Solid-phase synthesis of trisubstituted benzo[1,4]-diazepin-5-one derivatives

Article abstract of DOI:10.1021/co300124q

Solid-phase synthesis of 3,4-dihydro-benzo[e][1,4]diazepin-5-ones with three diversity positions is described. Various primary amines were used as the starting material and immobilized on the polystyrene resin equipped with different acid-labile linkers.

Full text of DOI:10.1021/co300124q

Research Article
pubs.acs.org/acscombsci
Solid-Phase Synthesis of Trisubstituted Benzo[1,4]-Diazepin-5-one
Derivatives
Veronika Fulopova,† Tomas Gucky,‡ Martin Grepl,§ and Miroslav Soural*,†
́
́
̌
́
̈
̈
†Department of Organic Chemistry, Institute of Molecular and Translational Medicine, Faculty of Science, Palacky University,
771 46 Olomouc, Czech Republic
‡Centre of the Region Hana for Biotechnological and Agricultural Research, Department of Growth Regulators, Faculty of Science,
̌
Palacky University, Slechtitelu 11, 783 71 Olomouc, Czech Republic
̊
§Farmak a.s., 771 17 Olomouc, Czech Republic
S
* Supporting Information
ABSTRACT: Solid-phase synthesis of 3,4-dihydro-benzo[e]-
[1,4]diazepin-5-ones with three diversity positions is described.
Various primary amines were used as the starting material and
immobilized on the polystyrene resin equipped with different
acid-labile linkers. Polymer-supported amines were converted to
α-aminoketones with the use of their sulfonylation with the 4-nitrobenzensulfonylchoride (4-Nos-Cl) and subsequent alkylation with
α-bromoketones. After the cleavage of the 4-Nos group, the corresponding α-aminoketones were acylated with various o-nitrobenzoic
acids. Reduction of the nitro group followed by spontaneous on-resin ring closure gave the target immobilized benzodiazepines. After
acid-mediated cleavage the products were obtained in very good crude purity and satisfactory yields, which makes the developed
method applicable for simple library synthesis of the corresponding derivatives in a combinatorial fashion.
KEYWORDS: benzodiazepines, solid-phase synthesis, α-aminoketones, nitrobenzoic acids, haloketones
recently used with slight modification for the preparation of
benzodiazepine β-turn mimetics.14
INTRODUCTION
■
In the entire history of 1,4-benzodiazepine scaffold containing
substances, 5-substituted-1,3-dihydro-benzo[e][1,4]diazepin-2-ones
I have been studied most extensively particularly because of their
influence on a central nervous system (CNS).1,2 The most frequently
observed effects which resulted in an introduction of more than 30
marketed benzodiazepine drugs3 are sedative-hypnotic,4 anxiolytic,5
muscle relaxant6 and anticonvulsant7 activities. The commercial
success of benzodiazepine drugs (such as clonazepam, diazepam,
bromazepam, or flunitrazepam, see Figure 1) caused exhaustive
research in this area and preparation of large number of
benzodiazepine derivatives have been described. In contrast,
structurally isomeric 2-phenyl-3,4-dihydro-benzo[e][1,4]diazepin-5-
ones II have been studied rarely and only a few articles dedicated to
the preparation and properties of such compounds have been
published.
Until now three general methods have been developed for the
preparation of 2-phenyl-3,4-dihydro-benzo[e][1,4]diazepin-5-ones
II and all of them take advantage of traditional solution-phase
synthesis. The oldest approach is based on the cyclization of
phenacyl anthranilamides which can be easily obtained by the
reaction of isatoic anhydride and α-aminoacetophenones.8−11 The
second method is based on the ring closure of β-ketoesters with
o-phenylendiamine and it has been used for the preparation of
derivatives which have been studied for their platelet-activating
factor (PAF) antagonist activity.12 The latest synthetic approach
takes advantage of the multicomponent Ugi reaction with use of
isocyanide chemistry.13 The last mentioned method has been
In our current research, we have been focused on an extension
of the method that uses α-aminoacetophenones as key synthons.
Preparation of such compounds in solution is well-known, the
traditional approach is based on the reaction of bromoacetophe-
nones with sodium azide15,16 or urotropine.17 Quite recently an
efficient method involving solid-support chemistry dealing with
the synthesis of α-aminoketones and their use for the diversity-
oriented synthesis of various nitrogenous heterocycles has been
developed.18,19
This article describes another application of polymer-supported
α-aminoketones for the preparation of 2-phenyl-3,4-dihydro-
benzo[e][1,4]diazepin-5-ones from commercially available build-
ing blocks. The solid-phase synthesis concept has been used to
introduce the methodology applicable for the future preparation of
chemical library and subsequent structure−activity relationship
studies of the target substances II that contain three diversity
positions: (i) substitution at position 2, (ii) substitution of the
nitrogen atom at position 4, (iii) substitution of a benzene ring of
the benzodiazepine scaffold.
RESULTS AND DISCUSSION
■
The key building blocks for the preparation of the target substances
were primary amines, α-bromoketones and o-nitrobenzoic acids.
Received: August 24, 2012
Revised: November 8, 2012
Published: November 12, 2012
© 2012 American Chemical Society
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Figure 1. General structure of target substances (II) and benzodiazepine drugs (I).
a
Scheme 1. General Synthetic Route Leading to the Target Benzodiazepines
a
Reagents: (i) 4-nitrobenzensulfonyl chloride, 2,6-lutidine, DCM, rt, 16 h; (ii) bromoketone, DIEA DMF, rt, 16 h; (iii) 2-mercaptoethanol, DBU, rt,
10 min; (iv) o-nitrobenzoic acids, DIC, DMF, rt, 16 h; (v) SnCl2·2H2O, DIEA, deoxygenated DMF, rt, 16 h (repeated); (vi) 50% TFA in DCM, rt, 30 min.
A general synthetic route leading to target compounds is described
in Scheme 1.
Alkylation with bromoketones 1−6 afforded sulfonamides
3{3, 1−6} with excellent purity (more than 90%, UPLC-UV
traces). A different result was obtained when chloroacetone 7 was
used. Alkylation with this agent gave the desired intermediate in
limited purity (up to 75%). First, we tried to optimize the reaction
conditions with the use of different solvents, bases and temperature
(see Table 2) but we did not manage to increase the overall purity.
Additionaly, repetition of reaction conditions was tested but the
purity decreased due to secondary products formation.
To expand the diversity of R1 position as much as possible,
starting amines of various structures were attached to the
polystyrene resin via suitable acid-labile linkers (Scheme 2). To
introduce an aliphatic chain with a terminal hydroxy group, amino
group or carboxy group respectively, Wang resin was used and
Fmoc-aminoethanol 1{1}, propylendiamine 1{2} and Fmoc-β-Ala
1{3} were immobilized. To introduce an aliphatic ligand with the
terminal unsubstituted carboxamide group, Rink amide resin was
used and acylated with Fmoc-β-Ala-OH to give aminoderivative
1{4}. To include N-substituted carboxamide the aminomethylated
polystyrene resin equipped with backbone amide linker (BAL)
was reductively aminated with two model amines (propylamine
and benzylamine) which were subsequently acylated with Fmoc-β-
Ala-OH to give intermediates 1{5} and 1{6}.
Denosylation of intermediates 3{R1,R2} afforded the corre-
sponding aminoketones 4{R1,R2} that were acylated with
o-nitrobenzoic acid. It should be noted that denosylation of
intermediate 3{6,1} required a significantly longer reaction time
(60 min instead of typical 10 min procedure). After the acylation
step, we observed formation of a side product, which was
identified with the use of UPLC-UV-MS as the dealkylated
byproduct 5-D{R1,-,1}. Further investigation (cleavage of resins
5{R1,R2,1} was performed for various time periods and identical
mixtures of compounds were obtained) showed that the side
product was not formed during the cleavage of intermediates 5
from the resin but during the acylation of intermediates 4{R1,R2}
with o-nitrobenzoic acid.
Following the previously described procedure19 the amines
1{R1} were transformed to the corresponding α-aminoketones
4{R1,R2}. Surprisingly, sulfonylation of aminoderivatives with 4-
nitrobenzensulfonyl chloride was not quantitative in most cases
(resins 1{1} and 1{3−5}) and the reaction had to be repeated
for completion. For verification of the subsequent alkylation we
used compound 2{3} and five aromatic bromoketones substituted
with electron-withdrawing as well as electron-donating groups
were tested. Also one heterocyclic and one aliphatic haloketone
was included (see Figure 2).
In most of the tested cases, the dealkylation did not decrease
the overall purity significantly and the intermediates 5{R1,R2,R3}
were obtained in a sufficient purity ranging from 72 to 93% (UPLC-
UV-traces). However, in the case of the intermediate 3{3,7}
prepared from chloroacetone, we obtained only a mixture of
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a
Scheme 2. Preparation of Immobilized Amines 1{R1}(for Resulting R1 See Also Table 3)
a
Reagents: (i) (a) CDI, pyridine, DCM, rt, 3 h, (b) ethylenediamine, DCM, rt, 3 h; (ii) (a) trichloroacetonitrile, DBU, anhydrous DCM, rt, 1 h, (b)
2-(Fmoc-amino)ethanol, boron trifluoride diethyl etherate, anhydrous THF, rt, 30 min; (iii) 50% piperidine in DMF, rt, 10 min; (iv) Fmoc-β-Ala-
OH, TPP, DIAD, anhydrous THF, rt, 5 h; (v) Fmoc-β-Ala-OH, HOBt, DIC, DMF, DCM, rt, 16 h; (vi) (a) amine, 10% acetic acid in anhydrous
DMF, rt, 16 h, (b) sodium triacetoxyborohydride, 10% acetic acid in anhydrous DMF, rt, 4 h, (c) 20% piperidine in DMF, rt, 10 min.
Figure 2. List of haloketones tested for R2 substitution.
Table 1. Various Reaction Conditions for the Preparation of
Intermediate 3{3,7}
Scheme 3. Dealkylation during Acylation with
o-Nitrobenzoic Acid
a
a
reaction conditions
DIEA, DMF, rt, 16 h
purity (%)
49
70
75
46
70
DIEA, DMF, rt, 16 h, repeated
DIEA, DMF, 40 °C, 16 h
DIEA, DMF, 40 °C, 16 h, repeated
proton sponge, DMF, rt, 16 h
DBU, DMF, rt, 16 h
a
Reagents: (i) o-nitrobenzoic acid, DIC, DMF, rt, 16 h.
mixture of compounds
mixture of compounds
DIEA, THF, rt, 16 h
a
compounds without the corresponding product. The use of
intermediate prepared from 3-bromo-2-bromoacetophenone led
Calculated from HPLC-UV traces (PDA 200−600 nm) after cleavage
from the polymer support.
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to quantitative dealkylation, intermediate prepared from 4-fluoro-
2-bromoacetophenone dealkylated from 60% so the both building
with the use of a symmetrical anhydride prepared in situ from the
corresponding o-nitrobenzoic acids in N,N-dimethylformamide.
To introduce the third diversity position various o-nitro-
benzoic acids were used (see Table 3) to acylate intermediates
4{R1,R2}. After subsequent reduction of the nitrogroup and
cleavage of the resulting material from the resin we did not
detect the linear intermediates 6{R1,R2,R3} but only the final
products 7{R1,R2,R3}. When the sample of the resin 6{1,1,1}
was treated with Fmoc-Cl and cleaved, the corresponding
N-Fmoc intermediate was not detected which indicates the ring
closure took place on-resin after reduction of the nitro group.
After the reduction step, we detected the appearance of a side
product in each case (10−30%, UPLC-UV traces). From
UPLC-MS traces, we have concluded that the structure of the
́
blocks were excluded. To suppress the side reaction we tested the
acylation of intermediate 4{1,1} with alternative agents (such as
isatoic anhydride or anthranilic acid) and species (HOBt ester,
symmetrical anhydride) but the purity of intermediates 5 was still
usually decreased (see Table 2). The best results were obtained
Table 2. Summary of Alkylated/Dealkylated Product Ratio
with Use of Intermediate 4{3,1} and Different Acylating
Methods and Species
alkylated
dealkylated
a
a
reagents
(%)
(%)
Scheme 4. Side Product Formation after First-Round
Reduction of Precursors 5
o-nitrobenzoic acid, DIC, HOBt, DMF/
64
30
a
DCM
o-nitrobenzoic acid, DIC, DMF
93
0
6
21
31
33
2
isatoic anhydride, DIPEA, DMF
anthranilic acid, DIC, HOBt, DMF/DCM
3-nitropthalic anhydride, DIEA, THF/dry
13
52
85
3-nitropyridine-2-carboxylic acid, DIC,
DMF
4-bromo-2-nitrobenzoic acid, DIC, DMF
4-methyl-2-nitrobenzoic acid, DIC, DMF
5-methoxy-2-nitrobenzoic acid, DIC, DMF
82
72
87
13
20
12
a
a
Reagents: (i) SnCl2·2H2O, DIEA, deoxygenated DMF, rt, 16 h; (ii)
50% TFA in DCM, rt, 30 min.
Calculated from HPLC-UV traces (PDA 200−600 nm) after the
cleavage from the polymer support.
Table 3. List of Final Compounds
*
Calculated from HPLC-UV traces (PDA 200−600 nm), NI = not isolated due to the decomposition during semiprep. HPLC isolation.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Details of experimental synthetic and screening procedures and
spectroscopic data for synthesized compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 3. Two possible tautomeric forms of the final products.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: +420 585634418. Fax: +420 585634465. E-mail:
soural@orgchem.upol.cz.
Notes
The authors declare no competing financial interest.
Figure 4. List of o-nitrobenzoic acids successfully used for the verification
of the synthetic route.
ACKNOWLEDGMENTS
■
The authors are grateful to project CZ.1.07/2.3.00/20.0009
coming from European Social Fund. The infrastructural part of
this project (Institute of Molecular and Translational Medicine)
was supported from the Operational Program Research and
Development for Innovations (project CZ.1.05/2.1.00/
01.0030).
side products corresponds to N-hydroxyderivatives 8{R1,R2,R3}
formed after incomplete reduction of the nitrogroup to
hydroxylamine derivative. After repeating the reduction step,
the side products 8{R1,R2,R3} were not detected, which is in
accordance with the theory of hydroxylamine intermediate
formation.
REFERENCES
The final compounds 7{R1,R2,R3} were generally obtained in
very good crude purity (see Table 3), and their final
purification was achieved by the use of flash chromatography
on reversed phase C18 cartridges and subsequent reverse phase
semipreparative HPLC. The use C18 cartridges was necessary to
remove tin(II) and tin(IV) salts otherwise HPLC column was
clogged during purification. During the isolation process, an
unexpected instability of amino group containing derivatives
7{1,R2,R3} was observed. Because of their decomposition, such
substances have not been isolated in a pure form. The structure
of the final products was confirmed with the help of 1H and 13C
NMR spectrometry and HRMS.
■
(1) Sternbach, L. H. The benzodiazepine story. J. Med. Chem. 1979,
22 (1), 1−7.
(2) Mohler, L. H.; Fritschy, J. M.; Rudolph, U. A new benzodiazepine
pharmacology. J. Pharmacol. Exp. Ther. 2002, 300 (1), 2−8.
(3) THER-20: Anxiolytic and THER-27: Sedative/hypnotic. The
Merck Index, 14th ed.; Merck: Whitehouse Station, NJ, 2007.
(4) McKernan, R. M.; Roosahl, T. W.; Reynolds, D. S.; Sur, C.;
Wafford, K. A.; Atack, J. R.; Farrar, S.; Myers, J.; Cook, G.; Ferris, P.;
Garrett, L.; Bristow, L.; Marshall, G; Macaulay, A.; Brown, N.; Howell,
O.; Moore, K. W.; Carling, R. W.; Street, L. J.; Castro, J. L.; Ragan, C.
I.; Dawson, G. R.; Whiting, P. J. Sedative but not anxiolytic properties
of benzodiazepines are mediated by the GABAA receptor_1 subtype.
Nat. Neurosci. 2000, 3, 587−592.
We also investigated the tautomerism of prepared 1,4-benzo-
diazepine-5-ones since at least two possible tautomeric forms (7A)
and (7B) have to be considered. We observed a broad singlet at
around 4.20 ppm in the 1H NMR spectra of the studied
compounds which corresponds to a methylene group of tautomeric
form (7A). The tautomers (7A) seem to be the only present form
of the studied compounds under the experimental conditions used.
(5) Low, K.; Crestani, F.; Keist, R.; Benke, D.; Brunig, I.; Benson, J.
̈
̋
A.; Fritschy, J. M.; Rulicke, T.; Bluethmann, H.; Mohler, H.; Rudolph,
̋
̈
U. Molecular and neuronal substrate for the selective attenuation of
anxiety. Science 2000, 290, 131−134.
(6) Crestani, F.; Low, K.; Keist, R.; Mandelli, M.; Mohler, H.;
̈
̈
Rudolph, U. Molecular targets for the myorelaxant action of diazepam.
Mol. Pharmacol. 2001, 59, 442−445.
(7) Rudolph, U.; Crestani, F.; Benke, D.; Brunig, I.; Benson, J. A.;
Fritschy, J. M.; Martin, J. R.; Bluethmann, H.; Mohler, H.
Benzodiazepine actions mediated by specific γ-aminobutyric acid A
receptor subtypes. Nature 1999, 401, 796−800.
(8) Santilli, A. A.; Osdene, T. S. 5H-1,4-Benzodiazepin-5-ones. Ring-
closure reactions of substituted 2-aminobenzamides. J. Org. Chem.
1964, 29, 1998−2003.
(9) Breslin, H. J.; Kukla, M. J.; Ludovici, D. W.; Mohrbacher, R.; Ho,
W.; Miranda, M.; Rodgers, J. D.; Hitchens, T. K.; Leo, G.; Gauthier, D.
A.; Ho, Ch. Y.; Scott, M. K.; De Clercq, E.; Pauwels, R.; Andries, K.;
Janssen, A. C.; Janssen, P. A. J. Synthesis and anti-HIV-1 activity of
4,5,6,7-tetrahydro-5-methylimidazo [4,5,1-jk][1,4]benzodiazepin-
2(1H)-one (TIBO) derivatives. J. Med. Chem. 1995, 38, 771−793.
(10) Hradil, P.; Grepl, M.; Hlavac, J.; Soural, M.; Malon, M.;
Bertolasi, V. Some new routes for the preparation of 3-amino-2-
phenyl-4(1H)-quinolinones from anthranilamides. J. Org. Chem. 2006,
71 (2), 819−822.
(11) Weber, K. H. Synthese und eigenschaften von 4-alkyl-2-phenyl-
3,4-dihydro-5H-1,4-benzodiazepin-5-onen. Arch. Pharm. 1969, 302
(8), 584−590.
(12) Fray, M. J.; Cooper, K.; Parry, M. J.; Richardson, K.; Steelet, J.
Novel antagonists of platelet-activating factor. 1. Synthesis and
structure−activity relationships of benzodiazepine and benzazepine
1
We have proven this suggestion with the help of H−1H COSY
̋
1
and H−13C edited HSQC experiments in the case of compound
̈
7{5,1,1}. The formation of these kind of tautomers is in accordance
with previously studied derivatives.9,10
In conclusion, we have developed a simple method for the
preparation of novel trisubstituted 3,4-dihydro-benzo[e][1,4]-
diazepin-5-ones with the use of solid-phase based synthesis. Various
building blocks have been successfully tested and many others are
commercially available. Final compounds were obtained in very
good crude purity containing usually only one side product
separable with the use of reverse phase chromatography. On the
other hand, some limitations were observed: (i) the reaction
pathway seems not to be well applicable for bromoketones
substituted only with electron withdrawing ligands due to a
formation of significant side-products, (ii) amino group
containing ligands in position 4 of the benzodiazepinone scaffold
exhibited unexpected unstability and decomposed during
purification procedure. Despite this fact the described chemistry
is still applicable for (semi)automated combinatorial synthesis of
chemical library to produce novel derivatives for biological
screening.
655
dx.doi.org/10.1021/co300124q | ACS Comb. Sci. 2012, 14, 651−656

ACS Combinatorial Science
Research Article
derivatives of 2-methyl-1-phenylimidazo[4,5-c]pyridine. J. Med. Chem.
1995, 38, 3514−3523.
(13) Marcaccini, S.; Miliciani, M.; Pepino, R. A facile synthesis of 1,4-
benzodiazepine derivatives via Ugi four-component condensation.
Tetrahedron Lett. 2005, 46, 711−713.
(14) Sannudo, M.; Garcıía-Valverde, M.; Marcaccini, S.; Delgado, J.
J.; Rojo, J.; Torroba, T. Synthesis of benzodiazepine β-turn mimetics
by an Ugi 4CC/Staudinger/aza-Wittig sequence. Solving the
conformational behavior of the Ugi 4CC Adducts. J. Org. Chem.
2009, 74, 2189−2192.
(15) Ackrell, J.; Muchowski, J. M. Alkylation of α-formamido ketone
enolate anions. A versatile synthesis of α-alkyl α-amino ketones. J. Org.
Chem. 1986, 51 (17), 3374−3376.
(16) Widler, L.; Green, J.; Missbach, M.; Susa, M.; Altmann, E.
Pyrrolidinyl- and 7-piperidinyl-5-aryl-pyrrolo[2,3-d]pyrimidinesPo-
tent inhibitors of the tyrosine kinase c-Src. Bioorg. Med. Chem. Lett.
2001, 11, 849−852.
(17) Bretschneider, H.; Hormann, H. ω-Azidoacetophenone und ihre
̈
katalytische reduktion. Monatsh. Chem. 1953, 84 (5), 1021−1032.
(18) Pudelova, N.; Krchnak, V. Efficient traceless solid-phase
synthesis of 3,4-dihydropyrazino[1,2-b]indazoles and their 6-oxides.
J. Comb. Chem. 2009, 11 (5), 851−859.
(19) Koci, J.; Pudelova, N.; Krchnak, V. Polymer-supported α-
acylamino ketones: Preparation and application in syntheses of 1,2,4-
trisubstituted-1H-imidazoles. J. Comb. Chem. 2009, 11 (3), 397−402.
656
dx.doi.org/10.1021/co300124q | ACS Comb. Sci. 2012, 14, 651−656