Piperazine amide linker for cyclative cleavage from solid support: Traceless synthesis of dihydroquinoxalin-2-ones

Article abstract of DOI:10.1021/co300023b

A piperazine amide linker for cyclative cleavage from solid support and its use in the traceless solid-phase synthesis of dihydroquinoxalinones are described. Piperazine was attached to Wang resin via a carbamate linkage and acylated with Fmoc-amino acids. Following Fmoc group removal, resin-bound amines were reacted with 1-fluoro-2-nitrobenzenes. The nitro group of the resulting 2-nitroanilines was reduced, and acyclic precursors, in contrast to traditional ester-type linkage, remained attached to the resin. Target dihydroquinoxalinones were obtained either by acid- or microwave-mediated cyclative cleavage. The synthesis provided crude compounds of high purity and enabled the preparation of stable immobilized linear intermediates. The linker is suitable for combinatorial synthesis of compound libraries.

Full text of DOI:10.1021/co300023b

Letter
pubs.acs.org/acscombsci
Piperazine Amide Linker for Cyclative Cleavage from Solid Support:
Traceless Synthesis of Dihydroquinoxalin-2-ones
Cleopatra Neagoie† and Viktor Krchnak*,†,‡
̌
́
†Department of Organic Chemistry, Faculty of Science, Palacky University, 17. Listopadu 12, 771 46 Olomouc, Czech Republic
́
‡Department of Chemistry and Biochemistry, 251 Nieuwland Science Center, University of Notre Dame, Notre Dame, Indiana
46556, United States
S
* Supporting Information
ABSTRACT: A piperazine amide linker for cyclative cleavage
from solid support and its use in the traceless solid-phase
synthesis of dihydroquinoxalinones are described. Piperazine
was attached to Wang resin via a carbamate linkage and
acylated with Fmoc-amino acids. Following Fmoc group
removal, resin-bound amines were reacted with 1-fluoro-2-
nitrobenzenes. The nitro group of the resulting 2-nitroanilines
was reduced, and acyclic precursors, in contrast to traditional
ester-type linkage, remained attached to the resin. Target
dihydroquinoxalinones were obtained either by acid- or microwave-mediated cyclative cleavage. The synthesis provided crude
compounds of high purity and enabled the preparation of stable immobilized linear intermediates. The linker is suitable for
combinatorial synthesis of compound libraries.
KEYWORDS: solid-phase synthesis, cyclative cleavage, linker, piperazine, dihydroquinoxalinone
Cyclative cleavage is a very effective methodology for the
synthesis of amide-containing heterocycles in the solid phase
(for a review, see ref 1). Typically, a nucleophile of an acyclic
precursor attacks an electrophilic carboxylate immobilized on a
solid support. Esters of the Merrifield resin are the most often
used precursors for cyclative cleavage (Scheme 1). This
synthetic strategy was used to prepare various five-, six-, and
seven-membered amide-containing heterocycles.1
Solid-phase syntheses of quinoxalinones have been reported
several times. The key intermediate for the synthesis was
frequently 1-fluoro-2-nitrobenzene.2−7 A synthetic route of
choice is ring closure by nucleophilic displacement of an ester
by aniline nitrogen of alkyl 2-((2-aminophenyl)amino) acetate.
Rink amide resin was acylated with 4-fluoro-3-nitrobenzoic acid
and the resin-bound fluorine was displaced using nucleophilic
substitution with amino acid esters. After reduction of the nitro
group, the six-membered ring was spontaneously closed. 4-
Fluoro-3-nitrobenzoic acid also was the key component in the
synthesis of a library of dihydroimidazolyl dihydroquinoxali-
nones.5 An analogous reaction sequence was reported with an
α-amino acid esterified to the Merrifield resin and 1-fluoro-2-
nitrobenzene in solution. After reduction of the nitro group, the
product was spontaneously released from the resin.6 Traceless
synthesis of quinoxalinones on BAL resin also was reported.7
All the above-mentioned six-membered ring closures were
between aniline nitrogen and carboxylate. The drawback of the
ester route is the spontaneous cyclization of alkyl 2-((2-
aminophenyl)amino)acetates to quinoxalinones, thus prevent-
ing further modification of alkyl 2-((2-aminophenyl)amino)-
acetates before cyclization.
We have recently observed that secondary amides can
perform in an analogous route. In this paper, we report the
expeditious synthesis of quinaxolinones by cyclative cleavage
from a tertiary amide rather than ester (Scheme 1). This
alternative allows the preparation of stable immobilized linear
intermediates and their cyclization triggered by specific reaction
conditions.
Scheme 1. Quinaxolinones by Cyclative Cleavage from an
Ester and Amide
To evaluate the tendency of an amide for cyclative cleavage
via amide nucleophilic displacement, we prepared three model
compounds (Figure 1) that differ by the type of amide. We
Received: February 21, 2012
Revised: May 9, 2012
Published: June 8, 2012
© 2012 American Chemical Society
399
dx.doi.org/10.1021/co300023b | ACS Comb. Sci. 2012, 14, 399−402

ACS Combinatorial Science
Letter
Figure 1. Model compounds designed for amide cyclative cleavage.
examined one secondary amide synthesized using Fmoc-β-Ala-
OH and two tertiary amides prepared from Fmoc-Pro-OH and
piperazine. Model compounds 1 and 2 containing Fmoc-β-Ala-
OH and Fmoc-Pro-OH were prepared on Rink amide resin; the
piperazine-containing compounds 3 were made on Wang resin
1 via carbamate linkage (Scheme 2). The starting amines 6
Scheme 2. Solid-Phase Synthesis of Dihydroquinoxalinones
4 via Acid-Mediated Cyclative Cleavage
Figure 2. Building blocks used for preparation of dihydroquinox-
alinones.
a
limitations of this synthetic route and prepared a set of
compounds with various R1 and R2 substitutions. The building
blocks used for the synthesis are portrayed in Figure 2.
From the preparative point of view, the disadvantage of the
acid-mediated cleavage is the presence of piperazine in the
cleaved sample. To eliminate its presence, we attempted
cyclization at elevated temperature to avoid the acidic
treatment that released piperazine from the resin and
contaminated the cyclized products.
We evaluated the cyclative cleavage of model compound
3(1,1) in polar solvents, such as DMF and DMSO, at an
elevated temperature using conventional as well as microwave
heating. The results of our experiments are summarized in
Table 1. Conventional heating required 72 h to obtain 89%
a
Reagents and conditions: (i) CDI, pyridine, DCM, rt, 3 h, then
piperazine, DCM, rt, 16 h; (ii) Fmoc amino acid, HOBt, DIC, DMF/
DCM, rt, 16 h; (iii) 50% piperidine, DMF, rt, 20 min; (iv) 1-fluoro-2-
nitrobenzene, DIEA, DMSO, see experimental procdures in
Supporting Information for time and temperature; (v) SnCl2·2H2O,
DIEA, DMF, rt, 16 h; (vi) 50% TFA, DCM, rt, 1 h.
Table 1. Reaction Conditions for Cyclative Cleavage
a
entry temperature
time
16 h
heating
solvent
conversion
1
2
3
4
5
6
7
8
80 °C
conventional
conventional
microwave
microwave
microwave
microwave
microwave
microwave
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
53%
89%
50%
75%
53%
75%
70%
88%
100 °C
100 °C
100 °C
120 °C
150 °C
150 °C
150 °C
72 h
5 min
60 min
5 min
5 min
60 min
5 min
were acylated with Fmoc-Phe-OH (R1 = Bn), the Fmoc group
was cleaved with piperidine, and the resin-bound intermediates
were reacted with 4-fluoro-3-nitrobenzotrifluoride (R2 = CF3).
After reduction of the nitro group with tin(II) chloride
dihydrate, the linear precursor 1−3 remained attached to the
resin, and the target compounds 4 were released from the resin
with 50% trifluoroacetic acid (TFA) in dichloromethane
(DCM).
The results indicated that piperazine model compound
3(1,1) (for building block numbering, cf. Figure 2) provided
complete conversion to the target dihydroquinoxalinone
4(1,1), whereas β-Ala and Pro-derived linear precursors
1(1,1) and 2(1,1) afforded the dihydroquinoxalinone 4(1,1)
in only 45% and 30% conversion, respectively. Encouraged by
the conversion of linear precursor 3(1,1) to dihydroquinox-
alinone 4(1,1), we decided to evaluate the scope and
a
Conversion is a relative amount of product released from the resin.
conversion of the acyclic precursor (entry 2). Microwave
heating at the same temperature (100 °C) afforded respectable
conversion (50%) after 5 min (entry 3). Increasing the
temperature to 150 °C provided 75% conversion after the
same time, but further prolonging the reaction time was not
beneficial. However, changing the solvent from DMF to DMSO
increased the yield to 88% (entry 8).
The set of our amino acids also contained those that have
functional groups in the side chain protected by the t-Bu group
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ACS Combinatorial Science
Letter
Table 2. Summary of Synthesized Compounds 4, 11, and 12
a
b
entry
R1
R2
cyclative cleavage
purity [%]
MS ESI−
yield [%]
4(1,1)
4(2,1)
4(2,3)
4(3,1)
4(4,1)
4(5,1)
11(7,1)
12(6,1)
12(6,2)
12(6,3)
12(6,4)
Bn
6-CF3
6-CF3
H
DMSO microwave
>99
>99
>99
>99
90
305
229
161
269
245
321
255
269
269
201
47
45
58
18
40
30
27
35
28
59
30
CH3
DMSO microwave
CH3
DMSO microwave
iBu
6-CF3
6-CF3
6-CF3
7-CF3
7-CF3
7,8-diCl
H
DMSO microwave
CH2OH
TFA
TFA
TFA
TFA
TFA
TFA
TFA
p-OH-Bn
75
c
c
c
c
c
90
90
80
90
8-Cl-7-pip
86
319
a
b
c
Purity of crude compounds calculated from LC traces integrated at 210−500 nm. Yield after HPLC purification. Not applicable.
a
Scheme 3. Synthesis of Pyrrolo[1,2-a]quinoxalin-4(5H)-one Derivative 11
a
Reagents and conditions: (i) 50% piperidine, DMF, rt, 20 min; (ii) Fmoc-Pro-OH, HOBt, DIC, DMF/DCM, rt, 16 h; (iii) 4-fluoro-3-
nitrobenzotrifluoride, DIEA, DMSO, rt, 16 h; (iv) SnCl2·2H2O, DIEA, DMF, rt, 16 h; (v) 50% TFA, DCM, rt, 1 h.
(Fmoc-Tyr(tBu)-OH, Fmoc-Ser(tBu)-OH, and Fmoc-Glu-
(OtBu)-OH). In these instances, we used TFA-mediated
cleavage to demask the protected side chain. Table 2 shows a
summary of the results obtained in the TFA-mediated
procedure used for amino acids with side chain protection
and in the microwave-mediated cyclative cleavage that provided
crude products of excellent purity.
to 3,3a-dihydropyrrolo[1,2-a]quinoxaline-1,4(2H,5H)-diones
12 (Scheme 4).
Scheme 4. Synthesis of 3,3a-Dihydropyrrolo[1,2-
a]quinoxaline-1,4(2H,5H)-diones 12
Synthesis on hydroxymethyl polystyrene resin using identical
sequence of reaction steps provided comparable results without
contamination of crude product by piperazine. However,
monitoring of individual steps by TFA cleavage of resin sample
was problematic because of enhanced acid stability of the linker.
Two amino acids used for the synthesis of dihydroquinox-
alinones deserved further attention. We observed that the Pro-
derived intermediate 3(7,1) built on the piperazine linker was
cleaved from the resin during reduction of the nitro group. The
premature cleavage was proven by treatment of a sample of a
resin with Fmoc-OSu and subsequent LC/MS analysis. The
only product was the corresponding Fmoc-piperazine, indicat-
ing that, in this case, the cyclization and cleavage occurred
spontaneously after the reduction of the nitro group. To avoid
the premature loss of the compound, we replaced the
piperazine with Fmoc-β-Ala-OH attached to the Rink resin
(Scheme 3). In this case, the Pro-derived intermediate 1(7,1)
did not undergo the unwanted cyclization after tin(II)-
mediated reduction of the nitro group.
Synthesis of compounds 4(1,1),8 4(2,1),9 5(7,1),10 and
12(6,3)11 using different synthetic strategies already has been
reported.
Reduction of the nitro group in 2-((2-nitrophenyl)amino)-
acetates triggers spontaneous cyclization to dihydroquinox-
alinones.2,3 Thus, when the 2-((2-nitrophenyl)amino)acetate is
attached to the resin via an ester bond and the nitro group is
reduced, the target compound is released from the resin into a
solution containing the reducing agent, typically tin(II) chloride
in DMF. The product has to be isolated from this complex
mixture as reported, for example, in ref 6. On the other hand,
the amide linker allows reduction of the nitro group without
spontaneous release of product. Thus, the resin can be
thoroughly washed after reduction, and the product is released
in a subsequent step. The purity of released target products is
high, and it is comparable for both types of linkers.
We also attempted to prepare dihydroquinoxalinones using
Fmoc-Glu(OtBu)-OH. After the acid-mediated cleavage of
intermediates 3(6,1−4), we observed spontaneous cyclization
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ACS Combinatorial Science
Letter
a
Scheme 5. On-Resin Transformation of Reduced Acyclic Precursor
a
Reagents and conditions: (i) 1-fluoro-2-nitrobenzene, DIEA, DMSO, rt, 2 days; (ii) 50% TFA, DCM, rt, 1 h.
(6) Laborde, E.; Peterson, B. T.; Robinson, L. Traceless, Self-
Cleaving Solid- and Solution-Phase Parallel Synthesis of 3,4,7-
Trisubstituted 3,4- Dihydroquinoxalin-2-ones. J. Comb. Chem. 2001,
3, 572−577.
(7) Krchnak, V.; Szabo, L.; Vagner, J. A solid phase traceless synthesis
of quinoxalinones. Tetrahedron Lett. 2000, 41, 2835−2838.
(8) Sanna, P.; Carta, A.; Loriga, M.; Zanetti, S.; Sechi, L. Preparation
and biological evaluation of 6/7-trifluoromethyl(nitro)-, 6,7-difluoro-3-
alkyl (aryl)-substituted-quinoxalin-2-ones. Part 3. Farmaco 1999, 54
(3), 169−177.
(9) Carta, A.; Loriga, M.; Piras, S.; Paglietti, G.; Ferrone, M.;
Fermeglia, M.; Pricl, S.; La Colla, P.; Collu, G.; Sanna, T.; Loddo, R.
Synthesis and anti-picornaviridae in vitro activity of a new class of
helicase inhibitors the N,N′-bis[4-(1H(2H)-benzotriazol-1(2)-yl)-
phenyl] alkyldicarboxamides. Med. Chem. 2007, 3 (6), 520−532.
(10) Xu, L. T.; Jiang, Y. W.; Ma, D. W. Synthesis of
Tetrahydropyrrolo[1,2-a]quinoxalines and Tetrahydropyrido[1,2-a]-
quinoxalines via a One-Pot CuI-Catalyzed Aryl Amination-Hydrol-
ysis-Condensation Process. Synlett 2010, No. 15, 2285−2288.
(11) TenBrink, R. E.; Im, W. B.; Sethy, V. H.; Tang, A. H.; Carter, D.
B. Antagonist, Partial Agonist, and Full Agonist Imidazo[1,5-
A]Quinoxaline Amides and Carbamates Acting Through the Gaba(A)
Benzodiazepine Receptor. J. Med. Chem. 1994, 37 (6), 758−768.
An additional benefit of the amide linker is potential for on-
resin modification of the linear precursor 3, exemplified by
synthesis of nitro derivative 13. The resin-bound amine 3(R1,4)
was reacted with 1-fluoro-2-nitrobenzene, and the target
compound 14 was released from the resin by acid-mediated
cyclative cleavage (Scheme 5).
In conclusion, a piperazine amide linker for cyclative cleavage
from solid support was developed, and its use was portrayed in
an efficient, traceless solid-phase synthesis of dihydroquinox-
alinones. The key step of this synthetic route is cyclative
cleavage from an amide rather than the typically used ester.
Target dihydroquinoxalinones were not cleaved from the resin
after reduction of the nitro group; they were released by acid-
or microwave-mediated cyclative cleavage. The synthesis
provided crude compounds of high purity and enabled the
preparation of stable immobilized linear intermediates, and
their cyclization was triggered by specific reaction conditions.
ASSOCIATED CONTENT
■
S
* Supporting Information
The details of the experimental procedures, the analytical data
for the synthesized compounds, and copies of the NMR
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
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 of the University of Notre Dame and by the
project CZ.1.07/2.3.00/20.0009 from the European Social
Fund. We gratefully acknowledge the use of the NMR facility at
the University of Notre Dame.
REFERENCES
■
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