Functional group requirements within the peptide H-Pro-Pro-Asp-NH2 as a catalyst for aldol reactions

Article abstract of DOI:10.1016/j.tet.2007.06.010

H-Pro-Pro-Asp-NH₂ 1 is a versatile catalyst for asymmetric aldol reactions. In this work, the functional group tolerance within the catalyst structure has been examined. Several analogs of H-Pro-Pro-Asp-NH₂ in which the N-terminal secondary amine or the carboxylic acid in the side chain of the aspartic acid residue is replaced by different functional groups were prepared. Evaluation of their catalytic properties revealed that both the N-terminal secondary amine and the carboxylic acid are important for catalysis. The implications for the reaction mechanism are discussed.

Full text of DOI:10.1016/j.tet.2007.06.010

Tetrahedron 63 (2007) 8420–8424
Functional group requirements within the peptide
H-Pro-Pro-Asp-NH2 as a catalyst for aldol reactions
*
Jefferson D. Revell and Helma Wennemers
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
Received 3 May 2007; revised 29 May 2007; accepted 2 June 2007
Available online 9 June 2007
Dedicated to Professor Hisashi Yamamoto
Abstract—H-Pro-Pro-Asp-NH2 1 is a versatile catalyst for asymmetric aldol reactions. In this work, the functional group tolerance within the
catalyst structure has been examined. Several analogs of H-Pro-Pro-Asp-NH2 in which the N-terminal secondary amine or the carboxylic acid
in the side chain of the aspartic acid residue is replaced by different functional groups were prepared. Evaluation of their catalytic properties
revealed that both the N-terminal secondary amine and the carboxylic acid are important for catalysis. The implications for the reaction mech-
anism are discussed.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
the peptides H-Pro-Pro-Asp-NH2 1 and H-Pro-D-Ala-D-
Asp-NH2 2 as catalysts for aldol reactions.7 Particularly, 1
is a highly active and selective catalyst, 1 mol % suffices
to catalyze aldol reactions between acetone and several alde-
hydes in high yields and enantioselectivities (Scheme 1).
The aldol reaction is one of the most important carbon–
carbon bond forming reactions. Many different protocols
have been established with which the aldol reaction can be
performed and likewise numerous excellent asymmetric
catalysts have been developed.1 Over recent years, proline
and other secondary amine based catalysts have become in-
creasingly popular organocatalysts for aldol and related
reactions.2 For many substrates, excellent selectivities are
achieved, however, often poor reactivity renders the use of
large catalyst loadings necessary. Since rigid small organo-
catalysts bear only a limited number of sites for modifica-
tions, peptides have been envisioned as useful alternative
catalysts.3,4 Short chain peptides of intermediate complexity
offer many sites for functional and structural modifications
that can be used to develop optimal catalysts.3,4 Work by
List, Reymond, Gong and others showed that peptides can
catalyze aldol reactions but also demonstrated that the
purely rational design of peptidic catalysts is not trivial.5
The many degrees of freedom of even short chain peptides
render a prediction of their structures and catalytic proper-
ties challenging. Smart combinatorial screening methods
that allow for the identification of catalysts within libraries
of different peptides are therefore attractive to aid the dis-
covery process.3,4 One such method is ‘catalyst–substrate
coimmobilization’, which allows for the identification of
catalysts for bimolecular reactions among the members of
split-and-mix libraries.6 Using this method we identified
O
H
N
1 mol%
NH2
N
O
CO2H
O
O
OH
*
O
NH
R
H
R
Acetone, 1 mol% NMM
RT or -20 °C
24-98 % yield, 66-91% ee
Scheme 1. Aldol reactions catalyzed by H-Pro-Pro-Asp-NH2 1.
These results represent a significant increase in catalytic
activity over proline, which, for the same reactions is re-
quired at 30 mol % to obtain comparable product yields
and selectivities within the same time. Thus, the higher com-
plexity of the peptidic catalyst proved to be an excellent
trade-off for its higher activity.7,8 A further enhancement
of the catalytic activity of H-Pro-Pro-Asp-NH2 1 was
achieved by attachment of a triethylene glycol chain to its
C-terminus.9 H-Pro-Pro-Asp-NH(CH2CH2O)3CH3 is signif-
icantly more soluble compared to 1, thereby only requiring
the use of 0.5 mol % for efficient catalysis. Furthermore,
immobilization of 1 on TentaGel or PEGA furnished solid
supported catalysts that can be reused multiple times.9
With these results in hand we wish to gain insight into the
mechanism by which H-Pro-Pro-Asp-NH2 catalyzes aldol
* Corresponding author. Tel.: +41 61 267 1146; fax: +41 61 267 0976;
e-mail: helma.wennemers@unibas.ch
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.06.010

J. D. Revell, H. Wennemers / Tetrahedron 63 (2007) 8420–8424
8421
reactions. We started by examining, which functional groups
are necessary for efficient catalysis and disclose the results
of these initial mechanistic studies here.
secondary amine is replaced by an amide, whereas in peptide
4 the basic functionality is retained. In addition, we prepared
H-Pro-Pro-Asp-OCH3 7 to investigate the role of the
C-terminal amide during catalysis.
2. Results and discussion
2.1. Synthesis of peptides 3–7
In common with proline, H-Pro-Pro-Asp-NH2 1 and H-Pro-
D-Ala-D-Asp-NH2 2containa secondaryamine anda carboxy-
lic acid. This suggests that the mechanism by which the
peptides catalyze intermolecular aldol reactions may be sim-
ilar to that proposed for proline by List and Houk.10–12 This
mechanism involves enamine formation followed by nucleo-
philic addition of the enamine to the aldehyde with simul-
taneous proton transfer from the carboxylic acid (Fig. 1).
Peptides 3–7 were either prepared on Wang resin (peptide 7)
or Rink-amide resin (peptides 3–6) following the general
Fmoc/tBu protocol for peptide synthesis. For the preparation
of peptides 3, 5, and 7, commercially available a-N-Fmoc
protected amino acids with acid sensitive protecting groups
(tBu for Asp and trityl for Asn) on the side chain functional
groups were used. N-Methylproline for the synthesis of pep-
tide 4 was prepared by reductive amination in quantitative
yield.13 For the synthesis of peptide 6, Fmoc-Cys(SO3H)-
OH was prepared starting from cystine (Scheme 2). Oxida-
tion of L-cystine with an aqueous bromine solution yielded
L-cysteic acid, which was isolated in 89% yield after re-
crystallization.14 Protection of the amine was accomplished
under standard Schotten–Baumann conditions using Fmoc-
Cl in a mixture of aqueous Na2CO3 solution and dioxane
following established procedures.14
R´´
R´
N
O
O
H
O
R
H
Figure 1. Transition state proposed by List and Houk for aldol reactions
catalyzed by proline.10,11
2.2. Catalytic properties of peptides 3–7
To test the importance of these two functional groups we
prepared analogs of H-Pro-Pro-Asp-NH2 1 in which the
secondary amine is acetylated (Ac-Pro-Pro-Asp-NH2 3) or
methylated (Me-Pro-Pro-Asp-NH2 4) and the carboxylic
acid replaced by an amide (H-Pro-Pro-Asn-NH2 5) or a sul-
fonic acid (H-Pro-Pro-Cys(SO3H)-NH2 6) (Fig. 2).
The catalytic properties of peptides 3–7 were evaluated using
the aldol reaction between acetone and p-nitrobenzaldehyde
as a test reaction. When using H-Pro-Pro-Asp-NH2 1 as a cat-
alyst, 1 mol % is sufficient to obtain the aldol product in
quantitative yield and 80% ee at room temperature within
4 h.7 At ꢀ20 ꢁC, the product forms with an enantiomeric
excess of 91% in 98% yield using 5 mol % of 1 within 24 h.7
Peptides 3–7 were evaluated under identical conditions to
those used with the parent catalyst 1. However, since only
peptide 7 provided the aldol product in good quantities
with a catalyst loading of 1 mol %, amounts of 5 or 10 mol %
were employed in the reactions with peptides 3–6 (Table 1).
O
O
H
N
H
N
NH2
CO2H
NH2
CONH2
N
R
N
O
O
O
O
N
NH
R = Ac: Ac-Pro-Pro-Asp-NH2
3
H-Pro-Pro-Asn-NH2
5
R = Me: CH3-Pro-Pro-Asp-NH2 4
Neither the N-terminally acetylated peptide 3 nor peptide 4
with the basic tertiary amine at the N-terminus catalyzed the
aldol reaction within 72 h (entry 3 and 4). Furthermore,
when the TFA salt of H-Pro-Pro-Asp-NH2 1 was used as cat-
alyst in the absence of a base such as N-methylmorpholine
(NMM), the reaction proceeded only slowly, demonstrating
that the protonation state of the secondary amine is crucial
(entry 5). These findings demonstrate the importance of
the secondary amine for catalysis and suggest that the forma-
tion of an enamine as an intermediate is crucial for catalysis.
O
H
O
H
N
N
NH2
OCH3
N
N
O
O
SO3H
CO2H
O
O
NH
H-Pro-Pro-Cys(SO3H)-NH2
Figure 2. Peptides 3–7.
NH
6
H-Pro-Pro-Asp-OCH3
7
Peptides 5 and 6 were chosen to bear functionalities in place
of the carboxylic acid that either lack an acidic proton (pep-
tide 5) or are more acidic (peptide 6). In peptide 3 the
Evaluation of peptide 5 with an amide functionality in place
of the carboxylic acid allowed for analysis of the role of the
O
H2N
O
O
OH
H2N
FmocNH
OH
SO3H
OH
Br2, H2O
89 %
Fmoc-Cl, Na2CO3
S
S
Dioxane, H2O
98 %
SO3H
OH
H2N
8
O
Scheme 2. Synthesis of Fmoc-Cys(SO3H)-OH (8).

8422
J. D. Revell, H. Wennemers / Tetrahedron 63 (2007) 8420–8424
Table 1. Aldol reaction between p-nitrobenzaldehyde and acetone catalyzed
by peptides 3–7
peptidic catalysts discussed here.) Given that the carboxylic
acid can be replaced by amide and sulfonic acid functional
groups without breakdown of catalysis raises the question
of whether nucleophilic addition of the enamine to the
aldehyde is accompanied by simultaneous proton transfer
as proposed for proline catalysis. One intriguing alternative
could be that the proton is transferred via the base, which
is held in place by coordination to the carboxylic acid or
related functional group.
O
OH O
x mol% catalyst•TFA
x mol% NMM
acetone, RT
H
O2N
Entry
O2N
Yielda
Catalyst
mol %
Time (h)
eeb
1 (Ref. 7)
1
1
3
4
1
5
6
7
1
5
10
10
5
10
10
1
4
24
72
72
24
24
24 (72)
12
99
98
ꢂ5
ꢂ5
67
80
2c (Ref. 7)
90
3
ndd
ndd
77
4
5e
6
4. Experimental
4.1. General
39
54
7f
8
26 (49) 53 (52)
87 73
a
Materials and reagents were of the highest commercially
available grade and used without further purification. Reac-
tions were monitored by thin layer chromatography using
Merck silica gel 60 F254 plates. Compounds were visualized
by UV, KMnO4 and ninhydrin. Flash chromatography was
performed using Merck silica gel 60, particle size 40–
63 mm. Ion exchange chromatography was performed
using DowexÒ 1ꢃ2-400 from Sigma—Aldrich. 1H and
13C NMR spectra were recorded on Bruker DPX 400 and
600 spectrometers. Chemical shifts are reported in parts
per million using the solvent residue signals as reference.
Finnigan MAT LCQ and Finnigan MAT 312 instruments
were used for electrospray ionization (ESI) and fast atom
bombardment (FAB) mass spectrometries. HPLC analyses
were carried out using a Chiralcel AS (250 mmꢃ4.6 mm)
from Daicel.
Yields are listed in %; in entries 3–7 unreacted aldehyde could be reiso-
lated, essentially no side products were observed.
The ee was determined by chiral stationary HPLC analysis and is listed
in %.
b
c
d
e
f
Reaction was performed at ꢀ20 ꢁC.
Not determined.
Reaction was performed in the absence of NMM.
Data in brackets after 72 h of reaction time.
acidic proton. Peptide 5 is significantly less active compared
to 1, however, catalytic turn over is still observed. This is re-
markable since the respective proline derivative H-Pro-NH2
mediates aldol reactions only very slowly. With 40 mol % of
H-Pro-NH2, the aldol product was obtained in ꢂ10% yield in
DMSO after 24 h.15 Peptide 6 with a sulfonic acid was de-
signed to test the effect on catalysis of a functionality with
higher acidity in place of the carboxylic acid.16 Peptide 6
proved to be a less competent catalyst compared to 1 both
in terms of catalytic activity and selectivity. However, like
5, peptide 6 allowed still for catalytic turn over. These results
demonstrate that peptides 1 and 7 bearing a carboxylic acid
functionality are optimal compared with those bearing an
amide or sulfonic acid. However, the acidic functional group
did not prove essential for catalytically active peptides.17
4.2. General procedures for peptide synthesis on solid
support
General procedure for peptide couplings: Fmoc-amino acid
(3 equiv) and 1-hydroxybenzotriazole (3 equiv) dissolved in
the minimum amount of DMF necessary were added to the
suspension of amino-functionalized resin in CH2Cl2. The
mixture was agitated for 2 min before adding diisopropyl-
carbodiimide (3 equiv) and agitating for another 2 h. The
suspension was washed with DMF (3ꢃ) and CH2Cl2 (5ꢃ).
General procedure for Fmoc-deprotections: to the resin
(pre-swollen in DMF) was added a solution of 20% piper-
idine in DMF and the reaction mixture was agitated for
5 min, drained, and the piperidine treatment repeated for
10 min. Finally the resin was washed with DMF (7ꢃ) and
CH2Cl2 (5ꢃ). Both the couplings and the deprotections
were monitored by the qualitative Kaiser test. Quantitative
Fmoc-monitoring was carried out as spot checks.
Finally we tested whether the C-terminal amide in catalyst 1
is crucial for catalysis, possibly by stabilizing the con-
formation of the peptide through H-bonding. Peptide 7
bearing a methyl ester in place of the amide performed
almost as well as 1. Thus, the N-terminal amide does not
play a key role for catalysis thereby allowing for modifica-
tions at this site.
3. Conclusions
In conclusion, the peptidic catalyst H-Pro-Pro-Asp-NH2
relies on both the N-terminal secondary amine and the car-
boxylic acid in the side chain of the aspartic acid residue
for efficient catalysis. Acetylation, alkylation, or protonation
of the secondary amine strongly diminishes or prevents cat-
alytic activity. Modifications at the site of the carboxylic acid
with stronger acids or functionalities lacking an acidic pro-
ton reduce both the catalytic activity and selectivity but
are not entirely detrimental to catalysis. These results sup-
port a mechanism relying on enamine catalysis. (Note:
oxazolidinones, which have recently been proposed as key
intermediates in proline catalysis,18 cannot form in the
Peptides 3–6 were prepared on Rink-amide resin following
the general protocol for solid phase synthesis and were
cleaved off the resin by stirring in a mixture of TFA/
CH2Cl2 (1:1) for 2 h. Filtration and removal of all volatiles
under reduced pressure followed by precipitation with
Et2O afforded the TFA salts of the peptides. Peptide 7 was
prepared on Wang resin following the general protocol for
peptide synthesis and cleaved off the resin by agitation in
a mixture of CH3OH/NEt3 4:1 for 3 days. Filtration and re-
moval of all volatiles under reduced pressure was followed
by purification of H-Pro-Pro-Asp(tBu)-OCH3 over a size
exclusion column (LH20, 1% MeOH in CH2Cl2). The

J. D. Revell, H. Wennemers / Tetrahedron 63 (2007) 8420–8424
8423
tBu-protecting group was removed as described for peptides
3–6 to afford the TFA salt of 7.
DMSO-d6, 25 ꢁC): d 173.7, 172.8, 171.5, 168.2, 61.0,
59.0, 50.5, 47.7, 46.6, 37.5, 29.8, 28.8, 25.3, 24.4. Minor
1
isomer: H NMR (400 MHz, DMSO-d6, 25 ꢁC): d 8.63 (d,
4.2.1. Ac-Pro-Pro-Asp-NH2 (3). s-cis/s-trans Conformers
around the Pro–Pro amide bond are observed in a ratio of
3:1 in the H NMR spectrum in DMSO-d6. Major isomer:
J¼8.3 Hz, 1H; CONH), 8.56 (br s, 1H; Pro-NH), 7.49 (s,
1H; CONH2), 7.37 (s, 1H; CONH2), 7.12 (s, 1H; CONH2),
6.93 (s, 1H; CONH2), 4.55 (m, 1H; Asn-Ha), 4.39 (m, 1H;
Pro-Ha), 3.98 (m, 1H; Pro-Ha), 3.62 (m, 1H; Pro-Hd),
3.45 (m, 1H; Pro-Hd), 3.18 (m, 2H; Pro-Hd), 2.58 (m, 1H;
Asn-Hb), 2.44 (m, 1H; Asn-Hb), 2.38 (m, 1H; Pro-Hb),
2.21 (m, 1H; Pro-Hb), 1.87 (m, 6H; 2H; Pro-Hb, 4H; Pro-
1
1H NMR (400 MHz, DMSO-d6, 25 ꢁC): d 10.50 (br s, 1H;
Asp-OH), 7.89 (d, J¼8.2 Hz, 1H; Pro-CONH), 7.15 (s,
1H; Asp-CONH2), 6.99 (s, 1H; Asp-CONH2), 4.51 (dd, J¼
8.3, 3.2 Hz, 1H; Pro-Ha), 4.40 (m, 1H; Asp-Ha), 4.20 (dd,
J¼8.3, 4.8 Hz, 1H; Pro-Ha), 3.68 (m, 1H; Pro-Hd), 3.52
(m, 1H; Pro-Hd, 1H; Pro-Hd), 3.34 (m, 1H; Pro-Hd), 2.68
(dd, J¼16.4, 5.5 Hz, 1H; Asp-Hb), 2.57 (dd, J¼16.4, 7.3 Hz,
1H; Asp-Hb), 2.30–1.55 (m, 8H; Pro-Hb, Pro-Hg), 1.94 (s,
13
Hg). C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d 173.6,
172.7, 171.5, 167.6, 59.7, 59.2, 50.7, 48.3, 46.3, 38.1,
32.4, 28.7, 24.3, 22.9. HRMS (ESI) m/z: calcd for
C14H24N5O4 326.1828; found, 326.1819.
13
3H; CH3). C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d
173.3, 173.1, 172.1, 171.7, 168.9, 61.0, 58.2, 50.1, 48.4,
47.7, 36.5, 29.5, 29.2, 25.5, 25.1, 22.9. Minor isomer: H
4.2.4. H-Pro-Pro-Cys(SO3H)-NH2 (6). s-cis/s-trans Con-
formers around the Pro–Pro amide bond are observed in a ra-
tio of 1:1 in the 1H NMR spectrum in DMSO-d6. Isomer 1:
1H NMR (600 MHz, DMSO-d6, 25 ꢁC): d 9.29 (1H; Pro-
NH2), 8.61 (1H; Pro-NH2), 8.52 (d, J¼8.2 Hz, 1H; Cys-
NH), 7.04 (1H; Cys-CONH2), 6.89 (1H; Cys-CONH2),
4.65 (m, 1H; Cys(SO3H), Ha), 4.49 (m, 1H; Pro-Ha), 4.48
(m, 1H; Pro0-Ha), 3.64 (m, 1H; Pro0-Hd), 3.53 (m, 1H;
Pro0-Hd), 3.24 (m, 1H; Pro-Hd), 3.17 (m, 1H; Pro-Hd),
2.98 (dd, J¼13.9, 3.1 Hz, 1H; Cys(SO3H), Hb), 2.83 (dd,
J¼13.9, 11.0 Hz, 1H; Cys(SO3H), Hb), 2.35 (m, 1H; Pro-
Hb), 2.18 (m, 1H; Pro-Hb), 2.26 (m, 1H; Pro0-Hb), 2.02
(m, 1H; Pro0-Hb), 1.97 (m, 1H; Pro0-Hg), 1.89 (m, 1H;
Pro0-Hg), 1.92 (m, 2H; Pro-Hg). 13C NMR (151 MHz,
DMSO-d6, 25 ꢁC): d 172.1, 171.0, 167.5, 59.6, 58.3, 51.6,
50.3, 46.7, 45.6, 31.0, 24.2, 23.2. Isomer 2: 1H NMR
(600 MHz, DMSO-d6, 25 ꢁC): d 8.99 (1H; Pro-NH2), 8.68
(1H; Pro-NH2), 8.35 (d, J¼6.4 Hz, 1H; Cys-NH), 7.41
(1H; Cys-CONH2), 7.13 (1H; Cys-CONH2), 4.35 (m, 1H;
Pro-Ha), 4.29 (m, 1H; Pro0-Ha), 4.18 (m, 1H; Cys(SO3H),
Ha), 3.51 (m, 1H; Pro0-Hd), 3.39 (m, 1H; Pro0-Hd), 3.17
(m, 2H; Pro-Hd), 2.85 (dd, J¼13.7, 6 Hz, 1H; Cys(SO3H),
Hb), 2.78 (dd, J¼13.7, 6 Hz, 1H; Cys(SO3H), Hb), 2.31
(m, 1H; Pro-Hb), 2.14 (m, 1H; Pro0-Hb), 1.94 (m, 2H;
Pro-Hg), 1.89 (m, 1H; Pro0-Hb), 1.81 (m, 1H; Pro-Hb),
1.81 (m, 2H; Pro0-Hg). 13C NMR (151 MHz, DMSO-d6,
25 ꢁC): d 172.1, 170.1, 167.3, 60.6, 57.4, 50.6, 50.0,
46.9, 45.8, 27.6, 22.3, 22.0. HRMS (ESI) m/z: calcd for
C13H23N4O6S 363.1338; found, 363.1331.
1
NMR (400 MHz, DMSO-d6, 25 ꢁC): 10.50 (br s, 1H; Asp-
OH), 8.36 (d, J¼8.2 Hz, 1H; Pro-CONH), 7.37 (s, 1H;
Asp-CONH2), 7.22 (s, 1H; Asp-CONH2), 4.72 (dd, J¼8.6,
2.5 Hz, 1H; Asp-Ha), 4.47 (m, 1H; Pro-Ha), 4.30 (dd,
J¼8.1, 4.8 Hz, 1H; Pro-Ha), 3.68 (m, 1H; Pro-Hd), 3.52
(m, 2H; Pro-Hd), 3.34 (m, 1H; Pro-Hd), 2.76, (dd, J¼16.4,
4.9 Hz, 1H; Asp-Hb), 2.68 (dd, J¼16.4, 5.5 Hz, 1H; Asp-
Hb), 2.30–1.55 (m, 8H; Pro-Hb, Pro-Hg), 1.76 (s, 3H;
13
CH3). C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d 173.3,
173.2, 172.7, 172.7, 169.3, 59.5, 59.2, 50.2, 47.5, 46.9,
36.5, 29.4, 29.2, 25.5, 25.2, 23.1. HRMS (ESI) m/z: calcd
for C16H24N4O6Na 391.1593; found, 391.1589.
4.2.2. TFA$Me-Pro-Pro-Asp-NH2 (4). s-cis/s-trans Con-
formers around the Pro–Pro amide bond were observed in
a ratio of 1.5:1 in the 1H NMR spectrum in DMSO-d6. Major
isomer: 1H NMR (400 MHz, DMSO-d6, 25 ꢁC): d 9.68 (br s,
1H; +NH), 8.22 (d, J¼8.0 Hz, 1H; NH), 7.16 (s, 1H;
CONH2), 7.11 (s, 1H; CONH2), 4.48 (m, 3H; Pro/Pro/Asp-
Ha), 3.59 (m, 2H; Pro-Hd), 3.41 (m, 1H; Pro-Hd), 3.11
(m, 1H; Pro-Hd), 2.79 (s, 3H; Me), 2.59 (m, 2H; Asp-Hb),
2.49 (m, 1H; Pro-Hb), 2.18–1.70 (m, 7H; Pro-Hb, Pro-
13
Hg). C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d 173.2,
173.0, 172.7, 171.5, 67.8, 60.8, 56.6, 50.3, 47.6, 36.9, 32.4,
29.8, 28.1, 25.2, 23.0. Minor isomer: d 9.74 (br s, 1H; +NH),
8.71 (d, J¼8.0 Hz, 1H; NH), 7.46 (s, 1H; NH2), 7.16 (s, 1H;
NH2), 4.53 (m, 1H; Ha), 4.33 (m, 1H; Ha), 4.85 (m, 1H;
Ha), 3.59 (m, 2H; Pro-Hd), 3.41 (m, 1H; Pro-Hd), 3.11
(m, 1H; Pro-Hd), 2.73 (s, 3H; Me), 2.59 (m, 2H; Asp-Hb),
2.25 (m, 1H; Pro-Hb), 2.18–1.70 (m, 7H; Pro-Hb, Pro-Hg).
13C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d 173.2, 173.1,
172.7, 171.7, 67.8, 59.9, 56.1, 50.4, 48.1, 36.9, 32.4, 29.8,
28.3, 25.2, 22.9. HRMS (ESI) m/z: calcd for C15H25N4O5
341.1824; found, 341.1823.
4.2.5. TFA$H-Pro-Pro-Asp-OCH3 (7). s-cis/s-trans Con-
formers around the Pro–Pro amide bond were observed in
a ratio of 4:1 in the 1H NMR spectrum in DMSO-d6. Major
1
isomer: H NMR (400 MHz, DMSO-d6, 25 ꢁC): d 9.89 (s,
1H; Asp-OH), 8.45 (br s, 1H; Pro-NH), 8.42 (d, J¼8.2 Hz,
1H; Pro-CONH), 4.55 (dd, J¼13.9, 6.4 Hz, 1H; Pro-Ha),
4.46 (m, 1H; Asp-Ha), 4.11 (dd, J¼8.3, 4.8 Hz, 1H; Pro-
Ha), 3.60 (m, 1H; Pro-Hd), 3.58 (s, 3H; Asp-OCH3), 3.42
(m, 1H; Pro-Hd), 3.23 (m, 1H; Pro-Hd), 3.16 (m, 1H; Pro-
Hd), 2.75 (m, 1H; Asp-Hb), 2.65 (m, 1H; Asp-Hb), 2.40
(m, 1H; Pro-Hb), 2.12 (m, 1H; Pro-Hb), 2.94–1.72 (m,
6H; Pro-Hb, Pro-Hg). 13C NMR (100.6 MHz, DMSO-d6,
25 ꢁC): d 172.4, 172.3, 172.2, 171.8, 60.2, 59.1, 52.9, 49.4,
47.6, 46.6, 36.7, 29.9, 28.7, 25.2, 24.4. Minor conformer:
1H NMR (400 MHz, DMSO-d6, 25 ꢁC): d 9.80 (s, 1H;
Asp-OH), 8.55 (br s, 1H; Pro-NH), 8.88 (d, J¼7.6 Hz, 1H;
Pro-CONH), 4.61 (m, 1H; Asp-Ha), 4.46 (m, 1H; Pro-
Ha), 4.11 (dd, J¼8.3, 4.7 Hz, 1H; Pro-Ha), 3.87 (m, 1H;
4.2.3. TFA$H-Pro-Pro-Asn-NH2 (5). s-cis/s-trans Con-
formers around the Pro–Pro amide bond are observed in a
ratio of 3:1 in the H NMR spectrum in DMSO-d6. Major
1
1
isomer: H NMR (400 MHz, DMSO-d6, 25 ꢁC): d 8.56 (br
s, 1H; Pro-NH), 8.16 (d, J¼8.3 Hz, 1H; CONH), 7.43 (s,
1H; CONH2), 7.08 (s, 1H; CONH2), 7.00 (s, 1H; CONH2),
6.91 (s, 1H; CONH2), 4.50 (m, 1H; Asn-Ha), 4.39 (m, 1H;
Pro-Ha), 4.32 (m, 1H; Pro-Ha), 3.62 (m, 1H; Pro-Hd),
3.45 (m, 1H; Pro-Hd), 3.18 (m, 2H; Pro-Hd), 2.44 (m, 2H;
Asn-Hb), 2.38 (m, 1H; Pro-Hb), 2.09 (m, 1H; Pro-Hb),
1.87 (m, 6H; Pro-Hb, Pro-Hg). 13C NMR (100.6 MHz,

8424
J. D. Revell, H. Wennemers / Tetrahedron 63 (2007) 8420–8424
3. For recent reviews, see: (b) Revell, J. D.; Wennemers, H. Curr.
Opin. Chem. Biol. 2007, 11, 269; (b) Berkessel, A. Curr. Opin.
Chem. Biol. 2003, 7, 409; (c) Fonseca, M. H.; List, B. Curr.
Opin. Chem. Biol. 2004, 8, 319 and references therein.
Pro-Hd), 3.61 (s, 3H; Asp-OCH3), 3.60 (m, 1H; Pro-Hd),
3.45 (m, 1H; Pro-Hd), 3.23 (m, 1H; Pro-Hd), 2.78, (m, 1H;
Asp-Hb), 2.60 (m, 1H; Asp-Hb), 2.40 (m, 1H; Pro-Hb),
2.26 (m, 1H; Pro-Hb), 2.94–1.72 (m, 6H; Pro-Hb, Pro-
13
Hg). C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d 172.36,
4. Miller, S. J. Acc. Chem. Res. 2004, 37, 601.
5. (a) Martin, H. J.; List, B. Synlett 2003, 1901; (b) Kofoed, J.;
Nielsen, J.; Reymond, J.-L. Bioorg. Med. Chem. Lett. 2003,
13, 2445; (c) Tang, Z.; Yang, Z.-H.; Cun, L.-F.; Gong, L.-Z.;
Mi, A.-Q.; Jiang, Y.-Z. Org. Lett. 2004, 6, 2285; (d) Tsogoeva,
S. B.; Wei, D. Tetrahedron: Asymmetry 2005, 16, 1947; (e)
Tanaka, F.; Fuller, R.; Barbas, C. F., III. Biochemistry 2005,
44, 7583; (f) Andreae, M. R. M.; Davis, A. P. Tetrahedron:
Asymmetry 2005, 16, 2487; (g) Akagawa, K.; Sakamoto, S.;
Kudo, K. Tetrahedron Lett. 2005, 46, 8185; (h) Zou, W.;
171.91, 171.67, 171.56, 60.37, 59.16, 53.12, 49.51, 47.15,
46.37, 36.38, 29.81, 28.85, 25.13, 24.41. HRMS (ESI) m/z:
calcd for C15H24N3O6 342.1665; found, 342.1654.
4.2.6. Fmoc-Cys(SO3H)-OH (8). Fmoc-Cys(SO3H)-OH 8
1
was prepared as described in Refs. 13 and 14. H NMR
(400 MHz, DMSO-d6, 25 ꢁC): d 12.69 (br s, 1H; CO2H),
7.86 (d, J¼7.7 Hz, 2H; ArH), 7.68 (d, J¼7.1 Hz, 2H;
ArH), 7.49 (d, J¼7.0 Hz, 1H; OCONH), 7.38 (t, J¼7.3 Hz,
2H; ArH), 7.31 (t, J¼7.0 Hz, 2H; ArH), 4.28 (m, 1H; Ha),
4.24 (m, 3H; Fmoc CH and CH2), 2.92 (d, J¼5.5 Hz, 2H;
ꢀ
Ibrahem, I.; Dziedzic, P.; Sunden, H.; Cordova, A. Chem.
Commun. 2005, 4946; (i) Cordova, A.; Zou, W.; Dziedzic, P.;
ꢀ
ꢀ
13
Hb). C NMR (100.6 MHz, DMSO-d6, 25 ꢁC): d 173.2,
Ibrahem, I.; Reyes, E.; Xu, Y. Chem.—Eur. J. 2006, 12, 5383;
(j) Dodda, R.; Zhao, C.-G. Synlett 2007, 1605.
6. Krattiger, P.; McCarthy, C.; Pfaltz, A.; Wennemers, H. Angew.
Chem., Int. Ed. 2003, 42, 1722.
156.5, 144.7, 144.6, 141.5, 128.5, 128.0, 126.1, 121.0,
66.7, 52.2, 47.4.
ꢁ
7. (a) Krattiger, P.; Kovasy, R.; Revell, J. D.; Ivan, S.; Wennemers,
H. Org. Lett. 2005, 7, 1101; (b) Krattiger, P.; Kovasy, R.;
4.3. General procedure for aldol reactions
ꢁ
Revell, J. D.; Wennemers, H. QSAR Comb. Sci. 2005, 24,
1158.
8. H-Pro-Pro-Asp-NH2$TFA can easily be prepared on a 10 g
scale within a day in an academic laboratory.
9. Revell, J. D.; Gantenbein, D.; Krattiger, P.; Wennemers, H.
Biopolymers (Pept. Sci.) 2006, 84, 105.
10. (a) Clemente, F. R.; Houk, K. N. Angew. Chem., Int. Ed. 2004,
43, 5766; (b) List, B.; Hoang, L.; Martin, H. J. Proc. Natl. Acad.
Sci. U.S.A. 2004, 101, 5839; (c) Bahmanyar, S.; Houk, K. N.;
Martin, H. J.; List, B. J. Am. Chem. Soc. 2003, 125, 2475; (d)
Hoang, L.; Bahmanyar, S.; Houk, K. N.; List, B. J. Am.
Chem. Soc. 2003, 125, 16.
11. For a review, see: Allemann, C.; Gordillo, R.; Clemente,
F. R.; Cheong, P. H.-Y.; Houk, K. N. Acc. Chem. Res. 2004,
37, 558.
The peptide (0.004 mmol, 1 mol % or 0.04 mmol,
10 mol %) was suspended in a solution of N-methylmorpho-
line (0.5 ml, 0.004 mmol or 4.6 ml, 0.04 mmol (standard
solution in acetone)) in acetone (2 ml, 27.2 mmol) and
4-nitrobenzaldehyde (60 mg, 0.40 mmol) was added. The
reaction mixture was stirred for 4–24 h, then quenched
with half saturated NH4Cl solution (2 ml). The mixture was
extracted with Et2O (4ꢃ5 ml), and the combined extracts
were washed with brine and dried over MgSO4. The filtered
solution was concentrated in vacuo and purified by flash
chromatography on silica gel (15 g, 40% EtOAc in pentanes)
to afford the aldol product. Enantiomeric excess was deter-
mined by chiral HPLC. For a full characterization of the
aldol product see Refs. 7 and 15.
12. Klussmann, M.; Iwamura, H.; Mathew, S. P.; Wells, D. H.;
Pandya, U.; Armstrong, A.; Blackmond, D. Nature 2006,
441, 621.
Acknowledgements
13. Han, Z.-j.; Wang, R.; Zhou, Y.-f.; Liu, L. Eur. J. Org. Chem.
2005, 934.
14. (a) Clarke, H. T. Org. Synth. 1940, 20, 23; (b) Nagase, T.;
Kumagai, U.; Niiyama, K.; Mase, T.; Ishikawa, K.
Tetrahedron Lett. 1993, 34, 1173.
This work was supported by the Swiss National Science
Foundation and BACHEM. H.W. is grateful to BACHEM
for an endowed professorship.
15. List, B.; Lerner, R. A.; Barbas, C. F., III. J. Am. Chem. Soc.
2000, 122, 2395.
References and notes
16. Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456; CH3CO2H:
pKa¼12.3 (DMSO), CH3SO3H: pKa¼1.6 (DMSO).
17. This is in agreement with results by Gong and co-workers
who described the peptide H-Pro-(Phe)4-OCH3 as a catalyst
for aldol reactions of hydroxyacetone and aldehydes, see
Ref. 5c.
18. Seebach, D.; Beck, A. K.; Badine, D. M.; Limbach, M.;
Eschenmoser, A.; Treasurywala, A. M.; Hobi, R.;
Priskoszovich, W.; Linder, B. Helv. Chim. Acta 2007, 90,
425.
1. Modern Aldol Reactions; Mahrwald, R., Ed.; Wiley-VCH:
Weinheim, 2004; Vols. 1 and 2.
2. For recent reviews, see: (a) Taylor, M. S.; Jacobsen, E. N.
Angew. Chem., Int. Ed. 2006, 45, 1520; (b) Gaunt, M. J.;
Johansson, C.; McNally, A.; Vo, N. T. Drug Discov. Today
2006, 12, 8; (c) List, B. Chem. Commun. 2006, 819; (d)
ꢀ
Guillena, G.; Ramon, D. J. Tetrahedron: Asymmetry 2006,
17, 1465; (e) Asymmetric Organocatalysis; Berkessel, A.,
€
Groger, H., Eds.; Wiley-VCH: Weinheim, 2005.