (m, 1 H), 2.34 (dt, J = 13.8,
4.4 Hz, 1 H), 2.23–2.07 (m, 5 H), 2.03 (s, 2 H), 1.95 (dq, J = 12.7, 7.2 Hz,
1 H).
13C NMR (CDCl3/CD3OH 9:1, 151 MHz): δ = 177.19, 172.85, 168.71,
167.57, 59.39, 57.35, 51.87, 51.34, 44.98, 41.48, 33.00, 28.69, 27.06,
23.11, 23.03, 13.12.
HRMS (ESI): m/z [M + H]+ calcd for C16H27N4O5S: 387.1697; found:
387.1690.
Peptide-Catalyzed Conjugate Addition Reaction of Butanal to
(E)-Nitrostyrene; General Procedure
The peptide TFA salt (10 μmol, 1 mol%) was added to a solution of
N-methylmorpholine (10 μmol, 1 mol%), (E)-nitrostyrene (149.2 mg,
1 mmol, 1 equiv), and butanal (135.4 μL, 1.5 mmol, 1.5 equiv) in
CHCl3/i-PrOH (9:1, 2 mL). The reaction mixture was stirred for 24 h.
The solvent was removed under reduced pressure and the crude mix-
ture was subjected to flash chromatography (1% → 50% EtOAc in hex-
ane, silica gel). The γ-nitroaldehyde was isolated as a light yellow solid
(Tables 1 and 2).
The enantiomeric excess was determined by chiral stationary phase
SFC [AD-3, 5% MeOH, 2.0 mL/min, 40 °C, 214 nm, 1.00 min (syn, mi-
nor), 1.21 min (syn, major)].
The analytical data are in agreement with the previously published
data.4
Funding Information
Swiss National Science Foundation (Grant No. 200020_169423) and
Fonds der Chemischen Industrie (Germany).
)(
Acknowledgment
(10) (a) Kuemin, M.; Nagel, Y. A.; Schweizer, S.; Monnard, F. W.;
Ochsenfeld, C.; Wennemers, H. Angew. Chem. Int. Ed. 2010, 49,
6324. (b) Erdmann, R. S.; Wennemers, H. Angew. Chem. Int. Ed.
2011, 50, 6835. (c) Erdmann, R. S.; Wennemers, H. J. Am. Chem.
Soc. 2012, 134, 17117.
We thank the Fonds der Chemischen Industrie (Germany) for a
Kekulé Fellowship for T. S. and the Swiss National Science Foundation
for financial support.
(11) Pandey, A. K.; Naduthambi, D.; Thomas, K. M.; Zondlo, N. J. J. Am.
Chem. Soc. 2013, 135, 4333.
Supporting Information
(12) Cadamuro, S. A.; Reichold, R.; Kusebauch, U.; Musiol, H.-J.;
Renner, C.; Tavam, P.; Moroder, L. Angew. Chem. Int. Ed. 2008,
47, 2143.
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1609547.
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(13) Siebler, C.; Maryasin, B.; Kuemin, M.; Erdmann, R. S.; Rigling, C.;
Grünenfelder, C.; Ochsenfeld, C.; Wennemers, H. Chem. Sci.
2015, 6, 6725.
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Synthesis
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–F