Parallel synthesis of 1,6-disubstituted-1,2,4-triazin-3-ones on solid-phase

Article abstract of DOI:10.1021/co400064d

A parallel solid-phase synthesis of 1,6-disubstituted-1,2,4-triazin-3-ones from MBHA resin is described. The reduction of resin-bound nitrosamino acids provides hydrazines efficiently without affecting the amide bond. The trityl protected hydrazine is then reduced with borane, and cyclized with 1,1-carbonyldiimidazole. The desired products are cleaved from their solid support and obtained in good yield and purity. This methodology is of value for the rapid parallel preparation of these potentially bioactive molecules.

Full text of DOI:10.1021/co400064d

Letter
pubs.acs.org/acscombsci
Parallel Synthesis of 1,6-Disubstituted-1,2,4-triazin-3-ones on Solid-
Phase
Miao Hu,† Wei Huang,† Marc A. Giulianotti,‡ Richard A. Houghten,‡ and Yongping Yu*,†,‡
†Institute of Materia Medica, College of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, P.R. China
‡Torrey Pines Institute for Molecular Studies, 11350 SW Village Parkway, Port St. Lucie, Florida 34987, United States
S
* Supporting Information
ABSTRACT: A parallel solid-phase synthesis of 1,6-disub-
stituted-1,2,4-triazin-3-ones from MBHA resin is described.
The reduction of resin-bound nitrosamino acids provides
hydrazines efficiently without affecting the amide bond. The
trityl protected hydrazine is then reduced with borane, and
cyclized with 1,1-carbonyldiimidazole. The desired products
are cleaved from their solid support and obtained in good yield
and purity. This methodology is of value for the rapid parallel preparation of these potentially bioactive molecules.
KEYWORDS: solid-phase, 1,2,4-triazinones, N-nitroso, hydrazine, reduction, cyclization
Herein, a synthetic strategy in which Red-Al is highly efficient
and selective at reducing N-NO over amides has been
developed (Scheme 1). From the LC-MS data, the crude
product A was obtained with 95% purity (tR = 5.214 min). Only
3% of the amide bond was reduced (tR = 6.227 min) (Figure 1).
The isolated yield of A was 88%.
Moreover, through the use of a reductive alkylation, diverse
substitutions were generated on the N1 position of hydrazine
(Table 1). As a note, it was observed that the 4-OBn-Bn group
was cleaved from the hydrazine moiety by HF (Scheme 2),
which provided us a new method for obtaining N1 (H)-
hydrazine. The isolated yield of B was 82%. Additionally, B was
characterized by LC-MS, HRMS, 1H NMR, and 13C NMR (see
Supporting Information).
To extend the practical value of this methodology for the
production of a hydrazine moiety, it was applied to the
synthesis of 1,6-disubstituted-1,2,4-triazin-3-ones (Scheme 3).
Starting from p-methylbenzhydrylamine (MBHA) resin 1, a
Boc-amino acid was coupled to the resin using a standard DIC/
HOBt procedure to generate the resin bound amino acid 2.
After removal of the Boc group with 55% TFA in DCM, a
reductive alkylation was performed in order to introduce a
secondary diversity15 (3). Subsequent reaction with tert-butyl
nitrite led to the resin-bound nitroso compound 4. The
hydrazine 5 was then generated by reducing 4 with Red-Al at
room temperature. The hydrazine was protected with a Trt
group because of its reported instability to BH3/THF.13 The
corresponding resin-bound product 7 was treated with 2%
TFA/DCM and cyclized with 1,1-carbonyldiimidazole, which
was then released from the resin by HF at 0 °C. Afterward, the
olid-phase organic synthesis (SPOS) is a pivotal technique
Sfor the rapid access to structurally diverse compounds in
the drug discovery process.1 One major focus of this field is on
the synthesis of small bioactive molecules and their derivatives
on the solid phase. In particular, heterocyclic compounds have
received special attention due to their high degree of structural
diversity and multiple applications in the treatment of human
diseases.2,3
1,2,4-Triazinones are analogues of the pyrimidine bases of
nucleic acids and an important class of molecules with several
biological properties. For example, 1,2,4-triazin-3(2H)-ones
have been shown to possess potential as anticancer,4 antiviral5
and antibacterial drugs.6 Moreover, 1-aryl-2H,4H-tetrahydro-
1,2,4-triazin-3-ones were reported as orally selective 5-lip-
oxygenase (5-LO) inhibitors.7 However, to the best of our
knowledge, no examples of 1,6-disubstituted-1,2,4-triazin-3-one
have been reported in the literature.
In the past decades, the N-nitroso group (N-NO) has
attracted much attention because of its pharmaceutical
properties and its synthetic utility.8 It can be used synthetically
to generate an N−N bond, as well as a hydrazine linker. There
are several reagents that can reduce the N-NO group in order
to obtain hydrazine, including Zn/AcOH,9 LiAH4,10 or
titanium trichloride (TiCl3)11 in solution phase, and
LiAH48b,12or DIBAL13 on solid-phase. We have found that
LiAH4 or DIBAL do not selectively afford hydrazine efficiently
when an amide bond is present, even at low temperatures, such
as 0 °C with DIBAL (data not shown), since the amide bond
will also be reduced. Red-Al is an alternative reductive reagent
to LiAH4, and it is much safer and more convenient for storage.
While Lee et al.14 have reported the reduction of amide bonds
with Red-Al as part of their effort to obtain [1,2]-diamines at
room temperature, it was believed that the conditions could be
optimized to selectively reduce N-NO over amide bonds.
Received: May 4, 2013
Revised: June 6, 2013
© XXXX American Chemical Society
A
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ACS Combinatorial Science
Letter
Scheme 1. Synthesis Approach for Hydrazine on Solid-Phase
Figure 1. LC-MS data of crude product A.
final products 9 (or 9′) were obtained in good yields and
purity.
EXPERIMENTAL PROCEDURES
■
General Methods. Reagents were obtained from commercial
sources and used without any further purification. Solvents were
purchased from Sigma-Aldrich and were used directly. The isolated
yields were based on the manufacturer’s loading of MBHA resin. LC-
MS (APCI and ESI) were recorded on a Shimadzu LCMS-2010EVat
LCQ mass spectrometer (ThermoQuest Corporation) using a
Phenomenex Luna 5 μ C18, 100 Å, 150 × 4.60 mm 5 μm column
running a 5−95 (H2O/CH3CN; 0.1% formic acid) gradient over 6
min. Preparative RP-HPLC was performed on a Shimadzu LC-8A
preparative HPLC using a Phenomenex Luna 5 μ C18 (2) 100 Å
column (75 × 21.2 mm, 5 μm). High resolution mass spectra were
measured on an Agilent 1290 HPLC-6224 Time of Fight Mass
As outlined in Table 1, several amino acids and different
aldehydes were accessed for viability under the synthetic
conditions. When R2 was Phenyl, amino acids with an aromatic
side chain generated products with slightly higher purity than
those with alkyl side chains according to the HPLC data of the
crude products (9a−9e to 9f−9i, except 9d, which may be due
to the 1-naphthyl hindering the cyclization). When R2
contained the electron-withdrawing group 2-fluoro on the
phenyl ring, the purity was excellent (9l and 9m). However, the
situation was different when 4-bromo was present (9j and 9k).
This may be the result of dehalogenation of the bromo during
the Red-Al reduction reaction. When the electron-donating
group 4-OBn was present on the phenyl ring (9′a−9′e), the
entire 4-OBn-Bn group was cleaved from the product during
the HF step. Low purity products were obtained for these
compounds (9′a−9′e). The overall yield for the analogs tested
under the described conditions ranged from 56−82% (Table
1).
In conclusion, the reduction of an N-NO group on the solid-
phase affords a facile approach for producing amino acids with
hydrazine termini and a reductive alkylation provides a second
diversity position. Moreover, these synthetic methods can be
applied to the efficient parallel synthesis of novel 1,6-
disubstituted-1,2,4-triazin-3-ones. This methodology is of
value for the rapid parallel preparation of these potentially
bioactive molecules in drug discovery.
1
Spectrometer. H NMR and 13C NMR spectra were recorded at 400
MHz on a Bruker Advance spectrometer with a 5-mm inverse gradient
probe.
General Procedures for the Synthesis 1, 6-Disubstituted-1,
2, 4-Triazin-3-Ones (9a−9o, 9′a−9′e). One hundred milligrams of
MBHA resin (1.1 mequiv/g) was contained in a polypropylene mesh
packet. Following neutralization with 5% diisopropylethylamine
(DIEA), the resin was coupled with Boc-amino acid, hydroxybenzo-
triazole (HOBt, 6.0 equiv, 0.1 M) and diisopropylcarbodiimide (DIC,
6.0 equiv, 0.1 M) in DMF at room temperature for 2 h. Upon removal
of the Boc group with 55% TFA in DCM (30 min), the resin was
washed with DCM (2 times), neutralized with a solution of 5% DIEA
in DCM. Reductive alkylation was performed according to the general
methods. AcOH (0.11 mL) and trimethyl orthoformate (TMOF)
(0.55 mL) were added to a mixture of resin in DCM (8.1 mL) and
MeOH (2.2 mL). The appropriate aromatic aldehyde (10 equiv) was
added to the above solution and shaken for 20 min. NaCNBH3 (69
mg, 10 equiv) in DMF (1.1 mL) was added and the mixture was
B
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ACS Combinatorial Science
Letter
Table 1. Individual Products of 1,6-Disubstituted-1,2,4-triazin-3-ones
a
b
c
d
compound
R1
R2
Phenyl
mass [M + H]+
tR (min)
purity (%)
HRMS [M + H]+
yield (%)
9a
9b
9c
9d
9e
9f
Benzyl
281.9
299.9
311.9
332.0
331.9
191.9
234.0
248.0
205.9
359.7
361.7
283.7
285.7
299.9
223.9
332.0
255.9
191.9
209.9
221.9
241.9
241.9
5.60
5.72
5.58
6.22
6.26
4.01
5.43
5.73
4.33
6.18
92
90
88
70
92
88
80
85
81
69
282.1606
300.1514
312.1645
332.1761
332.1759
192.1133
234.1604
248.1758
206.1294
360.0712
362.0693
284.0399
286.0381
300.1515
224.1197
332.1761
256.1447
192.1136
210.1042
222.1139
242.1293
64
71
69
56
64
64
60
68
70
60
4-F-Benzyl
4-OCH3Benzyl
1-Naphthyl-CH2
2-Naphthyl-CH2
H
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
4-Br-Phenyl
e
f
9g
9h
9i
CH(CH3)2
CH2CH(CH3)2
CH3
9j
Benzyl
9k
CH3
4-Br-Phenyl
5.03
55
65
9l
Benzyl
2-F-Phenyl
5.63
4.37
6.28
5.21
3.98
4.16
4.01
4.72
4.80
88
93
80
90
40
55
28
58
63
70
82
65
68
68
70
64
61
61
9m
9n
CH3
2-F-Phenyl
Benzyl
2-Naphthyl
9o
CH3
2-Naphthyl
9′a
9′b
9′c
9′d
9′e
Benzyl
4-OBnPhenyl
4-OBnPhenyl
4-OBnPhenyl
4-OBnPhenyl
4-OBnPhenyl
4-F-Benzyl
4-OCH3Benzyl
1-Naphthyl-CH2
2-Naphthyl-CH2
e
f
242.1298
a
b
c
Confirmed by mass spectra (ESI). Retention times from analytical RP-HPLC profile (UV detection at 214 and 254 nm). The purity of the crude
material was estimated by the peak area from analytical RP-HPLC traces at λ = 254 and 214 nm. Isolated yield based on manufacturer’s loading of
d
e
f
MBHA resin. 1-Naphthyl-CH2:
2-Naphthyl-CH2:
Scheme 2. Synthesis Approach for N1 (H)-Hydrazine on Solid-Phase
shaken for 1 h at room temperature. The resin was then washed with
DMF, DCM, 5% DIEA/DCM, DCM and MeOH, and dried in the
hood. The resulting resin bound amine was treated with 0.5 M tert-
Butyl nitrite in THF at 60 °C for 24 h. The bag was washed with THF,
DCM, MeOH, and dried in vacuo. The resulting resin-bound
nitrosamino acid was then reduced with 0.5 M Red-Al (70% in
toluene) in THF. The reduction solution was poured off and
quenched with methanol before adding to waste, and then washed
bags with THF and MeOH several times. The corresponding resin-
bound hydrazine was reacted with TrtCl (10 equiv) and DIEA (20
C
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a
Scheme 3. Synthesis of 1,6-Disubstituted-1,2,4-triazin-3-ones
a
Reagents and Conditions: (a) (i) prewash with 5% DIEA/DCM; (ii) Boc-AA-OH, DIC, HOBT, DMF, r.t., 2 h; (b) (i) 55%TFA/DCM; (ii)
reductive alkylation, R2CHO; (c) 0.5 M tert-butyl nitrite/THF, 60 °C, 24 h; (d) Red-Al, THF, r.t., overnight; (e) TrtCl, DIPEA, DMF, DCM; (f)
BH3/THF, 60 °C; (g) (i) 2% TFA/DCM; (ii) 1,1-carbonyldiimidazole, DCM; (h) HF, 0 °C, 1.5 h; (i) HF, 0 °C, 1.5 h.
equiv) in 0.1 M 10% DMF/DCM. The resin was washed with DMF
and DCM twice. After the Trt protection, the resin was reduced in
BH3/THF at 60 °C for four days and then with piperidine at 60 °C for
one day. The bags were then washed with DMF (twice), DCM (four
times) and MeOH (twice). After removal of the Trt group in 2%
TFA/DCM (3 × 10 min), the resin was cyclized with 1,1-
carbonyldiimidazole (6 equiv) in DCM. The final product was then
released from the resin by anhydrous HF in the presence of 0.5%
anisole at 0 °C for 1.5 h. The product was extracted by 95% acetic acid
in water. After lyophilization, the product of 1,6-disubstituted-1,2,4-
triazin-3-one was obtained. The crude product was purified by
preparative HPLC and characterized by LC-MS under ESI conditions,
HRMS and NMR spectroscopy.
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ASSOCIATED CONTENT
* Supporting Information
Experimental data for compounds A, B, 9a−9o, 9′a−9′b, and
1H, 13C NMR spectra. This material is available free of charge
via Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*Phone: +86 571 88208452. Fax: +86 571 88208452. E-mail:
yyu@zju.edu.cn.
Funding
This work was supported in part by The Project of Science
Technology Department of Zhejiang Province (2012C33065),
the Project of Education Department of Zhejiang Province
(Y201223193), NIH Grant 1R01DA031370, the State of
Florida, Executive Office of the Governor’s Office of Tourism,
Trade, and Economic Development, and the Osteoporosis and
Breast Cancer Research Center.
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Notes
The authors declare no competing financial interest.
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