Construction of 5-Aminotetrazoles via in Situ Generation of Carbodiimidium Ions from Ketones Promoted by TMSN3/TfOH

Article abstract of DOI:10.1021/acs.joc.9b00555

A novel synthetic approach for the synthesis of 5-aminotetrazoles has been developed by employing simple ketones as substrates. This methodology involved the N₂-extrusion/aryl migration of azido complexes as the key step for the in situ generation of carbodiimidium ion, which could further react with hydrazoic acid and cyclize intramolecularly to provide 5-aminotetrazoles in good to excellent yields. In addition, the regioselectivity of the reaction was studied and rationalized by density functional theory calculations.

Full text of DOI:10.1021/acs.joc.9b00555

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Article
Construction of 5-Aminotetrazoles via In Situ Generation of
Carbodiimidium ions from Ketones Promoted by TMSN3/TfOH
Phongprapan Nimnual, Jumreang Tummatorn, Bundet Boekfa, Charnsak
Thongsornkleeb, Somsak Ruchirawat, Pawida Piyachat, and Kunlayanee Punjajom
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00555 • Publication Date (Web): 04 Apr 2019
Downloaded from http://pubs.acs.org on April 4, 2019
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Construction of 5-Aminotetrazoles via In Situ Generation of
Carbodiimidium ions from Ketones Promoted by TMSN3/TfOH
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Phongprapan Nimnual,a Jumreang Tummatorn,*’a,b Bundet Boekfa,c
Charnsak Thongsornkleeb,a,b Somsak Ruchirawata,b, Pawida Piyachatb
and Kunlayanee Punjajoma
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a Program on Chemical Biology, Chulabhorn Graduate Institute, Center of Excellence on
Environmental Health and Toxicology (EHT), Ministry of Education, 54 Kamphaeng Phet 6,
Laksi, Bangkok 10210, Thailand
b Laboratory of Medicinal Chemistry, Chulabhorn Research Institute, 54 Kamphaeng Phet 6,
Laksi, Bangkok 10210, Thailand
c Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University,
Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand.
∗Email: jumreang@cri.or.th
N
N
O
N
N
TMSN3 (3.0 eq)
TfOH (3.0 eq)
R
R1
R1
R
R
N
C N
HN
R1
DCM
H
. Readily avialable
. High yields
. Electronically driven
regioselectivity
Direct carbodiimidium ion formation
via double N2-extrusion/aryl migration
Abstract
A novel synthetic approach for the synthesis of 5-aminotetrazoles has been developed
employing simple ketones as the substrates. This methodology involved the N2-extrusion/aryl
migration of azido complexes as the key step for the in situ generation of carbodiimidium ion
which could further react with hydrazoic acid and cyclize intramolecularly to provide 5-
aminotetrazoles in good to excellent yields. In addition, the regioselectivity of the reaction
was studied and rationalized by DFT calculations.
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■ INTRODUCTION
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9
5-Substituted tetrazoles are ubiquitous structures found in many applications
including pharmaceutical chemistry, organic and inorganic chemistry, and material sciences.1
Several methods are available for the preparation of 5-substituted tetrazoles.2 However, 5-
aminotetrazoles are relatively much less accessible even though they are crucial core
structures of many biologically active molecules.3 In particular, this class of compounds
shows several pharmacological properties such as anti-allergic,3a antitumor3c anti-
inflammatory,3d anti-neoplastic,3e-3f antimicrobial3g and anti-HIV3j activities. They also have
inhibitory effect in the secretion process of hepatitis B virus surface antigen (HBsAg)3b and
could act as inhibitors of iNOS,3h HIF-2,3i and STAT33k as shown in Figure 1. Besides the
biological activities, 5-aminotetrazoles could also be utilized as an energetic material. Due to
their importance, the development of efficient and practical methods for preparation of 5-
aminotetrazoles continues to be vital. Some examples are depicted in Scheme 1.4
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O
Ar
H
OEt
N
OMe
O
N
N
N
N
Me
NH
N
N
N
N
H
Cl
Selective STAT3 non-peptide inhibitor
NO2
Antimicrobial
Br
NH3+Cl-
NH3
Me
Br
Cl
H
N
N
N
N
N
N
N
N
H
N
H
NH
O
N
iNOS inhibitor
F
HIF-2 inhibitor
Figure 1. Bioactive compounds
Among a small number of synthetic methodologies, carbodiimide is the most
important key intermediate for the preparation of 5-aminotetrazole analogues through
nucleophilic addition of azide, followed by ring closure. The most prevalent precursors used
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in the preparation of carbodiimides are thioureas which could be synthesized from
isothiocyanates and amines.5 In the synthesis of 5-aminotetrazoles using these methods,
carbodiimide intermediate was generated in situ from the reaction of thiourea either with
6
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I2/NEt3 or hypervalent iodine/NEt3 which further reacted with azide in a one-pot manner. In
a related procedure, thioureas could be generated in situ from isothiocyanates and amines
which could be directly converted to carbodiimides by treatment with HgCl2 or CuBr in the
presence of base.7 However, the availability and variety of thiourea and isothiocyanate
derivatives are rather limited. Therefore, the development of a novel method for the
preparation of 5-aminotetrazoles using more readily available and structurally diverse
substrates is highly beneficial.
N
NH2
R1
R
HN
NaNO2
HN
R1
or
N
N
N
N
-
N
N
N3
N
N
H2N R1
N
HN R1
R
R
R1
HN
N
R
LG
LG
-
N3
S
R
N
H
R1
R
N C S
heat
+
N
R
N C N
R1
H
conditions
R1 NH2
Isolated or generated in situ
carbodiimide
Isolated thiourea
or generated in situ
Scheme 1. The reported methods for the synthesis of 5-aminotetrazole.
Herein, we report an alternative approach for the formation of 5-aminotetrazoles from
simple ketone precursor (1) which is easily accessible from inexpensive and abundant
materials. In this work, ketone substrate was converted to a carbodiimidium ion intermediate
(2) using TMSN3/TfOH via double rearrangement-nitrogen extrusion (Pathway I)8 as shown
in Scheme 2. Then, this intermediate was further reacted with HN3 to provide 5-
aminotetrazole product (3). However, the formation of tetrazoles could also compete with the
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generation of carbodiimidium ion intermediate when the rate of R1-migration is slower than
5
6
the cyclization (Pathway II).
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TMSN3 + TfOH
O
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HN3
HN3 + TMSOTf
R1
R
R1
R
N
-N2
1
 N2-extrusion
 R-rearrangement
N
N
N
N
N
N
Pathway II:
N
NH
R
R
cyclization
R1
4
R1
Pathway I:
 N2-extrusion
 R-rearrangement
-N2
N
N
-
N
N
N3
R
N
C
N
H
R1
R
Carbodiimidium ion
generated in situ
This work
R1
HN
2
3
Scheme 2. Proposed synthetic methodology for the synthesis of 5-aminotetrazole.
■ RESULTS AND DISCUSSION
In our study, benzophenone (1a) was employed as the screening substrate to optimize
reaction conditions as shown in Table 1. Based on the proposed strategy, at least 3.0
equivalents of TMSN3 were required. Therefore, the combination of 3.0 equiv of TMSN3 and
3.0 equiv of TfOH in dry DCM was examined in entry 1 to provide the desired product 3a in
90%. Interestingly, reducing the reaction time could increase the yield of 5-aminotetrazole to
94% (entry 2) whereas reducing the amount of TfOH to 1.0 and 2.0 equivalents provided the
desired product in lower yields (entries 3-4). Moreover, we investigated the effect of solvents
such as DCE, toluene and CH3CN (entries 5-7) and found that these solvents diminished the
yields of 5-aminotetrazole. Therefore, using 3.0 equivalents each of TfOH and TMSN3 in dry
DCM at room temperature was the optimal conditions for the transformation.
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Table 1. Screening for the optimal conditions.a
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N
N
O
N
N
TMSN3, TfOH
HN
3
Solvent, RT, 5 h
1
Entry
1
TMSN3
TfOH
(eq)
Solvent
Time
(h)
Yield (%)
(eq)
(eq)
3.0
3.0
3.0
3.0
3.0
3.0
1
2
3
4
5
6
1.0
1.0
1.0
1.0
1.0
1.0
3.0
3.0
1.0
2.0
3.0
3.0
DCM
DCM
DCM
DCM
DCE
o/n
5
90
94
45
78b
71
87
5
5
5
PhCH3
5
7
1.0
3.0
3.0
CH3CN
5
88
a Isolated yields b 1,3-Diphenylurea was isolated in 8% yield.
To evaluate the generality of our protocol, a variety symmetrical ketone precursors
was examined using the optimal conditions as shown in Table 2. The reaction of
benzophenone (1a) provided the desired product in 94%. Other analogues of benzophenone
were also smoothly converted to the desired products. The para-methylbenzophenone (1b)
gave 5-aminotetrazole product 3b in 88% yield whereas the ortho-methylbenzophenone (1c)
furnished the corresponding product 3c in slightly higher yield (92%). All para-
halobenzophenone derivatives (1d-1f) were converted to the corresponding aminotetrazole
products in good yields. Furthermore, regioisomeric methoxybenzophenone derivatives (1g-
1i) were also investigated under these conditions. All three substrates provided the
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corresponding products in good to excellent yields with the ortho-methoxybenzophenone
(1g) giving the desired product in highest yield among the three regioisomers. This was
possibly due to the steric effect of the ortho-methoxy group in facilitating the rearrangement
process. Unfortunately, the reaction of dicyclohexylketone (1j) provided tetrazole 4j as the
major product whereas the corresponding product 3j was obtained in only 12% yield. This
result demonstrated that dicyclohexylketone could be smoothly converted to azidonium imine
(1j') via cyclohexyl migration and nucleophilic addition of HN3. However, the rate of
cyclohexyl migration of this intermediate (Pathway I, Scheme 2) is slower than cyclization
(Pathway II, Scheme 2) which led to the formation of 5-aminotetrazole 3j as the minor
product (12%) and tetrazole 4j as the major product (80%).
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Table 2. Synthesis of 5-aminotetrazoles from symmetrical ketones.a
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N
N
N
O
N
TMSN3 (3.0 eq), TfOH (3.0 eq)
DCM, rt, 5 h
R
R
R
HN
R
1
3
6
7
8
9
N
N
N
N
N
N
N
N
HN
HN
Me
Me
3b
: 88%
3a
: 99%
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N
N
N
N
Me
N
N
N
N
Me
HN
HN
F
F
3c
: 92%
3d
: 83%
N
N
N
N
N
N
N
N
HN
HN
Cl
Br
Cl
Br
3e
: 88%
3f
: 84%
N
N
N
OMe
N
N
N
MeO
N
OMe
N
OMe
HN
HN
3h
: 80%
3g
: 92%
N
N
N
N
N
N
N
N
HN
HN
MeO
OMe
3j
: 12%
3i
: 87%
N
N
N
N
N
O
N
Partway II
N
NH
2 eq. HN3
fast
1j
1j'
4j:
80%
slow
Partway I
N
N
N
N
HN3
N
C N
H
HN
3j
: 12%
a Isolated yields
Table 3 Substrate Scope.a
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N
N
N
O
N
N
N
N
N
TMSN3 (3.0 eq), TfOH (3.0 eq)
R1
R1
R
R
+
HN
R1
HN
DCM, rt, 5 h
R
6
3
3'
1
7
8
9
3
3'
)
Product ( and
Sub
stra
Entry
Yield (%) and ratio (3:3')
91% (55:45)b
N
N
N
N
te
N
N
N
O
1
( )
N
1
10
11
12
13
14
15
16
17
18
19
20
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22
23
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25
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32
33
34
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
HN
HN
t-Bu
t-Bu
t-Bu
3k'
3k
1
k
N
N
N
N
N
N
N
N
O
90% (48:52)b
2
HN
HN
F
3l'
F
3l
F
1l
N
N
N
N
N
N
N
N
O
(
95% (49:51)
HN
3
4
HN
Cl
Cl
Br
Cl
Br
3m'
: 48%
3m
: 47%
1
m
N
N
N
N
O
N
N
N
N
82% (43:57)
HN
HN
Br
3n'
: 47%
3n
: 35%
1
n
N
N
N
N
O
N
N
N
94% (48:52)b
N
5
6
HN
HN
OMe
OMe
MeO
MeO
3o'
OMe
3o
1o
N
N
N
N
N
N
N
N
O
OMe
94% (53:47)b
HN
3p'
HN
3p
1p
N
N
N
N
N
N
O
N
7
8
N
84% (40:60)
HN
HN
CF3
F3C
CF3
1q
3q
3q'
: 50%
: 34%
N
N
O
N
N
N
N
N
N
75% (35:65)
HN
HN
MeO
CF3
F3C
MeO
1r
OMe
3r
: 26%
CF3
3r'
: 49%
N
N
O
O
N
N
N
N
MeO
N
N
9
OMe
MeO
HN
91% (40:60)
75% (28:72)
HN
3s'
OMe
OMe
1s
1t
3s
: 30%
OMe
: 45%
OMe
N
N
N
N
N
N
N
MeO
MeO
N
10
OMe
MeO
MeO
HN
HN
OMe 3t'
OMe
OMe
: 45%
3t
: 25%
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3
3'
)
Product ( and
1
Substrate ( )
Entry
11
Combined Yield (%) and ratio (3:3')
78% (64:36)
N
N
N
N
N
N
Br
N
N
O
Br
7
HN
HN
8
9
3u
: 50%
1u
3u'
: 28%
Br
N
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OMe N
N
O
OMe
t-Bu
N
N
N
N
68% (79:21)
OMe
12
HN
HN
1v
3v
: 54%
3v':
14%
t-Bu
N
N
N
N
77% (100:0)
N
N
O
HN
13
N
N
HN
3w'
: 0%
t-Bu
1w
3w
%
: 77
O
OMe
N
N
OMe N
N
N
N
N
N
OMe
99% (55:45)b
14
HN
OMe
MeO
HN
1x
3x'
3x
MeO
N
O
N
N
N
N
N
N
N
MeO
MeO
MeO
HN
-
Me
15
Me
1y
1z
MeO
MeO
MeO
MeO
3y:
25%
4y
: 74%
N
N
O
N
N
HN
77%
82%
16
3z
N
N
N
N
O
HN
17
18
3ab
1ab
N
O
N
N
Me
N
64%
Me
4ac
1ac
N
N
O
Me
Me
N
N
Me
Me
59%
69%
19
20
Me
Me
1ad
4ad
N
O
N
N
Me
Me
N
Me
Me
4ae
Me
Me
1ae
a Isolated yields bA mixture of inseparable products was isolated whose ratio was determined
using 1H NMR.
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Next substrate scope study is shown in Table 3, unsymmetrical ketones were
employed to generate unsymmetrical carbodiimidium ion intermediates which could lead to
the formation of regioisomeric 5-aminotetrazole products. A number of substituents on
unsymmetrical ketones was varied to study their influence on product distribution. Using 4-
tert-butylbenzophenone (1k), the reaction provided aminotetrazole 3k and 3k' in an almost
1:1 ratio. Similarly, a series of 4-halobenzophenone (1l-1n) also gave the corresponding
products as mixtures of regioisomeric products in ratios of nearly 1:1 in good combined
yields. Next investigation, derivatives of methoxybenzophenone (1o-1p) were examined as
shown in entries 5-6. Both meta- and para-methoxybenzophenone afforded mixtures of
aminotetrazoles in almost 1:1 ratios with excellent combined yields in both cases. The
regioselectivity for the formation of the two isomeric products was clearly observed when
both aryl substituents of ketone have great difference in electron density. For example,
compound 1q, having electronically neutral phenyl ring and strong electron-withdrawing 4-
trifluoromethylbenzene as substituents of ketone (entry 7), provided a mixture of 5-
aminotetrazoles 3q and 3q’ with a ratio of 40:60 in very good combined yield. The
regioselective conversion was achieved based on ring closure of the nitrogen atom on the
more electron-rich aniline moiety (phenyl ring) while the minor product was obtained from
the ring closure of nitrogen atom on the less electron-rich aniline unit (para-
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trifluoromethylbenzene). Furthermore, substrates having higher difference in electron density
on both aromatic rings were examined in entries 8-10. To increase the difference in electron
density on both aryl rings of ketone substrate, we then employed compound 1r containing
para-methoxybenzene and para-trifluoromethylbenzene as the ketone substituents (entry 8).
In this case, the improvement in regioselectivity was observed to give a mixture of 5-
aminotetrazoles 3r and 3r’ with a 35:65 ratio in good combined yield. In comparing entries 6
and 9, the substrate with one more methoxy group (entry 9), 1,3-dimethoxybenzene, could
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improve the regioselectivity providing a mixture of compounds 3s and 3s’ with a 40:60 ratio
in 91% combined yield. In addition, the result from entry 10 with 3,4,5-trimethoxybenzene as
the substituent provided even better regioselectivity with a 28:72 ratio of the corresponding
products 3t and 3t’ in good combined yield. From all results, the regioselective conversion
was achieved via cyclization of the nitrogen atom on the more electron rich aniline moiety
while the minor product was obtained via cyclization of the nitrogen atom on the lower
electron density aniline unit. These results also implied that when the electron densities on
both aryl rings were obviously different, the migratory aptitude of aryl rings were distinct
such that the corresponding products were obtained regioselectively. The reaction mechanism
was proposed in Scheme 3.
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Pathway A (R-migrate)
Electrondensity: R  R1 (R migrates first)
Proton exchange
N
TfO-
in strong acidic conditions
TfO-
TfO-
N
TfOH-
N
NH
O
HN3
HN3
-N2
N
R1
R
R1
R
N
C
N
R
N
C
N
R1
R1
R
R
X
H
H
-N2
R1
II
I
TfO-
V
Pathway A-1 (Favorable Pathway)
TfO-
Compound
1t
N
N
TfO-
TS1
N
N
N
N
HN3
N
Ea = 14.8 kcal/mol
NH
-H
-TfO-
N
fast
N
N
R
N C N
R1
TfO-
NH
R
R
R
H
R1
HN
R1
HN
II
HN
R1
III
IV
major product
R =
OMe
OMe
R1
=
Pathway A-2 (Unfavorable Pathway)
OMe
TfO-
H
N
N
N
HN3
N
H
-H
N
R
1t
Compound
Ea = 30.9 kcal/mol
N
N
fast
R
N
TS2
R
N
C
N
R1
R
N
N
N
HN
N
N
H
H
R1
R1
NH
TfO-
R1
TfO-
II
III
VIII
minor product
Pathway B (R1-migrate)
Electrondensity: R  R1 (R1 migrates first)
N
TfO-
TfO-
O
TfO-
N
TfOH-
HN3
HN3
N
NH
R1
N C N R
R1
N
R
R1
R
R1
H
-N2
R
IX
V
(Unfavorable Pathway)
N
TfO-
N
HN3
N
N
N
N
N
N
TfO-
TS3
N
NH
-H
fast
1t
Compound
Ea = 15.4 kcal/mol
R1
N C N R
N
NH
R1
R1
R1
H
HN
VII
HN
TfO-
R
R
HN
R
V
VI
minor product
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Scheme 3. Proposed reaction mechanism.
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The ketone substrate reacted with HN3, which was generated in situ, to give nitrilium
ion intermediate following one of the two possible N2-extrusion/migration Pathways, A and
B, depending on the migratory aptitude of R and R1 groups. In Pathway A, the higher
electron density aryl ring (R group) underwent the first aryl migration to give the nitrilium
ion intermediate. Then, this intermediate reacted with the second equivalent of HN3 to
provide intermediate I. The reaction subsequently underwent another nitrogen extrusion/aryl
migration of R1 (lower electron density) to generate carbodiiminium ion intermediate II.
According to experiments in the literature regarding rate of proton exchange, the NH
exchange rate was very slow under strong acidic conditions.10 Therefore, the proposed
mechanism for the generation of carbodiiminium ion intermediate V via proton exchange of
intermediate II could be ruled out. The proposed mechanism for the generation of major
product was proposed in Pathway A-1. The intermediate II could react with the third
equivalent of HN3 and immediately cyclize to obtain the major product. An alternative route
was the cyclization using N-6 atom of III to give the intermediate VIII as shown in Pathway
A-2. However, Pathway A-1 could give the more stable intermediate IV due to its higher
aromatic character and being less strained compared to intermediate VIII. Another
competitive mechanism is Pathway B which is the competing R1 aryl ring migration resulting
in the formation of minor product. In this pathway, the lower electron density (R1) underwent
aryl migration in the first step to obtain the nitrillium ion IX which further reacted with HN3
to give carbodiimidium ion intermediate V. Next, this intermediate reacted with another HN3
and directly cyclized to provide the aminotetrazole salt which could give the desired product
as the minor product after work up.
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The steric effect of ortho substituents on aryl ketone substrate was investigated. With
bromo substituent 1u, the reaction gave a mixture of the corresponding products 3u and 3u’
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in 78% yield with a ratio of 64:36. Increasing the size of substituents, the reaction could
significantly affect the regioselectivity of the corresponding products. Substrates 1v
containing ortho-methoxy substituent gave the desired products with high regioselectivity
(3v:3v’; 79:21). Interestingly, the regioselectivity of this reaction was opposite to the reported
method from which compound 3v was obtained as the minor product.6a Having more steric
hindrance in substrate 1w, containing ortho-(4-tBu-phenyl) group gave a single product 3w in
77% yield. These results indicated that the steric bulkiness of ortho substituent could
significantly increase a migratory aptitude of the aryl ring to provide high regioselectivity for
the formation of 5-aminotetrazole products. The results from entries 7-13 clearly showed that
the regioselectivity of the reaction depended on the migratory aptitude of the aryl groups.
Aryl ring with higher electron density and steric hindrance has lower migratory. Surprisingly,
low regioselectivity was observed with the substrate having symmetrical 2,6-
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dimethoxyphenyl groups 1x which offered the corresponding products in an almost 1:1 ratio
(55:45) in excellent yield. The results from the reactions of alkylarylketones in entries 15-17
were strongly supportive of our proposed mechanism. In case of methylarylketone (1y), the
reaction gave compound 3y in only 25% yield while longer side chain, butylarylketone,
provided a single regioisomer of compound 3z in 77% yield. In case of compound 1ab
containing cyclohexyl group as the substituent, the reaction smoothly provided
aminotetrazole 3ab as a single isomer in 82% yield. The results from entries 15-17
demonstrated that the order of migratory aptitude as aryl ring> bulky alkyl group > less bulky
alkyl group leading to the formation of a single regioisomeric product. These results further
confirmed that proton exchange under strong acidic conditions could be ruled out as
intermediate V was formed as only single regioisomeric products were obtained (Pathway A-
1).
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Furthermore, ketone substrates having dialkyl substituents were examined in entries
18-20. Unfortunately, 5-aminotetrazole products could not be generated; only the formation
of tetrazoles was resulted in all cases. These results implied that the migratory aptitude of
ketone substituents played an important role on the outcome of reactions. In case of ketone
having alkyl substituent on both sides, the reaction readily underwent cyclization after the
first alkyl migration and nucleophilic addition by HN3 at the nitrilium ion intermediate to
provide tetrazole products.
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Figure 2. The potential energy diagram of the reaction of compound 1t via Pathways
A-1, A-2, and B calculated with MP2/6-311+G(2df,2p) on the optimized structures with
M06-2X/6-311+G(2df,2p). Energies are in kcal/mol.
In this work, a computational study was also performed to assist in understanding the
regioselectivity of the reaction. The molecular structure and the reaction mechanism to
produce compound 1t were studied as a representative case using density functional theory
(DFT).11 The reaction mechanism in Pathway A-1 to produce compound 3t’ was studied as
shown in Figure 2. The formation of intermediate IV was a strongly exothermic reaction with
relative energy of -34.5 kcal/mol. This pathway led to the lowest activation energy (14.8
kcal/mol). The activation energy of Pathway A-2 was highest (30.9 kcal/mol) among the
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three pathways. Therefore, this was an unfavorable reaction pathway to form 5-
aminotetrazole product. For the formation of compound 3t (minor product, Pathway B), the
total activation energy of the pathway was 15.4 kcal/mol. The activation energy for TS3 was
higher than TS1 of 0.6 kcal/mol while the stability of the product VII is less stable than IV
by 1.0 kcal/mol. Product 3t’ was predicted to be the major product while product 3t was the
minor product. In summary, the results from computational study of a model system using the
reaction of compound 1t illustrated that the activation energy of the reaction in Pathway A-1
is lower than the reaction of Pathway A-2 and B. Therefore, Pathway A-1 became a more
favorable pathway in term of both kinetic and thermodynamic properties resulting in the
cyclization of nitrogen on electron-rich aniline unit to give the major product. The results
from the computational study corroborated well to our experimental results. Therefore, we
concluded that regioselectivity outcome of 5-aminotetrazoles depended on the difference in
migratory aptitude of ketone substituents. This implied that the more highly differentiated
electron density between the two aryl substituents would result in better regioselectivity in
products as summarized in Pathway A-1.
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■ CONCLUSIONS
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We have developed a new method for the synthesis of 5-aminotetrazoles employing
ketone precusors. This method involved N2-extrusion/aryl migration of ketone compounds to
generate carbodiimidium ion as the key intermediate, followed by cyclization with HN3 to
obtain 5-aminotetrazoles in good to excellent yields. The regioselectivity of this conversion
was controlled by the migratory aptitude of ketone substituents. A good regioselectivity could
be obtained when both substituents of ketone have a significant difference in migratory
aptitude. This method has the advantages in the ease of access to starting materials from
commercially available precursors, metal-free conditions and a broad range of substrate
scope.
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■ EXPERIMENTAL SECTION
General Procedure. The commercial grade chemicals were used without further purification,
unless otherwise specified. All solvents used were purified by the solvent purification system.
The oven-dried glassware (110 °C at least for 2 h) was used for all reactions. Crude reaction
mixtures were concentrated under reduced pressure by removing organic solvent with the
rotary evaporator. Column chromatography was performed using silica gel 60 (particle size
0.06−0.2 mm; 70−230 mesh ASTM). Analytical thin layer chromatography (TLC) was
performed with silica gel 60 F254 aluminum sheets. The nuclear magnetic resonance (NMR)
spectra were recorded in deuterochloroform (CDCl3), deuterated acetone (acetone-d6) and
deuterated dimethyl sulfoxide (DMSO-d6, 2.50 ppm) with 300, 400 and 600 MHz
spectrometers. Chemical shifts for 1H and 13C NMR spectra were reported in part per million
(ppm, δ), relative to tetramethylsilane (TMS, 0.00 ppm) as the internal reference. Coupling
constants (J) were reported in hertz (Hz). Infrared spectra were measured using an FT-IR
spectrometer and were reported in cm−1. High resolution mass spectra (HRMS) were obtained
using time-of-flight (TOF).
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General procedure for the synthesis of aminotetrazole: compound 1a (506.1 mg, 2.7775
mmol, 1.0 equiv) was dissolved in dry DCM (7.4 mL/mmol) under argon. A solution was
added TMSN3 (1.10 mL, 8.3324 mmol, 3.0 equiv), followed by TfOH (730 µL, 8.3324
mmol, 3.0 equiv). The reaction mixture was stirred at room temperature for 5 hr. After
completion, the reaction was quenched with sat. NaHCO3 and extracted with EtOAc. The
combined organic layers were washed with brine, dried over Na2SO4, filtered, and
concentrated under reduced pressure to provide crude product which was purified on silica
gel (EtOAc/Hexane: 1:1) to yield N,1-diphenyl-1H-tetrazol-5-amine (3a) 617.5 mg. (94%,
white solid); mp 163-164 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3200, 3052, 2922,
1605, 1571, 1496, 1317, 1089, 751 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.33 (s, 1H),
7.69-7.62 (m, 7H), 7.35-7.30 (m, 2H), 7.01 (t, 1H, J = 7.2 Hz); 13C{1H} NMR (75 MHz,
DMSO-d6) δ 152.3, 139.8, 133.1, 130.0, 129.9, 128.8, 125.5, 122.1, 118.3; HRMS (ESI-TOF)
m/z: [M+H]+ calcd for C13H12N5 238.1087; found 238.1083.
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1,3-Diphenylurea:12 white solid; mp 237-239 °C; 1H NMR (300 MHz, DMSO-d6) δ 8.64 (s,
2H), 7.44 (dd, 4H, J = 8.7, 1.2 Hz), 7.30-7.25 (m, 4H), 6.99-6.93 (m, 2H); 13C{1H} NMR (75
MHz, DMSO-d6) δ 152.5, 139.6, 128.7, 121.7, 118.1.
N,1-dip-tolyl-1H-tetrazol-5-amine (3b): Yield 118.1 mg (88%, white solid); mp 207-208 °C
(1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3180, 3021, 1603, 1562, 1508, 1389, 1231, 1079,
824, 808 cm−1; 1H NMR (300 MHz, CDCl3) δ 7.43-7.41 (m, 6H), 7.16-7.13 (m, 2H), 6.29 (s,
1H), 2.48 (s, 3H), 2.31 (s. 3H); 13C{1H} NMR (75 MHz, CDCl3) δ 151.8, 141.0, 135.6,
133.0, 131.1, 130.1, 129.8, 124.7, 118.2, 21.3, 20.7; HRMS (ESI-TOF) m/z: [M+H]+ calcd
for C15H16N5 266.1400; found 266.1401.
N,1-dio-tolyl-1H-tetrazol-5-amine (3c): Yield 90.5 mg (92%, pale yellow solid); mp 148-149
°C (3/7 EtOAc/hexane); IR (neat): υmax 3154, 2985, 2911, 1600, 1564, 1493, 1465, 1086,
746 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 8.58 (s, 1H), 7.55-7.40 (m, 4H), 7.31 (d, 1H, J =
7.2 Hz), 7.22-7.06 (m, 3H), 2.16 (s, 3H), 2.14 (s. 3H); 13C{1H} NMR (75 MHz, DMSO-d6)
δ 154.2, 137.5, 135.3, 132.4, 132.0, 131.4, 130.7, 130.6, 127.6, 127.3, 126.3, 125.3, 124.7,
17.8, 17.0; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C15H16N5 266.1400; found 266.1394.
N,1-bis(4-fluorophenyl)-1H-tetrazol-5-amine (3d): Yield 110.8 mg (88%, pale yellow solid);
mp 179-180 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3268, 3060, 1617, 1577, 1503,
1418, 1238, 1222, 1089, 822 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 8.56 (s, 1H), 7.80-7.66
(m, 4H), 7.48-7.40 (m, 2H), 7.14-7.06 (m, 2H); 13C{1H} NMR (75 MHz, DMSO-d6) δ 164.1
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(d, JCF = 246 Hz), 160.7, 155.6 (d, JCF = 296 Hz), 136.9 (d, JCF = 2.3 Hz), 140.4 (d, JCF = 2.4
Hz), 129.5 (d, JCF = 9 Hz), 120.8 (d, JCF = 8 Hz), 117.7 (d, JCF = 24 Hz), 116.1 (d, JCF = 22
Hz); HRMS (ESI-TOF) m/z: [M+H]+ calcd for C13H10F2N5 274.0899; found 274.0891.
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N,1-bis(4-chlorophenyl)-1H-tetrazol-5-amine (3e): Yield 102.2 mg (83%, pale yellow
solid); mp 194-195 °C (2/8 EtOAc/hexane); IR (neat): υmax 3267, 3064, 1606, 1563, 1490,
1089, 802 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.48 (s, 1H), 7.73-7.65 (m, 6H), 7.38 (d,
2H, J = 8.4 Hz); 13C{1H} NMR (75 MHz, DMSO-d6) δ 152.7, 139.2, 135.3, 132.2, 130.4,
129.1, 128.3, 126.2, 120.2; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C13H10Cl2N5 (Cl-35)
306.0308; found 306.0297.
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N,1-bis(4-bromophenyl)-1H-tetrazol-5-amine (3f): Yield 91.3 mg (84%, white solid); mp
175-176 °C (3/7 EtOAc/hexane); IR (neat): υmax 3262, 3190, 3043, 1606, 1561, 1492, 1070,
824, 804 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.50 (s, 1H), 7.85 (dd, 2H, J = 8.4, 1.8 Hz),
7.62 (td, 4H, J = 8.7, 2.1 Hz), 7.49 (dd, 2H, J = 9.0, 2.1 Hz); 13C{1H} NMR (75 MHz,
DMSO-d6) δ 152.2, 139.2, 133.0, 132.2, 131.6, 128.0, 123.6, 120.3, 113.9; HRMS (ESI-TOF)
m/z: [M+H]+ calcd for C13H10Br2N5 (Br-79) 393.9298; found 393.9304.
N,1-bis(2-methoxyphenyl)-1H-tetrazol-5-amine (3g): Yield 100.5 mg (97%, pale yellow
solid); mp 99-100 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3353, 2943, 2843, 1612,
1574, 1506, 1464, 1249, 1024, 749 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 7.92 (s, 1H), 7.80
(d, 1H, J = 7.8 Hz), 7.64-7.53 (m, 2H), 7.34 (d, 1H, J = 8.4 Hz), 7.18 (t, 1H, J = 7.8 Hz),
7.07-6.93 (m, 3H), 3.89 (s, 3H), 3.80 (s, 3H); 13C{1H} NMR (75 MHz, DMSO-d6) δ 152.7,
152.6, 148.9, 132.1, 128.1, 127.7, 123.6, 121.2, 121.1, 120.6, 119.4, 113.0, 111.0, 56.1, 55.9;
HRMS (ESI-TOF) m/z: [M+H]+ calcd for C15H16N5O2 298.1299; found 298.1309.
N,1-bis(3-methoxyphenyl)-1H-tetrazol-5-amine (3h): Yield 83.4 mg (80%, white solid); mp
129-130 °C (2/8 EtOAc/hexane); IR (neat): υmax 3078, 2997, 2835, 1608, 1575, 1490, 1158,
869, 794 cm−1; 1H NMR (300 MHz, Acetone-d6) δ 8.48 (s, 1H), 7.56 (t, 1H, J = 8.1 Hz), 7.42
(d, 1H, J = 2.1 Hz), 7.26-7.16 (m, 5H), 6.61 (dt, 1H, J = 6.6, 2.4 Hz), 3.91 (s, 3H), 3.78 (s,
3H); 13C{1H} NMR (75 MHz, Acetone-d6) δ 161.5, 161.2, 153.1, 141.8, 135.0, 131.7, 130.4,
118.5, 117.0, 112.0, 111.2, 108.5, 104.9, 56.0, 55.4; HRMS (ESI-TOF) m/z: [M+H]+ calcd
for C15H16N5O2 298.1299; found 298.1293.
N,1-bis(4-methoxyphenyl)-1H-tetrazol-5-amine (3i): Yield 79.2 mg (87%, pale yellow solid);
mp 137-138 °C (3/7 EtOAc/hexane); IR (neat): υmax 3259, 3104, 2835, 1612, 1578, 1508,
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1464, 1244, 1235, 1031, 837 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 8.98 (s, 1H), 7.57-7.52
(m, 4H), 7.17 (d, 2H, J = 8.7 Hz), 6.90 (d, 2H, J = 9.0 Hz), 3.86 (s, 3H), 3.72 (s, 3H);
13C{1H} NMR (75 MHz, DMSO-d6) δ 160.3, 154.6, 152.9, 133.0, 127.5, 125.6, 120.1,
115.0, 113.9, 55.6, 55.2; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C15H16N5O2 298.1299;
found 298.1298.
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N,1-dicyclohexyl-1H-tetrazol-5-amine (3j): Yield 12.7 mg (12%, pale yellow solid); mp 182-
183 °C (2/8 EtOAc/hexane); IR (neat): υmax 3313, 3261, 2926, 2852, 1594, 1448, 1101, 894
cm−1; 1H NMR (600 MHz, DMSO-d6) δ 6.50 (d, 1H, J = 7.4 Hz), 4.16 (tt, 1H, J = 11.5, 3.8
Hz), 1.95-1.58 (m, 11H), 1.39-1.12 (m, 10H); 13C{1H} NMR (125 MHz, DMSO-d6) δ 154.0,
53.7, 52.7, 32.4, 31.6, 25.2, 24.7, 24.64, 24.55; HRMS (ESI-TOF) m/z: [M+H]+ calcd for
C13H24N5 250.2026; found 250.2027.
1,5-dicyclohexyl-1H-tetrazole (4j): Yield 80.3 mg (80%, pale yellow solid); mp 171-172 °C
(2/8 EtOAc/hexane); IR (neat): υmax 2936, 1450, 1265, 733 cm−1; 1H NMR (300 MHz,
CDCl3) δ 4.16-4.09 (m, 1H), 2.81-2.73 (m, 1H), 2.11-1.72 (m, 14H), 1.51-1.21 (m, 6H); 13C
NMR (75 MHz, CDCl3) δ 157.5, 57.3, 33.4, 33.1, 31.4, 25.8, 25.4, 25.3, 24.8; HRMS (ESI-
TOF) m/z: [M+H]+ calcd for C13H23N4 235.1917; found 235.1917.
1-(4-tert-butylphenyl)-N-phenyl-1H-tetrazol-5-amine (3k), N-(4-tert-butylphenyl)-1-phenyl-
1H-tetrazol-5-amine (3k’): Yield 129.1 mg (91%, ratio 55:45, white solid); mp 154-155 °C
(1/19 to 3/7 EtOAc/hexane); IR (neat): υmax 3265, 3053, 2963, 1604, 1567, 1499, 1088, 837,
751 cm−1; 1H NMR (300 MHz, CDCl3) δ .7.68-7.52 (m, 5H, 4.1H minor), 7.48-7.44 (m, 2H,
1.64H minor), 7.39-7.32 (m, 2H, 1.64H minor), 7.12-7.06 (m, 1H), 6.35 (s, 1H), 6.31 (s,
0.82H minor), 1.40 (s, 9H), 1.31 (s, 7.38H minor); 13C{1H} NMR (75 MHz, CDCl3) δ 154.2,
151.9, 151.6, 146.7, 138.1, 135.4, 132.8, 130.6, 130.4, 129.95, 129.4, 127.6, 126.2, 124.8,
124.5, 123.4, 118.2, 118.1, 35.1, 34.3, 31.4, 31.2; HRMS (ESI-TOF) m/z: [M+H]+ calcd for
C17H20N5 294.1713; found 294.1721.
1-(4-fluorophenyl)-N-phenyl-1H-tetrazol-5-amine (3l), N-(4-fluorophenyl)-1-phenyl-1H-
tetrazol-5-amine (3l’): Yield 41.5 mg (90%, ratio 48:52, light yellow solid); mp 180-181 °C
(2/8 to 1/1 EtOAc/hexane); IR (neat): υmax 2924, 2854, 1618, 1572, 1509, 1499, 1224, 746
cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.36 (br s, 1H), 9.29 (br s, 0.91H minor), 7.76-7.74
(m, 1.82H minor), 7.68-7.61 (m, 7H, 1.82H minor), 7.53-7.48 (m, 1.82H minor), 7.35-7.30
(m, 1.82H minor), 7.20-7.14 (m, 2H), 7.03-6.98 (m, 1.82H minor); 13C{1H} NMR (75 MHz,
DMSO-d6) δ 162.7 (d, JCF = 246 Hz), 157.5 (d, JCF = 237 Hz), 152.5, 152.4, 139.7, 136.2,
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133.0, 130.1, 129.9, 129.4, 128.8, 128.6 (d, JCF = 9 Hz), 125.6, 122.1, 120.2 (d, JCF = 8 Hz),
118.2, 116.8 (d, JCF = 24 Hz), 115.3 (d, JCF = 23 Hz); HRMS (ESI-TOF) m/z: [M+H]+ calcd
for C13H11FN5 256.0993; found 256.0995.
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1-(4-chlorophenyl)-N-phenyl-1H-tetrazol-5-amine (3m): Yield 37.2 mg (47%, white solid);
mp 165-166 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3253, 3054, 2921, 2851, 1610,
1571, 1534, 1496, 1092, 1013, 831, 737 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.31 (s, 1H),
7.72 (s, 4H), 7.61 (d, 2H, J = 8.1 Hz), 7.32 (d, 2H, J = 7.8 Hz), 7.01 (t, 1H, J = 7.2 Hz);
13C{1H} NMR (75 MHz, DMSO-d6) δ 152.4, 139.7, 134.7, 131.8, 129.9, 128.8, 127.7,
122.1, 118.2; HRMS (ESI-TOF) m/z: [M+Na]+ calcd for C13H10ClN5Na (Cl-35) 294.0517;
found 294.0509.
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N-(4-chlorophenyl)-1-phenyl-1H-tetrazol-5-amine (3m’): Yield 38.2 mg (48%, white solid);
mp 195-196 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 2925, 1607, 1565, 1490, 1242,
1089, 804, 771, 691 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.49 (s, 1H), 7.68-7.66 (m, 7H),
7.38 (d, 2H, J = 7.2 Hz); 13C{1H} NMR (75 MHz, DMSO-d6) δ 152.1, 138.8, 132.9, 130.1,
129.9, 128.6, 125.7, 125.6, 119.8; HRMS (ESI-TOF) m/z: [M+Na]+ calcd for C13H10ClN5Na
(Cl-35) 294.0517; found 294.0516.
1-(4-bromophenyl)-N-phenyl-1H-tetrazol-5-amine (3n): Yield 41.5 mg (35%, white solid);
mp 164-165 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3185, 3030, 2916, 1601, 1563,
1487, 1068, 834, 746 cm−1; 1H NMR (300 MHz, acetone-d6) δ 8.56 (s, 1H), 7.88-7.83 (m,
2H), 7.79-7.66 (m, 4H), 7.33 (t, 2H, J = 7.5 Hz), 7.03 (t, 1H, J = 7.2 Hz); 13C{1H} NMR (75
MHz, acetone-d6) δ 140.6, 134.0, 133.5, 129.7, 128.8, 124.5, 123.2, 118.96, 118.89; HRMS
(ESI-TOF) m/z: [M+H]+ calcd for C13H11BrN5 (Br-79) 316.0192; found 316.0206.
N-(4-bromophenyl)-1-phenyl-1H-tetrazol-5-amine (3n’): Yield 57.3 mg (47%, white solid);
mp 181-182 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3258, 3083, 1606, 1563, 1486,
1238, 1073, 823, 762, 686 cm−1; 1H NMR (300 MHz, acetone-d6) δ 8.66 (s, 1H), 7.70-7.63
(m, 7H), 7.51-7.46 (m, 2H); 13C{1H} NMR (75 MHz, acetone-d6) δ 140.2, 134.2, 132.6,
131.2, 130.9, 126.7, 120.9, 120.8, 115.0; HRMS (ESI-TOF) m/z: [M+H]+ calcd for
C13H11BrN5 (Br-79) 316.0192; found 316.0206.
1-(4-methoxyphenyl)-N-phenyl-1H-tetrazol-5-amine (3o), N-(4-methoxyphenyl)-1-phenyl-1H-
tetrazol-5-amine (3o’): Yield 123.2 mg (94%, ratio 48:52, orange solid); mp 178-179 °C (2/8
to 1/1 EtOAc/hexane); IR (neat): υmax 3079, 2836, 1603, 1570, 1511, 1497, 1236 cm−1; 1H
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NMR (300 MHz, DMSO-d6) δ 9.14 (s, 0.94H minor), 9.06 (s, 1H), 7.60-7.45 (m, 6H, 5.64H
minor), 7.26 (t, 2H, J = 7.8 Hz), 7.13 (d, 1.88H, J = 8.7 Hz minor), 6.94 (t, 1H, J = 7.2 Hz),
6.86 (d, 1.88H, J = 9.0 Hz minor), 3.81 (s, 2.82H minor), 3.68 (s, 3H); 13C{1H} NMR (75
MHz, CDCl3) δ 160.9, 155.3, 153.2, 153.0, 140.4, 133.6, 133.4, 130.4, 129.2, 128.0, 126.03,
125.95, 122.4, 122.1, 120.9, 118.6, 115.5, 114.4, 56.1, 55.7; HRMS (ESI-TOF) m/z: [M+H]+
calcd for C14H14N5O1 268.1193; found 268.1194.
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1-(3-methoxyphenyl)-N-phenyl-1H-tetrazol-5-amine (3p), N-(3-methoxyphenyl)-1-phenyl-1H-
tetrazol-5-amine (3p’): Yield 123.2 mg (94%, ratio 53:47, white solid); mp 157-158 °C (2/8
to 1/1 EtOAc/hexane); IR (neat): υmax 3078, 2998, 1604, 1571, 1495, 1269 cm−1; 1H NMR
(300 MHz, DMSO-d6) δ 9.30 (s, 0.88H mimor), 9.27 (s, 1H), 7.67-7.61 (m, 5H), 7.56 (t, 1H,
J = 8.1 Hz), 7.34 (t, 2H, J = 5.4 Hz), 7.29-7.17 (t, 1H, J = 7.2 Hz), 6.62-6.55 (m, 0.88H
minor), 3.84 (s, 3H), 3.73 (s, 2.64H minor); 13C{1H} NMR (75 MHz, DMSO-d6) δ 160.1,
159.7, 152.3, 152.2, 141.0, 139.8, 133.9, 133.0, 130.7, 130.1, 139.9, 129.6, 128.8, 125.6,
122.1, 118.4, 117.6, 116.0, 111.2, 110.6, 107.7, 104.1, 55.6, 55.0; HRMS (ESI-TOF) m/z:
[M+Na]+ calcd for C14H13N5ONa 290.1012; found 290.1018.
N-phenyl-1-(4-(trifluoromethyl)phenyl)-1H-tetrazol-5-amine (3q): Yield 34.0 mg (34%, pale
yellow solid); mp 155-156 °C (1/9 EtOAc/hexane); IR (neat): υmax 3359, 2921, 2851, 1606,
1575, 1523, 1422, 1327, 1177, 1107, 1068, 846, 751 cm−1; 1H NMR (300 MHz, DMSO-d )
훿
6
9.46 (s, 1H), 8.05 (d, 2H, J = 8.7 Hz), 7.95 (d, 2H, J = 8.4 Hz), 7.62 (d, 2H, J = 7.8 Hz), 7.34
(t, 2H, J = 7.8 Hz), 7.02 (t, 1H, J = 7.5 Hz); 13C{1H} NMR (75 MHz, DMSO-d6) δ 152.4,
139.6, 136.5, 130.0 (q, JCF = 32 Hz), 128.8, 127.1 (q, JCF = 56 Hz), 126.4, 123.8 (q, JCF = 272
Hz), 122.3, 118.4; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C14H11F3N5 306.0961; found
306.0960.
1-phenyl-N-(4-(trifluoromethyl)phenyl)-1H-tetrazol-5-amine (3q’): Yield 49.5 mg (50%,
pink solid); mp 184-185 °C (1/9 EtOAc/hexane); IR (neat): υmax 3261, 2926, 1610, 1570,
1530, 1499, 1325, 1111, 1068, 839, 760, 687 cm−1; 1H NMR (300 MHz, DMSO-d ) 9.80 (s,
훿
6
1H), 7.84 (d, 2H, J = 8.7 Hz), 7.70-7.65 (m, 7H); 13C{1H} NMR (75 MHz, DMSO-d6) δ
152.8, 143.4, 132.8, 130.2, 129.9, 126.0 (q, JCF = 4 Hz), 125.5, 124.5 (q, JCF = 269 Hz),
122.0 (q, JCF = 32 Hz), 117.8; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C14H11F3N5
306.0961; found 306.0962.
N-(4-methoxyphenyl)-1-(4-(trifluoromethyl)phenyl)-1H-tetrazol-5-amine (3r): Yield 24.7 mg
(26%, white solid); mp 176-177 °C (1/9 EtOAc/hexane); IR (neat): υmax 3311, 2925, 2854,
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1604, 1508, 1325, 1139, 1125, 1071, 1033, 1014, 845, 829 cm−1; 1H NMR (600 MHz,
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DMSO-d ) 9.23 (s, 1H), 8.02 (d, 2H, J = 8.5 Hz), 7.92 (d, 2H, J = 8.4 Hz), 7.50-7.49 (m,
훿
6
2H), 6.92-6.90 (m, 2H), 3.72 (s, 3H); 13C{1H} NMR (150 MHz, DMSO-d6) δ 155.0, 152.8,
136.5, 132.7, 129.9 (q, JCF = 32 Hz), 127.1 (q, JCF = 4 Hz), 126.2, 123.8 (q, JCF = 271 Hz),
120.6, 114.1, 55.3; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C15H13F3N5O 336.1067; found
336.1070.
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1-(4-methoxyphenyl)-N-(4-(trifluoromethyl)phenyl)-1H-tetrazol-5-amine (3r’): Yield 46.2 mg
(49%, pale yellow solid); mp 174-175 °C (1/9 EtOAc/hexane); IR (neat): υmax 3260, 2927,
2854, 1609, 1571, 1514. 1323, 1252, 1112, 1068, 834 cm−1; 1H NMR (600 MHz, DMSO-d6)
9.63 (s, 1H), 7.80 (d, 2H, J = 8.5 Hz), 7.66 (d, 2H, J = 8.7 Hz), 7.55 (d, 2H, J = 8.9 Hz),
훿
7.16 (d, 2H, J = 8.9 Hz), 3.84 (s, 3H); 13C{1H} NMR (125 MHz, DMSO-d6) δ 160.7, 152.2,
143.6, 127.7, 126.2 (q, JCF = 4 Hz), 125.4, 124.6 (q, JCF = 270 Hz), 122.2 (q, JCF = 32 Hz),
118.0, 115.2, 55.8; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C15H13F3N5O 336.1067; found
336.1064.
N-(3,5-dimethoxyphenyl)-1-phenyl-1H-tetrazol-5-amine (3s): Yield 33.9 mg (30%, white
solid); mp 134-135 °C (2/8 EtOAc/hexane); IR (neat): υmax 3278, 2938, 1608, 1572, 1153,
692 cm−1; 1H NMR (400 MHz, DMSO-d ) 9.28 (s, 1H), 7.69-7.63 (m, 5H), 6.90 (d, 2H, J =
훿
6
2.0 Hz), 6.16 (t, 1H, J = 2.0 Hz), 3.71 (s, 6H); 13C{1H} NMR (100 MHz, DMSO-d6) δ 160.6,
152.1, 141.4, 133.0, 130.1, 129.9, 125.7, 96.6, 94.2, 55.1; HRMS (ESI-TOF) m/z: [M+H]+
calcd for C15H16N5O2 298.1298; found 298.1296.
1-(3,5-dimethoxyphenyl)-N-phenyl-1H-tetrazol-5-amine (3s’): Yield 50.8 mg (45%, white
solid); mp 149-150 °C (2/8 EtOAc/hexane); IR (neat): υmax 3259, 2938, 1570, 1158, 749
cm−1; 1H NMR (400 MHz, DMSO-d ) 9.27 (s, 1H), 7.65 (d, 2H, J = 8.0 Hz), 7.32 (t, 2H, J
훿
6
= 7.6 Hz), 7.01 (t, 1H, J = 7.6 Hz), 6.84 (d, 2H, J = 2.0 Hz), 6.74 (t, 1H, J = 2.4 Hz), 3.82 (s,
6H); 13C{1H} NMR (100 MHz, DMSO-d6) δ 161.1, 152.2, 139.8, 134.4, 128.8, 122.1,
118.4, 104.0, 102.0, 55.7; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C15H16N5O2 298.1298;
found 298.1296.
1-phenyl-N-(3,4,5-trimethoxyphenyl)-1H-tetrazol-5-amine (3t): Yield 31.5 mg (25%, white
solid); mp 185-186 °C (3/7 to 1/1 EtOAc/hexane); IR (neat): υmax 3270, 2924, 2850, 1608,
1570, 1453, 1226, 1128, 1002, 693 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.19 (s, 1H),
7.66-7.62 (m, 5H), 7.02 (s, 2H,), 3.74 (s, 6H), 3.61 (s, 3H); 13C{1H} NMR (75 MHz,
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DMSO-d6) δ 153.2, 152.7, 136.1, 133.2, 133.0, 130.6, 130.4, 126.0, 96.8, 60.6, 56.1; HRMS
(ESI-TOF) m/z: [M+Na]+ calcd for C16H17N5O3Na 350.1224; found 350.1228.
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N-phenyl-1-(3,4,5-trimethoxyphenyl)-1H-tetrazol-5-amine (3t’): Yield 82.1 mg (66%, white
solid); mp 172-173 °C (3/7 to 1/1 EtOAc/hexane); IR (neat): υmax 3316, 2926, 2854, 1600,
1571, 1499, 1232, 1126, 1087, 751 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.21 (s, 1H), 7.66
(d, 2H, J = 7.8 Hz), 7.32 (t, 2H, J = 7.8 Hz), 7.03-6.99 (m, 3H), 3.82 (s, 6H), 3.76 (s, 3H);
13C{1H} NMR (75 MHz, CDCl3) δ 153.6, 152.5, 139.8, 138.6, 128.8, 128.3, 122.2, 118.5,
104.1, 60.2, 56.4; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C16H18N5O3 328.1404; found
328.1401.
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1-(2-bromophenyl)-N-phenyl-1H-tetrazol-5-amine (3u): Yield 47.8 mg (50%, pale yellow
solid); mp 157-158 °C (1/9 EtOAc/hexane); IR (neat): υmax 3169, 3039, 2997, 1615, 1575,
1530, 1486, 1083, 748 cm−1; 1H NMR (300 MHz, DMSO-d ) 9.36 (s, 1H), 7.96 (d, 1H, J =
훿
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7.2 Hz), 7.79 (d, 1H, J = 7.5 Hz), 7.70-7.61 (m, 4H), 7.33 (t, 2H, J = 7.2 Hz), 7.01 (t, 1H, J =
7.2 Hz); 13C{1H} NMR (75 MHz, DMSO-d6) δ 152.8, 139.6, 133.7, 133.0, 131.9, 130.6,
129.3, 128.8, 122.2, 121.8, 118.2; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C13H11BrN5 (Br-
79) 316.0192; found 316.0195.
N-(2-bromophenyl)-1-phenyl-1H-tetrazol-5-amine (3u’): Yield 26.7 mg (28%, yellow solid);
mp 137-138 °C (1/9 EtOAc/hexane); IR (neat): υmax 3368, 3065, 2921, 1595, 1562, 1522,
1448, 1314, 1089, 1024, 748 cm−1; 1H NMR (300 MHz, DMSO-d ) 8.85 (s, 1H), 7.73-7.58
훿
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(m, 7H), 7.36 (td, 1H, J = 7.5, 1.2 Hz), 7.11 (td, 1H, J = 8.1, 1.5 Hz); 13C{1H} NMR (75
MHz, DMSO-d6) δ 152.8, 137.7, 133.2, 133.0, 129.9, 129.8, 128.6, 126.3, 124.7, 124.5,
117.4; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C13H11BrN5 (Br-79) 316.0192; found
316.0196.
1-(2-methoxyphenyl)-N-phenyl-1H-tetrazol-5-amine (3v): Yield 79.4 mg (54%, reddish
solid); mp 121-122 °C (2/8 to 1/1 EtOAc/hexane); IR (neat): υmax 3396, 3058, 2943, 1610,
1572, 1489, 1247, 1106, 1022, 745 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 9.10 (s, 1H),
7.67-7.61 (m, 3H), 7.54 (dd, 1H, J = 7.8, 1.5 Hz), 7.34-7.29 (m, 3H), 7.17 (td, 1H, J = 7.5,
0,9 Hz), 6.99 (t, 1H, J = 7.2 Hz), 3.79 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3) δ 154.6,
153.0, 139.8, 132.4, 128.9, 128.8, 122.0, 120.98, 120.89, 118.2, 113.2, 56.0; HRMS (ESI-
TOF) m/z: [M+H]+ calcd for C14H14N5O 268.1193; found 268.1202.
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N-(2-methoxyphenyl)-1-phenyl-1H-tetrazol-5-amine (3v’): Yield 20.9 mg (14%, brown
solid); mp 130-131 °C (2/8 to 1/1 EtOAc/hexane); IR (neat): υmax 3251, 3087, 2840, 1612,
1573, 1499, 1280, 1020, 753 cm−1; 1H NMR (400 MHz, DMSO-d6) δ 8.24 (s, 1H), 7.75 (dd,
1H, J = 8.0, 1.6 Hz), 7.68-7.56 (m, 5H), 7.08-6.94 (m, 3H), 3.74 (s, 3H); 13C{1H} NMR (100
MHz, CDCl3) δ 152.4, 149.6, 133.2, 129.9, 129.8, 128.0, 124.5, 124.0, 120.6, 120.2, 111.3,
55.8; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C14H14N5O 268.1193; found 268.1196.
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1-(4'-(tert-butyl)-[1,1'-biphenyl]-2-yl)-N-phenyl-1H-tetrazol-5-amine (3w): Yield 70.9 mg
(77%, pale yellow solid); mp 167-168 °C (1/9 to 1/1 EtOAc/hexane); IR (neat): υmax 3240,
3038, 2963, 2866, 1602, 1572, 1486, 1271, 1088, 742 cm−1; 1H NMR (300 MHz, DMSO-d6)
8.99 (s, 1H), 7.77-7.71 (m, 1H), 7.69-7.62 (m, 3H), 7.29-7.16 (m, 6H), 6.97-6.89 (m, 3H),
훿
1.14 (s, 9H); 13C{1H} NMR (75 MHz, DMSO-d6) δ 152.7, 150.0, 139.4, 138.98, 134.0,
131.2, 131.1, 130.3, 128.8, 128.5, 128.4, 127.7, 125.1, 121.8, 118.0, 34.1, 30.9; HRMS (ESI-
TOF) m/z: [M+H]+ calcd for C23H24N5 370.2026; found 370.2028.
1-(2,6-dimethoxyphenyl)-N-phenyl-1H-tetrazol-5-amine and N-(2,6-dimethoxyphenyl)-1-
phenyl-1H-tetrazol-5-amine (3x): Yield 68.6 mg (≥99%, white solid); mp 248-249 °C (3/7
EtOAc/hexane); IR (neat): υmax 3258, 1604, 1575, 1489, 1265, 1114, 1024, 777, 755 cm−1; 1H
NMR (300 MHz, DMSO-d ) 9.07 (s, 1H), 8.48 (s, 1H), 7.66 (d, 2H, J = 8.1 Hz), 7.59 (t,
훿
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1H, J = 8.4 Hz), 7.46-7.42 (m, 5H), 7.31 (t, 2H, J = 7.5 Hz), 7.07 (t, 1H, J = 8.4 Hz), 6.98 (t,
1H, J = 7.2 Hz), 6.91 (d, 2H, J = 8.4 Hz), 6.55 (d, 2H, J = 8.4 Hz), 3.76 (s, 6H), 3.67 (s, 6H);
13C{1H} NMR (75 MHz, DMSO-d6) δ 156.4, 155.1, 154.0, 153.3, 139.8, 133.4, 132.7,
129.1, 128.7, 127.0, 124.2, 121.8, 118.0, 115.6, 109.2, 104.9, 104.2, 56.2, 55.7; HRMS (ESI-
TOF) m/z: [M+Na]+ calcd for C15H15N5O2Na 320.1118; found 320.1116.
1-(4-methoxyphenyl)-5-methyl-1H-tetrazole (4y): Yield 98.1 mg (74%, white solid); mp 91-
92 °C (1/1 EtOAc/hexane) IR (neat): υmax 2924, 2851, 1607, 1523, 1509, 1449, 1273, 1254,
1021, 830 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 7.60-7.54 (m, 2H), 7.21-7.16 (m, 2H),
3.92 (s, 3H), 2.55 (s, 3H); 13C{1H} NMR (75 MHz, DMSO-d6) δ 161.7, 152.8, 127.8, 127.2,
115.6, 56.1, 9.5; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C9H11N4O 191.0927; found
191.0920.
1-(4-methoxyphenyl)-N-methyl-1H-tetrazol-5-amine (3y): Yield 35.7 mg (25%, white solid);
mp 145-146 °C (1/1 EtOAc/hexane); IR (neat): υmax 3333, 2923, 2853, 1609, 1518, 1463,
1247, 1017, 835 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 7.48-7.43 (m, 2H), 7.16-7.11 (m,
2H), 6.71 (d, 1H, J = 4.8 Hz), 3.83 (s, 3H), 2.85 (d, 3H, J = 4.8 Hz); 13C{1H} NMR (75
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MHz, DMSO-d6) δ 159.9, 155.8, 126.5, 125.8, 115.0, 55.6, 30.2; HRMS (ESI-TOF) m/z:
[M+H]+ calcd for C9H12N5O 206.1036; found 206.1041.
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N-butyl-1-(4-methoxyphenyl)-1H-tetrazol-5-amine (3z): Yield 82.6 mg (77%, white solid);
mp 113-114 °C (2/8 EtOAc/hexane); IR (neat): υmax 3270, 2937, 1605, 1519, 1256, 836, 735
cm−1; 1H NMR (300 MHz, DMSO-d6) δ 7.47-7.42 (m, 2H), 7.17-7.12 (m, 2H), 6.79 (t, 1H, J
= 5.7 Hz), 3.84 (s, 3H), 3.25 (q, 2H, J = 6.9 Hz), 1.31 (quint, 2H, J = 7.2 Hz), 1.30-1.24 (m,
2H), 0.88 (t, 3H, J = 7.2 Hz); 13C{1H} NMR (75 MHz, DMSO-d6) δ 159.9, 155.3, 126.5,
125.9, 115.0, 55.6, 43.3, 30.8, 19.4, 13.7; HRMS (ESI-TOF) m/z: [M+H]+ calcd for
C12H18N5O 248.1506; found 248.1502.
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N-cyclohexyl-1-(4-methoxyphenyl)-1H-tetrazol-5-amine (3ab): Yield 93.5 mg (82%, white
solid); mp 158-159 °C (2/8 to 1/1 EtOAc/hexane); IR (neat): υmax 3264, 2922, 2854, 1595,
1577, 1516, 1450, 1251, 1090, 838 cm−1; 1H NMR (300 MHz, DMSO-d6) δ 7.43 (d, 2H, J =
8.7 Hz), 7.13 (d, 2H, J = 8.7 Hz), 6.59 (d, 1H, J = 7.5 Hz), 3.83 (s, 3H), 3.48 (s, 1H), 1.90 (s,
2H), 1.71 (s, 2H), 1.6-1.56 (m, 1H), 1.35-1.20 (m, 4H), 1.09-1.06 (m, 1H); 13C{1H} NMR
(75 MHz, DMSO-d6) δ 159.8, 154.6, 126.6, 126.0, 114.9, 55.6, 53.2, 32.2, 25.2, 24.9; HRMS
(ESI-TOF) m/z: [M+H]+ calcd for C14H20N5O 274.1662; found 274.1660.
5-Methyl-6,7,8,9-tetrahydro-5H-tetrazolo[1,5-a]azepine (4ac): Yield 97.7 mg (64%,
colorless oil); (3/7 to 1/1 EtOAc/hexane); IR (neat): υmax 2937, 1450, 1428, 733 cm−1; 1H
NMR (300 MHz, CDCl3) δ 4.85-4.75 (m, 1H), 3.15-2.99 (m, 2H), 2.08-1.66 (m, 6H), 1.61 (d,
3H, J = 7.0 Hz); 13C{1H} NMR (75 MHz, CDCl3) δ 155.6, 55.9, 33.5, 25.9, 24.5, 24.0, 18.7;
HRMS (ESI-TOF) m/z: [M+H]+ calcd for C7H13N4 153.1135; found 153.1133.
5-Isopropyl-8-methyl-6,7,8,9-tetrahydro-5H-tetrazolo[1,5-a]azepine (4ad): Yield 87.8 mg
(59%, colorless oil) (3/7 to 1/1 EtOAc/hexane); IR (neat): υmax 2969, 1523, 1403, 1141 cm−1;
1H NMR (300 MHz, CDCl3) δ 4.42-4.35 (m, 1H), 3.10 (d, 2H, J = 4.5 Hz), 2.52-2.42 (m,
1H), 2.17-2.11 (m, 2H), 2.06-1.84 (m, 2H), 1.70-1.60 (m, 1H), 1.10 (d, 3H, J = 6.6 Hz), 0.89
(d, 3H, J = 6.9 Hz), 0.83 (d, 3H, J = 6.6 Hz); 13C{1H} NMR (75 MHz, CDCl3) δ 153.8, 65.4,
31.2, 29.8, 28.5, 28.2, 23.5, 19.6, 18.5, 18.4; HRMS (ESI-TOF) m/z: [M+H]+ calcd for
C10H19N4 195.1604; found 195.1602.
1-(sec-Butyl)-5-methyl-1H-tetrazole (4ae): Yield 117.7 mg (69%, colorless oil) (3/7 to 1/1
EtOAc/hexane); IR (neat): υmax 2969, 1523, 1403, 1384, 1141 cm−1; 1H NMR (300 MHz,
CDCl3) δ 4.38-4.26 (m, 1H), 2.56 (s, 3H), 2.14-1.78 (m, 2H), 1.59 (d, 3H, J = 6.9 Hz), 0.84
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(t, 3H, J = 7.5 Hz); 13C{1H} NMR (75 MHz, CDCl3) δ 150.8, 56.2, 29.4, 20.4, 10.3, 8.9;
HRMS (ESI-TOF) m/z: [M+H]+ calcd for C6H13N4 141.1135; found 141.1133.
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■ ACKNOWLEDGMENTS
This research work was supported by Chulabhorn Research Institute, Mahidol University,
and the Center of Excellence on Environmental Health and Toxicology, Science &
Technology Postgraduate Education and Research Development Office (PERDO), Ministry
of Education Education, and Thailand Research Fund through the Royal Golden Jubilee
(RGJ) Ph.D. Program (Grant number PHD/0217/2558) and Thailand Research Fund
(RSA6080085).
■ ASSOCIATED CONTENT
Supporting Information
1H and 13C NMR spectra of all prepared products. This material is available free of charge via
the Internet at http://pubs.acs.org/.
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