Structural insight into the aggregation of l-prolyl dipeptides and its effect on organocatalytic performance

Article abstract of DOI:10.1039/c4ob02003k

NMR and organocatalytic studies of four dipeptides derived from l-proline are described. Results indicate that important conformational changes around the catalytic l-proline moiety are observed for free dipeptides upon changing the adjacent amino acid. Also, an aggregation process is detected as the concentration increases. The self-association of the dipeptides has been fitted to a cooperative binding model. All the compounds have been assayed as catalysts for the conjugated addition of cyclohexanone to trans-β-nitrostyrene in toluene. In agreement with the structural studies, noticeable changes in the catalytic performance are detected upon changing the catalyst concentration, as the catalyst is activated by self-aggregation. This journal is

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RSCPublishing
ARTICLE
Structural Insight into the Aggregation of L-
Prolyl Dipeptides and its Effect on
Organocatalytic Performance
Cite this: DOI: 10.1039/x0xx00000x
Cristina Berdugo,a BeatriuEscuder,a* and Juan F. Miraveta*
Received 00th January 2012,
Accepted 00th January 2012
NMR and organocatalytic studies of four dipeptides derived from Lꢀproline are described. Results indicate
that important conformational changes around the catalytic Lꢀproline moiety are observed for free
dipeptides upon changing the adjacent amino acid. Also, an aggregation process is detected as the
concentration increases. Selfꢀassociation of the dipeptides has been fitted to a cooperative binding model.
DOI: 10.1039/x0xx00000x
www.rsc.org/
All the compounds have been assayed as catalysts for the conjugated addition of cyclohexanone to trans
ꢀ
βꢀnitrostyrene in toluene. In agreement with the structural studies, noticeable changes in the catalytic
performance are detected upon changing catalyst concentration, being the catalyst activated by selfꢀ
aggregation.
Introduction
process.9 Nonꢀlinear effects in organocatalysis have been in
some cases correlated with catalyst selfꢀaggregation.10
Organocatalysis has become a major topic of research in
organic chemistry during the last decades. The extensive work
in this area has been reviewed very often and examples can be
found of organocatalysts for quite a variety of reactions and
media.1 An outstanding family of organocatalysts is that derived
from peptides and in particular from Lꢀproline. This kind of
catalysts have been reported successfully in asymmetric
catalysis for a wide range of synthetically reactions.2 Focusing
on the reaction studied in the present work, Lꢀproline
derivatives have been used extensively as catalysts for
conjugate addition reactions. Seminal work by List and
coworkers reported first the use of Lꢀproline3 and then of Lꢀ
prolylꢀpeptides4 to catalyse efficiently the conjugate addition of
Recently, the aggregation of catalytic Lꢀproline containing
block copolymers was studied.11 Previous work reported in our
group shows how the aggregation of bolaamphiphilic
organogelators containing Lꢀproline catalytic moieties into
fibrilar gel networks modified the enantioselectivity of the
reaction when compared to solution.12 Also in that work the
dipeptide soluble catalyst ProValPr (see Figure 1) was
preliminary studied.
cyclohexanone to
βꢀnitrostyrene. Wennemers and coworkers
have studied in detail peptide catalysed conjugated additions to
nitroalkanes with excellent efficiency in terms of conversion
and enantioselectivity.5 Advances in this line of research
comprise the achievement of highly efficient 1,4ꢀaddition of
aldehydes to nitroolefins using a continuous flow system
containing solidꢀsupported peptidiccatalysts.6 Not long ago, a
detailed mechanistic study including the determination of the
stereoselectivityꢀdetermining step for this type of reaction has
been described.7
Although catalyst aggregation is sometimes neglected, there is
a growing interest in addressing this behaviour and how it
affects to catalytic performance. Aggregation effects have been
analysed especially for thiourea catalysts8 and smart strategies
have been developed to avoid this undesired assembled
Figure 1.Structure of the catalytic dipeptides.
Here we report a detailed study of compound ProValPr and
three other dipeptide analogues as catalysts, focusing on their
structural analysis. Special attention is paid to the
conformational preferences of these analogues and to their selfꢀ
association in solution. Additionally, the efficiency of these
molecules as catalysts in the conjugated addition of
cyclohexanone to β
ꢀnitrostyrene is evaluated.13
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The present study aims to highlight the complexity of Lꢀprolyl exemplified in Figure 3, species distribution diagram reveals
catalysts associated to conformational mobility and selfꢀ that upon going from ca. 1 mM to 5 mM a very important linear
aggregation rather than to develop new catalysts with high increase of dimeric species takes place. Also exponential
enantioselectivity, which have been reported elsewhere as cited growth of oligomeric species is observed.
above.
Molecular mechanics calculations (AMBER* force field) for
the free catalysts predict in all the cases two energetically close
folded conformations (ꢀ
E ca. 7 kJ molꢀ1) near the global
Results and Discussion
minimum. In both cases the conformers contain a Hꢀbond
between propylamide NH and CO of the proline moiety, in
accordance with the experimental results described below (see
anti and syn conformers in Scheme 1; consult Supporting
Information for pictures of the molecular models; details of
computations in Experimental Section).
Four different Lꢀproline derivative dipeptides were studied
containing respectively Lꢀalanine, Lꢀphenylalanine, Lꢀvaline
and Lꢀisoleucine. All of them were capped at Cꢀterminus as
propylamides (Figure 1). The selfꢀassembly of the dipeptide
catalysts in toluene was studied from a thermodynamic and
structural point of view using NMR experiments. The
compounds were fully soluble in this solvent in all the
Table 1. Thermodynamic constants for the selfꢀaggregation of the catalysts at
concentration range studied. Firstly, selfꢀassociation of the 30 ºC in toluene.
molecules was monitored by NMR in D8ꢀtoluene following the
Compound
K2
Kn
Aggregation
Aggregation
chemical shift of the amide signals. It can be seen in Figure 2
degree at 1 mM
degree at 5 mM
(%) a
(%) a
28
that both amide resonances of ProIlePr were shifted downfield
ProValPr
as the concentration increases, revealing intermolecular
ProIlePr
29
20
17
21
100
106
38
6
4
3
4
24
16
21
association by means of hydrogen bonding. A similar behaviour
was observed for the other compounds.
ProPhePr
ProAlaPr
72
[a] Calculated using K2 and Kn values. Estimated error is ca. 3%.
4
dimeric species
3
oligomeric species
20 mM
15 mM
10 mM
2
1
0
5 mM
2 mM
1 mM
0
2
4
6
8
10
total [ProIlePr] / mM
20 mM
15 mM
Figure 3. Species distribution diagram for the aggregation of ProIlePr in toluene.
10 mM
5 mM
2 mM
1 mM
Figure 2. Partial1H NMR spectra of ProIlePr and ProAlaPr in D8-toluene at
different concentrations.
Data corresponding to the shift of amide resonances upon
increasing concentration could be fitted to a supramolecular
polymerization model with a dimerization constant, K2, and
equivalent successive aggregation constants, Kn (see
Experimental Section and Supporting Information). In all the
cases moderate cooperativity was observed being Kn> K2
(Table 1). The association constants K2 were similar for the
four dipeptides but higher values of Kn were obtained for
ProValPr and ProIlePr. However these differences do not
affect very significantly to the proportion of aggregated species
in the range of concentrations studied for catalysis. As
Scheme 1. Conformational and aggregation equilibria for the studied peptides.
Similar conformations can be obtained with semiempirical
AM1 calculations (not shown). The main difference among the
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found conformations is the dihedral angle NꢀCꢀC=O of the
proline unit, giving place to syn and anti conformations which
present respectively dihedral angles below and above 90º.
Additionally anti conformation presents an intramolecular
hydrogen bond between Lꢀproline amine and the NH of the
peptidic linkage as described previously (see Scheme 1).12, 14
The existence of this strong intramolecular hydrogen bond can
be demonstrated by comparison of the NMR spectra recorded
in different solvents. As shown in Figure 4, in difference with
NHꢀa signal, chemical shift of NHꢀb, which is adjacent to
proline ring, is insensitive to solvent polarity, indicating its
involvement in intramolecular Hꢀbonding. In this way its
interaction with solvent molecules is precluded.
N-Hb
N-Ha
a
Figure 5. Amide signals shift variation with the temperature of 1 mM ProIlePr
derivative in D8-toluene.
b
Finally, NOE detected between the NH of the propylamide
chain and the proton of the chiral carbon of proline (Scheme 1
and Supporting Information, Figure S5) also fits with anti
conformations which present a shorter distance between these
protons in the models (3.8 and 4.5 Å for anti and syn
dispositions respectively). It has to be noted that although NMR
data indicate that ProValPr and ProIlePr present almost
exclusively anti conformation, ProAlaPr and ProPhePr
derivatives present a detectable amount of molecules in syn
conformation. As seen in Figure 6, peptidic NH signals of
ProAlaPr and ProPhePr present significantly lower chemical
shift values than ProValPr and ProIlePr (7.85, 7.92, 8.04 and
8.02 ppm respectively).
b
a
a
b
D6-DMSO
δ / ppm
Figure 4.Partial 1H NMR spectra of ProValPr (1 mM) in solvents of different
polarity.
On the other hand, syn conformation presents an intramolecular
hydrogen bond between Lꢀproline amine and the carbonyl CO
unit of the peptide linkage.
Structural studies using NMR were carried out to evaluate the
presence of these conformations in solution. A whole set of
evidences were collected which pointed to the majoritarian
presence of folded anti conformations in diluted solutions.
Firstly, for all the compounds, VTꢀNMR experiments (c = 1
mM, almost no aggregation) revealed a significant variation of
both amide signals with temperature indicating their
participation in intramolecular Hꢀbonding (see data for
ProIlePr in Figure 5). This fact fits with the presence of anti
conformations which have two intramolecular Hꢀbonds formed
by amide NHs, as depicted in Scheme 1.
3
ProValPr
ProIlePr
ProPhePr
ProAlaPr
Secondly, as shown in the Figure 6, the splitting of the
resonances of geminal protons in position 3 of the proline ring
also might point to the presence of anti disposition. This type of
conformation provokes the spatial proximity of C3 protons to
Figure 6. Chemical shift of the amide (left) and the protons at position 3 of the L-
proline ring (right) for the different studied compounds (1 mM, D8-toluene, 30
ºC).
the carbonyl, experiencing to
a
different degree its
shielding/deshielding effect. Although this splitting is common
in Lꢀproline rings, its magnitude seems to be related to the
particular conformation present, as shown below.
These data point to the weakening of the intramolecular Hꢀbond
between the nitrogen atom of proline and the hydrogen atom of
the peptide bond that results from syn type conformation.
Furthermore, the geminal protons at C3 position of the proline
ring in ProAlaPr and ProPhePr, although splitted, present a
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reduced difference in chemical shift when compared to
ProValPr and ProIlePr (ca. 0.07 ppm for the former peptides
and ca. 0.15 ppm for the latter compounds) pointing again to
the coexistence of fast exchanging syn and anti conformations
for the alanine and phenylalanine derivatives (see Scheme 1).
The higher steric demand of Val and Ile side chains (secondary
carbon atom attached to chiral centre), compared to Ala and
Phe (methyl and primary carbon atom attached to the chiral
centre respectively) can explain the observed differences. In the
case of anti conformations steric interactions between Val or
Ile side chains and the Lꢀproline ring would arise as indicated
by molecular models. (see Supporting Information, Figure S10,
for schematic Newman projection).
A
B
NOE
2.69 Å
2.55 Å
NOE
Figure 8. Molecular models for the dimers formed by ProIlePr (A) and ProAlaPr
(B) obtained from molecular mechanics calculations. Non polar hydrogen atoms
have been omitted by means of clarity except those involved in NOE contacts.
This reaction has been shown to be catalysed by proline
moieties which forms an enamineꢀtype intermediate by reaction
with cyclohexanone (Scheme 2).13 We reported previously that
dipeptide structures provide improvement of catalyst
performance when compared with the simple analogue Nꢀ
propylꢀLꢀprolinamide.12 For this work it was hypothesized that
the different steric demand of the amino acid side chains could
influence on the catalytic behaviour by means of different
conformational preferences as those discussed above.
In the current work, the syn diastereoisomer of the conjugated
addition product was very majoritarian in all the cases (see
Scheme 2). To evaluate the effect of catalyst aggregation,
reaction rates were studied at catalyst concentrations of 1 mM
and 5 mM. The studied systems contain in the former case
mostly free, nonꢀaggregated catalyst and, in the latter case, a
significant amount of aggregated species (ca. 25 %, see Table
1).
Analysis of the aggregates by NMR was carried out at a
concentration of 70 mM (ca. 80 % of aggregated species). It
was found that in all the cases NOE correlations were obtained
between protons of propyl chain moiety and protons of proline
ring, pointing to the formation of antiparallel aggregates (see
Scheme 1 and Supporting Information, Figure S6 and S7).
Interestingly, 1HꢀNMR spectra of the aggregates formed by
ProPhePr and ProAlaPr show a reduction of the splitting
observed for the geminal protons at C3 in the proline ring when
compared to spectra from diluted samples (Figure 7). However,
for ProValPr and ProIlePr derivatives the mentioned splitting
is maintained upon aggregation. This behaviour points to a
conformational change around the Lꢀproline moiety upon
aggregation of ProPhePr and ProAlaPr which might be
associated to a transition from anti to syn type conformations
upon aggregation as suggested in Scheme 1.
ProPhePr
ProIlePr
70 mM
40 mM
10 mM
2 mM
f1 (ppm)
Figure 7. Partial 1H NMR spectra of ProPhePr and ProIlePr in D8-toluene at
different concentrations.
Molecular mechanics calculations (AMBER* force field)
permit to obtain energy minimized models for the dipeptide
dimers studied. In these simulations several intermolecular Hꢀ
bonds were found. In addition, the spatial proximity of the
propyl and proline moieties in the model agrees with NMR
NOE experiments. As an example, the structures obtained for
ProIlePr and ProAlaPr are shown in Figure 8.
Scheme 2.Catalytic cycle for the conjugated addition of cyclohexanone to trans-
β
-nitrostyrene.
In order to assess how the conformational preferences and
aggregation behaviour affect the catalytic activity of the four
dipeptides, the conjugate addition of cyclohexanone to transꢀβꢀ
nitrostyrene was studied in toluene.
The reactions were carried out in the presence of an excess of
cyclohexanone and therefore by means of simplicity the system
was analysed in terms of pseudo first order kinetics (see
equations 1ꢀ3; in equation (3) [alkene]f = concentration at the
end of the reaction time, [alkene]0 = initial concentration, k’ = k
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[ketone][catalyst]). Indeed the reaction catalysed by ProIleP
r
the hinted conformational differences mentioned above
was monitored in situ at regular time intervals by NMR for ca. between aggregated ProPhePr and ProAlaPr
(anti
60 h and the data fitted well to a first order kinetic model (see conformation) when compared to ProValPr and ProIlePr (syn
Supporting Information, Figure S2). Using the reaction yields conformation) could be argued to explain in part the changes in
under different conditions of catalyst concentration and reaction selectivity. Enantioselectivity was also measured when the
time the values shown in Figure 9 were obtained.
catalysts concentration was varied in the range of 5ꢀ30 mM,
affording only a slight increase of selectivity upon increasing
concentration (see Supporting Information).
d[alkene]
−
−
= k[catalyst][ketone][alkene] (1);
dt
Table 2. Enantioselectivity obtained in the addition of cyclohexanone to
transꢀβꢀnitrostyrene at 25 ºC in toluene.a
d
[
alkene
dt
]
= k'[alkene
]
(2); (pseudo first order kinetics)
Catalyst
Enantiomeric
excessb
[alkene]
ProValPr
ProIlePr
ProPhePr
ProAlaPr
30
29
8
f
ln
= −k't (3); (integrated rate equation)
[alkene]
0
4
Noticeable it was observed that upon increasing catalyst
concentration from 1 to 5mM, the kinetic constant of the
reaction increased significantly for all the cases, being the
catalyst almost inactive for diluted solutions. For example, a 15
fold and 10 fold increase are observed respectively for
ProValPr and ProPhePr when the catalyst concentration
grows from 1 to 5 mM. These results point to catalyst activation
upon increasing concentration and agree with the fact that
aggregates as those shown in Scheme 1 are much more active
than nonꢀaggregated species. A rationale for this behaviour is
that upon aggregation the catalytic amino centre of proline ring
is liberated from the intramolecular Hꢀbonding which prevents
its nucleophilic activity (see Scheme 1 and 2).
[a] [Cyclohexanone] = 110 mM, [transꢀβꢀnitrostyrene] = 55 mM, [catalyst] =
10 mM. Reaction time 72 h.[b] syn (2R, 1'S) enantiomer was majoritary in all
the cases.
Conclusions
The structural studies carried out agree well with the
experimentally observed catalytic activity of the different
dipeptides. In first place, the catalyst activation observed upon
increasing the concentration is clearly associated to the
aggregation through hydrogen bonding. This process
presumably activates the Lꢀproline moiety as a result of the
conformational changes associated to the aggregation. NNR
studies point to the majoritarian presence of the so called anti
conformations both for diluted and concentrated solutions of
ProValPr and ProIlePr. In the case of ProPhePr and
ProAlaPr a conformational change from anti to syn type
conformation might be operating upon aggregation. Overall, the
results support that organocatalysis can be rather sensitive to
concentration and conformational effects associated to minor
changes in the catalyst structure. Structurally simple
compounds such as the reported dipeptides present a broad
scope of species in solution arising both from aggregation and
intrinsic conformational mobility.
30
1mM
5 mM
20
10
0
Experimental Section
General Considerations
.
ProValPr
ProIlePr
ProPhePr
ProAlaPr
NMR spectra were recorded at 500 MHz, 300 MHz (1H NMR)
and 125 MHz, 75 MHz (13C NMR) in different solvents at 30ºC
with the solvent signals as internal reference. Mass spectra were
run in the electrospray (ESMS) mode.
Figure
9
.
kinetic constants (k, see equation
)
determined for different
concentrations of catalyst in the addition of cyclohexanone to trans-β-
nitrostyrene at 25 ºC in toluene. See details in Experimental Section and
Supporting Information.
Synthesis.
Additionally, the enantiomeric excess for the reactions carried
out with a catalyst concentration of 10 mM are disclosed in
Table 2. The reaction showed moderate to poor
enantioselectivity but a difference was found for compounds
The catalysts were prepared with conventional peptide
chemistry methodology. See Supporting Information for
synthetic scheme.
The compound ProValPr was synthetized as reported
previously.15
General procedure for the preparation of N-
hydroxysuccinimide esters of amino acids. Synthesis of
ProValPr and ProIlePr (ee values ca. 30) when compared to
ProPhePr and ProAlaPr derivatives (ee values ca. 6). These
effects are quite small in terms of transition state energy (ca
0.3ꢀ0.4 kcal molꢀ1) and therefore difficult to rationalize. Perhaps
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ZPheOSu
:
N
ꢀCbzꢀLꢀphenylalanine(5.21 g, 16.4 mmol) and
N
ꢀ
General procedure to the synthesis of ZAlaPr: To an iceꢀ
ꢀCbzꢀLꢀalanine (5.03 g,
THF (50 mL) at 0°C. Once a clear solution had been obtained, 22.5 mmol) and triethylamine (3.65 mL, 26.3 mmol) in THF
N,N'ꢀdicyclohexylcarbodiimide DCC) (3.62 g, 17.3 mmol) in (40 mL), a solution of ethyl chloroformate (2.05 mL, 25.7
hidroxysuccinimide (1.97 g, 16.9 mmol) were dissolved in dry cooled solution of the corresponding
N
(
anhydrous THF (10 mL) was added in several aliquots and the mmol) in THF (10 mL) was added dropwise with vigorous
resulting solution was stirred at 0ꢀ5°C for3 h. The stirring, a white precipitate was observed. After 30 min stirring
dicyclohexylurea formed was filtered off and the filtrate was in an iceꢀcold bath, a solution of propylamine (2.15 mL, 26.1
concentrated to dryness. The crude product was recrystallized mmol) in THF (10 mL) was added. The mixture was stirred at 0
from2ꢀpropanol to furnish the pure product crystals. (Yield ºC for 1 h and was left overnight at room tempertature. The
93%). ZPheOSu compound was previously described in resulting white solid solution was filtered and the solvent was
literature and both 1H and 13C NMR spectra were in good evaporated under vacuum. The resulting viscous solid was
agreement with the literature spectra.16
dissolved in dichloromethane (20 mL) and washed with HCl
Synthesis of ZIleOSu: A similar procedure to that described 0.1 M (3 x 20 mL), KOH 0.1 M (3 x 20 mL), NaHCO3 (1 x 20
for ZPheOSu was used. Pure crystals were obtained (Yield mL) and water (1 x 20 mL). The organic phase was dried with
1
89%). H NMR (500 MHz, DMSOꢀd6) δ 8.04 (d,
J = 8.2 Hz, magnesium sulfate anhydrous and the solvent was evaporated
1H), 7.48–7.27 (m, 5H), 5.07 (s, 2H), 4.43–4.30 (m, 1H), 2.81 under vacuum. The solid obtained was purified by column
(s, 4H), 2.04 – 1.78 (m, 1H), 1.52 (ddd, = 14.5, 9.4, 5.8 Hz, chromatography (silica gel, hexane/ ethyl acetate; 1:1). A white
1H), 1.30 (qd, = 15.3, 7.4 Hz, 1H), 1.01 (dd,
= 25.8, 3.8 Hz, solid compound was obtained. (Yield 35%). 1H NMR (300
3H), 0.86 (t,
= 7.4 Hz, 3H); 13C NMR (75 MHz, DMSOꢀd6) δ MHz, DMSOꢀd6) δ 7.76 (s, 1H), 7.45 – 7.15 (m, 5H), 7.15 (m,
170.3, 168.2, 156.5, 137.1, 128.8, 128.3, 128.2, 66.2, 57.5, 1H), 4.99 (s, 2H), 3.98 (p, = 7.1 Hz, 1H), 3.11 – 2.85 (m, 2H),
36.8, 25.9, 24.9, 15.3, 11.4; (ESIꢀTOF, positive mode) m/z exp 1.50 – 1.30 (m, 2H), 1.18 (d, = 7.1 Hz, 3H), 0.81 (t, = 7.4
[M + Na]+calcd for C18H22N2NaO6+ 385.1370 ; found, 385.1381 Hz, 3H); 13C NMR (75 MHz, DMSOꢀd6) δ 172.6, 156.0, 137.5,
J
J
J
J
J
J
J
[M + Na]+, (ꢁ = 1.3 ppm).
128.7, 128.1, 65.7, 50.5, 40.6, 22.7, 18.8, 11.7; (ESIꢀTOF,
General procedure for the preparation of N-N´-Bis(N-Cbz- positive mode) m/z exp [M + Na]+calcd for C14H20N2O3Na+
L-aminoacyl) amines. Synthesis of ZPhePr: The Nꢀ 287.1366 ; found, 287.1372 [M + Na]+, (ꢁ = 0 ppm).
hydroxysuccinimide ester, ZPheOSu, (6.09 g, 15.3 mmol) was General procedure for the deprotection of N
-Cbz groups.
dissolved in DME (100 mL). The propylamine (1.0 g, 16.9 Synthesis of AlaPr: The corresponding ꢀbenzyloxycarbonyl
N
mmol) dissolved in DME (20 mL) was added dropwise and the protected peptide derivative (0.48 g, 2.15 mmol) and catalytic
resulting solution was stirred at room temperature for 18 hours amount of Pd over activated carbon (5ꢀ10% w/w) were placed
and then was warmed for 2 hours at 40ꢀ50ºC. The solvent was in a two necked round bottom flask and suspended in MeOH
evaporated under vacuum. The resulting solid was dissolved in (50 mL). The system was purged to remove the air with N2 and
dichloromethane (25 mL) and washed three times with HCl 0.1 connected to H2 atmosphere. The pasty grey suspension was
M (3 x 25 mL) and water (3 x 25 mL). The organic phase was stirred for several hours until it turned completely black (also
dried with magnesium sulfate anhydrous and the solvent was checked with TLC, MeOH: CH2Cl2 (1:4) and revealed with
evaporated under vacuum. A white solid was obtained (Yield ninhydrin). The black suspension was filtered over Celite and
1
93%). H NMR (500 MHz, DMSOꢀd6) δ 7.92 (t,
J
= 5.5 Hz, the solvent was evaporated under reduced pressure. The
= 15.6 Hz, 1H), 7.38 – 7.10 (m, 10H), 4.99 – resulting oil was dried in vacuum pump for 24 hours. (Yield
4.89 (s, 2H), 4.21 (m, 1H), 3.10 – 2.86 (m, 3H), 2.77 (dd,
86%). 1H NMR (300 MHz, DMSOꢀd6) δ 7.72 (s, 1H), 3.20 (dd,
13.6, 10.1 Hz, 1H), 1.45 – 1.30 (m, 2H), 0.80 (t, = 7.4 Hz, = 13.7, 6.9 Hz, 1H), 3.00 (dd, = 13.6, 6.5 Hz, 2H), 1.52 –
= 6.9 Hz, 3H), 0.82 (t, = 7.4 Hz, 3H);
1H), 7.45 (d,
J
J
=
J
J
J
3H). 13C NMR (126 MHz, DMSOꢀd6) δ 171.5, 156.2, 138.5, 1.27 (m, 2H), 1.09 (d,
J
J
137.5, 129.6, 128.7, 128.4, 128.0, 127.8, 126.6, 65.6, 56.7, 13C NMR (75 MHz, DMSOꢀd6) δ 176.0, 50.7, 40.4, 22.8, 22.1,
40.7, 38.2, 22.7, 11.7. (ESIꢀTOF, positive mode) m/z exp [M + 11.7; (ESIꢀTOF, positive mode) m/z exp [M + H]+calcd for
H]+calcd for C20H25N2O3 341.1865 ; found, 341.1860 [M + C6H15N2O+ 131.1179 ; found, 131.1181 [M + H]+, (ꢁ = 2.3
+
H]+, (ꢁ = 1.5 ppm).
ppm).
Synthesis of ZIlePr: A similar procedure to that described for Synthesis of PhePr: A similar procedure to that described for
1
ZPhePr was used starting from the
of Cbzꢀisoleucine (ZIleOSu). A white solid was obtained NMR (300 MHz, DMSOꢀd6) δ 7.74 (s, 1H), 7.41 – 6.98 (m,
(Yield 84%). H NMR (300 MHz, DMSOꢀd6) δ 7.85 (s, 1H), 10H), 3.39 (m, 1H), 3.11 – 2.80 (m, 3H), 2.60 (dd,
7.32 (m, 5H), 7.18 (d,
8.1 Hz, 1H), 3.11–2.86 (m, 2H), 1.66 (s, 1H), 1.38 (dd,
14.3, 7.4 Hz, 3H), 1.03 (m, 1H), 0.95 – 0.66 (m, 9H); 13C NMR 129.7, 128.4, 126.4, 56.7, 41.7, 40.5, 22.7, 11.7; (ESIꢀTOF,
(126 MHz, DMSOꢀd6) δ 171.4, 156.4, 137.5, 128.7, 128.1, positive mode) m/z exp [M + H]+calcd for C12H19N2O+
128.0, 65.7, 59.7, 40.6, 36.7, 24.8, 22.6, 15.8, 11.8, 11.3; (ESIꢀ 207.2915 ; found, 207.1498 [M + H]+, (ꢁ = 0.5 ppm).
Nꢀhydroxysuccinimide ester AlaPr was used. A yellow oil was obtained (Yield 96%). H
N
-
1
J
= 13.3, 8.0
= 7.2 Hz, 2H), 0.77 (t, = 7.4
Hz, 3H); 13C NMR (126 MHz, DMSOꢀd6) δ 174.5, 139.2,
J
= 8.4 Hz, 1H), 5.01 (s, 2H), 3.80 (t,
J
J
=
=
Hz, 1H), 1.61 (s, 1H), 1.34 (h,
J
J
TOF, positive mode) m/z exp [M
C17H26N2NaO3 329.1836 ; found, 329.1840 [M + Na]+, (ꢁ = AlaPr was used. An oil was obtained (Yield 95%). H NMR
+
Na]+ calcd for Synthesis of IlePr: A similar procedure to that described for
+
1
0.3 ppm).
(300 MHz,DMSOꢀd6) δ 7.74 (s, 1H), 3.14 – 2.87 (m, 3H), 1.57
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4OB02003K
(dd,
1H), 0.88 – 0.69 (m, 9H); 13C NMR (75 MHz, DMSOꢀd6) δ NaOH (pH =12), and extracted with chloroform (3 x 15 mL).
174.9, 59.8, 40.5, 38.9, 24.2, 22.8, 16.2, 11.9, 11.8; (ESIꢀTOF, The organic layers were washed with water and dried (Na SO ).
positive mode) m/z exp [M + H]+calcd for C9H21N2O+ 173.1648 The solvent was evaporated under vacuum to yield a white
; found, 176.1656 [M + H]+, (ꢁ = 1.2 ppm). 1H NMR (500 MHz, DMSOꢀd6
solid compound. (Yield 66%)
General procedure for the preparation of N-Boc-protected δ 8.09 (d, = 7.7 Hz, 1H), 7.84 (d, = 97.2 Hz, 1H), 4.37 –
J = 8.9, 3.6 Hz, 1H), 1.45 – 1.28 (m, 3H), 1.15 – 0.91 (m, wasdissolved in water (50 mL). The solution was treated with
2
4
.
)
J
J
compounds. Synthesis of BocProAlaPr: A solution of amino 4.14 (m, 1H), 3.60 – 3.38 (m, 1H), 3.09 – 2.96 (m, 2H), 2.90 –
amide AlaPr (0.64 g, 4.8 mmoles) in dry DME (10 mL) was 2.80 (m, 1H), 2.77 – 2.67 (m, 1H), 2.06 – 1.85 (m, 1H), 1.70 –
added dropwise over a solution of BocꢀLꢀProꢀOSu (1.87 g, 5.8 1.60 (m, 1H), 1.61 – 1.52 (m, 2H), 1.46 – 1.31 (m, 2H), 1.28 –
mmol) in dry DME (100 mL). The mixture was stirred at room 1.04 (m, 3H), 0.83 (td,
J
= 7.4, 2.1 Hz, 3H); 13C NMR (75
temperature for 24 h and then at 40 ºC for 5 h. The solvent was MHz, DMSOꢀd6) δ 174.4, 172.5, 61.0, 48.3, 47.3, 41.3, 30.9,
evaporated under vacuum and the resulting white solid was 26.2, 22.8, 19.8, 11.8;(ESIꢀTOF, positive mode) m/z exp [M +
+
dissolved in dichloromethane (50 mL) and washed with H]+calcd for C11H22N3O2 228.1707 ; found, 228.1712 [M +
NaHCO3 (3 x 15 mL). Afterwards, the organic layers were H]+, (ꢁ = 0.9 ppm).
dried (Na2SO4) and the solvent was evaporated under vacuum Synthesis of ProPhePr: A similar procedure to that described
1
to yield a white solid product. (Yield 60%). H NMR (300 for ProAlaPr was used. A white solid product was obtained
MHz, DMSOꢀd6) δ 7.87 (s, 1H), 7.70 (d,
J
= 50.9 Hz, 1H), 4.55 (Yield 68%)
= 8.2, 3.4 Hz, 1H), 3.34 14.5, 7.4 Hz, 2H), 7.30 – 7.02 (m, 5H), 4.48 (td,
= 12.8, 6.8 Hz, 2H), 2.38 1H), 3.43 (dt, = 23.6, 11.7 Hz, 1H), 3.12 – 2.86 (m, 3H), 2.85
= 7.9, 7.5 Hz,
= 7.4 Hz, 3H);13C NMR
= 7.4 Hz, 3H);13C NMR (75 MHz, DMSOꢀd6) δ172.3, (126 MHz, DMSOꢀd6) δ 174.1, 170.9, 137.7, 129.6, 128.3,
. J =
1H NMR (300 MHz, DMSOꢀd6) δ 8.00 (dd,
(dd,
(dd,
(dd,
J
J
J
= 8.9, 3.7 Hz, 1H), 4.08 (dd,
= 14.3, 7.4 Hz, 2H), 3.00 (dd,
= 14.7, 7.3 Hz, 1H), 2.15 – 1.94 (m, 1H), 1.93 – 1.59 (m, – 2.69 (m, 3H), 2.65 – 2.53 (m, 1H), 1.81 (tt,
= 7.0 Hz, 3H), 1H), 1.52 – 1.20 (m, 5H), 0.79 (t,
J
J
J = 8.6, 5.5 Hz,
J
J
2H), 1.50 – 1.32 (m, 9H), 1.30 (s, 2H), 1.17 (d,
0.80 (t,
J
J
J
153.7, 78.8, 59.7, 48.4, 46.9, 40.6, 31.3, 28.4, 25.7, 22.7, 19.1, 126.7, 60.4, 53.2, 46.9, 40.7, 38.9, 30.6, 26.0, 22.6, 11.7; (ESIꢀ
11.68, 11.7; (ESIꢀTOF, positive mode) m/z exp [M + H]+calcd TOF, positive mode) m/z exp [M + H]+calcd for C17H26N3O+
+
for C16H30N3O4 328.2231 ; found, 328.2237 [M + H]+, (ꢁ = 304.2020; found, 304.2027 [M + H]+, (ꢁ = 0.7 ppm).
0.3 ppm).
Synthesis of BocProPhePr: A similar procedure to that for ProAlaPr was used. A white solid product was obtain (Yield
described for BocProAlaPr was used. A white solid product 52%)
1H NMR (500 MHz, DMSOꢀd6) δ 8.13 – 7.89 (m, 2H),
was obtained (Yield 95%). H NMR (300 MHz, DMSOꢀd6) δ 4.11 (ddd, = 25.7, 13.2, 7.8 Hz, 1H), 3.53 (dd, = 9.0, 5.0 Hz,
7.88 (s, 1H), 7.80 (d, = 8.2 Hz, 1H), 7.22 (q, = 8.7 Hz, 5H), 1H), 3.14 – 2.91 (m, 2H), 2.91 – 2.84 (m, 1H), 2.72 (dt,
4.01 (dd, = 8.7, 3.3 Hz, 1H), 3.41 (m, 1H), 3.24 – 2.89 (m, 10.2, 6.4 Hz, 1H), 1.94 (ddd, = 16.3, 12.4, 7.5 Hz, 1H), 1.78 –
6H), 2.00 (s, 1H), 1.64 (d, = 12.8 Hz, 3H), 1.36 (dd, = 14.1, 1.64 (m, 2H), 1.63 – 1.48 (m, 2H), 1.48 – 1.28 (m, 3H), 1.08 –
5.9 Hz, 6H), 1.22 (d, = 11.9 Hz, 5H), 0.76 (t, = 7.4 Hz, 3H); 0.91 (m, 1H), 0.81 (dq,
= 11.8, 7.3 Hz, 9H).13C NMR (300
Synthesis of ProIlePr: A similar procedure to that described
.
1
J
J
J
J
J =
J
J
J
J
J
J
J
13C NMR (75 MHz, DMSOꢀd6) δ 172.4, 171.1, 153.8, 138.2, MHz, dmso) δ 174.2, 171.0, 60.6, 56.2, 47.1, 40.5, 37.8, 31.0,
129.5, 128.4, 126.6, 78.8, 60.1, 54.3, 46.9, 38.4, 31.2, 28.3, 26.3, 24.6, 22.6, 15.8, 11.8, 11.3;(ESIꢀTOF, positive mode)
23.3, 22.6, 11.7;(ESIꢀTOF, positive mode) m/z exp [M + m/zexp [M + H]+calcd for C14H28N3O+ 270.2176; found,
Na]+calcd for C22H33N3O4Na+ 426.2363; found, 426.2365 [M + 270.2186 [M + H]+, (ꢁ = 1.5 ppm).
Na]+, (ꢁ = 0.9 ppm).
Synthesis of BocProPhePr: A similar procedure to that General procedure for the 1,4-conjugated Michael addition
described for BocProAlaPr was used. A white solid product reaction.12 Catalyst (0.033 mmol) was dissolved in a vial using
was obtained (Yield 89%)
8.02 – 7.79 (m, 1H), 7.61 (d,
1H), 3.51 – 3.30 (m, 1H), 3.09 – 2.87 (m, 2H), 2.76 (d,
.
1H NMR (500 MHz, DMSOꢀd6) δ the amount of toluene required to reach the targeted final
J
= 20.3 Hz, 1H), 4.29 – 3.98 (m, concentration (for diluted systems, 1 mM and 2.5 mM, 0.006
= 27.1 mmol of catalyst were added). Then, transꢀβꢀnitrostyrene (0.16
J
Hz, 2H), 2.16 – 1.95 (m, 1H), 2.01 – 1.57 (m, 4H), 1.48 – 1.20 mmol) and cyclohexanone (3.29 mmol) were added and the
(m, 12H), 1.17 – 1.00 (m, 1H), 0.93 – 0.68 (m, 9H); 13C NMR reacction was left at room temperature the required time. The
(126 MHz, DMSOꢀd6) δ 172.4, 171.1, 153.8, 80.2, 59.9, 57.2, reaction was quenched by addition of a 0.25 M aqueous acetic
46.9, 40.6, 37.1, 31.5, 28.1, 25.9, 24.9, 22.6, 15.8, 11.7, 11.2 ; acid solution (2 mL) and toluene (1 mL). The aqueous layer
(ESIꢀTOF, positive mode) m/z exp [M + Na]+calcd for was extracted with toluene (2 x 2 mL). Then the solvent of the
C19H35N3O4Na+ 392.2520 ; found, 392.2527 [M + Na]+, (ꢁ = combined organic extracts were removed until dryness. The
1
0.5 ppm).
reaction crude was analyzed by HꢀNMR in CDCl3 in order to
General procedure for the deprotection of N-Boc groups. determine the yield and the diastereoselectivity (syn= 3.76 ppm;
Synthesis of ProAlaPr: ꢀprotected
compound anti= 4.01 ppm)17 and was further purified by column
N
BocProAlaPr(2.00 g, 7.8 mmol) was dissolved in chromatography (silica gel, mixture of hexane /ethyl acetate,
dichloromethane (50 mL) andafter addition of TFA (15 mL) the 3:1) to isolate the pure product used to determine
mixture was stirred at room temperature for 2 h. The solvent enantioselectivity.
was evaporated under vacuum and then the resulting crude oil
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J. Name., 2012, 00, 1-3 | 7

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ARTICLE
Journal Name
DOI: 10.1039/C4OB02003K
Determination of the enantiomeric excess.12 The Berdugo thanks Ministry of Science and Innovation of Spain
enantiomeric excess was determined by HPLC using a Chiral for a PhD fellowship (FPI program).
Pack IA column, hexane/IPA (v/v: 85/15), flow rate= 1
mL/min, λ= 210 nm, t1= 7.76 min (2S, 1'R), t2= 8.76 min (2R, Notes and references
aDepartament de Química Inorgànica i Orgànica, Universitat Jaume I,
Avda. SosBaynat s/n, Castelló, 12071 Spain
Fax: +34964728214
1'S).
Kinetic studies. Kinetic constants were estimated from the
reaction yield at a given final time. The concentration of
cyclohexanone was approximated to be constant and the system
analyzed as having a pseudo first order kinetics.
Eꢀmail: miravet@uji.es; escuder@uji.es
Homepage: http://supra.uji.es
†Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
NMR studies 1H and 13C NMR spectra were recorded in a
Varian Mercury 300 MHz or Varian Inova 500 MHz
spectrometer at 30 ºC. NOESYꢀ1D experiments were recorded
using a mixing time of 800 ms at 30 ºC.
1
(a) S. Mukherjee, J. W. Yang, S. Hoffmann and B. List, Chem. Rev.,
2007, 107, 5471ꢀ5569; (b) W. Notz, F. Tanaka and C. F. Barbas III,
Acc. Chem. Res., 2004, 37, 580ꢀ591.
2
3
4
5
H. Wennemers, Chem. Commun., 2011, 47, 12036ꢀ12041.
Determination of aggregation constants. The NMR chemical
shift of the NH signal from the propylamide unit in D8ꢀ
toluenewas used as a probe for the determination of the
association constants. The samples were prepared dissolving
the required amount of catalyst in toluene and were left
overnight at r.t. A set of data corresponding to at least 10
different concentrations were acquired at 30 ºC. The results
could be fitted to a cooperative binding model using the
equations reported previously in the literature18 (see Supporting
Information). The equations were solved by nonꢀlinear
regression with Solver (Microsoft Excel) to afford K2
(dimerization constant, see equation (4)), Kn (successive
oligomerization constant, see equation (5)) and maximum
B. List, P. Pojarliev and H. J. Martin, Org. Lett., 2001,
H. J. Martin and B. List, Synlett, 2003, 1901ꢀ1902.
3, 2423ꢀ2425.
(a) R. Kastl, Y. Arakawa, J. Duschmale, M. Wiesner and H.
Wennemers, Chimia, 2013, 67, 279ꢀ282; (b) J. Duschmale and H.
Wennemers, Chem. Eur. J., 2012, 18, 1111ꢀ1120; (c) M. Wiesner, J.
D. Revell and H. Wennemers, Angew. Chem. Int. Ed., 2008, 47
,
1871ꢀ1874; (d) M. Wiesner, M. Neuburger and H. Wennemers,
Chem. Eur. J., 2009, 15, 10103ꢀ10109.
6
(a) S. B. Otvos, I. M. Mandity and F. Fulop, ChemSusChem, 2012,
266ꢀ269; (b) Y. Arakawa and H. Wennemers, ChemSusChem, 2013,
, 242ꢀ245.
5,
6
7
8
F. Bachle, J. Duschmale, C. Ebner, A. Pfaltz and H. Wennemers,
Angew. Chem. Int. Ed., 2013, 52, 12619ꢀ12623.
chemical shift (δmax).
(a) H. S. Rho, S. H. Oh, J. W. Lee, J. Y. Lee, J. Chin and C. E. Song,
Chem. Commun., 2008, 1208ꢀ1210; (b) H. B. Jang, H. S. Rho, J. S.
Oh, E. H. Nam, S. E. Park, H. Y. Bae and C. E. Song, Org. Biomol.
A + A = A2; K2 (4)
Chem., 2010, 8, 3918ꢀ3922; (c) G. Tarkanyi, P. Kiraly, T. Soos and
An + A = An+1; Kn (5)
S. Varga, Chem. Eur. J., 2012, 18, 1918ꢀ1922.
9
(a) S. H. Oh, H. S. Rho, J. W. Lee, J. E. Lee, S. H. Youk, J. Chin and
C. E. Song, Angew. Chem. Int. Ed., 2008, 47, 7872ꢀ7875; (b) J. S.
Oh, J. W. Lee, T. H. Ryu, J. H. Lee and C. E. Song, Org. Biomol.
Chem., 2012, 10, 1052ꢀ1055.
Molecular modeling studies. The models reported were
obtained by molecular mechanics calculations performed with
MacroModel using AMBER* as force field. Exhaustive Monte
Carlo conformational search (1000 steps of torsional angles
variation) was carried out for the isolated molecules. The
structures described for the folded conformations in scheme 1
and Supporting Information correspond to energy minimized
structures near the global minimum (within ca. 10 kJmolꢀ1). The
models for the aggregates shown in Figure 8 were built
manually from unfolded energy minimized conformers. Then
the dimeric structures were energy minimized to the nearest
local energy minimum. The feasibility of the molecular
mechanics models was checked with AM1 semiempirical
energy minimizations which afforded basically similar
conformations within a similar energy range.
10 T. Satyanarayana, S. Abraham and H. B. Kagan, Angew. Chem. Int.
Ed., 2009, 48, 456ꢀ494.
11 E. G. Doyagüez, G. Corrales, L. Garrido, J. RodríguezꢀHernández, A.
Gallardo and A. FernándezꢀMayoralas, Macromolecules, 2011, 44
,
6268ꢀ6276.
12 F. RodriguezꢀLlansola, J. F. Miravet and B. Escuder, Chem. Eur. J.,
2010, 16, 8480ꢀ8486.
13 (a) M. E. Kuehne and L. Foley, J. Org. Chem., 1965, 30, 4280ꢀ4284;
(b) A. P. Carley, S. Dixon and J. D. Kilburn, Synthesis-Stuttgart
,
2009, 2509ꢀ2516; (c) N. Mase, K. Watanabe, H. Yoda, K. Takabe, F.
Tanaka and C. F. Barbas III, J. Am. Chem. Soc., 2006, 128, 4966ꢀ
4967.
14 F. RodriguezꢀLlansola, J. F. Miravet and B. Escuder, Chem.
Commun, 2009, 209.
Acknowledgments
Ministry of Science and Innovation of Spain (grant CTQ2012ꢀ
37735) and Universitat Jaume I (grants P1.1B2012ꢀ25 and
P1.1B2013ꢀ57) are thanked for financial support. Cristina
15 (a) B. Escuder, S. Marti and J. F. Miravet, Langmuir, 2005, 21, 6776ꢀ
6787; (b) F. RodriguezꢀLlansola, B. Escuder and J. F. Miravet, J. Am.
Chem. Soc., 2009, 131, 11478ꢀ11484.
8 | J. Name., 2012, 00, 1-3
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Journal Name
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DOI: 10.1039/C4OB02003K
16 J. Becerril, M. Bolte, M. I. Burguete, F. Galindo, E. GarcíaꢀEspaña,
S. V. Luis and J. F. Miravet, J. Am. Chem. Soc., 2003, 125, 6677ꢀ
6686.
17 A. J. Cobb, D. M. Shaw, D. A. Longbottom, J. B. Gold and S. V.
Ley, Org. Biomol. Chem., 2005, 3, 84ꢀ96.
18 (a) M. de Loos, J. van Esch, R. M. Kellogg and B. L. Feringa,
Angew. Chem. Int. Ed., 2001, 40, 613ꢀ616; (b) M. Akiyama and T.
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50, 317ꢀ324.
This journal is © The Royal Society of Chemistry 2012
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