Solid-phase synthesis of anagrelide sulfonyl analogues

Article abstract of DOI:10.1021/co400119c

Simple solid-phase synthesis of 3,10-dihydro-2H-benzo[e]imidazo[1,2-b][1,2, 4]thiadiazin-2-one 5,5-dioxides is described, with Fmoc-α-amino acids and 2-nitrobenzenesulfonyl chlorides (2-NosCls) being the key building blocks. Fmoc-α-amino acids were immobi

Full text of DOI:10.1021/co400119c

Research Article
pubs.acs.org/acscombsci
Solid-Phase Synthesis of Anagrelide Sulfonyl Analogues
Claire McMaster,† Veronika Fulopova,† Igor Popa,† Martin Grepl,‡ and Miroslav Soural*,†
́
̈
̈
†Department of Organic Chemistry, Faculty of Science, Institute of Molecular and Translational Medicine, University of Palacky, 771
46 Olomouc, Czech Republic
‡Farmak a.s., 771 17 Olomouc, Czech Republic
S
* Supporting Information
ABSTRACT: Simple solid-phase synthesis of 3,10-dihydro-2H-
benzo[e]imidazo[1,2-b][1,2,4]thiadiazin-2-one 5,5-dioxides is de-
scribed, with Fmoc-α-amino acids and 2-nitrobenzenesulfonyl
chlorides (2-NosCls) being the key building blocks. Fmoc-α-amino
acids were immobilized on Wang resin and transformed to the
corresponding 2-nitrobenzenesulfonamides in two steps. After
reduction of the nitro group, Fmoc-thioureas were synthesized followed by cyclization of the 1,2,4-benzothiadiazine-1,1-dioxide
scaffold with diisopropylcarbodiimide (DIC). Cleavage of the Fmoc protecting group followed by spontaneous cyclative cleavage
gave the target products in excellent crude purity.
KEYWORDS: anagrelide analogues, cyclic guanidines, Fmoc-amino acids, 2-nitrobenzenesulfonyl chlorides, solid-phase synthesis,
cyclative cleavage
Scheme 1. Target Scaffold and Synthesis of Analogueical
Compounds by Saito et al.
INTRODUCTION
■
2-Nitrobenzenesulfonyl chloride (2-NosCl) and 4-nitrobenze-
nesulfonyl chloride (4-NosCl) are frequently used reagents in
organic synthesis. They have typically been used for the
preparation of the corresponding nitrobenzenesulfonamides
which allows monoalkylation of a primary amino group with
use of either alkyl halides or alcohols.1 After alkylation, the
nitrobenzenesulfonyl group can be smoothly removed by
treatment with ethanethiol/DBU cleavage cocktail. In our
previous research, we used this strategy for the preparation of
some benzodiazepinones2 (use of 2-NosCl) or tetrahydroben-
zodiazepinones3 (use of 4-NosCl). Alternatively, 2-NosCl and
4-NosCl can be used as common building blocks for small
molecules synthesis. Quite recently Krchnak et al. reported an
interesting synthetic pathway leading to indazol derivatives via
polymer supported N-(2-nitrobenzenesulfonyl)-amino acids.4,5
Inspired by this contribution we proposed the use of N-(2-
nitrobenzenesulfonyl)-α-amino acids) for the preparation of
3,10-dihydro-2H-benzo[e]imidazo[1,2-b][1,2,4]thiadiazin-2-
one 5,5-dioxides (Figure 1). The target scaffold was initially
synthesized in 2010 by Saito et al. with use of traditional
solution-phase synthesis (Scheme 1).6
esters. While being efficient, this synthetic approach relies on
the somewhat problematic availability of starting azidobenze-
nesulfonyl chlorides, and also relies on purification by silica gel
chromatography throughout the reaction sequence. Biological
properties of 3,10-dihydro-2H-benzo[e]imidazo[1,2-b][1,2,4]-
thiadiazin-2-one 5,5-dioxides have never been reported, but the
structure is very closely related to Anagrelide (Agrylid/
Xagrid),7,8 a drug used for the treatment of essential
thrombocytosis,9,10 overproduction of blood platelets and
chronic meyloid leukemia (Figure 1).11
Because of this similarity, we presumed promising
pharmacological properties of the target compounds and we
focused on the development of a convenient and modular solid-
phase12 method for the preparation of libraries to study
potential structure−activity relationships. To make the method-
ology generally applicable, we used only commercially available
building blocks and common coupling reagents and procedures.
A key element is the cyclative release of the completed products
from the synthesis resin at the last step.
The single published strategy is based on aza-Wittig reaction
using iminophosphoranes as the key synthons, prepared from
2-azidobenzenesulfonyl chlorides and α-amino acids methyl
Received: September 18, 2013
Revised: March 29, 2014
Published: April 11, 2014
Figure 1. General structure of target compounds and Anagrelide.
© 2014 American Chemical Society
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Research Article
a
Scheme 2. Synthetic Pathway Leading to Target Compounds
a
Reagents and conditions: (i) Fmoc-amino acid, DIC, HOBt, DMAP, DCM, DMF, rt, overnight; (ii) piperidine, DMF, rt, 30 min; (iii) 2-
nitrobenzenesulfonyl chloride, 2,6-lutidine, DCM, rt, overnight; (iv) sodium dithionite, potassium carbonate, TBAHS, water, rt, 2 h; (v) Fmoc-NCS,
THF, rt, overnight; (vi) DIC, DMF, rt, overnight; (vii) piperidine, DMF, rt, 30 min.
RESULTS AND DISCUSSION
■
The general synthetic approach is shown in Scheme 2. The first
two steps are based on traditional solid-phase peptide synthesis
using Wang resin and Fmoc-α-amino acids. To verify the
synthetic route, Wang resin was acylated with Fmoc-L-Ala-OH
followed by cleavage of the Fmoc protecting group with
piperidine. The intermediate 2{2} was reacted with 2-
nitrobenzenesulfonyl chloride and subsequently the nitro
group of sulfonamide 3{2,1} was reduced using the sodium
dithionite method reported recently.13 Reaction of 4{2,1} with
Fmoc-NCS gave the corresponding Fmoc-thiourea 5{2,1}
Figure 2. Building blocks successfully used for verification of the
which after treatment with DIC, furnished intermediate
synthetic route.
6{2,1} in excellent crude purity (96%, calculated from LC/
UV traces). Cleavage of the Fmoc protecting group was
followed by spontaneous intramolecular aminolysis of ester
bond and the target product 7{2,1} was released from the
polymer support by a cyclative cleavage.14 Separation of the
product from the fluorenylmethylpiperidine byproduct formed
after Fmoc protecting group removal was easily accomplished
by reverse phase semipreparative HPLC. The model compound
7{2,1} was obtained in a very good overall yield of 45%.
To evaluate the scope and limitations of our method we
selected representative building blocks with variable electronic
and steric properties: five 2-nitrobenzenesulfonyl chlorides with
electron-donating or electron-withdrawing ligands were tested,
as well as four Fmoc-amino acids with different substitution of a
side chain (Figure 2). With use of the split-and-split method12
and polypropylene fritted syringes, we prepared the full 4 × 5
library of 20 compounds. The purities of crude intermediates
6{R1,R2} ranged from 63% to 98% and the final compounds
were obtained in good overall yields (Table 1).
compounds 7 by replacing Fmoc-NCS with benzyl-NCS.
Surprisingly the reaction with 4{2,1} did not take place, even at
elevated temperature. Benzyl-NCO provided the intermediate
10 in excellent purity but unfortunately the following
cyclization with DIC was not successful (Scheme 3) and only
the starting material was recovered. When the resin 10 was
heated to reflux in toluene, the compound decomposed to give
only a mixture of unknown compounds.
In the preparation of 5-noranagrelide derivatives based on a
similar solid-phase approach we observed complete racemiza-
tion of the single stereocenter.15 For this reason, we evaluated
the effect of the cyclization reaction on the potential
racemization of 7{R1,R2} by synthesizing compound 7DL{2,1}
from an equimolar mixture of Fmoc-D-Ala-OH and Fmoc-L-
Ala-OH. The compounds 7DL{2,1} and 7{2,1} were subjected
to chiral HPLC analysis (Chiralpak IF column), revealing that
partial racemization occurred (enantiomer ratio 62:38,
Supporting Information) during the synthesis of 7{2,1} from
the enantiopure starting material. The effect of the stereocenter
In addition, we investigated the possible use of the reaction
sequence for the preparation of N1-substituted derivatives of
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Research Article
Table 1. List of Prepared Compounds 7{R1,R2}
natural) are commercially available, the developed method can
be applied for the simple preparation of sizable chemical
libraries of this privileged molecular scaffold.
ASSOCIATED CONTENT
■
a
b
S
* Supporting Information
compound
R1
R2
crude purity
yield (%)
Details of experimental synthetic and analytical procedures and
spectroscopic data for synthesized compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
7{1,1}
7{1,2}
7{1,3}
7{1,4}
7{1,5}
7{2,1}
7{2,2}
7{2,3}
7{2,4}
7{2,5}
7{3,1}
7{3,2}
7{3,3}
7{3,4}
7{3,5}
7{4,1}
7{4,2}
7{4,3}
7{4,4}
7{4,5}
H
H
90
92
70
98
85
96
91
98
93
98
64
65
72
92
63
70
90
93
87
98
69
22
14
22
10
45
34
26
35
36
24
28
35
33
43
36
47
16
62
40
H
CF3
Cl
H
H
Me
OMe
H
H
AUTHOR INFORMATION
■
Me
Me
Me
Me
Me
Corresponding Author
CF3
Cl
*Phone: +420 585634418. Fax: +420 585634465. E-mail:
soural@orgchem.upol.cz.
Me
OMe
H
Notes
CH2Ph
The authors declare no competing financial interest.
CH2Ph
CF3
Cl
CH2Ph
ACKNOWLEDGMENTS
■
CH2Ph
Me
OMe
H
The authors are grateful to projects CZ.1.07/2.3.00/20.0009,
CZ.1.07/2.3.00/30.0060 and CZ.1.07/2.4.00/31.0130 coming
from European Social Fund and from Palacky University
(internal grants No. PrF_2012_023 and PrF_2013_027). 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). The special thanks belong to Teva
CH2Ph
CH2OtBu
CH2OtBu
CH2OtBu
CH2OtBu
CH2OtBu
CF3
Cl
Me
OMe
a
Crude purity of precursors 6{R1,R2} after cleavage from the polymer
support calculated from LC-UV traces (215 nm). Overall yields after
six steps and preparative HPLC purification calculated from loading of
Fmoc amino acids (resins 1).
b
́ ́
Gyogyszergyar Zrt. for providing valuable experience in the
purification of the final compounds (project CZ.1.07/2.4.00/
17.0015).
Scheme 3. Attempt to Increase Diversity of Target
a
Compounds: N1 Substitution.
REFERENCES
■
(1) Fukuyama, T.; Jow, Ch.-K.; Cheung, M. 2- and 4-Nitro-
benzenesulfonamides: Exceptionally Versatile Means for Preparation
of Secondary Amines and Protection of Amines. Tetrahedron Lett.
1995, 36 (36), 6373−6374.
(2) Fulopova, V.; Gucky, T.; Grepl, M.; Soural, M. Solid-Phase
Synthesis of Trisubstituted Benzo[1,4]-Diazepin-5-one Derivatives.
ACS Comb. Sci. 2012, 14 (12), 651−656.
(3) Lemrova, B.; Soural, M. Solid-Phase Synthesis of 4,7,8-
Trisubstituted 1,2,3,4-Tetrahydro-benzo[e][1,4]diazepin-5-ones. ACS
Comb. Sci. 2012, 14 (12), 645−650.
(4) 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 (3), 370−374.
(5) Bouillon, I.; Zajicek, J.; Pudelova, N.; Krchnak, V. Remarkably
Efficient Synthesis of 2H-Indazole 1-Oxides and 2H-Indazoles via
Tandem Carbon−Carbon Followed by Nitrogen−Nitrogen Bond
Formation. J. Org. Chem. 2008, 73 (22), 9027−9032.
(6) Hirota, S.; Sakai, T.; Kitamura, N.; Kubokawa, K.; Kutsumura, N.;
Otani, T.; Saito, T. Synthesis of Nitrogen Heterocycle-Fused 1,2,4-
Benzothiadiazine-1,1-dioxide, Quinazolinone, and Pyrrolidinone De-
rivatives with a Guanidine Joint via Sequential aza-Wittig Reaction/
Intramolecular NH-Addition Cyclization/Nucleophilic Substitution
Ring Closure Methodology, Using Functionalyzed Carbodiimides As
Key Intermediates. Tetrahedron 2010, 66 (3), 653−662.
(7) The Merck Index, 11th ed.; Merck: Rahway, NJ, U.S.A., 1989; p
98.
(8) Fleming, J. S.; Buyniski, J. P. A Potent New Inhibitor of Platelet
Aggregation and Experimental Thrombosis, Anagrelide (BL-4162A).
Thromb. Res. 1979, 15 (3−4), 373−388.
(9) Reilly, J. T. Anagrelide for the Treatment of Essential
Thrombocythemia: A Survey among European Hematologists/
Oncologists. Hematology 2009, 14 (1), 1−10.
a
Reagents and conditions: (i) Bn-NCS, THF, rt or 50 °C, overnight;
(ii) Bn-NCO, THF, rt, overnight; (iii) DIC, DMF, rt to 50 °C,
overnight or toluene, reflux, overnight.
substitution (different R1 ligands) and reaction conditions on
the resulting stereochemistry will be studied in the near future.
In conclusion, we have developed a convenient high-
throughput synthesis of Anagrelide sulfonyl analogues with
two diversity positions based on solid-phase chemistry on
Wang resin. The reaction sequence can be accomplished in 4
days with only ca. 60 min total hands-on time. Various building
blocks were tested and a model library of 20 compounds was
prepared and fully characterized. The target substances were
isolated in good overall yields. Since a number of building
blocks (particularly Fmoc-amino acids, both natural and non-
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Research Article
(10) Pescatore, S. L.; Lindley, C. Anagrelide: A Novel Agent for the
Treatment of Myeloproliferative Disorders. Expert Opin. Pharmacother.
2000, 1 (3), 537−546.
(11) Trapp, O. M.; Beykirch, M. K.; Petrides, P. E. Anagrelide for
Treatment of Patients with Chronic Myelogenous Leukemia and a
High Platelet Count. Blood Cells, Mol. Dis. 1998, 24 (1), 9−13.
́ ̌ ̌ ́
(12) Hlavac, J.; Soural, M.; Krchnak, V. In Solid-Phase Organic
Synthesis; Toy, P. H., Lam, Y., Eds.; Wiley: Hoboken, NJ, U.S.A., 2012,
95−130.
́ ̌ ́
(13) Kaplanek, R.; Krchnak, V. Fast and Effective Reduction of
Nitroarenes by Sodium Dithionite under PTC Conditions: Applica-
tion in Solid-Phase Synthesis. Tetrahedron Lett. 2013, 54 (21), 2600−
2603.
(14) Ganesan, A. Cyclative Cleavage as the Solid-Phase Strategy. In
Linker Strategies; Scott, P. J. H., Ed.; Wiley: Chichester, West Sussex,
U.K., 2012; p 135.
(15) Messina, I.; Popa, I.; Maier, M.; Soural, M. Solid-Phase
Synthesis of 5-Noranagrelide Derivatives. ACS Comb. Sci. 2014, 16 (1),
33−38.
224
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