Circular dichroism studies on absolute configuration assignment of peptidomimetics with a terminal chiral 3-arylpropionic acid unit

Article abstract of DOI:10.1016/j.tetasy.2006.08.024

The relationship between the molecular structure of peptidomimetics 1 and their chiroptical properties has been studied using circular dichroism spectroscopy. It was demonstrated that the sign of the Cotton effects occurring around 230 and 270 nm enables the direct assignment of stereochemistry at C-1 and C-3 carbon atoms. The TD-DFT calculations of the circular dichroism (CD) spectrum of one of the model compounds confirm the absolute stereochemistry assignment made experimentally. Thus, CD spectroscopy proved to be a simple, reliable and general tool for the determination of the absolute configuration of the stereogenic centers of peptidomimetics with a terminal 3-arylpropionic group.

Full text of DOI:10.1016/j.tetasy.2006.08.024

Tetrahedron: Asymmetry 17 (2006) 2469–2478
Circular dichroism studies on absolute configuration assignment
of peptidomimetics with a terminal chiral 3-arylpropionic acid unit
Jadwiga Frelek,a,* Anna Fryszkowska,b Marcin Kwitc and Ryszard Ostaszewskia,b,
*
aInstitute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
bFaculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
cFaculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland
Received 31 January 2006; revised 24 August 2006; accepted 29 August 2006
Abstract—The relationship between the molecular structure of peptidomimetics 1 and their chiroptical properties has been studied using
circular dichroism spectroscopy. It was demonstrated that the sign of the Cotton effects occurring around 230 and 270 nm enables the
direct assignment of stereochemistry at C-1 and C-3 carbon atoms. The TD-DFT calculations of the circular dichroism (CD) spectrum of
one of the model compounds confirm the absolute stereochemistry assignment made experimentally. Thus, CD spectroscopy proved to
be a simple, reliable and general tool for the determination of the absolute configuration of the stereogenic centers of peptidomimetics
with a terminal 3-arylpropionic group.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, we reported a novel approach to the synthesis of
non-racemic peptidomimetics of structure 1a–d (Fig. 1)
with a combination of enzymatic desymmetrization of 3-
phenylglutaric anhydrides with the Ugi multi-component
reaction.7,8
Currently, medicinal chemists have dedicated much atten-
tion to the novel classes of peptide-like compounds, pepti-
domimetics, which exhibit therapeutic activity.1,2 It is well
known that there is a close relationship between their bio-
logical activity and stereostructure. In this context, there is
a need for new synthetic methods leading to chiral pepti-
domimetics with defined absolute configuration. Therefore,
a reliable assignment of configuration at all stereogenic
centers present in peptidomimetics is rapidly becoming a
key issue.
R2
O
H
N
R1
N
R3
C-1
C-3
O
Peptidomimetics possessing a terminal 3-arylpropionic acid
are known to exhibit biological activity.3–6 According to
the literature, the configuration at C-3 of this group has
a significant influence on their biological activity.4–6 Never-
theless, the problem of the absolute configuration assign-
ment often remains unsolved in the literature. For
example, Tucker5 and Brady6 designed a series of potent
thrombin inhibitors possessing a 3-arylpropionic unit. A
clear difference in the potency between the diastereoisomers
was observed, although, the stereochemistry at the C-3
atom was not established.
1a, R =R3= CH2CO2Et, R2 = H
1b, R1 = Bn, R2 = H, R3= CH2CO2Et
1c, R1 = Bn, R2 = Cl, R3= CH2CO2Et
1d, R1 = Bn, R2 = OMe, R3= CH2CO2Et
Figure 1. Peptidomimetics of structure 1.
Since over the course of the aforementioned reaction the
diastereoisomers are formed, we needed to solve the prob-
lem of the assignment of their relative and absolute config-
urations. The initial circular dichroism (CD) studies of
compounds 1a–d, proved that the less polar diastereoiso-
mers of 1a–d possessed the same relative configurations
*
Corresponding authors. Tel.: +48 22 343 21 20; fax: +48 22 632 66 81
(R.O.); e-mail: rysza@icho.edu.pl
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.08.024

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J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
at C-1 and C-3.8 Similarly, all the more polar diastereoiso-
mers possessed the opposite relative configuration to their
less polar counterparts. We initially assumed that the
Smith’s benzene sector rule9 could be used to assign abso-
lute configuration at the C-3 carbon atom. A direct assign-
ment of absolute configuration at the C-1 carbon atom was
a more complex problem, which could not be resolved with
the data we had in our hands. To unambiguously deter-
mine the absolute configuration at this stereogenic center
we decided to undertake extended chiroptical studies. For
this purpose we synthesized all the possible stereoisomers
of peptidomimetics 1a and 1b (four for the a series and four
for the b series, respectively) as model compounds (Fig. 1)
for CD analysis. On the basis of the obtained data we in-
tended to establish a general correlation between Cotton
effects (CEs) present in the CD spectra and the stereo-struc-
ture of peptidomimetics possessing a terminal 3-arylprop-
ionic acid unit.
side reactions for the glycine series, the compounds 6a, ent-
6a and ent-6b were obtained without isolation of the
respective intermediates 5. After deprotection of the amine
group, the resulting crude compounds 5 were used directly
in subsequent aminoalkylation reaction to give secondary
amines 6 in 29–47% yield. Chiral monoesters 7 (80% ee)
and ent-7 (77% ee) were obtained following the enzymatic
desymmetrization procedure reported earlier by us.10
A key synthetic step was the coupling of amines 6 with
monoesters 7. This was achieved by mixed anhydride meth-
odology. This approach, in comparison to the coupling
with DCC/HOBt, gave comparable yields (Table 1),
although the purification procedure was simplified. The
diastereoisomers of peptidomimetics 1a and 1b were syn-
thesized in yields ranging from 16% to 45% (Table 1). No
attempt was made to optimize the reaction.
Table 1. Synthesis of model compounds 1a and 1b by coupling of amines
6 with monoacids 7
2. Results and discussion
2.1. Chemistry
Entry
Amine 6
Acid 7
Reaction
time [h]
Yield
[%]
Product
1
2
3
4
5
6
7
8
ent-6a
6a
6a
7
24
16
5.5
24
24
24
24
24
45
24
16
32
(1R,3S)-1a
(1S,3R)-1a
(1S,3S)-1a
(1R,3R)-1a
(1R,3S)-1b
(1S,3R)-1b
(1S,3S)-1b
(1R,3R)-1b
The diastereoisomers with a defined configuration at C-1
and C-3 carbons were obtained by the four-step functional-
ization of L- and D-leucine 2 and ent-2, respectively, leading
to the respective derivatives 6 followed by their coupling
with optically active 3-phenylglutaric acid monoethyl esters
7 and ent-7, as shown in Scheme 1.
ent-7
7
ent-6a
ent-6b
6b
ent-7
7
28
ent-7
7
26 (28)a
31 (26)a
17
6b
ent-6b
ent-7
a By coupling using DCC and HOBt in CH2Cl2.
Coupling of the Cbz-protected amino acids 3 with glycine
ethyl ester hydrochloride using DCC in the presence of
hydroxybenzotriazole led to the formation of amides 4a
and ent-4a in 61% and 78% yield, respectively. Analogous
coupling with benzylamine gave the compounds 4b in
80% yield. Amine 6b was obtained in a two-reaction
sequence. Catalytic hydrogenation of amide 4b gave
L-leucine benzylamide 5b in 81% yield. The reductive alkyl-
ation with butanal in the presence of NaBH3CN afforded
amine 6b in 47% yield. The overall yield for the two steps
was 38%. To simplify the reaction procedure and to avoid
2.2. Configuration assignment by CD
The above synthetic approach gave us an access to the
model compounds 1 with unambiguously defined stereo-
chemistry at C-1 and C-3. The respective CD and UV data
are presented in Table 2. The CD and UV spectra of the
compounds (1S,3S)-1a and (1R,3R)-1a are shown in Figure
2.
b or c
d,e
a
H
H
R1
BuNH
R1
OH
OH
N
N
H2N
CbzNH
CbzNH
O
O
3
O
O
2
4a, R1 = CH2CO2Et
4b, R1 = Bn
6a, R1 = CH2CO2Et
6b, R1 = Bn
O
O
+ 7
f or g
OH
EtO
7
(S,S)-1a-b
Scheme 1. The synthetic strategy leading to model compounds 1a and 1b with a defined stereochemistry at C-1 and C-3. Reagents and conditions: (a)
CbzCl, NaOH, 0 ꢁC, ꢀ90%; (b) Gly-OEtÆHCl, DCC, HOBt, Et3N, CH2Cl2, 68–78%; (c) BnNH2, DCC, HOBt, CH2Cl2, 80%; (d) H2, Pd/C, MeOH; (e)
PrCHO, NaBH3CN, Na2SO4, THF, 29–47% for two steps d and e; (f) ClCO2Et, Et3N, acetone, 16–45%; (g) DCC, HOBt, CH2Cl2, 26–28%.

J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
Table 2. UV and CD data of the model compounds 1a and 1b recorded in MeCN
2471
Entry
Compound
UV e (k)
9000 (209)
CD De (k)
+1.29 (206.0)
1
2
3
4
5
6
7
8
(1R,3S)-1a
(1S,3R)-1a
(1S,3S)-1a
(1R,3R)-1a
(1R,3S)-1b
(1S,3R)-1b
(1S,3S)-1b
(1R,3R)-1b
330 (257)
350 (257)
475 (256)
300 (257)
510 (257)
580 (257)
520 (257)
480 (258)
+2.79 (191.0)
ꢁ2.20 (190.0)
+8.07 (190.0)
ꢁ7.98 (191.5)
ꢁ3.16 (197.5)
+2.00 (196.5)
+10.85 (193.0)
ꢁ13.21 (194.0)
+7.71 (230.0)
ꢁ7.41 (230.0)
ꢁ9.73 (229.0)
+10.12 (230.0)
+10.83 (230.0)
ꢁ8.23 (230.5)
ꢁ9.22 (229.0)
+12.73 (230.0)
ꢁ0.03 (269.0)
14,100 (208sh
11,700 (206sh
9600 (208)
)
)
ꢁ1.64 (207.0sh
ꢁ1.28 (208.5sh
+1.25 (207.5sh
+2.18 (208.5)
ꢁ1.75 (210.5)
ꢁ1.36 (210.5sh
+2.55 (210.0)
)
)
)
+0.03 (266.0)
ꢁ0.09 (269.0)
+0.10 (267.5)
ꢁ0.03 (269.5)
+0.02 (267.5)
ꢁ0.02 (269.5)
+0.08 (267.5)
25,000 (208sh
23,600 (207sh
20,300 (208sh
)
)
)
)
27,000 (206sh a
)
The UV and CD values are given as e (k) and De (k), respectively.
a An additional strong absorption band at 192 nm was observed.
As can be seen from Tables 1 and 2, four CD bands are
present in the CD spectra of the compounds investigated.
However, only two CD bands of lowest energy, occurring
around 270 and 230 nm were of interest for the purpose
of our studies.
The next CD band at 230 nm does not possess any corre-
sponding electronic absorption (vide Table 2). This band
most probably represents a sum of contributions from
the ester, amide, and benzene (1La) chromophores present
in the molecules. A relatively strong intensity of this
absorption suggests the presence of different electronic
transitions of approximately the same energy.
The long-wavelength CD band at approximately 268 nm is
associated with the electronic absorption observed at ca.
257 nm (vide Fig. 2). This Cotton effect (CE) can be attrib-
Regarding this CD band, the investigated compounds fall
into two different classes. In the first class, consisting of
compounds (1R,3S)-1a, (1R,3R)-1a, (1R,3S)-1b, and
(1R,3R)-1b, the sign of the 230 nm CD band is positive,
whereas in the second class, represented by compounds
(1S,3R)-1a, (1S,3S)-1a, (1S,3S)-1b, and (1S,3R)-1b, the
sign of this band is negative. Both classes represent the
opposite configurations at stereogenic center C-1: the abso-
lute configuration is (1R) for the first class and (1S) for the
second class. Thus, it can be concluded that the stereo-
chemistry at C-1 is responsible for the sign of this CD
band, independent of the R1 substituent (Fig. 1). Conse-
quently, it can be stated that despite other differences in
the molecular structure, the negative CE at 230 nm corre-
lates to the (1S) and the positive CE corresponds to
the (1R) absolute configuration in the investigated
peptidomimetics.
1
uted to the Lb benzene transition. Although its amplitude
is low, as expected, the band is well developed and can be
undoubtedly used for a correlation between the structure
and CE signs. The circular dichroic assignment for this
band is based on the benzene sector rule.9 According to
the rule, the absolute configuration of a stereogenic center
contiguous to the benzene ring can be established unambig-
uously on the basis of the sign of this excitation. Therefore,
we can use this regularity to assign the configuration at the
C-3 stereogenic center of compounds 1. The presence of an
additional phenyl substituent in the 1b series should not
1
affect the assignment because the sign of the Lb CE de-
pends exclusively on the stereochemistry of the stereogenic
center directly connected to the benzene ring.9 In the case
of peptidomimetics 1, the rule predicts a negative sign of
the CE around 268 nm for diastereoisomers with an (S)-
absolute configuration and a positive one for diastereo-
isomers with an (R)-absolute configuration at the C-3
carbon atom. The spectroscopic data are in excellent agree-
ment with the predicted ones, as evident from Table 2 and
Figure 2.
The regularities above allow the assignment of the absolute
configuration at both C-1 and C-3 stereogenic centers of
the peptidomimetics obtained by the Ugi reaction 1a–d.
Since the negative CE around 230 nm points out unambig-
uously to the (S)-absolute configuration at C-1, the config-
uration at this carbon atom in compounds 1a, 1b, 1c, and
1d displaying a negative sign of this band (Table 3) should
be (1S). Conversely, compounds 1a, 1b, 1c, and 1d with a
positive sign of the 230 nm CD band possess a (1R)-abso-
lute configuration (Table 3).
All compounds obtained by the Ugi reaction should pos-
sess an absolute (S)-configuration at C-3, based on enzyme
enantioselectivity.8,10 Thus, on the basis of the benzene sec-
tor rule, the sign of the 268 nm CD band should be nega-
tive. As can be seen from Table 3, the majority of
compounds studied fulfill this criterion. In the case of prod-
ucts 1a, 1b, and 1d, a negative sign of the band points out
to (3S) absolute configuration, which very nicely corrobo-
rates our expectation. Two stereoisomers of compound 1c,
however, deviate from the regularity, displaying a positive
268 nm CD band for the assumed (3S) absolute configura-
tion. Nevertheless, it is well documented that the benzene
Figure 2. CD spectra of compounds (1S,3S)-1a (
) recorded in MeCN.
) and (1R,3R)-1a
(

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J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
Table 3. UV and CD data of the compounds 1a, 1b, 1c, and 1d obtained by the Ugi reaction, recorded in MeCN
Entry
Compound
UV e (k)
15,900 (205sh
CD De (k)
1
2
3
4
5
6
7
8
(1S,3S)-1a
(1R,3S)-1a
(1S,3S)-1b
(1R,3S)-1b
(1S,3S)-1c
(1R,3S)-1c
(1S,3S)-1d
(1R,3S)-1d
280 (257)
260 (257)
450 (257)
480 (257)
490 (266)
590 (268)
1630 (275)
1530 (275)
)
)
)
)
)
)
)
)
+3.10 (193.0)
ꢁ0.69 (197.0)
+12.1 (191.5)
ꢁ1.68 (197.5)
+4.61 (197.0)
+2.37 (197.0)
+7.70 (193.0)
ꢁ1.47 (195.5)
ꢁ0.46 (210.5)
ꢁ4.56 (228.5)
+6.92 (229.5)
ꢁ9.23 (229.5)
+6.96 (230.5)
ꢁ5.72 (232.0)
+6.38 (230.0)
ꢁ6.29 (235.0)
+4.25 (234.0)
ꢁ0.05 (268.0)
ꢁ0.03 (268.0)
ꢁ0.03 (268.0)
ꢁ0.02 (268.0)
+0.09 (265.5)
+0.05 (265.5)
ꢁ0.09 (278.0)
ꢁ0.08 (281.5)
14,700 (205sh
24,500 (207sh
24,300 (207sh
11,200 (225sh
14,200 (226sh
14,000 (225sh
12,800 (225sh
+1.17 (207.0sh
ꢁ2.40 (210.5sh
+1.16 (209.5)
ꢁ1.54 (209.0sh
+1.91 (210.0sh
ꢁ1.56 (209.0)
+1.57 (207.5)
)
)
)
)
The UV and CD values are given as e (k) and De (k), respectively.
ring substituents, which have a positive spectroscopic mo-
6-31g(d) level was calculated. In the case of (S,S)-1a all
AM1 optimized structures converged to the two conform-
ers, which were fully optimized at b3lyp/6-31g(d) level of
theory. Frequency calculations at the b3lyp/6-31g(d) level
proved that those conformations are stable. For both con-
formers, percentage populations were calculated on the
basis of DE and DG values, using Boltzmann statistics and
T = 298 K (see Table 4). Due to the significant difference
between them, both DE and DG conformer distribution val-
ues were taken under further considerations.24
ment, for example, a chlorine atom, especially at the para
1
position, lead to an opposite sign of the Lb CEs in com-
parison to the unsubstituted counterpart.11,12
Thus, the CD data for 1c validates our assumption as well.
It should be noted, however, that for enantiomers or local
enantiomers of chiral benzene compounds possessing ring
substituents, the benzene sector rule for sign prediction
has to be used carefully. The vibronic and induced contri-
butions to the 1Lb CEs should be considered to fully ascer-
tain a sign prediction.
Table 4. Calculated B3LYP/6-31g(d) relative energies (DE and DG) and
populations of conformers of (S,S)-1a
A lower amplitude of the 230 nm CD band was observed
for the majority of Ugi compounds in comparison to the
model compounds (Tables 2 and 3). This may be due to
the use of enantiomerically pure amino acids (>98% ee)
as substrates in the synthesis of the latter.
Conformer
DE (population)
[kcal molꢁ1
DG (population)
]
[kcal molꢁ1
]
A
B
0.00 (58.9%)
0.21 (41.1%)
1.42 (8.0%)
0.00 (92.0%)
3. TD-DFT calculations
The DFT calculated structures of (S,S)-1a are shown in
Figure 3. Both conformers show structural similarities: (i)
the presence of intramolecular hydrogen NHꢂ ꢂ ꢂO@C
bonds; (ii) the bent structure (as a consequence of stabiliz-
ing hydrogen bond), and (iii) position of all bulky substit-
uents outside the ‘center’ of the molecule. The lengths of
the hydrogen NHꢂ ꢂ ꢂO@C bonds range from 2.013 to
In order to verify the absolute configuration determined
experimentally, we performed a theoretical determination
of the absolute configuration by a calculation of CD spec-
tra of the model compound (S,S)-1a using time-dependent
density functional theory (TD-DFT). Methods based on
density functional theory were found to be attractive in
terms of accuracy and computational effort, which hold
true not only for ground states but also for electronically
excited states.13–15 Therefore the TD-DFT method, in
which excited-state properties are determined from the lin-
ear response of the molecules to an external continuous
wave field, has became an important tool for the theoretical
treatment of molecular electronic excitation spectra.16–21
˚
3.066 A. In the case of the conformer A, the shortest
hydrogen bond occurs between the NH hydrogen atom
and C@O group of the tertiary amide, the second one—be-
tween the NH and C@O ester group is slightly longer,
whereas in case of conformer B the shortest one involves
a NH hydrogen atom and C@O ester carbonyl group,
opposite to NH group. The hydrogen bond between a
NH and tertiary amide C@O group is longer by about
˚
A fundamental prerequisite for the computational calcula-
tion of CD spectra is required for all CD-relevant confor-
mational species of the respective molecule. In our case,
the conformational analysis of (S,S)-1a was carried out
in two steps. From the results of a conformational search
using both MM3 molecular mechanics force field and CON-
FLEX software,22 we obtained the set of conformers of
(S,S)-1a accessible at ambient temperature. From these
conformers we selected those having the relative energies
in the range from 0.0 to 20 kcal molꢁ1, and their structures
were fully optimized with the use of the AM1 semi-empir-
ical method implemented in the Gaussian package.23 For
each conformer, the single point energy at the b3lyp/
1 A and does not affect the conformation. In both cases,
the distance between the NH hydrogen atom and neighbor-
˚
ing C@O oxygen atom is over 3 A and for this reason there
are no hydrogen bonds between NH and terminal ester
groups.
It is well known that the combination of b3lyp hybrid func-
tional with a large basis set augmented with diffuse func-
tions (e.g., aug-cc-pvtz) provided good or excellent results
for TD-DFT calculations of chiroptical properties. Due
to the size of (S,S)-1a, we chose even smaller split-
valence basis set augmented with the diffuse functions 6-
311++g(d,p) and for each conformer we calculated singlet

J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
2473
Figure 4. Calculated by TD DFT/b3lyp/6-311++g(d,p) method CD for
conformers of (S,S)-1a: A—upper panel; B—middle panel, and the
comparison between measured CD spectrum and Boltzman averaged
calculated CD spectra of (S,S)-1a—the lowest panel (note that in this case
the calculated spectra were scaled by the factor of 0.97 to match
experimental bands). Vertical bars represent the calculated rotatory
strengths.
Figure 3. Computed low-energy conformers A (upper) and B (lower) of
(S,S)-1a, with intramolecular hydrogen bonds shown. All CH hydrogen
atoms were omitted for clarity.
higher energy region. However, the Boltzmann averaged
spectra (shown in Fig. 4) are in good agreement with the
measured ones, independent of the method of population
calculations used. A good overall agreement has now been
found between the experimental and computed signs of the
Cotton effects. This correlation applies not only to
the long-wavelength Cotton effects but also to those in
the range of 200–215 nm, which has not been invoked
previously for the absolute configuration-spectra correlation.
states at b3lyp/6-311++g(d,p) level of theory. The theoret-
ical calculation of the CD and UV spectra of low-energy
conformers of (S,S)-1a by TD-DFT/b3lyp/6-311++g(d,p)
method afforded the curves illustrated in Figure 4.
The TD-DFT singlet states calculations well reproduce the
experimental data in the region between 250 and 200 nm,
although the excitation energies were underestimated by
about 5 nm. The calculated spectra for both conformers
show that the first negative Cotton effect at around
235 nm originated mainly from HOMO(ꢁ1) or HOMO-
(ꢁ2)–LUMO(+1) excitations. This band represents a sum
of contributions from the ester, amide, and benzene chro-
mophores present in the molecules. Note that in the case
of conformer B, a small difference in conformation caused
a decrease of the amplitude of the first Cotton effect and an
increase of the amplitude of the second Cotton effect at
Unfortunately the singlet states calculations do not repro-
duce the spectral region at lower energies. According to the
UV experimental data, the lowest energy transitions are
forbidden due to their low intensity and posses the vibronic
character. In order to verify this hypothesis, we calculated
the triplet states for the investigated molecules. Although
the calculated transition energies are in fair agreement
with the experimental data, the calculated oscillator and

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J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
rotatory strengths have zero values and for this reason, the
theoretical confirmation of Smith’s benzene sector rule for
these compounds was not possible at this level of theory.
centrations of approximately 2 · 10ꢁ4 M. The UV and
CD data for all the compounds 1 are summarized in Tables
2 and 3. NMR spectra were recorded in CDCl3 with TMS
as an internal standard using a 200 MHz Varian Gemini
200 or a Bruker DMX 500 Avance (500 MHz) spectrome-
ters. Chemical shifts are reported in parts per million and
coupling constants (J) are given in hertz (Hz). MS spectra
were recorded using an API-365 (SCIEX) apparatus. Opti-
cal rotations were measured in a 1 dm cell of 1 mL capacity
using a Jasco DIP-360 polarimeter operating at 589 nm.
Concentrations c are given in 10 mg/mL. Elemental analy-
ses were performed using a CHN Perkin–Elmer 240 appa-
ratus. Melting points are uncorrected. All reactions were
monitored by TLC on Merck 60 F254 silica gel plates. Pre-
parative TLC was performed on 0.5 mm Kieselgel 60 F254
plates. Column chromatography was performed using
Merck 60/230–400 mesh silica gel.
It is evident that the basic pattern of the CD and UV spec-
tral curves, that is, the sign, position, intensity, and shape
of the bands, is well reproduced by the calculation. It is
noteworthy that the calculated structures and the theory
data points out the presence of intramolecular hydrogen
bonding. This observation is in perfect agreement with
the bent structure for peptidomimetics (S,S)-1 that we pos-
1
tulated previously on the basis of H NMR data.8 Thus,
our absolute configurational assignment is now theoreti-
cally proven. Since the absolute configuration of the model
compound (S,S)-1a is fixed as (1S,3S), the comparison of
the present calculated and observed CD data leads to the
unambiguous determination of the absolute configuration
of the other compounds discussed here. Accordingly, the
absolute stereochemistry of the other compounds 1 is the-
oretically confirmed.
5.2. Computational methods
In our computations all the excited-state calculations have
been performed based upon the ground state geometries of
single molecules with the use of a Gaussian program pack-
age.23 Thus the results correspond to vertical transitions,
and the excitation energies can be compared with the band
maxima in the experimental spectra. Rotatory strengths
were calculated using length and velocity representations.
Length and velocity rotatory strengths are equal and ori-
gin-independent at the complete basis set limit. Herein
the differences between the length and velocity calculated
values of rotatory strengths were quite small and for this
reason only velocity rotatory strengths were taken into
considerations. The CD and UV spectra were simulated
by overlapping Gaussian functions for each transition
according to the procedure described by Grimme et al.25
No correlation for the medium dielectric constant was
implemented. The conformational analysis has been per-
formed with the use of CaChe program package.22
4. Conclusions
In conclusion, the chiroptical correlation for peptidomi-
metics 1a–d obtained via an enzymatic/Ugi reaction combi-
nation has been carried out. A direct assignment of the
absolute configuration at the C-1 and C-3 atoms was pos-
sible on the basis of the signs of the Cotton effects around
268 m and 230 nm only. The CD method proposed here al-
lowed the assignment of the absolute configuration in this
class of peptidomimetics independently of the synthetic
method used. The CD spectroscopy appeared to be a con-
venient, reliable and very fast technique for peptidomimetic
configurational assignment. It was demonstrated that the
CD spectra are highly sensitive to changes in the structure
of the investigated compounds as well. These effects cannot
be studied easily by any other spectroscopic method. The
TD-DFT calculations of the theoretical CD proved the
empirical assignment to be correct. Considering the fact
that the molecule computed here possesses an enormous
flexibility and complexity in relation to the number of chro-
mophores, the agreement between theory and experiment
has to be judged as excellent. This agreement again con-
firms the configuration previously determined and, in addi-
tion, demonstrates the efficiency of the method.
5.3. The general procedure for the synthesis of the model
compounds 1a and 1b
To a cooled solution of monoester 7 (0.08 mmol, 18 mg) in
acetone (0.5 mL), Et3N (0.09 mmol, 12 lL) was added.
After stirring for 5 min, the solution of ethyl chloroformate
(0.09 mmol, 9 lL) in acetone (0.5 mL) was added dropwise
over 10 min. After 10 min, a solution of amine 6
(0.07 mmol) in acetone (0.5 mL) was added dropwise.
The mixture was stirred at 0 ꢁC for 1 h and at rt overnight.
The solvent was evaporated, the residue was dissolved in
Et2O and washed subsequently with citric acid (1 M aque-
ous solution), NaHCO3satd and brine. The organic layer
was dried (MgSO4) and the solvent was evaporated
in vacuo. The product was purified by flash chromato-
graphy (silica gel, hexane/EtOAc as eluents).
We would like to emphasize the fact that the chiroptical
correlation between stereostructure and the CD bands
signs, proposed here, can be applied to a broad variety of
peptidomimetics or naturally occurring products possess-
ing terminal 3-arylpropionic acid unit in their structure.
It should be noted that the benzene sector rule for sign pre-
diction has to be used with due care.
5. Experimental
5.1. General
5.3.1. Peptidomimetic (1R,3S)-1a: (3S)-4-{butyl-[(1R)-1-
(ethoxycarbonylmethyl-carbamoyl)-3-methyl-butyl]-carbam-
oyl}-3-phenyl-butyric acid ethyl ester. Yield 45%: trans-
parent oil; Rf = 0.35 (hexane/EtOAc, 6:4); 1H NMR
CD spectra were measured using a JASCO J-715 spectro-
polarimeter in 1 cm and 1 mm cells in acetonitrile at con-
(500 MHz, CDCl3)
J = 7.1 Hz, 3H), 1.26 (t, J = 7.1 Hz, 3H), 1.25–1.32 (m,
d
0.84–0.96 (m, 9H) 1.15 (t,

J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
2475
2H), 1.35–1.45 (m, 2H), 1.48–1.56 (m, 2H), 1.75–1.81 (m,
1H), 2.66 (dd, J = 7.7 Hz, J = 15.4 Hz, 1H), 2.71–2.77
(m, 2H), 2.80 (dd, J = 7.1 Hz, J = 15.4 Hz, 1H), 3.05–
3.20 (m, 2H), 3.76 (dd, J = 5.7 Hz, J = 18.0 Hz, 1H),
3.77–3.82 (m, 1H), 3.85 (dd, J = 6.0 Hz, J = 18.0 Hz,
1H), 4.04 (dq, J = 2.6 Hz, J = 7.1 Hz, 2H), 4.17 (q,
J = 7.1 Hz, 2H), 4.96 (t, J = 7.0 Hz, 1H), 6.55 (br s, 1H),
7.17–7.21 (m, 1H), 7.22–7.30 (m, 4H); 13C NMR (125
MHz, CDCl3) 13.6, 14.0, 14.1, 20.2, 22.2, 22.5, 22.8, 24.7,
32.2, 36.7, 38.9, 39.7, 40.5, 41.1, 45.5, 60.4, 61.1, 126.8,
127.4, 128.5, 143.1, 169.4, 171.8, 172.7; MS (ESI):
m/z = 513 ([M+Na]+, 64%), 491 ([M+H]+, 100%); ESI-
MS HR: m/z calcd for C27H43N2O6: 491.3121; found:
m/z: 491.3134.
(hexane/EtOAc, 6:4); 1H NMR (500 MHz, CDCl3) d
0.83–0.92 (m, 9H), 1.14 (t, J = 7.1 Hz, 3H), 1.17–1.33 (m,
4H) 1.36–1.48 (m, 1H), 1.49–1.61 (m, 1H), 1.69–1.84 (m,
1H), 2.63 (dd, J = 7.5 Hz, J = 15.5 Hz, 1H), 2.66–2.72
(m, 2H), 2.75 (dd, J = 7.3 Hz, J = 15.5 Hz, 1H), 3.05–
3.15 (m, 2H), 3.73 (quintet, J = 7.3 Hz, 1H), 4.01 (dq,
J = 3.6 Hz, J = 7.1 Hz, 2H), 4.17 (dd, J = 5.6 Hz,
J = 14.9 Hz, 1H), 4.33 (dd, J = 6.4 Hz, J = 14.9 Hz, 1H),
4.94 (br s, 1H), 6.66 (br s, 1H), 7.13–7.31 (m, 10H); 13C
NMR (125 MHz, CDCl3) d 13.6, 14.0, 20.2, 22.2, 22.9,
24.7, 32.0, 36.7, 38.8, 39.7, 40.5, 43.2, 45.2, 60.4, 126.8,
126.9, 127.1, 127.2, 127.3, 127.5, 127.7, 128.4, 128.5,
128.6, 138.4, 143.0, 171.4, 171.8, 172.5; ESI-MS: m/
z = 517 ([M+Na]+, 100%); ESI-MS HR: m/z calcd for
[M+Na]+, C30H42N2O4Na: 517.3037; found: 517.3062.
5.3.2. Peptidomimetic (1S,3R)-1a: (3R)-4-{butyl-[(1S)-1-
(ethoxycarbonylmethyl-carbamoyl)-3-methyl-butyl]-carb-
amoyl}-3-phenyl-butyric acid ethyl ester. Yield 24%: trans-
parent oil; Rf = 0.35 (hexane/EtOAc, 6:4); 1H NMR
5.3.6. Peptidomimetic (1S,3R)-1b: (3R)-4-[((1S)-1-benzyl-
carbamoyl-3-methyl-butyl)-butyl-carbamoyl]-3-phenyl-butyric
acid ethyl ester. Yield 27%: transparent oil; Rf = 0.60
(hexane/EtOAc, 6:4); 1H NMR (200 MHz, CDCl3) d
0.79–1.00 (m, 9H), 1.14–1.47 (m, 7H), 1.53–1.69 (m, 2H),
1.70–1.90 (m, 1H), 2.55–2.82 (m, 4H), 3.05–3.15 (m, 2H),
3.74 (quintet, J = 7.4 Hz, 1H), 4.01 (q, J = 7.1 Hz, 2H),
4.16 (dd, J = 6.5 Hz, J = 14.8 Hz, 1H), 4.34 (dd,
J = 5.6 Hz, J = 14.8 Hz, 1H), 4.94 (t, J = 7.6 Hz, 1H),
6.63 (br s, 1H), 7.12–7.39 (m, 10H). Analytical data were
almost the same as for the compound (1R,3S)-1b.
(200 MHz, CDCl3)
d 0.85–0.95 (m, 9H). 1.08 (dt,
J = 2.2 Hz, J = 7.1 Hz, 3H), 1.19 (t, J = 7.1 Hz, 3H),
1.25–1.50 (m, 5H), 1.55–1.80 (m, 2H), 2.55–2.77 (m, 4H),
3.00–3.15 (m, 2H), 3.50–3.60 (m, 1H), 3.65–3.80 (m, 2H),
3.97 (q, J = 2.6 Hz, J = 7.1 Hz, 2H), 4.10 (q, J = 7.1 Hz,
2H), 4.90 (m, 1H), 6.48 (br s, 1H), 7.10–7.30 (m, 5H). Ana-
lytical data were almost the same as for the compound
(1R,3S)-1a.
5.3.3. Peptidomimetic (1S,3S)-1a: (3S)-4-{butyl-[(1S)-1-
(ethoxycarbonylmethyl-carbamoyl)-3-methyl-butyl]-carb-
amoyl}-3-phenyl-butyric acid ethyl ester. Yield 16%: trans-
parent oil; Rf = 0.31 (hexane/EtOAc, 6:4); 1H NMR
(500 MHz, CDCl3) d 0.79 (d, J = 6.5 Hz, 3H), 0.85 (d,
J = 6.6 Hz, 3H), 0.88–0.97 (m, 3H), 1.15 (t, J = 7.1 Hz,
3H), 1.26 (t, J = 7.1 Hz, 3H), 1.24–1.32 (m, 2H), 1.35–
1.55 (m, 4H), 1.68–1.75 (m, 1H), 2.60–2.80 (m, 4H),
3.05–3.20 (m, 2H), 3.73–3.84 (m, 1H), 3.85 (d,
J = 5.3 Hz, 1H), 3.98 (d, J = 6.3 Hz, 1H), 4.00–4.08 (m,
2H), 4.17 (q, J = 7.1 Hz, 2H), 4.97 (t, J = 7.4 Hz, 1H),
7.00 (t, J = 5.3 Hz, 1H), 7.17–7.30 (m, 5H); 13C NMR
(125 MHz, CDCl3) d 13.6, 14.0, 14.1, 20.3, 22.1, 22.8,
24.5, 31.9, 36.5, 38.8, 39.6, 40.7, 41.1, 60.4, 61.1, 126.8,
127.2, 127.4, 128.5, 143.1, 169.5, 172.0, 172.1, 172.8; MS
(ESI): m/z = 513 ([M+Na]+, 50%), 491 ([M+H]+, 100%);
ESI-MS HR: m/z calcd for C27H43N2O6: 491.3121; found:
m/z: 491.3105.
5.3.7. Peptidomimetic (1S,3S)-1b: (3S)-4-[((1S)-1-benzyl-
carbamoyl-3-methyl-butyl)-butyl-carbamoyl]-3-phenyl-butyric
acid ethyl ester. Yield 31%: transparent oil; Rf = 0.58
1
(hexane/EtOAc, 6:4); H NMR (500 MHz, CDCl3) d 0.79
(d, J = 6.5 Hz, 3H), 0.82–0.97 (m, 6H), 1.12 (t,
J = 7.1 Hz, 3H), 1.18–1.33 (m, 4H), 1.35–1.45 (m, 1H),
1.47–1.57 (m, 1H), 1.70–1.77 (m, 1H), 2.64 (dd, J =
5.4 Hz, J = 7.3 Hz, 2H), 2.69 (dd, J = 7.3 Hz, J = 8.5 Hz,
2H), 3.05–3.15 (m, 2H), 3.75 (quintet, J = 7.3 Hz, 1H),
3.98 (dq, J = 4.3 Hz, J = 7.1 Hz, 2H), 4.35 (d, J = 6.0 Hz,
2H), 4.94 (t, J = 7.4 Hz, 1H), 6.95 (br s, 1H), 7.14–7.31
(m, 10H); 13C NMR (125 MHz, CDCl3) d 13.6, 14.0,
20.2, 22.1, 22.2, 22.9, 24.6, 31.9, 36.6, 38.8, 39.5, 40.8,
43.3, 45.0, 60.4, 126.8, 127.1, 127.2, 127.7, 128.5, 128.6,
138.4, 140.1, 171.5, 172.0, 172.6; ESI-MS: m/z = 517
([M+Na]+, 100%); ESI-MS HR: m/z calcd for [M+Na]+,
C30H42N2O4Na: 517.3037; found: 517.3020.
5.3.8. Peptidomimetic (1R,3R)-1b: (3R)-4-[((1R)-1-benzyl-
carbamoyl-3-methyl-butyl)-butyl-carbamoyl]-3-phenyl-butyric
acid ethyl ester. Yield 28%: transparent oil; Rf = 0.58
(hexane/EtOAc, 6:4); 1H NMR (200 MHz, CDCl3) d
0.83–1.00 (m, 9H), 1.14 (t, J = 7.1 Hz, 3H), 1.05–1.80 (m,
7H), 2.60–2.77 (m, 4H), 3.03–3.18 (m, 2H), 3.73 (quintet,
J = 7.4 Hz, 1H), 3.99 (q, J = 7.1 Hz, 2H), 4.28 (d,
J = 5.8 Hz, 2H), 4.76 (dd, J = 6.5 Hz, J = 8.8 Hz, 1H),
6.96 (br s, 1H), 7.14–7.30 (m, 10H). Analytical data were
almost the same as for the compound (1S,3S)-1b.
5.3.4. Peptidomimetic (1R,3R)-1a: (3R)-4-{butyl-[(1R)-1-
(ethoxycarbonylmethyl-carbamoyl)-3-methyl-butyl]-carb-
amoyl}-3-phenyl-butyric acid ethyl ester. Yield 32%:
1
transparent oil; Rf = 0.31 (hexane/EtOAc, 6:4); H NMR
(200 MHz, CDCl3)
d 0.71–0.97 (m, 9H), 1.08 (t,
J = 7.1 Hz, 3H), 1.19 (t, J = 7.1 Hz, 3H), 1.20–1.23 (m,
3H), 1.24–1.99 (m, 7H), 2.60–2.80 (m, 4H), 3.05–3.20 (m,
2H), 3.73–3.84 (m, 1H), 3.76–3.89 (m, 2H), 4.03 (q,
J = 7.1 Hz, 2H), 4.17 (q, J = 7.1 Hz, 2H), 4.97 (m, 1H),
7.00 (br s, 1H), 7.17–7.30 (m, 5H). Analytical data were al-
most the same as for the compound (1S,3S)-1a.
5.3.9. N-Benzyloxycarbonyl-L-leucine 3: (2S)-benzyloxycar-
bonylamino-4-methyl-pentanoic acid.26 To a solution of
L-leucine (2, 8.15 mmol, 1.07 g) in 2 M NaOHaq (10 mL),
cooled to 0 ꢁC, benzyl chloroformate (9.45 mmol,
1.40 mL) was added dropwise over 30 min. The mixture
5.3.5. Peptidomimetic (1R,3S)-1b: (3S)-4-[((1R)-1-benzyl-
carbamoyl-3-methyl-butyl)-butyl-carbamoyl]-3-phenyl-butyric
acid ethyl ester. Yield 27%: transparent oil; Rf = 0.60

2476
J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
was stirred at rt for another 2 h, then acidified with
HClconcd. The aqueous phase was extracted with EtOAc
(3 · 15 mL) and the combined organic layers were dried
(MgSO4). The solvent was evaporated in vacuo to give
5.3.13. N-Benzyloxycarbonyl-D-leucine benzylamide ent-4b:
(2R)-2-benzyloxycarbonylamino-4-methyl-pentanoic acid
benzylamide. Cbz-D-leucine ent-3 (4.00 mmol, 1.06 g),
benzylamine (4.02 mmol, 440 lL), and HOBt (4.07 mmol,
550 mg) were dissolved in dry CH2Cl2 (10 mL) and cooled
to 0 ꢁC. DCC (4.10 mmol, 846 mg) was then added. The
mixture was stirred for 15 min at 0 ꢁC and at rt overnight.
The precipitated urea was filtered off and the solution was
concentrated in vacuo. The residue was dissolved in EtOAc
(5 mL) and washed successively with citric acid (1 M aque-
ous solution), NaHCO3satd and brine. The organic layer
was dried over MgSO4 and the solvent evaporated in vacuo
to give a crude solid product. This was recrystallized from
Et2O/hexane to give product ent-4b in 80% yield: white
crude N-benzyloxycarbonyl-L-leucine 3 as a transparent
24
oil in 92% yield: ½aꢃD ¼ ꢁ16:8 (c 2.03, EtOH), (lit. ꢁ16.7
(c 2.0, EtOH)26); 1H NMR (200 MHz, CDCl3) d 0.80–
1.00 (m, 6H), 1.50–1.80 (m, 3H), 4.30–4.50 (m, 1H), 5.11
(s, 2H), 5.26 (d, J = 8.6 Hz, 1H), 7.34 (s, 5H).
5.3.10. N-Benzyloxycarbonyl-D-leucine ent-3: (2R)-benzyl-
oxycarbonylamino-4-methyl-pentanoic acid.27 The com-
pound ent-3 was synthesized as described for the
22
compound 3. 90% yield: transparent oil; ½aꢃD ¼ þ16:9 (c
26
1
2.00, EtOH), (lit. +14.7 (c 1.0, MeOH)27); 1H NMR d
(200 MHz, CDCl3) 0.80–1.00 (m, 6H), 1.50–1.80 (m, 3H),
4.30–4.50 (m, 1H), 5.11 (s, 2H), 5.26 (d, J = 8.6 Hz, 1H),
7.34 (s, 5H).
crystals, mp 103–108 ꢁC; ½aꢃD ¼ þ18:5 (c 1.26, EtOH); H
NMR (200 MHz, CDCl3) d 0.92 (d, J = 6.0 Hz, 6H),
1.20–1.80 (m, 3H), 4.10–4.30 (m, 1H), 4.40–4.49 (m, 2H),
5.08 (s, 2H), 5.23–5.42 (m, 1H), 6.80 (br s, 1H), 7.20–7.40
(m, 10H); 13C NMR (50 MHz, CDCl3) d 23.0, 24.8,
41.6, 43.5, 53.7, 67.1, 127.5, 127.7, 128.1, 128.2, 128.6,
128.7, 135.2, 138.1, 156.4, 172.4; Anal. Calcd for
C21H26N2O3: C, 71.16; H, 7.39; N, 7.90. Found: C, 70.45;
H, 7.56; N, 8.11.
5.3.11. N-(N-Benzyloxycarbonyl-D-leucyl)-glycine ethyl
ester ent-4a: ((2R)-2-benzyloxycarbonylamino-4-methyl-pen-
tanoylamino)-acetic acid ethyl ester. Cbz-D-leucine ent-3
(3.95 mmol, 1.048 mg), glycine ethyl ester hydrochloride
(4.03 mmol, 553 mg), and HOBt (4.05 mmol, 547 mg) were
suspended in dry CH2Cl2 (10 mL) and cooled to 0 ꢁC. DCC
(4.00 mmol, 540 mg) was added, and the mixture stirred for
15 min at 0 ꢁC. Then Et3N (2.50 mmol, 350 lL) was added
dropwise over 15 min. The mixture was stirred at 0 ꢁC for
another 30 min, and then at rt overnight. The precipitated
urea was filtered off and the solution concentrated in
vacuo. The residue was dissolved in Et2O (5 mL) and
washed successively with citric acid (1 M aqueous solu-
tion), NaHCO3satd and brine. The organic layer was dried
over MgSO4 and the solvent evaporated in vacuo to give
the crude product. Recrystallization from Et2O/hexane
5.3.14. N-Benzyloxycarbonyl-L-leucine benzylamide 4b:
acid
(2S)-2-benzyloxycarbonylamino-4-methyl-pentanoic
benzylamide.29 The compound 4b was synthesized as
described for compound ent-4b. 81% yield: white crystals
22
(Et2O/hexane); 110 ꢁC (lit. 112–11329); ½aꢃD ¼ ꢁ14:1 (c
1.20, EtOH) (lit. [a]D = ꢁ16.6 (c 1.20, EtOH)29); 1H
NMR (200 MHz, CDCl3) d 0.85–0.92 (m, 6H), 1.20–1.90
(m, 3H), 4.10–4.30 (m, 1H), 4.39–4.49 (m, 2H), 5.07 (s,
2H), 5.21–5.38 (m, 1H), 6.80 (br s, 1H), 7.20–7.40 (m,
10H); 13C NMR (50 MHz, CDCl3) d 23.0, 24.8, 41.6,
43.5, 53.7, 67.1, 127.5, 127.7, 128.1, 128.2, 128.6, 128.7,
135.2, 138.1, 156.4, 172.4; Anal. Calcd for C21H26N2O3:
C, 71.16; H, 7.39; N, 7.90. Found: C, 71.02; H, 7.41; N,
8.02.
gave compound ent-4a in 78% yield: white crystals; mp
25
96 ꢁC; ½aꢃD ¼ þ27:6 (c 1.00, MeOH); 1H NMR
(200 MHz, CDCl3) d 0.93 (d, J = 5.8 Hz, 6H), 1.27 (t,
J = 7.1 Hz, 3H), 1.50–1.80 (m, 3H), 4.00 (d, J = 5.1 Hz,
2H), 4.19 (q, J = 7.1 Hz, 2H), 4.15–4.30 (m, 1H), 5.10 (d,
J = 2.1 Hz, 2H), 5.37 (d, J = 8.4 Hz, 1H), 6.70 (br s, 1H),
7.34 (s, 5H); 13C NMR (50 MHz, CDCl3) d 14.1, 23.1,
24.8, 41.5, 53.6, 61.7, 67.3, 128.2, 128.4, 128.6, 148.5,
156.4, 169.9, 172.5; Anal. Calcd for C18H26N2O5: C,
61.70; H, 7.48; N, 7.99. Found: C, 61.97; H, 7.58; N,
8.04.
5.3.15. L-Leucine benzylamide 5b: (2S)-2-amino-4-methyl-
pentanoic acid benzylamide.30 Hydrogen gas was bubbled
through the stirred solution of Cbz-D-leucine benzylamide
4b (1.84 mmol, 654 mg) and palladium (10% Pd/C,
360 mg) in methanol (20 mL). After 2 h, the catalyst was
filtered off and the solution was evaporated to give product
1
5b as a transparent oil in 81% yield: H NMR (200 MHz,
CDCl3) d 0.85–0.92 (m, 6H), 1.20–1.90 (m, 3H), 3.30–
3.49 (m, 1H), 4.42 (d, J = 5.9 Hz, 2H), 7.20–7.40 (m,
5H), 7.70 (br s, 1H); ESI-MS: m/z 221 ([M+H]+,100%);
ESI-MS HR: m/z calcd for C13H21N2O ([M+H]+):
221.1648; found: 221.1662.
5.3.12. N-(N-Benzyloxycarbonyl-L-leucyl)-glycine ethyl
ester 4a: ((2S)-2-benzyloxycarbonylamino-4-methyl-penta-
noylamino)-acetic acid ethyl ester.28 The compound 4a
was synthesized as described for the compound ent-4a.
61% yield: white crystals (Et2O/hexane); mp 98 ꢁC (lit.
25
23
10115); ½aꢃD ¼ ꢁ27:5 (c 1.05, MeOH) (lit. ½aꢃD ¼ ꢁ26:4 (c
5.3.16. N-Butyl-L-leucine benzylamide 6b: (2S)-2-(butyl-
amino)-4-methyl-pentanoic acid benzylamide. To a cooled
0 ꢁC suspension of NaBH3CN (1.84 mmol, 116 mg),
Na2SO4 (166 mg), and L-leucine benzylamide 5b
(0.38 mmol, 83 mg) in dry THF, butanal (0.66 mmol,
60 lL) was added dropwise over 30 min. After stirring
for 2.5 h at 0 ꢁC, 10 mL of water was added and the aque-
ous slurry extracted with CH2Cl2 (3 · 10 mL). The com-
bined organic phases were dried over MgSO4 and the
solvent evaporated in vacuo. The product was purified by
1
1.94, EtOH)28); H NMR (200 MHz, CDCl3) d 0.93 (d,
J = 5.8 Hz, 6H), 1.27 (t, J = 7.1 Hz, 3H), 1.50–1.80 (m,
3H), 4.00 (d, J = 5.1 Hz, 2H), 4.19 (q, J = 7.1 Hz, 2H),
4.15–4.30 (m, 1H), 5.10 (d, J = 2.1 Hz, 2H), 5.37 (d,
J = 8.4 Hz, 1H), 6.70 (br s, 1H), 7.34 (s, 5H); 13C NMR
(50 MHz, CDCl3) d 14.1, 23.1, 24.8, 41.5, 53.6, 61.7, 67.3,
128.2, 128.4, 128.6, 148.5, 156.4, 169.9, 172.5; Anal. Calcd
for C18H26N2O5: C, 61.70; H, 7.48; N, 7.99. Found: C,
61.74; H, 7.76; N, 7.72.

J. Frelek et al. / Tetrahedron: Asymmetry 17 (2006) 2469–2478
2477
flash chromatography (hexane ! hexane/EtOAc, 6:4) to
NMR d 13.9, 14.1, 20.3, 21.8, 23.2, 25.1, 32.4, 40.8, 48.6,
61.1, 61.4, 169.8, 175.6; ESI-MS HR: m/z calcd for
[M+H]+, C14H29N2O3 ([M+H]+): 273.2178; found:
273.2181.
give compound 6b as a transparent oil that solidified upon
23
standing. Yield 47%: mp 39 ꢁC; ½aꢃD ¼ ꢁ7:3 (c 1.01,
MeOH); 1H NMR d (200 MHz, CDCl3) 0.82–0.93 (m,
9H), 1.20–1.49 (m, 4H), 1.50–1.90 (m, 3H), 2.41–2.63 (m,
2H), 3.06 (dd, J = 9.6 Hz, J = 4.8 Hz, 1H), 4.45 (d,
J = 5.9 Hz, 2H), 7.20–7.40 (m, 5H), 7.64 (t, J = 5.0 Hz,
1H); ESI-MS: m/z 277 ([M+H]+, 100%); ESI-MS HR:
m/z calcd for [M+H]+, C17H29N2O ([M+H]+): 277.2274;
found: 277.2280.
Acknowledgments
This work was supported by the Polish State Committee
for Scientific Research, grant PZB-MIN-07/P04/2003. All
´
calculations were performed at Poznan Supercomputing
Center, Poland.
5.4. The general procedure for the synthesis of compound 6 in
a one-pot procedure from the respective compound 4
References
Hydrogen gas was bubbled through a stirred solution of
Cbz-protected leucine amide 4 (1.10 mmol) and palladium
(10% Pd/C, 166 mg) in absolute EtOH (10 mL). After 2 h,
the catalyst was filtered off and the ethanolic solution
cooled to 0 ꢁC. Dry Na2SO4 (ꢀ400 mg) was added, fol-
lowed by butanal (2.22 mmol, 200 lL), and NaBH3CN
(2.15 mmol, 135 mg) was added portionwise. After stirring
for 1.5 h at 0 ꢁC, the solvent was evaporated and the resi-
due suspended in water (10 mL). The aqueous slurry was
extracted with CH2Cl2 (3 · 10 mL). The combined organic
layers were dried over MgSO4 and the solvent was evapo-
rated in vacuo. The product 6 was purified by flash chro-
matography (hexane ! hexane/EtOAc, 6:4).
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23
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23
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