Efficient macrocyclization of U-turn preorganized peptidomimetics: The role of intramolecular H-bond and solvophobic effects

Article abstract of DOI:10.1021/ja0284759

Simple peptidomimetic molecules derived from amino acids were reacted with meta- and parabis(bromomethyl) benzene in acetonitrile to very efficiently yield macrocyclic structures. The cyclization reaction does not require high dilution techniques and seems to be insensitive to the size of the formed macrocycle. The analysis of data obtained by ¹H NMR, single-crystal X-ray diffraction, fluorescence measurements, and molecular mechanics indicate that folded conformations can preorganize the system for an efficient cyclization. The role played by intramolecular hydrogen-bonding and solvophobic effects in the presence of folded conformations is analyzed.

Full text of DOI:10.1021/ja0284759

Published on Web 05/10/2003
Efficient Macrocyclization of U-Turn Preorganized
Peptidomimetics: The Role of Intramolecular H-Bond and
Solvophobic Effects
Jorge Becerril,† Michael Bolte,‡ M. Isabel Burguete,† Francisco Galindo,†
Enrique Garc´ıa-Espan˜a,§ Santiago V. Luis,*,† and Juan F. Miravet*,†
Contribution from the Department of Inorganic and Organic Chemistry,
UniVersity Jaume, I. E-12080 Castello´n, Spain, Department of Inorganic Chemistry,
UniVersity of Valencia, c/Dr. Moliner 50, 46100, Burjassot, Valencia, and Institut fu¨r
Organische Chemie, J. W. Goethe-UniVersita¨t Frankfurt, Marie-Curie-Str. 11,
60439 Frankfurt am Main, Germany
Received September 9, 2002; E-mail: luiss@uji.es.
Abstract: Simple peptidomimetic molecules derived from amino acids were reacted with meta- and para-
bis(bromomethyl)benzene in acetonitrile to very efficiently yield macrocyclic structures. The cyclization
reaction does not require high dilution techniques and seems to be insensitive to the size of the formed
macrocycle. The analysis of data obtained by 1H NMR, single-crystal X-ray diffraction, fluorescence
measurements, and molecular mechanics indicate that folded conformations can preorganize the system
for an efficient cyclization. The role played by intramolecular hydrogen-bonding and solvophobic effects in
the presence of folded conformations is analyzed.
Introduction
the observation of important biomedical activities of some
natural and synthetic peptidomimetics, as has been the case for
Much work has been devoted in recent years to the prepara-
tion and study of peptidomimetics. The interest on this class of
compounds has evolved from different areas of research. The
analysis of the molecular basis of structural features in proteins,
in particular structural motifs associated with folding, has been
one of the main driving forces for this interest.1 A second factor
has been the development of novel receptors in the fields of
Molecular Recognition and Supramolecular Chemistry. In this
area, molecules containing peptides or peptide-related fragments
have found applications in the self-assembly of nanotubes and
rosettes,2a-c modulation of protein-protein interactions,2d-e
catalysis 2f-k cation, and recognition.2l-n A third point has been
the synthesis of novel compounds with anti-HIV activities3a,b
or antibacterial properties.3c To stabilize “active” conformations,
many of the former examples are based on the preparation of
cyclic peptidomimetic structures. In particular, to achieve
additional conformational constraints, special attention is focused
on the incorporation of aromatic rings into the peptidomimetic
backbone. Therefore, different strategies have been described
associated to the preparation of cyclic peptides and analogues.
For these compounds, the cyclization step is usually the main
challenge and their preparation often involves complex synthetic
methodologies which require the use of specific reactions,
selective protection/deprotection steps and high dilution tech-
niques.4 Under some circumstances, the use of the appropriate
templates, in particular metal cations, can greatly improve
cyclization steps.5 In connection with those approaches, a simple
analysis of the structures of some peptidomimetic molecules
being able to form U-turns, suggests that this preorganization
† Department of Inorganic and Organic Chemistry, University Jaume.
‡ Institut fu¨r Organische Chemie, J. W. Goethe-Universita¨t Frankfurt.
§ Department of Inorganic Chemistry, University of Valencia.
(1) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (b) Gardner, R. R.;
Liang, G.-G. B.; Gellman, S. H. J. Am. Chem. Soc. 1995, 117, 3280. (c)
Nowick, J. S.; Insaf, S. J. Am. Chem. Soc. 1997, 119, 10 903.
(2) (a) Bong, D. T.; Ghadiri, M. R. Angew. Chem., Int. Ed. Engl. 2001, 40,
2163. (b) Ranganthan, D. Acc. Chem. Res. 2001, 34, 919. (c) Kerchoffs, J.
M. C.; Crego-Calama, M.; Luyten, I.; Timmerman, P.; Reinhoudt, D. N.
Org. Lett. 2000, 2, 4123. (d) Peczuh, M. W.; Hamilton, A. D. Chem. ReV.
2000, 100, 2479. (e) Park, H. S.; Hamilton, A. D. Proc. Natl. Acad. Sci.
U.S.A. 2002, 99, 5105. (f) Scarso, A.; Scheffer, U.; Go¨bel M.; Broxterman,
Q.; Kaptein, B.; Formaggio, F.; Toniolo, C.; Scrimin, P. Proc. Natl. Acad.
Sci. U.S.A. 2002, 99, 5144. (g) Gilbertson, S. R.; Collibee, S. E.; Agrakov,
A. J. Am. Chem. Soc. 2000, 122, 6522. (h) Dangel, B.; Clarke, M.; Haley,
Sames, D.; Polt, R. J. Am. Chem. Soc. 1997, 119, 10 865. (i) Jarvo, E. R.,
Miller, S. J. Tetrahedron 2002, 52, 2481. (j) Adria´n, F.; Burguete, M. I.;
Garc´ıa, J. I.; Garc´ıa, J.; Garc´ıa-Espan˜a, E.; Luis, S. V.; Mayoral, J. A.;
Royo, A. J.; Sa´nchez, M. C. Eur. J. Inorg. Chem. 1999, 2347. (k) Sansone,
F.; Baldini, L.; Casnati, A.; Lazzarotto, M.; Ugozzoli, F.; Ungaro, R. Proc.
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9
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J. AM. CHEM. SOC. 2003, 125, 6677-6686
6677

A R T I C L E S
Becerril et al.
Scheme 1 a
Table 1. Yields Obtained for the Cyclization Reaction after
Chromatographic Purification
compd
R
n
yield (%)
13a
14a
15a
16a
17a
13b
14b
15b
16b
17b
18a
19a
20a
21a
22a
18b
19b
20b
21b
22b
-CH2Ph
0
1
2
4
6
0
1
2
4
6
0
1
2
4
6
0
1
2
4
6
65
61
66
65
52
49
61
69
52
55
63
52
63
67
55
64
56
65
60
65
-CH2Ph
-CH2Ph
-CH2Ph
-CH2Ph
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH2Ph
a (i) DCC, N-hydroxysuccinimide, THF; (ii) H2NCH2(CH2)nCH2NH2,
DME; (iii) HBr/AcOH; (iv) aq. NaOH
-CH2Ph
-CH2Ph
-CH2Ph
could favor intramolecular processes over the intermolecular
ones and provide simple routes for the preparation of macro-
cyclic peptidomimetic structures. As a matter of fact, the
presence of folded conformations found for acyclic peptidomi-
metics have been associated by us and other groups to the
outcome of macrocyclization reactions.6 The predisposition of
different peptidomimetic molecules to fold has been widely
studied and several structural units such as urea, amino acids,
proline residues, etc., have been associated with the formation
of â-γ- or related U-turns in peptides or peptidomimetics.1,7
Here, we report how simple peptidomimetics with C2 symmetry
such as 8-12, show a strong tendency to form folded structures
under some conditions and how this can be used advantageously
for the synthesis of cyclophane peptidomimetics.
-CH2Ph
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
bromide as a phase transfer catalyst but this reagent did not af-
fect the yield of isolated macrocyclic product. The results were
quite good for the meta and para substituted aromatic subunits
and no significant differences were observed when derivatives
of phenylalanine or valine were used. In most cases, yields for
the expected product of ca. 80% could be estimated from the
crude reaction mixture. The formation of the corresponding 2+2
cyclization compounds was detected in some cases as minor
side products. In the case of ortho disubstituted aromatic deriv-
atives the cyclization reactions did not work and 1,2-dihy-
droisoindol derivatives were obtained, a situation that has been
described previously.9
The good results obtained when the components of the
reaction were 1,3- or 1,4-bis(bromomethyl)benzene and the dia-
mines 8, 9, and 10 (n ) 0, 1, 2) could, in a first analysis, be
ascribed to the above-mentioned preorganization induced by
intramolecular H-bonding as shown in I (Scheme 3). Surpris-
ingly, although longer aliphatic chains should disfavor this kind
of intramolecular hydrogen-bond formation, and therefore lead
to a decrease of the efficiency of macrocycle formation, for com-
pounds 11 and 12, with aliphatic spacers containing six and
eight methylene units respectively, cyclization yields were also
good and comparable to those obtained for compounds with
shorter aliphatic spacers. Those results prompted us to study in
more detail the importance of hydrogen bonding in the possible
preorganization.
Results and Discussion
Open-chain peptidomimetic molecules 8-12 can be easily
prepared starting from the corresponding diamines and the N-
Cbz protected amino acids 1 through the initial formation of
their activated N-hydroxysuccinimide esters 2 (see Scheme 1).8
Overall yields for the preparation of compounds 8-12, after
the final deprotection step, were in the range 60-80%, and did
not depend very much on the nature and length of the aliphatic
spacer.
Preliminary molecular mechanics calculations on compounds
8-10 suggested that folded conformations such as I (see Scheme
3) could be prevalent as a result of intramolecular H-bonds and
contribute to the appropriate preorganization of the peptidomi-
metic chain for macrocyclization. Consequently, we evaluated
the cyclization reaction shown in the Scheme 2 to test for the
preorganization present in those molecules and its dependence
on different factors, in particular the amino acid side chain and
the length of aliphatic spacer. With this purpose, compounds
8-12 were reacted with different bis(bromomethyl)arenes in
order to obtain the corresponding cyclic molecules 13-22.
The yields (obtained after chromatographic purification) for
the macrocyclization reactions carried out in acetonitrile and
using potassium carbonate as a base are shown in Table 1. The
reaction can be accelerated by addition of tetrabutylammonium
1
In first place, H NMR experiments were carried out. The
chemical shifts observed in CDCl3 and CD3CN for the signals
of the amide protons in compounds 8-12 correspond with those
expected for strongly hydrogen-bonded systems (see Table 2).
The described chemical shift values were almost insensitive
to changes in concentration (see Figure 1) indicating that
hydrogen bonding is taking place intramolecularly, as indicated
also by the changes observed with temperature.10 It is noticeable
that, both in chloroform and acetonitrile, the amide proton
resonances appear at lower ppm values as the length of the
(6) (a) Miller, S. C.; Blackwell, H. E.; Grubbs, R. H. J. Am.Chem. Soc. 1996,
118, 9606. (b) Adria´n, F.; Burguete, M. I.; Luis, S. V.; Miravet, J. F.;
Querol, M. Tetrahedron Lett. 1999, 40, 1039. (c) Prabhakaran, E. N.;
Rajesh, V.; Dubey, S.; Iqbal, J. Tetrahedron Lett. 2001, 42, 339. (d)
Pattarawarapan, R. S.; Roy, S.; Burgess, K. Tetrahedron 2000, 56, 9809.
(7) (a) Nakanishi, H.; Kahn, M. In Bioorganic Chemistry: Peptides and
Proteines; Hecht, S. M., Ed; Oxford University Press: New York, 1998;
Chapter 12. (b) Liu, Z.-P.; Rizo, J.; Gierasch, L. M. In Bioorganic
Chemistry: Peptides and Proteines; Hecht, S. M., Ed; Oxford University
Press: New York, 1998; Chapter 6.
(9) Yaounanc, J. J.; Le Bris, N.; Le Gall, G.; Cle¨ment, J. C.; Handel, H.; Des
Abbayes, H. J. Chem. Soc., Chem. Commun. 1991, 206.
(10) (a) Gellman, S. H.; Dado, G. P.; Liang, G. B.; Adams, B. R. J. Am. Chem.
Soc., 1991, 113, 1164. (b) Gaemman, K.; Bernhard, J.; Seebach, D.;
Perozzo, R.; Scapozza, L.; Folkers, G. HelV. Chim. Acta 1999, 82, 1.
(8) Wagler, T. R.; Fang, Y.; Burrows, C. J. J. Org. Chem. 1989, 54, 1584.
9
6678 J. AM. CHEM. SOC. VOL. 125, NO. 22, 2003

Macrocyclization of U-Turn Peptidomimetics
A R T I C L E S
Table 2. (a) 1H NMR Chemical Shifts for the Amide Protons of
Acyclic Compounds 8-12 in CDCl3 and CD3CN
Scheme 2 a
compd
R
n
δ (ppm, CDCl3)
δ (ppm, CD CN)
3
8a
9a
10a
11a
12a
8b
-CH2Ph
0
1
2
4
6
0
1
2
4
6
7.55
7.56
7.30
7.24
7.21
7.70
7.67
7.27
7.31
7.29
7.34
7.40
7.21
7.18
7.16
7.52
7.54
7.34
7.30
7.28
-CH2Ph
-CH2Ph
-CH2Ph
-CH2Ph
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
-CH(CH3)2
9b
10b
11b
12b
a The concentration was 10-2 M in all cases.
a Reagents: (i) p-bis(bromomethyl)benzene, CH3CN, Bu4NBr, K2CO3,
reflux 12 h; (ii) m-bis(bromomethyl)benzene, CH3CN, Bu4NBr, K2CO3,
reflux 12 h.
Scheme 3
Figure 1. Plot of 1H NMR chemical shift of amide hydrogen atoms of
compounds 8b and 12b in CDCl3 vs logarithm of concentration (M).
Scheme 4
aliphatic spacer increases from ethylenic to butylenic spacers
(see the values for 8-10) and remains almost invariable for
longer ones (10-12). These data indicate that intramolecularly
hydrogen bonded conformations must be predominant in solu-
tion but, according to previous work in this field, they are very
unlikely to correspond to folded structures such as I (Scheme
3) due to an unfavorable entropy factor.10 To better understand
this behavior the model compounds 24 and 25 were studied
(see Scheme 4).
1
For compound 24 H NMR experiments discarded intermo-
lecular association and the chemical shift value for the amide
protons (6.2 ppm in CDCl3) shows that hydrogen bonding is
not so strong as that found for compounds 8-12. On the other
hand, in compound 25 the only intramolecular H-bond interac-
tion that can take place connects the lone pair of the amine
group and the amide NH. In this case, the chemical shift of the
amide protons appears at 7.2 ppm in CDCl3 and dilution experi-
ments indicate the intramolecular nature of the interaction.
Therefore, two types of hydrogen bonds would explain the
observed spectra of compounds 8-12 in CDCl3 and CD3CN.
The first type would involve the two amide groups and result
in a folding in the molecule such as in I (Scheme 3). Its impor-
tance would decrease as the aliphatic spacer becomes longer.
The second one involves the amine lone pair and the amide
NH. This one must be present in all of the studied peptidomi-
metics independently of the spacer size and would explain the
chemical shift values for the amide proton resonance in the
molecules with long aliphatic spacers. Hence, the values reported
in Table 2 can be rationalized if it is assumed that for com-
pounds 8 and 9 both types of intramolecular H-bonding take
Scheme 5
place, whereas for larger compounds 10-12 only the last type
is present. Consequently, intramolecular hydrogen bonding can
be argued as a mean to understand the preorganization that
would yield preorganization favorable for the macrocyclization
with 8-9 but the same case cannot be done for longer molecules
10-12.
Because the key step for the efficiency of the cyclization is
the intramolecular reaction of the intermediate species II (see
Scheme 5) a computational study about the preorganization of
such species was performed. It has been shown previously how
9
J. AM. CHEM. SOC. VOL. 125, NO. 22, 2003 6679

A R T I C L E S
Becerril et al.
Table 3. Average (C-N) Distances between the Primary Amino
Group and Bromomethyl Units in Intermediate Compounds (II)
(see Scheme 5) Calculated by Molecular Dynamics Simulationsa
precursor of the
intermediate (II)
d (Å) in CHCl3
d(Å) in H O
2
8a
9a
10a
11a
12a
8b
10.7
11.8
12.5
13.4
15.6
10.7
13.4
13.3
14.7
15.5
8.2
9.7
8.7
9.0
8.2
7.2
8.7
7.4
7.9
8.3
9b
10b
11b
12b
a The molecular dynamics simulations were carried out with MACRO-
MODEL 7.0 (5 ns, AMBER* as force field and GB/SA simulation of
solvation).
Figure 2. Distribution of distances (see d in structure II, Scheme 5) found
in the molecular dynamics simulations of the intermediate compound derived
from 12a using simulations of chloroform and water as the solvent.
(MACROMODEL 7.0, AMBER* as force field and GB/SA simulation of
solvation).
molecular dynamics simulations can reproduce well the folding
induced by intramolecular H-bonding.11
The presence of intramolecular H-bonding such as that shown
in II was studied by analysis of 200 structures sampled during
a 5 ns molecular dynamics simulation. It was found, in
accordance with related studies described previously,10a that
hydrogen bonding between the two amido groups in chloroform
is only important for the shorter aliphatic species. For the
compounds derived from phenylalanine containing spacers with
2, 3, 4, 6, and 8 methylene units the percentage of H-bonded
structures found was respectively 25, 10, 7, 1 and 1. Similar
results were obtained for the compounds derived from valine.
As expected, intramolecular hydrogen bonding was found to
be almost negligible when a simulation of water as a solvent
was used for the calculations. Additionally, to analyze the ease
of the cyclization reaction in these species, the distance between
the nitrogen atom of the primary amino group and the carbon
atom of the bromomethyl subunit (see distance d in structure
II, Scheme 5) was monitored during the MD calculation (see
Table 3) both in chloroform and water (acetonitrile parametriza-
tion was not available in the molecular mechanics program.)
Figure 3. Representative structures found in the MD simulation of
intermediate II (see Scheme 5) derived from 12b using simulation of water
(up) and chloroform (bottom). (Surface area was calculated with MAC-
ROMODEL 7.0 assuming a probe radius of 1.4 Å. The dotted line indicate
the atoms whose distance was monitored during the simulation).
It can be seen that the average distances were always shorter
when a simulation of water was employed as a solvent. The
differences are significant, especially for the larger molecules,
and a shorter C-N distance should correlate with an increased
probability for the intramolecular cyclization reaction. Moreover,
the distances distribution was always much narrower for the
simulations performed in water where a range of preferred
distances was found (see as an example Figure 2).
to take place in acetonitrile.12 To illustrate the effect of the
solvent in the preorganization, representative molecules from
the simulations for the intermediate derived from 12b in
chloroform and in water are shown in Figure 3.
It can be seen that in chloroform the molecule tends to be
extended and fully exposed to the solvent. However, in water
the molecule tends to fold back and minimize the exposition to
the solvent due the presence of hydrophobic subunits.
To get more evidences of the proposed solvent-induced
folding, additional experiments were perfomed. First, the
reaction to obtain compounds 18a and 22a was carried out in
dimethylformamide as the solvent. The change of acetonitrile
a polar solvent (ꢀ ) 37.5), that at the same time allows for the
presence of fairly strong intramolecular hydrogen bonding
interactions, for dimethylformamide, a strongly hydrogen bond-
ing solvent with almost same polarity (ꢀ ) 36.7), resulted in a
significative decrease in reaction yields (due to an important
formation of side-products, no starting material was left) both
Therefore, the calculations suggest that solvophobic effects
seem to be a key issue for the efficiency of macrocyclization
and could explain the invariance of the yield with the length of
the aliphatic spacer. It can be recalled here that hydrophobic
effects are accepted to be the driving force for the protein folding
in water which results in actiVe conformations.7b Solvophobic
effects, although not as strong as in water, are expected to be
significant in acetonitrile, the solvent used for the studied
reactions. As a matter of fact, significant folding of apolar
molecules to yield ordered helical structures has been shown
(11) (a) McDonald, D. Q.; Still, W. C. J. Am. Chem. Soc. 1994, 116, 11 550.
(b) Burguete, M. I.; Escuder, B.; Garc´ıa-Espan˜a, E.; Lo´pez, L.; Luis, S.
V.; Miravet, J. F.; Querol, M. Tetrahedron Lett. 2002, 43, 1817. (c) For
reviews on molecular dynamics simulations of biomolecules see the special
issue on this topic: Acc. Chem. Res. 2002, 35, 321-489.
(12) Lahiri, S.; Thompson, J. L.; Moore, J. S. J. Am. Chem. Soc. 2000, 122,
11 315.
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Macrocyclization of U-Turn Peptidomimetics
A R T I C L E S
for compound 18a containing a ethylenic spacer (65% yield in
acetonitrile, 31% in dimethylformamide) and 22a containing
an octamethylenic spacer (53% yield in acetonitrile, 29% in
dimethylformamide). Second, the reaction was performed in
tetrahydrofurane, a quite less polar solvent (ꢀ ) 7.6), where
intramolecular hydrogen bonding is expected to be important
(the amide proton chemical shift values obtained for compounds
8a and 12a in this solvent are comparable to those obtained in
chloroform). In this case, the preparation of compounds 18a
and 22a resulted in poor cyclization yields (<20%). Other
experiments using mixtures acetonitrile-water or methanol as
solvent resulted in bromide solvolysis as an important side
reaction and could not be used to analyze the importance of
polarity and hydrogen bonding in the macrocylization. The
results in dimethylformamide indicate that the hydrogen-bonding
acceptor characteristics of this solvent disfavor the macrocy-
clization reaction. The strong interaction of the amino and amido
groups with this solvent probably disrupts the folded conforma-
tions favorable for the cyclization. The chemical shifts of amide
protons of compounds 8b and 12b in DMF-d8 were found to
be 8.00 and 7.84 respectively, in agreement with a strong
hydrogen bonding interaction with this solvent. On the other
hand, the low yield for the preparation of 18a in tetrahydrofurane
suggest that hydrogen bonding alone does not preorganize
enough the molecule for cyclization. This is supported by the
MD simulations in chloroform for the corresponding intermedi-
ate II (Scheme 5), that shows a poor preorganization as inferred
from the distances distribution which was found to be analogous
to that shown for 12a (see Figure 2 and Table 3).
Figure 4. Variation of fluorescence intensity for compounds 13a-17a (“n”
in the horizontal axis indicates the length of the aliphatic spacer) in
chloroform (a), acetonitrile (b) and acetonitrile-water 1:1 (c). The vertical
axis values indicate the variation of intensity (percentage) relative to the
emission of compound 13a. Emission was measured at 285 nm after
irradiation at 265 nm.
the aromatic subunits are, on average, more exposed to the
solvent as the aliphatic spacer becomes longer.
The next stage in the described study was the structural
analysis of the of cyclic compounds. Because of the loss of
conformational freedom, intramolecular hydrogen bonds are
more likely to be found in the cyclic derivatives provided that
the size of the macrocycle does not impose strong conforma-
tional constraints and the two types of intramolecular H-bond
mentioned previously are expected.
More information regarding the solvophobic contribution to
the folding of molecules 13a-17a was obtained by fluorescence
studies. It has been shown that variations in fluorescence
intensity of different subunits can be used to monitor polarity
changes in the environment of protein residues.13 This prompted
us to test the presence of solvophobic interactions by analysis
of the intensity of fluorescent emission of 285 nm light
(produced after irradiation with 268 nm light) from the phenyl
subunits of 13a-17a. As it is shown in Figure 4, when the
experiments were carried out in acetonitrile the emission
intensity showed a dependence on the length of aliphatic spacer
and increased for longer molecules, being noticeable a sharp
variation when changing from the ethylenic to the propylenic
spacer. This behavior can be ascribed to a less efficient
quenching of excited states by the solvent as the molecule
becomes more hydrophobic. This should be a consequence of
the presence of folded structures that minimize the exposure of
hydrophobic units to the solvent (by measurament of NMR
relaxation times it was found that unspecific aggregation was
not taking place at the concentration used in these experiments).
The results measured when a mixture of acetonitrile and water
is used as a solvent are also in accordance with a solvophobic
folding because in this case the change in emission intensity is
even more important. The opposite tendency was obtained when
experiments were carried out in chloroform. In this solvent, the
decrease of emission intensity with spacer length suggests that
The NMR study for the cyclic compounds indicated that the
importance of intramolecular hydrogen bonds between the two
amido groups cannot be studied properly using as a probe the
1H signals assigned to the amide proton resonance since the
two types or intramolecular H-bonds described above affect its
chemical shift. Moreover, for the para derivatives aromatic ring
shielding effects can be very important. However, the study of
the hydrogen atoms corresponding to the methylene attached
to the amide nitrogen atom are valuable for those purposes The
two nonequivalent geminal protons show increased differences
in their chemical shifts as a result of the loss of conformational
freedom imposed by the above-mentioned H-bond between
amido groups. For example, as can be seen in Figure 5 the
signals of Ha and Hb appear as multiplet in 1H NMR of
compound 14b (n ) 1). These signals split up into two well
separated multiplets for the compound 15b (n ) 2). Therefore
the presence of an additional carbon in the central spacer eases
the conformational constraints of the macrocycle in such a way
that amide groups can orient relative to each other in a proper
way to yield a stronger intramolecular hydrogen bond becoming
more rigid species. For each case the proper assignment of those
signals could be confirmed by 2-D NMR experiments.
Assuming that the difference in chemical shift for those
protons gives a measure of the amount of intramolecular amide-
amide H-bond, the analysis of the graphic in Figure 6 indicates
that this interaction is maximum when the spacer is a butylene
subunit. For shorter chains the macrocycle is probably too stiff
to orient appropriately both amido groups, whereas for longer
subunits entropic factors and steric constraints may diminish
the stability of this interaction. The analysis of data correspond-
(13) Gibney, B. R., Rabanal, F., Skalicky, J. J., Wand, A. J., Dutton, P. L J.
Am. Chem. Soc. 1999, 121, 4952
(14) (a) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (b) Sheldrick, G.
M. SHELX-97: A program for the refinement of crystal structures;
University of Go¨ttingen, Germany, 1997.
9
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A R T I C L E S
Becerril et al.
crystalline packing. Interestingly, one of the amide groups, N-
(14), is taking part in a hydrogen bond with the lone pair of N-
(17) as described above for the model compound 25. No other
intramolecular hydrogen bond is formed in the crystal presum-
ably due to the constraints imposed by the para-substituted
spacer. However, for compound 18b, in addition to different
intermolecular hydrogen bonds, three intramolecular ones were
found. The first one between the two amido groups, confirming
the higher flexibility of the macrocycles with a meta-type spacer,
and the other two between the amine lone pairs and the amide
hydrogen atoms, again in accordance with the previous discus-
sion.
Conclusions
In conclusion, the present method represents a simple and
efficient procedure for the preparation of cyclic peptidomimetics
which does not require the use of high dilution techniques or
the use of complex synthetic methodologies. The good yields
obtained are explained by the existence of a high preorganized
intermediate which favors the intramolecular reaction. As
evidenced from our experimental results and MD simulations,
the folding can be provided by intramolecular H-bonding and
solvophobic effects, being the latter important even for the cases
were intramolecular H-bonding between amide groups is
expected to be present. Therefore, the election of the appropriate
solvent is a key issue for the efficiency of the formation of cyclic
structures. The best results are obtained in acetonitrile which
at the same time provides solvophobic effects and is a weak
H-bond acceptor when compared with other polar solvents as
DMF. Further work is in progress in order to study the scope
of the method and to use this methodology which could be used
for the preparation of other structures of interest for catalytic,
biomimetic or biological applications.
Figure 5. 1H NMR spectra of 14b (up) and 15b (down) in CDCl3, 300
MHz.
Figure 6. Plot of 1H NMR chemical shift difference found for the geminal
protons Ha and Hb (see Figure 5).
Experimental Section
ing to molecules with a propylenic spacer reveals, as expected,
that meta-macrocycles are more flexible than para-substituted
ones as demonstrated by the change in chemical shift differences
for the studied geminal protons.
Single-crystals suitable for X-ray analysis were obtained of
the compounds 13b and 18b by slow diffusion of ether into a
solution of the compound in dichloromethane. Their analysis
by X-ray diffraction confirmed the structural hypotheses that
have been described above. (See Figure 7).
General Procedure for the Preparation of N-Hydroxysuccin-
imide Esters of Amino Acids. Synthesis of 2a. N-Cbz-L-phenyla-
lanine (25.14 g, 84.0 mmol) and N-hidroxysuccinimide (9.67 g, 84.0
mmol) were dissolved in dry THF at 0 °C. Once a clear solution had
been obtained, DCC (17.45 g, 84.6 mmol) in anhydrous THF was added
in several aliquots and the resulting solution was stirred at 0-5 °C for
3 h. The dicyclohexylurea formed was filtered off and the filtrate was
concentrated to dryness. The crude product was recrystallized from
2-propanol to furnish the pure product. Yield (28.47 g, 86%); mp 143-
144 °C; [R]25D ) -11.1° (c ) 0.1, CHCl3); IR (KBr) 3297, 1814, 1785,
1
1747, 1679, 1541 cm-1; H NMR (300 MHz, CDCl3) δ ) 2.76 (br s,
For compound 13b, the aliphatic spacer and the amide protons
are located over the aromatic ring and the carbonyl oxygen
atoms are hydrogen bonded with the neighbor molecules in the
4H), 3.20 (dd, 1H, J ) 14.2, 6.6 Hz), 3.33 (dd, 1H, J ) 14.2, 5.6 Hz),
5.00-5.13 (m, 3H), 5.39 (d, 1H, J ) 8.6 Hz), 7.24-7.38 (m, 10H);
Figure 7. Molecular structure of 13b (left) and 18b (right) obtained by X-ray diffraction of single crystals. Dotted lines indicate the intramolecular H-bonds
found.
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Macrocyclization of U-Turn Peptidomimetics
A R T I C L E S
13C NMR (75 MHz, CDCl3) δ ) 25.5, 37.8, 52.9, 67.1, 127.2, 127.9,
128.0, 128.3, 128.5, 129.4, 134.3, 135.7, 155.1, 167.3, 168.6; Anal.
Calcd. for C21H20N2O6: C, 63.6; H, 5.1; N, 7.1. Found: C, 63.5; H,
5.5; N, 7.4.
MHz, DMSOd6, 30 °C) δ 26.3, 28.7, 29.0, 37.9, 38.6, 56.2, 65.2, 126.1,
127.2, 127.5, 127.8, 128.1, 129.0, 136.9, 137.9, 155.5, 170.8; Anal.
Calcd. for C42H50N4O6: C, 71.4; H, 7.1; N, 7.9. Found: C, 71.1; H,
7.4; N, 7.9.
Synthesis of N-Hidroxysuccinimide Ester of N-Cbz-L-Valine
Synthesis of 3b. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-valine
(2b) and ethylenediamine. Yield 79%; mp 248-249 °C; IR (KBr) 3293,
1691, 1651, 1539 cm-1; 1H NMR (300 MHz, DMSOd6, 30 °C) δ 0.81
(d, 12H, J ) 6.6 Hz), 1.92 (m, 2H), 3.11 (br s, 4H), 3.77 (t, 2H, J )
7.4 Hz), 5.01 (s, 4H), 7.17 (d, 2H, J ) 8.4 Hz), 7.33 (br s, 10H), 7.92
(br s, 2H); 13C NMR (75 MHz, DMSOd6, 30 °C) δ 18.4, 19.4, 30.3,
38.4, 60.5, 65.5, 127.6, 127.7, 128.3, 137.1, 156.1, 171.2; Anal. Calcd.
for C28H38N4O6: C, 63.9; H, 7.3; N, 10.6. Found: C, 63.5; H, 7.4; N,
10.5.
2b. Yield 87%; mp 119-120 °C; [R]25 ) -20.1° (c ) 0.1, CHCl3);
D
IR (KBr) 3360, 1741, 1526 cm-1; 1H NMR (300 MHz, CDCl3) δ 1.02
(d, 6H, J ) 7.4 Hz), 1.06 (d, 6H, J ) 7.7 Hz), 2.31 (m, 1H), 2.77 (s,
4H), 4.66 (dd, 1H, J ) 4.7 Hz), 5.13 (s, 2H), 5.43 (d, 1H, J ) 9.5 Hz),
7.28-7.41 (m, 5H);13C NMR (75 MHz, CDCl3) δ 17.2, 18.7, 25.5,
31.5, 57.4, 67.2, 128.0, 128.1, 128.3, 135.8, 155.6, 167.5, 168.6; Anal.
Calcd. for C17H20N2O6: C, 58.6; H, 5.8; N, 7.7. Found: C, 58.1; H,
5.9; N, 8.0.
General Procedure for the Preparation of N-N′-Bis(N-Cbz-L-
aminoacyl)diamines. Synthesis of 3a. The N-hydroxysuccinimide ester
of N-Cbz-L-phenylalanine (12.00 g, 30.3 mmol) was dissolved in
anhydrous DME (150 mL) cooled in an ice bath. Ethylenediamine (0.92
g, 15.3 mmol) dissolved in dry DME (20 mL) was added in several
times. The reaction mixture was stirred at room temp for 18 h and
then was warmed for 6 h at 40-50 °C. The white solid was filtered
off and washed with cold water and cold methanol. Yield (8.79 g, 93%);
Synthesis of 4b. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-valine
(2b) and 1,3-diaminopropane. Yield 90%; mp 225-226 °C; IR (KBr)
1
3294, 1691, 1645, 1537 cm-1; H NMR (300 MHz, DMSOd6, 30 °C)
δ 0.83 (d, 12H, J ) 6.9 Hz), 1.51 (m, 2H), 1.91 (m, 2H), 2.99-3.11
(m, 4H), 3.76 (t, 2H, J ) 8.0 Hz), 5.01 (s, 4H), 7.19 (d, 2H, J ) 8.4
Hz), 7.28-7.35 (m, 10H), 7.87 (t, 2H, J ) 5.0 Hz); 13C NMR (75
MHz, DMSOd6, 30 °C) δ 18.4, 19.4, 29.3, 30.3, 36.4, 60.5, 65.5, 127.6,
128.3, 137.1, 156.0, 171.0; Anal. Calcd. for C29H40N4O6: C, 64.4; H,
7.5; N, 10.4. Found: C, 64.1; H, 7.4; N, 10.4.
mp 247-248 °C; IR (KBr) 3310, 3297, 1686, 1666, 1552, 1528 cm-1
;
1H NMR (300 MHz, DMSOd6, 40 °C) δ 2.80 (dd, 2H, J ) 13.7, 10.1
Hz), 2.99-3.16 (m, 6H), 4.21 (m, 2H), 4.89-5.00 (m, 4H), 7.18-
7.31 (m, 22H), 7.90 (br, s 2H); 13C NMR (75 MHz, DMSOd6, 40 °C)
δ 37.8, 38.4, 56.3, 65.3, 126.1, 127.2, 127.5, 127.9, 128.1, 129.0, 136.9,
137.9, 155.6, 171.2; Anal. Calcd. for C36H38N4O6: C, 69.4; H, 6.2; N,
9.0. Found: C, 69.3; H, 6.6; N, 8.9.
Synthesis of 5b. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-valine
(2b) and 1,4-diaminobutane. Yield 79%; mp 240-241 °C; IR (KBr)
1
3294, 1691, 1646, 1537 cm-1; H NMR (300 MHz, DMSOd6, 30 °C)
Synthesis of 4a. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of N-Cbz-phenylalanine
(2a) and 1,3-diaminopropane. Yield 93%; mp 232-233 °C; IR (KBr)
δ 0.81 (d, 12H, J ) 6.3 Hz), 1.37 (br s, 4H), 1.90 (m, 2H), 2.99-3.05
(m, 4H), 3.78 (t, 2H, J ) 6.9 Hz), 5.01 (s, 4H), 7.14 (d, 2H, J ) 8.4
Hz), 7.33 (br s, 10H), 7.86 (br s, 2H); 13C NMR (75 MHz, DMSOd6,
30 °C) δ 18.4, 19.4, 26.7, 30.5, 38.3, 60.5, 65.5, 127.6, 127.7, 128.3,
137.1, 156.0, 170.8; Anal. Calcd. for C30H42N4O6: C, 65.0; H, 7.6; N,
10.1. Found: C, 65.4; H, 8.0; N, 10.2.
1
3298, 1694, 1645, 1537 cm-1; H NMR (300 MHz, DMSOd6, 30 °C)
δ 1.47 (m, 2H), 2.78 (dd, 2H, J ) 13.4, 10.0 Hz), 2.95-3.07 (m, 6H),
4.20 (m, 2H), 4.95 (s, 4H), 7.18-7.40 (m, 22H), 7.87 (t, 2H, J ) 4.8
Hz); 13C NMR (75 MHz, DMSOd6, 30 °C) δ 29.1, 36.3, 37.8, 56.3,
65.3, 126.1, 127.2, 127.5, 127.9, 128.1, 129.0, 136.9, 137.9, 155.5,
170.9; Anal. Calcd. for C37H40N4O6: C, 69.8; H, 6.3; N, 8.8. Found:
C, 69.3; H, 6.6; N, 8.7.
Synthesis of 6b. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-valine
(2b) and 1,6-diaminohexane. Yield 73%; mp 223-224 °C; IR (KBr)
1
3292, 1690, 1648, 1538 cm-1; H NMR (300 MHz, DMSOd6, 30 °C)
Synthesis of 5a. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-
phenylalanine (2a) and the 1,4-diaminobutane. Yield 93%; mp 240-
241 °C; IR (KBr) 3311, 1690, 1656, 1528 cm-1; 1H NMR (300 MHz,
DMSOd6, 50 °C) δ 1.32 (br s, 4H), 2.77 (dd, 2H, J ) 13.5, 9.6 Hz),
2.93-3.04 (m, 6H), 4.21 (m, 2H), 4.94 (s, 4H), 7.15-7.33 (m, 22H),
7.79 (t, 2H, J ) 5.1 Hz); 13C NMR (75 MHz, DMSOd6, 50 °C) δ 26.3,
37.9, 38.3, 56.2, 65.2, 126.0, 127.1, 127.4, 127.8, 128.0, 128.9, 136.8,
137.8, 155.4, 170.7; Anal. Calcd. for C38H42N4O6: C, 70.1; H, 6.5; N,
8.6. Found: C, 69.7; H, 6.8; N, 8.9.
δ 0.82 (d, 12H, J ) 6.6 Hz), 1.22 (br s, 4H),1.35 (br s, 4H), 1.90 (m,
2H), 2.94-3.09 (m, 4H), 3.77 (t, 2H, J ) 9.0 Hz), 5.01 (s, 4H), 7.15
(d, 2H, J ) 8.4 Hz), 7.28-7.33 (m, 10H), 7.83 (br s, 2H); 13C NMR
(75 MHz, DMSOd6, 30 °C) δ 18.4, 19.3, 26.2, 29.1, 30.4, 38.5, 60.5,
65.4, 127.6, 127.7, 128.3, 137.1, 156.0, 170.8; Anal. Calcd. for
C32H46N4O6: C, 66.0; H, 8.0; N, 9.6. Found: C, 66.0; H, 8.1; N, 9.6.
Synthesis of 7b. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-valine
(2b) and 1,8-diaminooctaane. Yield 86%; mp 207-208 °C; IR (KBr)
1
3297, 1691, 1649, 1538 cm-1; H NMR (300 MHz, DMSOd6, 30 °C)
Synthesis of 6a. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-
phenylalanine (2a) and 1,6-diaminohexane. Yield 87%; mp 194-195
δ 0.81 (d, 12H, J ) 6.3 Hz), 1.20 (br s, 8H), 1.25 (br s, 4H), 1.89 (m,
2H), 2.95-3.08 (m, 4H), 3.76 (t, 2H, J ) 8.0 Hz), 5.00 (s, 4H), 7.13
(d, 2H, J ) 8.4 Hz), 7.33 (br s, 10H), 7.81 (br s, 2H); 13C NMR (75
MHz, DMSOd6, 30 °C) δ 18.4, 19.4, 26.4, 28.8, 29.1, 30.4, 38.5, 60.4,
65.4, 127.6, 127.7, 128.3, 137.1, 156.0, 170.8; Anal. Calcd. for
C34H50N4O6: C, 66.9; H, 8.3; N, 9.2. Found: C, 66.8; H, 8.4; N, 9.3.
General Procedure for the Deprotection of N-Cbz Groups.
Synthesis of 8a. N,N′-bis(N-Cbz-L-phenylalanyl)-1,2-diaminoethane
(3a) (8.01 g, 12.9 mmol) were added to HBr/AcOH (33%) (20 mL)
and the mixture was stirred at room temp. until CO2 evolution ceased.
At this point, diethyl ether was added to the clear solution, which led
to the deposition of a white precipitate. This was filtered off, washed
with additional ether and dissolved in distilled water, the resulting
solution was extracted with chloroform (30 mL, 3×). Solid NaOH was
then added up to a pH value of 12 and the resulting solution was
saturated with NaCl and extracted with chloroform (30 mL, 3×). The
°C; IR (KBr) 3306, 1690, 1653, 1536 cm-1 1H NMR (300 MHz,
;
DMSOd6, 30 °C) δ 1.17 (bs s, 4H), 1.32 (br s, 4H), 2.74 (dd, 2H, J )
13.4, 10.1 Hz), 2.90-3.07 (m, 6H), 4.19 (m, 2H), 4.93 (s, 4H), 7.18-
7.29 (m, 20H), 7.49 (d, 2H, J ) 8.6 Hz), 7.96 (br s, 2H); 13C NMR
(75 MHz, DMSOd6, 30 °C) δ 26.1, 29.0, 38.0, 38.6, 56.3, 65.3, 126.1,
127.3, 127.5, 127.9, 128.1, 129.1, 137.0, 137.9, 155.6, 170.9; Anal.
Calcd. for C40H46N4O6: C, 70.8; H, 6.8; N, 8.3. Found: C, 71.3; H,
7.1; N, 8.3.
Synthesis of 7a. This compound was obtained as described above
starting from the N-hydroxysuccinimide ester of the N-Cbz-L-
phenylalanine (2a) and 1,8-diaminooctane. Yield 97%; mp 202-204
°C; IR (KBr) 3316, 3298, 1687, 1657, 1536 cm-1; 1H NMR (300 MHz,
DMSOd6, 30 °C) δ 1.19 (bs s, 8H), 1.33 (br s, 4H), 2.74 (dd, 2H, J )
13.3, 10.2 Hz), 2.89-3.06 (m, 6H), 4.17 (m, 2H), 4.92 (s, 4H), 7.16-
7.31 (m, 20H), 7.50 (d, 2H, J ) 8.6 Hz), 8.02 (br s 2H); 13C NMR (75
organic phase was dried over MgSO4 and evaporated under vacuum to
25
obtain a white solid. Yield (3.8 g, 84%); mp 118-119 °C; [R]D
)
9
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A R T I C L E S
Becerril et al.
-86.9 ° (c ) 0.1, CHCl3); IR (KBr) 3358, 3299, 1655, 1529 cm-1; 1H
NMR (300 MHz, CDCl3) δ 1.45 (s, 4H), 2.66 (dd, 2H, J ) 13.7, 9.3
Hz), 3.19 (dd, 2H, J ) 13.7, 4.2 Hz), 3.32 (m, 4H), 3.55 (dd, 2H, J )
9.2, 4.3 Hz), 7.17-7.30 (m, 10H), 7.55 (br s, 2H); 13C NMR (75 MHz,
CDCl3) δ 39.1, 40.9, 56.2, 126.3, 128.2, 128.8, 137.4, 174.7; ESI-MS
m/z ) 355.1 (M + H+), 377.1 (M + Na+); Anal. Calcd. for
C20H26N4O2: C, 67.8; H, 7.4; N, 15.8. Found: C, 67.9; H, 7.8; N, 16.0.
Synthesis of 9a. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-phenylalanyl)-1,3-diaminopropane
(4a). Yield 91%; mp 121-122 °C; [R]D25 ) -85.3° (c ) 0.1, CHCl3);
Synthesis of 10b. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-valinyl)-1,4-diaminobutane (5b). Yield
95%; mp 109-110 °C; [R]D ) -48.4° (c ) 0.1, CHCl3); IR (KBr)
25
3294, 1641, 1547 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.71 (d, 6H, J
) 6.8 Hz), 0.85 (d, 6H, J ) 7.1 Hz), 1.39-1.43 (m, 8H), 2.08 (m,
2H), 3.05 (d, 2H, J ) 4.2 Hz), 3.14 (m, 4H), 7.27 (br s, 2H); 13C NMR
(75 MHz,CDCl3) δ 16.2, 19.5, 27.0, 30.9, 38.4, 60.2, 174.1; ESI-MS
m/z ) 143.2 (M + 2H+); Anal. Calcd. for C14H30N4O2: C, 58.7; H,
10.6; N, 19.6. Found: C, 58.9; H, 10.8; N 19.7.
Synthesis of 11b. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-valinyl)-1,6-diaminohexane (6b). Yield
1
IR (KBr) 3345, 3302, 1651, 1528 cm-1; H NMR (300 MHz, CDCl3)
25
δ 1.39 (s, 4H), 1.57 (m, 2H), 2.69 (dd, 2H, J ) 13.7, 9.3 Hz), 3.13-
3.24 (m, 6H), 3.57 (dd, 2H, J ) 9.0, 3.9 Hz), 7.18-7.31 (m, 10H),
7.56 (m, 2H); 13C NMR (75 MHz,CDCl3) δ 29.5, 35.5, 41.0, 56.5,
126.5, 128.4, 129.0, 137.6, 174.4; ESI-MS m/z ) 369.1 (M + H+),
391.1 (M + Na+), 407.1 (M + K+); Anal. Calcd. for C21H28N4O2: C,
68.4; H, 7.7; N, 15.2. Found: C, 68.5; H, 8.0; N, 15.4.
92%; mp 101-102 °C; [R]D ) -45.6° (c ) 0.1, CHCl3); IR (KBr)
1
3366, 3286, 1640,1552 cm-1; H NMR (300 MHz, CDCl3) δ 0.68 (d,
6H, J ) 7.1 Hz), 0.84 (d, 6H, J ) 6.8 Hz), 1.20 (br s, 8H), 1.37 (m,
4H), 2.11 (m, 2H), 3.03-3.12 (m, 6H), 7.31 (br s, 2H); 13C NMR (75
MHz,CDCl3) δ 15.9, 19.5, 26.2, 29.3, 30.7, 38.4, 59.9, 173.9; ESI-MS
m/z ) 315.3 (M + H+); Anal. Calcd. for C16H34N4O2: C, 61.1; H,
10.9; N, 17.8. Found: C, 61.3; H, 11.2; N, 17.9.
Synthesis of 10a. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-phenylalanyl)-1,4-diaminobutane (5a).
Synthesis of 12b. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-valinyl)-1,8-diaminooctane (7b). Yield
25
Yield 90%; mp 134-135 °C; [R]D ) -86.1° (c ) 0.1, CHCl3); IR
1
(KBr) 3358, 3303, 1648, 1534 cm-1; H NMR (300 MHz, CDCl3) δ
25
94%; mp 103-104 °C; [R]D ) -43.7° (c ) 0.1, CHCl3); IR (KBr)
3285, 1645, 1556 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.72 (d, 6H, J
) 6.8 Hz), 0.89 (d, 6H, J ) 7.1 Hz), 1.21 (s, 8H), 1.38-1.43 (m, 8H),
2.17 (m, 2H), 3.09-3.18 (m, 6H), 7.30 (br s, 2H); 13C NMR (75 MHz,
CDCl3) δ 15.9, 19.6, 26.7, 29.0, 29.5, 30.7, 38.8, 60.0, 174.0; ESI-MS
m/z ) 172.2 (M + 2H+); Anal. Calcd. for C18H38N4O2: C, 63.1; H,
11.2; N, 16.4. Found: C, 63.3; H, 11.6; N, 16.3.
1.13 (s, 4H), 1.26 (m, 4H), 2.47 (dd, 2H, J ) 13.7, 9.3 Hz), 3.02 (m,
6H), 3.36 (dd, 2H, J ) 9.2, 4.0 Hz), 6.98-7.16 (m, 10H), 7.30 (br s,
2H); 13C NMR (75 MHz, CDCl3) δ 26.9, 38.6, 41.0, 56.4, 126.6, 128.5,
129.1, 137.7, 173.9; ESI-MS m/z ) 383.0 (M + H+), 405.0 (M +
Na+), 421.0 (M + K+); Anal. Calcd. for C22H30N4O2: C, 69.1; H, 7.9;
N, 14.7. Found: C, 69.2; H, 8.1; N, 14.9.
Synthesis of 11a. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-phenylalanyl)-1,6-diaminohexane (6a).
General Procedure for the Preparation of the para-Macrocycles.
Synthesis of 13a (Method a). Compound 8a (0.507 g, 1.43 mmol),
anhydrous K2CO3 (1.97 g, 14.3 mmol) and 1,4-bis(bromomethyl)-
benzene (0.377 g, 1.43 mmol) were placed in a flask containing dry
CH3CN (175 mL) and the mixture was refluxed for 24 h under argon
atmosphere. The reaction was filtered and the solvent evaporated under
reduced pressure. The product was purified by silica flash chromatog-
raphy using MeOH/CH2Cl2 (1:40) as the eluent to yield a white solid
25
Yield 88%; mp 131-132 °C; [R]D ) -80.4° (c ) 0.1, CHCl3); IR
(KBr) 3365, 3289, 1650, 1628, 1547, 1525 cm-1; 1H NMR (300 MHz,
CDCl3) δ 1.27-1.48 (m, 12H), 2.66 (dd, 2H, J ) 13.7, 9.3 Hz), 3.18-
3.26 (m, 6H), 3.56 (dd, 2H, J ) 9.3, 4.2 Hz), 7.18-7.32 (m, 12H); 13
C
NMR (75 MHz,CDCl3) δ 26.3, 29.4, 38.8, 41.0, 56.4, 126.6, 128.5,
129.1, 137.8, 173.9; ESI-MS m/z ) 411.1 (M + H+) 433.1 (M + Na+),
206.2 (M + 2H+); Anal. Calcd. for C24H34N4O2: C, 70.2; H, 8.3; N,
13.7. Found: C, 70.5; H, 8.2; N, 13.8.
25
(13a). Yield (0.360 g, 55%); mp 77-80 °C; [R]D ) -32.5° (c )
0.01, CHCl3); IR (KBr) 3376, 1655, 1518 cm-1; 1H NMR (300 MHz,
CDCl3) δ 2.09 (br s, 2H), 2.62 (dd, 2H, J ) 13.4, 9.0 Hz), 3.03 (m,
4H), 3.10 (d, 2H, J ) 13.1 Hz), 3.21-3.32 (m, 4H), 3.82 (d, 2H, J )
13.2 Hz), 6.52 (br s, 2H), 7.03 (dd, 2H, J ) 7.9, 1.7 Hz), 7.13 (dd, 2H,
J ) 7.5, 1.5 Hz), 7.17-7.30 (m, 10H); 13C NMR (75 MHz,CDCl3) δ
39.1, 39.5, 54.4, 64.4, 126.4, 128.3, 128.5, 128.6, 129.0, 137.1, 139.0,
174.5; ESI-MS m/z ) 457.7 (M + H+); Anal. Calcd. for C28H32N4O2•‚1/2
H2O C, 72.2; H, 7.1; N, 12.0. Found: C, 72.3; H, 7.5; N, 11.7.
Synthesis of 13a. Method b. Compound 8a (0.521 g, 1.47 mmol),
anhydrous K2CO3 (2.041 g, 14.8 mmol), tetrabutylammonium bromide
(0.245 g, 0.76 mmol) and 1,4-bis(bromomethyl)benzene (0.388 g, 1.47
mmol) were placed in a flask containing dry CH3CN (175 mL) and
the mixture was refluxed for 12 h under argon atmosphere. The reaction
was filtered and the solvent evaporated under reduced pressure. The
crude product was dissolved in CHCl3 (50 mL) and extracted with
aqueous NaOH 0.01M (50 mL, 3×). The organic phase was dried over
anhydrous MgSO4 and the solvent was evaporated under reduced
pressure. The product was purified by silica flash chromatography using
MeOH/CH2Cl2 (1:40) as the eluent to give 0.436 of 13a as a white
solid. Yield 65%; mp 77-80 °C; [R]D25 ) -32.5° (c ) 0.01, CHCl3);
IR (KBr) 3376, 1655, 1518 cm-1; 1H NMR (300 MHz, CDCl3) δ 2.09
(br s, 2H), 2.62 (dd, 2H, J ) 13.4, 9.0 Hz), 3.03 (m, 4H), 3.10 (d, 2H,
J ) 13.1 Hz), 3.21-3.32 (m, 4H), 3.82 (d, 2H, J ) 13.2 Hz), 6.52 (br
s, 2H), 7.03 (dd, 2H, J ) 7.9, 1.7 Hz), 7.13 (dd, 2H, J ) 7.5, 1.5 Hz),
7.17-7.30 (m, 10H); 13C NMR (75 MHz,CDCl3) δ 39.1, 39.5, 54.4,
64.4, 126.4, 128.3, 128.5, 128.6, 129.0, 137.1, 139.0, 174.5; ESI-MS
Synthesis of 12a. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-phenylalanyl)-1,8-diaminooctane (7a).
Yield 92%; mp 114-115 °C; [R]D ) -70.1° (c )0.1, CHCl3); IR
25
(KBr) 3309, 1643, 1535 cm-1; 1H NMR (300 MHz, CDCl3) δ 1.28 (br
s, 12H), 1.45 (m, 4H), 2.67 (dd, 2H, J ) 13.7, 9.3 Hz), 3.19-3.29 (m,
6H), 3.58 (dd, 2H, J ) 9.4, 4.0 Hz), 7.18-7.33 (m, 12H); 13C NMR
(75 MHz,CDCl3) δ 26.9, 29.2, 29.6, 39.0, 41.1, 56.5, 126.6, 128.5,129.2,
137.9, 173.8; ESI-MS m/z ) 439.1 (M + H+), 461.0 (M + Na+), 219.9
(M + 2H+); Anal. Calcd. for C26H38N4O2: C, 71.2; H, 8.7; N, 12.8.
Found: C, 71.3; H, 8.6; N, 12.7.
Synthesis of 8b. This compound was obtained as described above
starting N,N′-bis(N-Cbz-L-valinyl)-1,2-diaminoethane (3b). Yield 91%;
mp 136-137 °C; [R]D25 ) -66.1° (c )0.01, CHCl3); IR (KBr) 3289,
1643, 1553 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.75 (d, 6H, J ) 7.1
Hz), 0.90 (d, 6H, J ) 6.8 Hz), 1.26 (s, 4H), 2.15 (m, 2H), 3.13 (d, 2H,
J ) 4.2 Hz), 3.33 (m, 4H), 7.69 (br s, 2H); 13C NMR (75 MHz,CDCl3)
δ 16.1, 19.6, 30.9, 39.4, 60.1, 175.2; ESI-MS m/z ) 259.0 (M + H+),
281.1 (M + Na+); Anal. Calcd. for C12H26N4O2: C, 55.8; H, 10.1; N,
21.7. Found: C, 55.2; H, 10.1; N, 21.5.
Synthesis of 9b. This compound was obtained as described above
starting from N,N′-bis(N-Cbz-L-valinyl)-1,3-diaminopropane (4b).
25
Yield 89%; mp 100-101 °C; [R]D ) -54.5° (c ) 0.1, CHCl3); IR
(KBr) 3298, 1651, 1538 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.64 (d,
6H, J ) 7.0 Hz), 0.80 (d, 6H, J ) 6.8 Hz), 1.23 (s, 4H), 1.45 (m, 2H),
2.04 (m, 2H), 3.01 (d, 2H, J ) 4.2 Hz), 3.11 (m, 4H), 7.58 (m, 2H);
13C NMR (75 MHz,CDCl3) δ 15.8, 19.2, 29.3, 30.6, 35.1, 59.8, 174.3;
ESI-MS m/z ) 273.1 (M + H+), 295.1 (M + Na+); Anal. Calcd for
C13H28N4O2: C, 57.3; H, 10.4; N, 20.6. Found: C, 57.2; H, 10.5; N,
20.6.
m/z ) 457.7 (M + H+); Anal. Calcd. for C28H32N4O2 ‚1/2 H2O C, 72.2;
•
H, 7.1; N, 12.0. Found: C, 72.3; H, 7.5; N, 11.7.
Synthesis of 14a (Method a). This compound was obtained as
25
described above starting from 9a. Yield 61%; mp 212-213 °C; [R]D
9
6684 J. AM. CHEM. SOC. VOL. 125, NO. 22, 2003

Macrocyclization of U-Turn Peptidomimetics
A R T I C L E S
) -58.5° (c ) 0.01, CHCl3); IR (KBr) 3348, 3302, 1648, 1530 cm-1
;
2H, J ) 3.9 Hz), 3.34-3.39 (m, 4H), 4.19 (d, 2H, J ) 14.6 Hz), 7.04
(m, 2H), 7.21 (s, 4H); 13C NMR (75 MHz,CDCl3) δ 17.2, 19.9, 26.1,
31.1, 38.3, 53.7, 69.7, 128.0, 139.6, 173.2; ESI-MS m/z ) 389.5 (M +
H+), 423.3 (M + Cl-); Anal. Calcd. for C22H36N4O2: C, 68.0; H, 9.3;
N, 14.4. Found: C, 68.1; H, 9.2; N, 14.3.
1H NMR (300 MHz, CDCl3) δ 1.19 (m, 2H), 2.06 (br s, 2H), 2.55 (dd,
2H, J ) 14.0, 10.6 Hz), 2.75 (m, 4H), 3.18 (d, 2H, J ) 14.2 Hz), 3.27
(dd, 2H, J ) 14.2, 3.4 Hz), 3.44 (dd, 2H, J ) 10.5, 3.6 Hz), 3.88 (d,
2H, J ) 14.2 Hz), 6.64 (t, 2H, J ) 5.5 Hz), 7.09-7.36 (m, 14H); 13
C
NMR (75 MHz,CDCl3) δ 28.4, 36.1, 40.1, 54.5, 66.9, 126.8, 128.5,
128.7, 128.8, 129.0, 137.2, 139.4, 173.9; ESI-MS m/z ) 471.2 (M +
H+), 509.2 (M + K+); Anal. Calcd. for C29H34N4O2: C, 74.0; H, 7.3;
N, 11.9. Found: C, 73.7; H, 7.7; N, 11.5.
Synthesis of 15b (Method b). Yield 69%.
Synthesis of 16b (Method a). This compound was obtained as
25
described above starting from 11b. Yield 52%; mp 216-217 °C; [R]D
) -14.1° (c ) 0.01, CHCl3); IR (KBr) 3303, 1633, 1547 cm-1; H
1
Synthesis of 14a (Method b). Yield 58%.
NMR (300 MHz, CDCl3) δ 0.85 (d, 6H, J ) 7.1 Hz), 0.94-1.16 (m,
14H), 1.75 (s, 2H), 2.18 (m, 2H), 2.73 (m, 2H), 2.98 (d, 2H, J ) 3.9
Hz), 3.32 (m, 2H), 3.47 (d, 2H, J ) 14.4 Hz), 4.04 (d, 2H, J ) 14.2
Hz), 7.14 (m, 2H), 7.23 (s, 4H); 13C NMR (75 MHz,CDCl3) δ 17.4,
19.8, 26.5, 29.2, 31.1, 38.3, 53.6, 69.1, 127.5, 138.9, 173.1; ESI-MS
m/z ) 417.5 (M + H+), 451.4 (M + Cl-); Anal. Calcd. for
C24H40N4O2: C, 69.2; H, 9.7; N, 13.5. Found: C, 69.4; H, 9.6; N,13.4.
Synthesis of 17b (Method a). This compound was obtained as
Synthesis of 15a (Method a). This compound was obtained as
25
described above starting from 10a. Yield 66%; mp 149-151 °C; [R]D
) -83.9° (c ) 0.01, CHCl3); IR (KBr) 3328, 3308, 1651, 1637, 1540
1
cm-1; H NMR (300 MHz, CDCl3) δ 0.90 (m, 4H), 1.96 (br s, 2H),
2.61 (m, 4H), 3.20 (d, 2H, J ) 14.7 Hz), 3.27-3.46 (m, 6H), 3.90 (d,
2H, J ) 14.7 Hz), 7.01 (dd, 2H, J ) 7.7, 4.0 Hz), 7.14-7.35 (m, 14H);
13C NMR (75 MHz,CDCl3) δ 25.9, 38.4, 39.4, 52.8, 65.3, 126.7, 127.7,
128.6, 128.7, 137.4, 139.3, 173.2; ESI-MS m/z ) 485.5 (M + H+),
507.5 (M + Na+); Anal. Calcd. for C30H36N4O2: C, 74.4; H, 7.5; N,
11.6. Found: C, 74.2; H, 7.8; N, 11.3.
25
described above starting from 12b. Yield 55%; mp 212-214 °C; [R]D
) -27.1° (c ) 0.01, CHCl3); IR (KBr) 3319, 1628, 1547 cm-1; H
1
NMR (300 MHz, CDCl3) δ 0.89 (d, 6H, J ) 6.8 Hz), 1.04 (d, 6H, J )
6.8 Hz), 1.26 (br s, 8H), 1.45 (m, 4H), 1.62 (br s, 2H), 2.22 (m, 2H),
3.01 (d, 2H, J ) 4.2 Hz), 3.09 (m, 2H), 3.41 (m, 2H), 3.57 (d, 2H, J
Synthesis of 16a (Method a). This compound was obtained as
25
described above starting from 11a. Yield 65%; mp 121-123 °C; [R]D
) -100.4° (c ) 0.01, CHCl3); IR (KBr) 3310, 1661, 1646, 1538 cm-1
;
) 13.4 Hz), 3.91 (d, 2H, J ) 13.2 Hz), 7.28 (s, 4H), 7.46 (m, 2H); 13
C
1H NMR (300 MHz, CDCl3) δ 1.11 (m, 8H), 1.86 (br s, 2H), 2.69 (dd,
2H, J ) 13.9, 10.0 Hz), 2.79 (m, 2H), 3.28-3.43 (m, 8H), 3.81 (d,
2H, J ) 14.4 Hz), 7.17-7.35 (m, 16H); 13C NMR (75 MHz,CDCl3) δ
26.3, 28.9, 38.3, 39.2, 52.5, 64.5, 126.6, 127.3, 128.5, 128.7, 137.3,
138.5, 173.0; ESI-MS m/z ) 513.5 (M + H+); Anal. Calcd. for
C32H40N4O2: C, 75.0; H, 7.9; N, 10.9. Found: C, 74.6; H, 8.2; N, 10.5.
Synthesis of 17a (Method a). This compound was obtained as
described above starting from (12a). Yield 52%; mp 182-184 °C;
NMR (75 MHz,CDCl3) δ 17.5, 19.8, 26.8, 29.3, 29.5, 31.2, 38.1, 53.4,
68.4, 127.8, 138.8, 173.0 ESI-MS m/z ) 445.6 (M + H+),479.5 (M +
Cl-); Anal. Calcd. for C26H44N4O2: C, 70.2; H, 10.0; N, 12.6. Found:
C, 70.6; H, 10.0; N, 12.2.
General Procedure for the Preparation of the meta-Macrocycles.
Synthesis of 18a (Method b). Compound 8a (0.546 g, 1.54 mmol),
anhydrous K2CO3 (2.13 g, 15.41 mmol), tetrabutylammonium bromide
(0.260 g, 0.81 mmol) and 1,3-bis(bromomethyl)benzene (0.406 g, 1.54
mmol) were placed in a flask containing dry CH3CN (175 mL) and
the mixture was refluxed for 12 h under argon atmosphere. The reaction
was filtered and the solvent evaporated under reduced pressure. The
crude product was dissolved in CHCl3 (50 mL) and extracted with
aqueous NaOH 0.01 M (50 mL, 3×). The organic phase was dried
over anhydrous MgSO4 and the solvent was evaporated under reduced
pressure. The product was purified by silica flash chromatography using
MeOH/CH2Cl2 (1:40) as the eluent to give a white solid (18a). Yield
(0.445 g, 63%); mp 157-158 °C; [R]D25 ) -80.1° (c ) 0.01, CHCl3);
[R]D25 ) -103.8° (c ) 0.01, CHCl3); IR (KBr) 3304, 1644, 1544 cm-1
;
1H NMR (300 MHz, CDCl3) δ 1.22 (br, s 8H), 1.42 (m, 4H), 1.79 (br,
s 2H), 2.77 (dd, 2H, J ) 13.9, 9.3 Hz), 3.06 (m, 2H), 3.28 (dd, 2H, J
) 13.8, 3.8 Hz), 3.34-3.48 (m, 6H), 3.69 (d, 2H, J ) 13.4 Hz), 7.11-
7.34 (m, 14H), 7.43 (t, 2H, J ) 5.4 Hz); 13C NMR (75 MHz,CDCl3)
δ 26.5, 29.0, 29.3, 38.1, 38.7, 52.1, 63.4, 126.6, 127.6, 128.5, 128.8,
137.2, 138.3, 172.8; ESI-MS m/z ) 541.6 (M + H+), 575.7 (M +
Cl-); Anal. Calc for C34H44N4O2: C, 75.5; H, 8.2; N, 10.4;. Found:
C, 75.9; H, 8.4; N, 10.2.
1
IR (KBr) 3328, 3302, 1648, 1521 cm-1; H NMR (300 MHz, CDCl3)
Synthesis of 13b (Method a). This compound was obtained as
25
δ 1.98 (br s, 2H), 2.75 (dd, 2H, J ) 13.9, 9.5 Hz), 3.12-3.28 (m, 6H),
3.33 (d, 2H, J ) 13.2 Hz), 3.42 (dd, 2H, J ) 9.4, 3.8 Hz), 3.82 (d, 2H,
J ) 13.4 Hz), 6.97 (d, 2H, J ) 7.6 Hz), 7.16-7.36 (m, 12H), 7.42 (br
s, 2H); 13C NMR (75 MHz, CDCl3) δ 38.9, 39.0, 53.7, 64.5, 126.8,
127.3, 128.4, 128.5, 128.6, 128.9, 137.2, 139.9, 174.4; ESI-MS m/z )
457.5 (M + H+), 479.4 (M + Na+); Anal. Calcd. for C28H32N4O2: C,
73.7; H, 7.1; N, 12.3. Found: C, 73.8; H, 7.1; N, 12.1.
described above starting from 8b. Yield 49%; mp 182-183 °C; [R]D
) 26.0° (c ) 0.01, CHCl3); IR (KBr) 3340, 3325, 3308, 1661, 1650,
1631, 1548 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.74 (d, 6H, J ) 7.1
Hz), 1.03 (d, 6H, J ) 7.1 Hz), 1.94 (s, 2H), 2.40 (m, 2H), 2.94 (d, 2H,
J ) 3.7 Hz), 3.11 (m, 4H), 3.41 (d, 2H, J ) 12.9 Hz), 4.13 (d, 2H, J
) 12.9 Hz), 6.64 (br s, 2H), 7.14 (dd, 2H, J ) 7.8, 1.7 Hz), 7.23 (dd,
2H, J ) 7.7, 1.6 Hz); 13C NMR (75 MHz,CDCl3) δ 16.6, 19.8, 31.1,
39.3, 55.5, 68.6, 129.1, 129.4, 139.6, 175.1; ESI-MS m/z ) 361.2 (M
+ H+), 395.1 (M + Cl-); Anal. Calcd. for C20H32N4O2: C, 66.6; H,
9.0; N, 15.5. Found: C,66.8; H, 8.8; N,15.6.
Synthesis of 19a (Method b). This compound was obtained as
25
described above starting from 9a. Yield 53%; mp 197-199 °C; [R]D
) -67.3° (c ) 0.01, CHCl3); IR (KBr) 3325, 3306, 1629, 1545 cm-1
;
Synthesis of 14b (Method a). This compound was obtained as
described above starting from 9b. Yield 61%; mp 223-224 °C; [R]D
1H NMR (300 MHz, CDCl3) δ 1.61 (m, 2H), 2.16 (br s, 2H), 2.84 (dd,
2H, J ) 13.9, 8.8 Hz), 3.06-3.32 (m, 6H), 3.46 (m, 4H), 3.82 (d, 2H,
J ) 13.9 Hz), 6.99 (d, 2H, J ) 7.3 Hz), 7.17-7.34 (m, 12H), 7.43 (br
s, 2H); 13C NMR (75 MHz, CDCl3) δ 28.5, 36.7, 38.5, 52.4, 63.5,
126.8, 126.8, 127.2, 128.5, 128.6, 129.0, 137.1, 140.0, 173.6; ESI-MS
m/z ) 471.6 (M + H+), 236.4 (M + 2H+); Anal. Calcd. for
C29H34N4O2: C, 74.0; H, 7.3; N, 11.9. Found: C, 73.9; H, 7.3; N, 11.6.
Synthesis of 20a (Method b). This compound was obtained as
25
) 16.1° (c ) 0.01, CHCl3); IR (KBr) 3345, 3320, 3302, 1662, 1649,
1637, 1544, 1527 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.84 (d, 6H, J
) 7.1 Hz), 1.01 (d, 6H, J ) 7.1 Hz), 1.16 (m, 2H), 2.03 (s, 2H), 2.12
(m, 2H), 2.72 (m, 4H), 3.03 (d, 2H, J ) 4.2 Hz), 3.37 (d, 2H, J ) 13.9
Hz), 4.12 (d, 2H, J ) 13.7 Hz), 6.67 (m, 2H), 7.17 (br s, 4H); 13C
NMR (75 MHz,CDCl3) δ 17.6, 19.8, 31.4, 36.1, 55.2, 71.4, 128.9,
139.6, 173.7; ESI-MS m/z ) 375.7 (M + H+); Anal. Calcd. for
C21H34N4O2: C, 67.4; H, 9.2; N, 15.0. Found: C, 67.5; H, 9.0; N, 15.0.
Synthesis of 15b (Method a). This compound was obtained as
25
described above starting from 10a. Yield 63%; mp 211-213 °C; [R]D
) -151.5° (c ) 0.01, CHCl3); IR (KBr) 3329, 3309, 1650, 1638, 1540
cm-1; 1H NMR (300 MHz, CDCl3) δ 1.53 (br s, 4H), 1.64 (br s, 2H),
2.70 (dd, 2H, J ) 13.9, 9.8 Hz), 2.96 (m, 2H), 3.26 (dd, 2H, J ) 13.9,
3.7 Hz), 3.40-3.45 (m, 4H), 3.57-3.69 (m, 4H), 6.94 (d, 2H, J ) 7.6
Hz), 7.14-7.33 (m, 12H), 7.44 (dd, 2H, J ) 7.3, 2.7 Hz); 13C NMR
(75 MHz,CDCl3) δ 26.8, 38.2, 39.0, 52.8, 64.0, 126.6, 126.8, 127.9,
25
described above starting from 10b. Yield 45%; mp 185-186 °C; [R]D
) 11.4° (c ) 0.01, CHCl3); IR (KBr) 3301, 1636, 1555 cm-1; 1H NMR
(300 MHz, CDCl3) δ 0.84 (d, 6H, J ) 6.5 Hz), 0.92 (m, 4H), 1.04 (d,
6H, J ) 7.1 Hz), 1.88 (br s, 2H), 2.25 (m, 2H), 2.59 (m, 2H), 3.04 (d,
9
J. AM. CHEM. SOC. VOL. 125, NO. 22, 2003 6685

A R T I C L E S
Becerril et al.
128.5, 128.6, 128.8, 137.2, 139.9, 173.2; ESI-MS m/z ) 485.7 (M +
H+), 243.4 (M + 2H+); Anal. Calcd. for C30H36N4O2: C, 74.4; H, 7.5;
N, 11.6. Found: C, 74.2; H, 7.4; N, 11.2.
) 417.5 (M + H+), 439.5 (M + Na+), 453.4 (M+ HCl + H+); Anal.
Calcd. for C24H40N4O2: C, 69.2; H, 9.7; N, 13.5. Found: C, 69.3; H,
10.0; N, 13.3.
Synthesis of 21a (Method b). This compound was obtained as
described above starting from 11a. Yield 67%; mp 200-202 °C; [R]D
Synthesis of 22b (Method b). This compound was obtained as
25
25
described above starting from 12b. Yield 65%; mp 222-223 ° C; [R]D
) -134.6° (c ) 0.01, CHCl3); IR (KBr) ν 3312, 1635, 1548 cm-1; 1H
NMR (300 MHz, CDCl3) δ 1.35-1.57 (m, 10H), 2.68 (dd, 2H, J )
13.9, 10.0 Hz), 3.04 (m, 2H), 3.28 (dd, 2H, J ) 13.9, 3.4 Hz), 3.42
(dd, 2H, J ) 9.9, 3.5 Hz), 3.50-3.61 (m, 6H), 6.89 (d, 2H, J ) 7.6
Hz), 7.14-7.36 (m, 14H); 13C NMR (75 MHz, CDCl3) δ 26.5, 29.4,
38.0, 39.1, 52.9, 64.0, 126.4, 126.7, 127.4, 128.5, 128.6, 128.7, 137.1,
139.4, 172.8; ESI-MS m/z ) 513.6 (M + H+); Anal. Calcd. for
C32H40N4O2: C, 75.0; H, 7.9; N, 10.9. Found: C, 74.8; H, 7.9; N, 10.6.
Synthesis of 22a (Method b). This compound was obtained as de-
) -47.3 ° (c 0.01, CHCl3); IR (KBr) 3322, 3297, 1628, 1551 cm-1
;
1H NMR (300 MHz, CDCl3) δ 0.90 (d, 6H, J ) 6.8 Hz), 1.00 (d, 6H,
J ) 7.1 Hz), 1.31-1.55 (m, 14H), 2.15 (m, 2H), 2.98 (d, 2H, J ) 4.4
Hz), 3.14 (m, 2H), 3.41 (m, 2H), 3.72 (s, 4H), 7.19-7.35 (m, 6H); 13
C
NMR (75 MHz, CDCl3) δ 17.9, 19.7, 26.5, 29.2, 29.3, 31.4, 38.3, 53.7,
68.6, 126.5, 127.8, 128.8, 139.9, 172.9; ESI-MS m/z ) 445.4 (M +
H+), 467.4 (M + Na+); Anal. Calcd for C26H44N4O2: C, 70.2; H, 10.0;
N, 12.6. Found: C, 69.8; H, 10.2; N, 12.3.
25
Fluorescence Measurements. Steady-state fluorescence measure-
ments were carried out in a VARIAN Cary Eclipse Fluorescence
Spectrophotometer equipped with a xenon flash lamp in a rectangular
quartz cell with an optical path of 10 mm. The concentration of the
samples was 10-3 M and emission intensity was measured at 285 nm
after excitation at 268 nm.
scribed above starting from 12a. Yield 55%; mp 207-208 °C; [R]D
) -100.4° (c ) 0.01, CHCl3); IR (KBr) 3347, 3280, 1650, 1528 cm-1
;
1H NMR (300 MHz, CDCl3) δ 1.29 (br s, 4H), 1.43-1.66 (m, 10H),
2.69 (dd, 2H, J ) 13.67, 9.76 Hz), 3.09 (m, 2H), 3.27 (dd, 2H, J )
13.92, 3.18 Hz), 3.36 (dd, 2H, J ) 9.77, 3.67 Hz), 3.45-6.65 (m, 6H),
6.72 (d, 2H, J ) 7.81 Hz), 7.07-7.34 (m, 14H); 13C NMR (75 MHz,
CDCl3) δ 26.45, 29.16, 29.27, 38.36, 39.34, 52.65, 63.54, 126.08,
126.77, 127.48, 128.62, 128.93, 137.33, 139.24, 172.96; ESI-MS m/z
) 541.5 (M + H+), 563.5 (M + Na+), 579.5 (M + K+); Anal. Calcd.
for C34H44N4O2: C, 75.5; H, 8.2; N, 10.4. Found: C, 75.0; H, 8.6; N,
10.4.
Molecular Mechanics Calculations. Monte Carlo conformational
search and molecular dynamics studies were performed with MAC-
ROMODEL 7.0 using AMBER* as the force field and GB/SA
simulation of chloroform as solvent.The Monte Carlo conformational
search involved the modification of the torsional angles automatically
setup by the program and convergence was obtained after performing
2 cycles of 1000 steps departing from different conformers in each
case and using simulation of chloroform. The global minima obtained
in this way were used as starting structures for the molecular dynamics
calculations which were carried out using the following conditions:
simulation temperature ) 300 K, length of simulation ) 5 ns with 1.5
fs steps, SHAKE was used to constraint the length of bonds to hydrogen
and 200 structures were sampled to analyze the percentage of hydrogen-
bonded structures.
Synthesis of 18b (Method b). This compound was obtained as
25
described above starting from 8b. Yield 64%; mp 156-157 °C; [R]D
) -27.9° (c ) 0.01, CHCl3); IR (KBr) 3314, 1650, 1524 cm-1; H
1
NMR (300 MHz, CDCl3) δ 0.82 (d, 6H, J ) 6.8 Hz), 0.97 (d, 6H, J )
6.8 Hz), 1.83 (br s, 2H), 2.12 (m, 2H), 2.97 (d, 2H, J ) 4.2 Hz), 3.07
(m, 4H), 3.45 (d, 2H, J ) 13.2 Hz), 4.03 (d, 2H, J ) 13.2 Hz), 7.02
(d, 2H, J ) 7.3 Hz), 7.17 (t, 1H, J ) 7.1 Hz), 7.27 (s, 1H), 7.32 (br s,
2H); 13C NMR (75 MHz, CDCl3) δ 17.5, 19.7, 31.3, 39.0, 54.8, 69.2,
127.2, 128.2, 128.7, 140.2, 174.5; ESI-MS m/z ) 361.4 (M + H+);
Anal. Calcd. for C20H32N4O2: C, 66.6; H, 9.0; N, 15.5. Found: C,
66.4; H, 9.5; N, 15.2.
Crystal Structure Analysis. Compound 13b. C20H32N4O2, ortho-
rhombic, space group P212121, a ) 6.4222(4), b ) 15.171(1), c )
Synthesis of 19b (Method b). This compound was obtained as
20.007(2) Å, V ) 1949.3(3) Å3, Z ) 4, µ ) 0.081 mm-1, Fcalc.
)
25
described above starting from 9b. Yield 56%; mp 225-226 °C; [R]D
) 22.2° (c ) 0.01, CHCl3); IR (KBr) ν 3343, 3313, 1636, 1545 cm-1
;
1.228 gcm-1, Siemens-SMART-CCD-three-circle diffractometer, Mo
KR-radiation, 2θ-range ) 3.36-54.20°, 27875 reflections collected,
4256 independent reflections (Rint) 0.077), empirical absorption
correction (SADABS, Sheldrick, 1996), structure solution with SHELXS-
90,13a refinement on F2 with SHELXL-97,13b R1[I > 2σ(I)]) 0.064,
wR2 ) 0.119, data-parameter-ratio ) 17.0, max. peak in final electron
density map ) 0.25 e Å3.
1H NMR (300 MHz, CDCl3) δ 0.85 (d, 6H, J ) 6.8 Hz), 0.97 (d, 6H,
J ) 7.1 Hz), 1.53 (m, 2H), 1.85 (br s, 2H), 2.09 (m, 2H), 2.93-3.02
(m, 4H), 3.31 (m, 2H), 3.44 (d, 2H, J ) 13.4 Hz), 3.99 (d, 2H, J )
13.7 Hz), 7.06 (d, 2H, J ) 8.1 Hz), 7.20 (t, 1H, J ) 6.8 Hz), 7.27 (s,
1H), 7.38 (t, 2H, J ) 5.4 Hz); 13C NMR (75 MHz, CDCl3) δ 17.7,
19.6, 28.6, 31.2, 35.5, 53.7, 68.8, 126.8, 127.8, 128.5, 140.4, 173.6;
ESI-MS m/z ) 375.5 (M + H+); Anal. Calcd. for C21H34N4O2: C,
67.4; H, 9.2; N, 15.0. Found: C, 67.2; H, 9.4; N, 15.0.
Compound 18b. C20H32N4O2, monoclinic, space group P21, a )
10.1580(6), b ) 18.8852(15), c ) 10.8306(6) Å, â ) 92.105(5)°, V )
2076.3(2) Å3, Z ) 4, µ ) 0.076 mm-1, Fcalc. ) 1.153 gcm-1, STOE-
IPDS-II-two-circle diffractometer, Mo KR-radiation, 2θ-range )
3.76-53.96°, 20 452 reflections collected, 8830 independent reflections
(Rint) 0.047), structure solution with SHELXS-90, refinement on F2
with SHELXL-97, R1[I > 2σ(I)]) 0.038, wR2 ) 0.066, data-para-
meter-ratio ) 17.6, max. peak in final electron density map ) 0.16 e
Å3.
Synthesis of 20b (Method b). This compound was obtained as
25
described above starting from 10b. Yield 65%; mp 237-238 °C; [R]D
) -94.3° (c ) 0.01, CHCl3); IR (KBr) 3301, 1636, 1543 cm-1; H
1
NMR (300 MHz, CDCl3) δ 0.87 (d, 6H, J ) 6.8 Hz), 1.02 (d, 6H, J )
6.8 Hz), 1.57 (br s, 6H), 2.15 (m, 2H), 2.95-3.04 (m, 4H), 3.51-3.64
(m, 4H), 3.88 (d, 2H, J ) 12.9 Hz), 7.15 (d, 2H, J ) 6.8 Hz), 7.28 (t,
1H, J ) 6.8 Hz), 7.39 (s, 1H), 7.46 (m, 2H); 13C NMR (75 MHz,
CDCl3) δ 17.7, 19.7, 26.9, 31.1, 38.1, 53.9, 68.8, 126.7 128.6, 128.6,
140.3 173.3; ESI-MS m/z ) 389.5 (M + H+), 423.5 (M + Cl-); Anal.
Calcd. for C22H36N4O2: C, 68.0; H, 9.3; N, 14.4. Found: C, 68.5; H,
9.5; N, 14.4.
Acknowledgment. Financial support was provided by Min-
isterio de Ciencia y Tecnolog´ıa of Spain (proyect BQU2000-
1424) and BANCAIXA (P1.1A2000-10). J.B. thanks Ministerio
de Educacio´n for a PhD fellowship.
Synthesis of 21b (Method b). This compound was obtained as
25
described above starting from 11b. Yield 60%; mp 240-242 °C; [R]D
) -60.8° (c ) 0.01, CHCl3); IR (KBr) 3305, 1632, 1551 cm-1; H
1
NMR (300 MHz, CDCl3) δ 0.88 (d, 6H, J ) 6.8 Hz), 1.00 (d, 6H, J )
7.1 Hz), 1.40-1.54 (m, 10H), 2.12 (m, 2H), 3.00-3.10 (m, 4H), 3.52
(m, 2H), 3.59 (d, 2H, J ) 12.5 Hz), 3.76 (d, 2H, J ) 12.5 Hz), 7.18-
7.36 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 17.7, 19.7, 26.7, 29.7,
31.3, 37.9, 54.0, 68.9, 126.9, 128.2, 128.8, 140.0, 172.9; ESI-MS m/z
Supporting Information Available: X-ray crystallographic
data (.cif files) for the compounds 13b and 18b. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA0284759
9
6686 J. AM. CHEM. SOC. VOL. 125, NO. 22, 2003