Privileged structures as peptide backbone constraints: Polymer-supported stereoselective synthesis of benzimidazolinopiperazinone peptides

Article abstract of DOI:10.1021/co500023k

A molecular scaffold comprising a privileged structure was designed and synthesized to serve as a peptide backbone conformational constraint. The synthesis of highly functionalized 2,3,10,10a-tetrahydrobenzo[4,5]imidazo[1,2-a] pyrazin-4(1H)-ones on a solid-phase support was performed via a tandem N-acyl-N-aryliminium ion cyclization-nucleophilic addition reaction. The synthesis proceeded with full stereocontrol of the newly formed stereogenic center. Conventional and microwave-assisted syntheses were compared with respect to efficiency and the optical integrity of the target compounds. Significant epimerization was observed during acylation with (S)- and (R)-2-bromopropionic acids under microwave conditions.

Full text of DOI:10.1021/co500023k

Research Article
pubs.acs.org/acscombsci
Privileged Structures as Peptide Backbone Constraints:
Polymer-Supported Stereoselective Synthesis of
Benzimidazolinopiperazinone Peptides
Pilar Ventosa-Andres,† Ludmila Hradilova,‡ and Viktor Krchnak*,†,§
́
́
̌
́
†Department of Organic Chemistry, Institute of Molecular and Translational Medicine, Faculty of Science, Palacky University,
́
17 Listopadu 12, 771 46 Olomouc, Czech Republic
‡Farmak, Na Vlci
̌ ́ ̌
nci 16/3, Klasterní Hradisko, 779 00 Olomouc, Czech Republic
§Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Center, Notre Dame,
Indiana 46556, United States
S
* Supporting Information
ABSTRACT: A molecular scaffold comprising a privileged structure was designed and synthesized to serve as a peptide backbone
conformational constraint. The synthesis of highly functionalized 2,3,10,10a-tetrahydrobenzo[4,5]imidazo[1,2-a]pyrazin-4(1H)-
ones on a solid-phase support was performed via a tandem N-acyl-N-aryliminium ion cyclization−nucleophilic addition reaction.
The synthesis proceeded with full stereocontrol of the newly formed stereogenic center. Conventional and microwave-assisted
syntheses were compared with respect to efficiency and the optical integrity of the target compounds. Significant epimerization was
observed during acylation with (S)- and (R)-2-bromopropionic acids under microwave conditions.
KEYWORDS: solid-phase synthesis, iminiums, privileged structure, peptidomimetics
considered peptidomimetics of conformationally constrained
turns,2 and their incorporation into traditional Merrifield solid-
phase synthesis is considered to be one of the most successful
approaches.4
INTRODUCTION
■
The three-dimensional structures of peptides and proteins are
substantially different. Whereas proteins form defined secondary
and tertiary structures, peptides tend to be conformationally
flexible, and their 3D architecture is highly dependent on their
sequence and environment. The determination of receptor-
bound conformations of peptide ligands not only contributes to
the understanding of receptor−ligand interactions but also allows
for the design and synthesis of conformationally constrained
analogues of peptides with “frozen,” biologically active conforma-
tions.1−4 Moreover, a large number of cellular processes involve
peptide−protein and protein−protein interactions (PPIs), such
as antigen−antibody interactions, intercellular communication,
and programmed cell death.5−7 Therefore, these interactions
have become valuable targets of investigation in the search for
new therapeutic agents.8 A promising approach in the search for
modulators of protein−protein interactions is the design and
synthesis of molecules that are able to mimic the electronic and
conformational properties of the interaction surfaces, such as the
mimetics of peptide secondary structure elements. Of the several
types of reverse turns that have been identified,9 β-turns
have been postulated to be essential for the interaction of linear
peptides with receptors, enzymes and antibodies.10 Conse-
quently, the preparation of fused heterocycles, which could be
The traditional approach to the design and synthesis of
constrained peptides involves the synthesis of a scaffold that
provides the desired conformational restrictions without
considering the pharmacological relevance of the scaffold.
The aim of our present work was to design and synthesize a
privileged-structure-containing scaffold that could serve as a
peptide backbone constraint.11 A privileged structure, a term
introduced by Ben Evans,12 is a molecular scaffold that possesses
the ability to bind to multiple targets (i.e., proteins). Binding to
individual targets can be tuned through the decoration of the
scaffold (i.e., modification of functional groups).13 The fused ring
system of benzo[4,5]imidazo[1,2-a]pyrazin-4(1H)-one (I) is
considered a hybrid of benzimidazoline and hexahydroimidazo-
[1,2-a]pyrazin-5(1H)-one; both scaffolds are present in bioactive
compounds and natural products. The benzimidazole ring is a
privileged scaffold12,13 present in numerous pharmacologically
Received: February 14, 2014
Revised: April 11, 2014
© XXXX American Chemical Society
A
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Figure 1. Fused and bridged heterocycles synthesized via iminium chemistry.
a
Scheme 1. Synthesis of Benzo[4,5]imidazo[1,2-a]pyrazin-4(1H)-ones 9 on a Solid-Phase Support
a
Reagents and conditions: (i) 50% piperidine in DMF (v/v), rt, 15 min; (ii) Fmoc-amino acid (5 equiv), HOBt (5 equiv), DIC (5 equiv), NMP;
(iii) 4,5-disubstituted o-fluoronitrobenzenes (10 equiv), DIEA (10 equiv), DMSO; for R2 = piperidin-1-yl: piperidine (10 equiv), DMSO; (iv)
Na2S2O4 (10 equiv), K2CO3 (14 equiv), TBAHS (1 equiv), DCM/H2O (1:1); (v) α-bromocarboxylic acids (2 equiv), DIC (1 equiv), DCM; (vi)
aminoacetaldehyde dimethyl acetal (5 equiv), DIEA (5 equiv), DMF; (vii) 4-nitrobenzenesulfonyl chloride (4-Nos-Cl) or 4-methylbenzenesulfonyl
chloride (Tos-Cl), 2,6-lutidine, DCM, or acid chloride (2 equiv), DIEA (2 equiv), THF; (viii) 50% TFA in DCM, rt, 30 min; (ix) 50% TFA in
DCM, rt, 16 h.
relevant molecules, such as antagonists for the angiotensin II
receptor,14 inhibitors of trypsin-like serine protease (factor Xa),15
antibacterial agents,16 anticancer agents,17,18 anthelmintic agents,19
and antiulcer agents.20,21 The hexahydroimidazo[1,2-a]pyrazin-
5(1H)-one ring is present in inhibitors of HIV-1 integrase22 and
antiviral drugs.23
Herein, we describe the synthesis of the pharmacologically
relevant nitrogenous scaffold I, which is designed to serve
as a conformational constraint for the peptide backbone (the
backbone constraint is shown in red). This scaffold, consisting
of fused heterocycles, was prepared via tandem N-acyliminium
ion cyclization−nucleophilic addition, a very powerful strategy
that has been successfully applied to the synthesis of numerous
diverse heterocycles.24−27 We have previously reported the
synthesis of several fused heterocycles (Figure 1, structures
II−V)28−31 as peptide backbone conformational constraints.
shown in red). The reported dramatic increases in the reaction
rates, yields, and purities of diverse products prepared under
nonconventional and energy-efficient heating in a micro-
wave cavity32−34 prompted us to compare the synthesis under
traditional reaction conditions (room temperature or conven-
tional heating) with microwave-assisted synthesis and to
evaluate the potential benefits of synthesis under microwave
conditions.
We applied the conventional reaction conditions reported in
our previous communications28−31 for the individual steps of
the synthetic route outlined in Scheme 1. The microwave-
assisted synthesis was performed under the following reaction
conditions. Rink amide resin35 1 was acylated with Fmoc-α-
amino acids (BB1; for structures and numbering of building
blocks, refer to Figure 2) activated by N,N′-diisopropylcarbo-
diimide (DIC) in the presence of HOBt in 1-methyl-2-
pyrrolidone (NMP) at 86 °C for 10 min using the microwave
heating conditions developed for peptide synthesis.36 The
Fmoc protecting group was removed with piperidine at room
temperature, and resin-bound amine 2 was reacted with three
different 1-fluoro-2-nitrobenzenes to yield resin-bound nitro-
aniline 3. Whereas substitution of fluorine in the activated
RESULTS AND DISCUSSION
■
Synthesis. To evaluate both the scope and limitations
of the proposed synthetic routes, with a particular focus on
stereochemistry, a set of model compounds was prepared as
shown in Scheme 1 (the constraint for the peptide backbone is
B
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Figure 2. Building blocks used for the synthesis of 9.
Table 1. Comparison of Conventional and Microwave-Assisted Syntheses
conventional conditions
microwave conditions
a
b
a
step
building blocks
BB1 (2−7)
time (h) T (°C) conversion (%) time (min) T (°C) conversion (%)
coupling with Fmoc-amino acids
2−16
16
16
16
16
72
72
2
rt
rt
rt
50
rt
rt
rt
rt
rt
rt
rt
99
99
99
99
99
99
99
99
95
90
90
10
5
86
80
99
99
99
99
99
99
99
99
86
80
90
substitution with 2-fluoronitrobenzenes
BB2 (1)
BB2 (2)
5
80
BB2 (4)
60
1
100
50
acylation with bromocarboxylic acids
BB3 (1)
BB3 (2)
10
10
1
86
BB3 (3)
86
nucleophilic substitution
nosylation
aminoacetaldehyde dimethyl acetal
90
BB4 (1)
BB4 (2)
BB4 (5)
4
15
15
10
150
150
86
tosylation
4
acylation with Fmoc-amino acids
16
a
b
Conversion calculated via LC/MS. Maximum power 100 W.
1-fluoro-2-nitrobenzenes BB2(1) and BB2(2) proceeded at
80 °C in DMSO within 5 min, the reaction with 1-fluoro-2-
nitrobenzene required a temperature of 100 °C for 1 h. To
increase the diversity of the final compound, the 4,5-dichloro-2-
nitrobenzene derivative 3(R1,2) was treated with piperidine in
DMF to yield the piperidine-substituted intermediate 3(R1,3).
Reduction of the nitro group was accomplished using sodium
dithionite (Na2S2O4) in the presence of K2CO3; tetrabuty-
lammonium hydrogen sulfate (TBAHS) was used as a phase-
transfer catalyst in DCM:H2O at room temperature37 and
afforded polymer-supported primary amine 4.
The acylation of resin 4 using bromoacetic acid and DIC in
the presence of DIEA did not afford intermediate 5; LC/MS
analysis revealed the presence of the primary amine. Micro-
wave-assisted reaction conditions without DIEA as the base
provided the desired intermediate 5 within 1 min at 50 °C.
When these reaction conditions were applied to the acylation
with (R)- and (S)-2-bromopropionic acids, only the primary
amine 4 was detected via LC/MS. Complete acylation was
observed when the reaction was conducted at 86 °C for
10 min. Subsequent nucleophilic displacement of the bromine
with aminoacetaldehyde dimethyl acetal was carried out under
microwave conditions at 90 °C in DMF for 1 min. Reaction
of 6 with 4-Nos-Cl or Tos-Cl at 150 °C for 15 min afforded
sulfonamide 7. Acylation with either Fmoc-Ala-OH via the
symmetric anhydride or benzoyl chlorides (4-methoxy, 4-bromo)
was utilized to provide derivatives of the amino group.
The comparison of the reaction conditions for the individual
synthetic steps is summarized in Table 1. The completion of all
steps of this solid-phase synthesis of acyclic precursor 7 using
traditional reaction conditions (room temperature or conven-
tional heating) required 3 to 6 days, depending on the reactivity
of the individual building blocks. Microwave-assisted synthesis
allowed a substantial reduction in the reaction times, and the
LC/MS analysis indicated excellent conversion for each step.
However, the LC/MS analysis did not provide any information
C
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Table 2. Effects of R1 and R2 on Diastereomer Formation
a
b
c
entry
cmpd
R1
R2
R2′
R4
isolation
R/S
% de
purity
yield
1
2
3
4
5
6
7
8
9
9(1,1,1,1)
9(2,1,1,1)
9(2,1,1,2)
9(2,2,1,1)
9(2,3,1,2)
9(3,1,1,1)
9(4,1,1,1)
9(5,1,1,1)
9(6,1,1,1)
H
H
H
H
Cl
Cl
H
H
H
H
CF3
CF3
CF3
Cl
Nos
Nos
Tos
Nos
Tos
Nos
Nos
Nos
Nos
HPLC
HPLC
HPLC
Precip
HPLC
precip
HPLC
precip
HPLC
46
64
77
76
56
88
77
73
60
22
58
45
52
32
55
86
27
34
(S)-CH3
(S)-CH3
(S)-CH3
(S)-CH3
(R)-CH3
(S)-iPr
60:40
77:23
65:35
66:34
57:43
65:35
47:53
64:36
20
54
30
32
14
30
6
piperidin-1-yl
CF3
CF3
(S)-CH2OH
(S)-Bn
CF3
CF3
28
a
b
c
1
R/S ratio calculated via H NMR of the crude product. Purity of the crude product. Total yield after purification via HPLC or precipitation in
MeOH of products prepared using a 9- to 10-step synthesis.
Table 3. Synthesized Derivatives of Benzimidazolinopiperazinones 9
a
b
c
e
d
e
entry
cmpd
R1
R2
R2′
R3
R4
method
isol
R/S
d
purity
yield
1
9(1,1,2,1)
9(1,1,3,1)
9(2,1,3,2)
9(2,1,3,4)
9(2,3,3,1)
9(3,1,2,1)
9(3,1,3,1)
9(2,1,2,1)
9(2,1,3,1)
9(2,1,3,3)
9(2,2,3,1)
9(6,1,2,1)
9(6,1,3,1)
H
H
H
H
H
H
Cl
H
H
H
H
H
Cl
H
H
CF3
CF3
CF3
CF3
(S)-CH3
(R)-CH3
(R)-CH3
(R)-CH3
(R)-CH3
(S)-CH3
(R)-CH3
(S)-CH3
(R)-CH3
(R)-CH3
(R)-CH3
(S)−CH3
(R)−CH3
Nos
Nos
Tos
B
B
A
A
A
A
A
B
B
B
B
B
B
HPLC
HPLC
HPLC
HPLC
HPLC
HPLC
HPLC
PREC
PREC
PREC
HPLC
PREC
PREC
75
77
68
45
54
79
68
77
78
70
62
78
69
19
21
27
37
58
57
42
56
39
39
37
23
24
2
3
(S)-CH3
(S)-CH3
(S)-CH3
(R)-CH3
(R)-CH3
(S)-CH3
(S)-CH3
(S)-CH3
(S)-CH3
(S)-Bn
58:42
55:45
53:47
37:62
52:48
11:89
77:33
73:27
55:45
16:84
76:24
16
10
6
4
CO-Ph-OMe
Nos
5
piperidin-1-yl
CF3
6
Nos
25
5
7
CF3
Nos
8
CF3
Nos
78
44
46
20
68
52
9
CF3
Nos
10
11
12
12
CF3
CO-Ph-Br
Nos
Cl
CF3
Nos
(S)-Bn
CF3
Nos
a
b
Configuration of building block used in the synthesis. Acylation with 2-bromopropionic acid. Method A: Microwave conditions (86 °C, 100 W,
c
d
e
10 min). Method B: Conventional protocol (rt, 72 h). R/S ratio calculated from 1H NMR of a crude sample. Purity of the crude product. Total
yield after purification via HPLC or precipitation in MeOH of products prepared using a 9- to 10-step synthesis.
regarding the optical integrity of the chiral intermediates and
the final products.
of the synthesized benzo[4,5]imidazo[1,2-a]pyrazin-4(1H)-ones
9, the purity of the crude products and the yields after
purification are summarized in Tables 2 and 3.
After acid-mediated unmasking of the aldehyde and cleavage
of the acyclic intermediates from the resin, cyclic N-acyl-N-
aryliminium ions were formed. Although N-acyl-N-alkyliminium
chemistry has been widely studied and utilized in numerous
syntheses, N-acyl-N-aryliminium ion chemistry has been
investigated much less frequently.38,39 After 30 min and cleavage
with 50% TFA in DCM, LC/MS analysis showed the formation
of hydroxy intermediate 8 (not isolated or characterized).
The presence of the aromatic amine as an internal nucleophile
facilitated formation of the fused ring, and benzo[4,5]imidazo-
[1,2-a]pyrazin-4(1H)-one (9) was obtained after 16 h of
treatment with the cleavage cocktail. The substitution patterns
All selected building blocks yielded the expected final
products. However, the yield of the compounds prepared
using 1-fluoro-2-nitrobenzene (BB2 #4) was very low because
of the premature release of dihydroquinaxolinone 10 from resin
4 via a cyclative cleavage mechanism (Scheme 2). Therefore,
we did not utilize this building block in further syntheses.
Analogous release from an amide linkage has previously been
reported.40,41
Stereoselectivity. The synthesis of target compound 9 via
N-acyl-N-aryliminium ion cyclization−nucleophilic addition
generated a new chiral center. The cyclization of an achiral
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NMR spectra. Thus, to address epimerization during synthesis,
we prepared a set of model precursors with two stereogenic
centers in both the R1 (a chiral amino acid) and R3 positions.
The results revealed significant epimerization of the R3 center
when acylation of the (S)- and (R)-2-bromopropionic acids
was performed at elevated temperatures in a microwave cavity
(Table 3, entries 3−7). The epimerization was not influenced by
the chirality of the amino acid, substitution on the aromatic ring
or the R4 substituent, indicating that the epimerization occurred
during the activation step. Acylation at room temperature
required long reaction times and showed less epimerization;
however, we still observed the formation of diastereomers
(entries 8−13). To address the epimerization of (S)- and
(R)-2-bromopropionic acids during acylation on a simple model
compound, we acylated Ala attached to the Rink resin with both
(S)- and (R)-2-bromopropionic acids under standard conditions
(acid, DIC, HOBt (1:1:1), 0 °C, and acid, DIC (2:1)). The
NMR of the crude products indicated 5% epimerization (de 90%,
manufacturer specification de > 85%).
The epimerization prompted us to explore an alternative
synthetic route: the acyclic resin-bound precursor 7(2,1,2,1)
was prepared via acylation of the resin-bound amine 4(2,1)
with Fmoc-Ala-OH, followed by incorporation of the protected
aldehyde via the Mitsunobu reaction. Reaction with Fmoc-Ala-
OH via the symmetric anhydride method in THF yielded
acylated compound 11 (Scheme 3). After removal of the
Fmoc protective group, the resin-bound primary amine was
activated with 4-Nos-Cl in DCM in the presence of 2,6-lutidine
to yield sulfonamide 12. Subsequent Mitsunobu alkylation with
glycolaldehyde dimethyl acetal afforded the acyclic intermediate
7(2,1,2,1). Treatment with 50% TFA in DCM for 16 h afforded
product 9(2,1,2,1) without epimerization.
Scheme 2. Release of Dihydroquinoxalinone via Cyclative
Cleavage
precursor yielded a mixture of enantiomers (Table 2, entry 1).
To stereoselectively obtain target compound 9, we evaluated
the effects of chiral R1 and R3 substituents on the stereo-
chemical outcome of the fused ring formation.
First, we describe the effect of a stereogenic center located
on the R1 substituent, that is, outside of the fused ring. The
stereochemistry of the fused ring was investigated using a set
of model compounds with different α-amino acids (Table 2).
(S)-Amino acids in the R1 position favored an (R)-configuration
for the new stereogenic center with a de of 54% (Table 2, entry 3).
The chirality of the amino acid and the substituents on the
aromatic ring (Table 2, entries 4−6) did not markedly affect the
reaction. This poor stereocontrol was expected and is dependent
on the steric hindrance of the N-acyliminium ion intermediate.
Interestingly, a 6% excess of the (S)-epimer was obtained with
Ser (Table 2, entry 8). This change in stereoselectivity could be
attributed to H-bond formation between the amide of Ser and
the carbonyl group and between the hydroxy group of the lateral
chain and the sulfonyl of 4-Nos in the cyclic intermediate, thus
favoring nucleophilic attack on the opposite face.
The next set of compounds was synthesized using (S)- and
(R)-2-bromopropionic acids. We observed complete stereo-
control of the newly formed chiral center. The compounds with
(R)- and (S)-configurations in the R3 position of the acyclic
precursor 7 exclusively induced the (R)- and (S)-configurations
of the new stereogenic center, respectively (Table 3, entries 1
and 2). The stereocontrol was dependent on the chirality of the
substituent in the R3 position, and it was not influenced by
the R1 substituent. Our results agree with those of previous
studies; there is precedent in the literature for complete stereo-
selectivity in syntheses via N-acyl-N-aryliminium ion cyclization−
nucleophilic addition reactions.28−30,42−44
Assignment of Configuration. The 10a-H protons of
compounds 9 were observed as a doublet of doublets at 5.37−
6.12 ppm in the 1H NMR spectra, which is a diagnostic signal for
a fused ring system. Accordingly, the 13C NMR spectra revealed
the corresponding fusion carbon C10a at 73.6−77.0 ppm.
The assignment of the absolute configuration of the new
stereogenic center in the Ala derivatives of 9 was based on
1
the NOE effect observed in their H NOESY 1D spectra.
The C3 chiral carbon was introduced by the (S)- or (R)-2-
bromopropionic acid; thus, its configuration was known. The
observed NOE correlations displayed the same relative configura-
tion for 3-CH3 and 10a-H in both pairs of diastereoisomers.
We have described the effects of N-acyl-N-aryliminium ion
cyclization−nucleophilic addition reactions on the stereo-
chemistry of the newly formed stereogenic center. However,
these results do not address the stereochemical integrity of
resin-bound intermediates, particularly the potential epimeriza-
tion of (S)- and (R)-2-bromopropionic acids during the acylation
reaction. Because the fused ring formation is stereoselective, the
epimerization of (S)- and (R)-2-bromopropionic acids would
yield a mixture of (S,S) and (R,R) enantiomers with identical
1
Comparing the H NMR data of each pair of epimers of
benzo[4,5]imidazo[1,2-a]pyrazin-4(1H)-one 9, 10a-H appeared
at a higher chemical shift (0.1−0.44 ppm) in the (R)-epimer
than in the (S)-epimer. Furthermore, the coupling constants
J
10a‑1 were 9−9.5 and 3.5−3.7 Hz in the (R)-epimer, whereas the
a
Scheme 3. Alternate Synthetic Strategy
a
Reagents and conditions: (i) Fmoc-Ala-OH (2 equiv), DIC (1 equiv), THF, 40 °C, 16 h; (ii) 50% piperidine in DMF (v/v), rt, 15 min; (iii) 4-Nos-Cl,
2,6-lutidine, DCM, rt, 16 h; (iv) 0.1 M glycolaldehyde dimethyl acetal, 0.1 M PPh3, anhydrous THF, −20 °C, then 0.1 M DIAD in anhydrous
THF, 40 °C, 16 h.
E
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coupling constants in the (S)-epimer were 7.2−8.6 and 4.0−
4.8 Hz. On the other hand, C10a and C3 appeared at a higher
field (0.8−1.4 ppm) in the (R)-epimers than in the (S)-epimers
in the 13C NMR spectra. These tendencies changed when R1
was Bn 9(6,1,3,1), in which case the signals of the diagnostic
proton and carbon in position 10a appeared at a higher chemical
shift (0.51 ppm in 1H NMR and 0.98 ppm in 13C NMR) in the
(S)-epimer than in the (R)-epimer (Figure 3). These differences
bicycle-Ala-NH2), and hexapeptides (Ac-Gly-Fused bicycle-Ala-
Ala-NH2) in which our fused cycle 9 mimicked two amino acids
of the sequence. The tetrapeptide model 14 was obtained from
acyclic intermediate 13 without any changes in the outcome of
the reaction, as expected. The extension of the peptide chain
toward the amino terminus by one amino acid yielded resin-
bound intermediates 15 and 17. The penta- and hexapeptide
precursors were prepared from the resins 6(2,1,1) and 6(7,1,1),
respectively, via coupling with Fmoc-Ala-OH, followed by
removal of the Fmoc protecting group and further acyla-
tion with Ac2O (Scheme 4). In these cases, the formation of
cyclic iminium is possible in two directions, that is, either
N-aryliminium (red arrow) or N-acyliminium (blue arrow).
After treatment of the penta- and hexapeptide precursors with
50% TFA in DCM, cyclic N-acyliminium was formed exclusively
and led to only one regioisomer, 16 or 18, respectively, which
was isolated and then purified via HPLC. The structures of
the regioisomers were confirmed via NMR. In conclusion, the
N-acyliminium intermediate was exclusively formed in the west-
bound direction, in accordance with our previous observations
of analogous intermediates.28,29 We are currently evaluating
extension of the peptide at the N-terminus after the formation
of the fused ring system 9 on a resin support.
Figure 3. NMR NOEs of 9(R1,R2,2,R4) derivative diastereomers.
CONCLUSION
between epimers were used to tentatively assign the configura-
tion of the remainder of the synthesized compounds. The NMR
spectral data of the diagnostic protons and carbons are tabulated
in the Supporting Information.
Extension of Peptide Backbone. To evaluate the in-
corporation of nitrogenous fused benzimidazolinopiperazinones
9 as peptide backbone constraints (β-turns) during traditional
peptide synthesis, the peptide backbone was extended toward
the amino and carboxy termini. We prepared model systems of
tetra-(Nos-Fused bicycle-Ala-Ala-NH2), penta-(Ac-Gly-Fused
■
We describe the synthesis of a peptide backbone conforma-
tional constraint that contains a privileged structure. Stereo-
selective solid-phase synthesis of benzimidazolinopiperazinones
was performed via N-acyl-N-aryliminium ion cyclization−
nucleophilic addition using commercially available building
blocks. The synthesis proceeded under full stereocontrol of the
newly formed stereogenic center. The effects of the individual
building blocks and reaction conditions on stereochemical
integrity were evaluated. Microwave-assisted synthesis resulted
a
Scheme 4. Incorporation of Bicyclic Constraints 9 during Peptide Synthesis
a
Reagents and conditions: (i) 4-nitrobenzenesulfonyl chloride (4-Nos-Cl), 2,6-lutidine, DCM, rt, 16 h; (ii) Fmoc-Ala-OH (3 equiv),
DIC (1.5 equiv), THF, rt, 16 h; (iii) 50% piperidine in DMF (v/v), rt, 15 min; (iv) Ac2O, rt, 1 h; (v) 50% TFA in DCM, rt, 16 h.
F
dx.doi.org/10.1021/co500023k | ACS Comb. Sci. XXXX, XXX, XXX−XXX

ACS Combinatorial Science
Research Article
in a substantial reduction in reaction times; however, we
observed significant epimerization during microwave-assisted
acylation with (S)- and (R)-2-bromopropionic acids.
ratio of the purified product = 60:40. HPLC: RT = 2.06 min.
1H NMR (400 MHz, DMSO-d6): δ (ppm) R-epimer (60%)
8.42 (d, J = 8.9 Hz, 1H, 4-Nos), 8.18 (d, J = 8.9 Hz, 1H,
4-Nos), 7.73 (d, J = 1.8 Hz, 1H, 6-H), 7.57 (bs, 1H, NH),
7.33−7.26 (m, 2H, 8-H and NH), 6.65 (d, J = 8.2 Hz, 1H,
9-H), 5.70 (dd, J = 9.4, 3.6 Hz, 1H, 10a-H), 4.46 (dd, J = 12.0,
3.6 Hz, 1H, 1-H), 4.32 (d, J = 16.8 Hz, 1H, 3-H), 4.27 (q, J =
7.1 Hz, 1H, α-H-Ala), 3.73 (d, J = 17.0 Hz, 1H, 3-H), 3.43−
3.28 (m, 1H, 1-H), 1.34 (d, J = 7.4 Hz, 3H, CH3-Ala); S-epimer
(40%) 8.39 (d, J = 8.9 Hz, 2H, 4-Nos), 8.14 (d, J = 8.9 Hz, 1H,
4-Nos), 7.71 (d, J = 1.9 Hz, 1H, 6-H), 7.66 (bs, 1H, NH), 7.36
(bs, 1H, NH), 7.32−7.26 (m, 1H, 8-H), 6.73 (d, J = 8.3 Hz,
1H, 9-H), 5.59 (dd, J = 9.2, 3.6 Hz, 1H, 10a-H), 4.36 (dd, J =
9.4, 3.6 Hz, 1H, 1-H), 4.33 (d, J = 7.4 Hz, 1H, α-H-Ala), 4.29
(d, J = 17.0 Hz, 1H, 3-H), 3.80 (d, J = 17.0 Hz, 1H, 3-H),
3.41−3.30 (m, 1H, 1-H), 1.32 (d, J = 7.5 Hz, 3H, CH3-Ala).
13C NMR (100 MHz, DMSO-d6): δ (ppm) R-epimer 172.1 (C,
CONH2-Ala), 161.3 (C, C4), 150.2 (C, 4-Nos), 143.6 (C, C9a),
141.7 (C, 4-Nos), 131.8 (C, C6a), 129.1 (2CH, 4-Nos), 124.8
(2CH, 4-Nos), 124.6 (q, J = 270.8 Hz, C, CF3), 122.9 (q, J =
4.2 Hz, CH, C8), 118.9 (q, J = 32.0 Hz, C, C7), 110.7 (q, J =
3.6 Hz, CH, C6), 108.4 (CH, C9), 75.9 (CH, C10a), 53.1 (CH,
Cα-Ala), 47.8 (CH2, C3), 47.0 (CH2, C1), 12.8 (CH3, CH3−Ala);
S-epimer 172.2 (C, CONH2-Ala), 161.4 (C, C4), 150.1 (C, 4-
Nos), 144.4 (C, C9a), 141.9 (C, 4-Nos), 130.9 (C, C6a), 129.0
(2CH, 4-Nos), 124.8 (2CH, 4-Nos), 124.6 (q, J = 270.5 Hz, C,
CF3), 122.9 (q, J = 4.4 Hz, CH, C8), 118.5 (q, J = 32.0 Hz, C,
C7), 110.5 (q, J = 3.8 Hz, CH, C6), 107.0 (CH, C9), 74.8
(CH, C10a), 53.7 (CH, Cα-Ala), 47.8 (CH2, C3), 46.9 (CH2,
C1), 13.3 (CH3, CH3-Ala). HRMS (ESI-TOF) m/z calcd for
C20H19F3N5O6S [M + H]+ 514.1003, found 514.1008.
EXPERIMENTAL PROCEDURES
■
The solid-phase syntheses were performed in plastic reaction
vessels (syringes, each equipped with a porous disc) using a
manually operated synthesizer.45 The volume of the wash
solvent was 10 mL per 1 g of resin. For washing, the resin slurry
was shaken with fresh solvent for at least 1 min before changing
the solvent. Commercially available Rink resin (100−200 mesh,
0.66 mmol/g) and Wang resin (100−200 mesh, 1.0 mmol/g)
were used. The yields of the crude products were calculated
with respect to the loading of the first building block. The
conventional reaction conditions for the individual steps of the
synthesis have been reported in previous communications.28−31
The microwave-assisted synthesis was performed in a CEM
Focused Microwave Synthesis System, Model Discover, at
100 W; the reaction temperatures and times are summarized in
Table 1.
Analytical Data of Representative Compounds.
(10aRS)-2-(2-((4-Nitrophenyl)sulfonyl)-4-oxo-7-(trifluoro-
methyl)-1,3,4,10a-tetrahydrobenzo[4,5]imidazo[1,2-a]-
piprazin-10(2H)-yl)acetamide 9(1,1,1,1).
ASSOCIATED CONTENT
* Supporting Information
Copies of NMR spectra associated with this article. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
Yield: 4.9 mg (22%) of amorphous solid. Purified by
semipreparative HPLC. HPLC: RT = 1.86 min. H NMR
S
1
(400 MHz, DMSO-d6): δ (ppm) δ 8.41 (d, J = 9.0 Hz, 2H,
4-Nos), 8.15 (d, J = 9.0 Hz, 2H, 4-Nos), 7.69 (d, J = 1.9 Hz, 1H,
6-H), 7.60 (bs, 1H, NH), 7.29 (d, J = 8.1 Hz, 1H, 8-H), 7.28
(s,1H, NH), 6.65 (d, J = 8.2 Hz, 1H, 9-H), 5.55 (dd, J = 9.4,
3.7 Hz, 1H, 10a-H), 4.46 (dd, J = 12.1, 3.7 Hz, 1H, 1-H), 4.30
(d, J = 16.9 Hz, 1H, 3-H), 3.98 (q, J = 17.4 Hz, 2H, Gly), 3.72
(d, J = 16.9 Hz, 1H, 3-H), 3.22 (dd, J = 12.1, 9.5 Hz, 1H, 1-H).
13C NMR (100 MHz, DMSO-d6): δ (ppm) 169.7 (C, CONH2-
Ala), 161.2 (C, C4), 150.2 (C, 4-Nos), 145.7 (C, C9a), 141.6
(C, 4-Nos), 130.9 (C, C6a), 129.1 (2CH, 4-Nos), 124.8 (2CH,
4-Nos), 123.7 (m, CH, C8), 110.7 (m, CH, C6), 106.2 (CH,
C9), 77.0 (CH, C10a), 47.9 (CH2, Cα-Gly), 46.6 (CH2, C3),
45.9 (CH2, C1). HRMS (ESI-TOF) m/z calcd for
C19H16F3N5NaO6S [M + Na]+: 522.0666, found 522.0662.
(S)-2-((RS)-2-((4-Nitrophenyl)sulfonyl)-4-oxo-7-(trifluoro-
methyl)-1,3,4,10a-tetrahydrobenzo[4,5]imidazo[1,2-a]-
pyrazin-10(2H)-yl)propanamide 9(2,1,1,1).
AUTHOR INFORMATION
Corresponding Author
*E-mail: vkrchnak@nd.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Department of Chemistry
and Biochemistry, University of Notre Dame and by the
projects P207/12/0473 from the Czech Science Foundation
(GACR) and the project CZ.1.07/2.3.00/30.0060 supported by
the European Social Fund. We gratefully appreciate the use of
the NMR facility at the University of Notre Dame.
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