A silver(i)-catalyzed cascade bicyclization strategy for synthesis of 5: H -benzo [d] tetrazolo[5,1- b] [1,3]thiazines

Article abstract of DOI:10.1039/c8ob03220c

A simple and efficient protocol for silver(i)-catalyzed tandem reaction of o-alkynylphenyl isothiocyanates with sodium azide has been developed, affording a series of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines in moderate to good yields. In this transformation, a [3 + 2] cycloaddition reaction mechanism was involved and two new rings were formed in one pot.

Full text of DOI:10.1039/c8ob03220c

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DOI: 10.1039/C8OB03220C
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The Silver(I)-catalyzed Cascade Bicyclization Strategy for Synthesis
of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines
Received 00th January 20xx,
Accepted 00th January 20xx
Jiankang Miao, Yahui Zhang, Xiaoyan Sang and Wenyan Hao*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple and efficient protocol for silver(I)-catalyzed tandem
reaction of o-alkynylphenyl isothiocyanates with sodium azide has
been developed, affording a series of 5H-benzo[d]tetrazolo[5,1-
b][1,3]thiazines in moderate to good yields. In this transformation,
a [3+2] cycloaddition reaction mechanism was involved and two
new rings were formed in one-pot.
1,3-thiazines is via the transition-metal-catalyzed tandem 6-
exo-dig cyclization reactions of 2-ethynylanilines to
isothiocyanates,
carbon
disulfide,
tetraalkylthiuram
disulfides.11 In 2014, Reddy and co-workers12 reported the
intramolecular C-O/C-S bond insertion of α-diazoesters for
synthesis of
a variety of 2-aryl-4H-benzo[d][1,3]thiazine
derivatives in high yields. Although these routes can work,
reports involving synthesis of the polycyclic skeleton 5H-
benzo[d]tetrazolo[5,1-b][1,3]thiazine is quite rare. To the best
of our knowledge, only one published example is known.9c
Recently, a copper-catalyzed cascade bicyclization of o-
alkynylphenyl isothiocyanates with isocyanides to synthesis of
5H-benzo[d]imidazo[5,1-b][1,3]thiazines has been reported by
our group (Scheme 1)14c. We envisioned that sodium azide also
could be applied in the reaction with o-alkynylphenyl
isothiocyanates by using this Strategy13. As our continuous
interest for the synthesis of biologically relevant heterocyclic
compounds, 14herein, we wish to introduce our recent effort
for the generation of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines
via the silver(I)-catalyzed cascade reaction of o-alkynylphenyl
isothiocyanates with sodium azide.
The development of efficient methods for the preparation of
azoles has become one of the most important research topics
in heterocyclic chemistry.1 Among these various azoles,
tetrazoles, the isostere of carboxyl acid group,2 are one of the
most promising compounds in medicinal chemistry due to its
3
important biological activities such as antituberculosis, anti-
allergy,4 antiinflammatory,5 antitumor,6 and other medicinal
properties. Furthermore, tetrazole derivatives are also widely
used as testing materials, explosives, metal coordination
compounds, chemical catalysts and other aspects of materials
and have attracted significant attention.7 By far, the prevalent
strategies for constructing tetrazole derivatives are the
addition of azides and isothiocyanates8 or the [3+2]
cycloaddition of an azide to a nitrile by using the Lewis acid as
9a
the catalyst.9 For instance, Hantzsch and Vagt first reported
Our previous work
the addition of hydrazoic acid to the cyanide group resulting in
the formation of 5-substituted tetrazole derivatives. In 2001,
R2
R2
S
Sharpless and co-workers described
a convenient, and
CuCl. Cs2CO3
R1
R3
+
CN
R1
R3
S
THF, reflux
environmentally friendly synthetic route to 5-substituted 1H-
tetrazoles, which can be accomplished with water as a solvent
and zinc salts as catalyst.9b On the other hand, Benzo-1,3-
thiazines are very important sulfur-containing N-heterocyclic
compounds which are found in many bioactive natural
products and pharmaceuticals.10 A common route to benzo-
N
C
N
N
This work
R2
S
R2
S
CF3COOAg, CH3CO2H
R1
NaN3
+
R1
MeCN, 80 o
C
N
C
N
N
N
N
Scheme 1 Comparison between previous and present work
a. College of Chemistry＆ Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, People’s Republic of China, Email: wenyanhao@jxnu.edu.cn
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
The starting o-alkynylphenyl isothiocyanates were prepared
via Sonogashira coupling of 2-iodoanilines with terminal
alkynes,15 followed by the treatment with thiophosgene
according to the literature procedure16. At the outset, we used
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Journal Name
22c
23d
AgOOCCF3
AgOOCCF3
CH3CO2H
CH3CO2H
MeCN
MeCN
80 oC
80 oC
45
o-phenylethynylphenyl isothiocyanate 1a and sodium azide 2
as the substrates in a model reaction to optimize the
conditions, and the results are summarized in Table 1. The
reaction was carried out in the presence of CuI (20 mol%), and
View Article Online
80
DOI: 10.1039/C8OB03220C
aReaction was performed with 1a (0.2 mmol), 2 (0.4 mmol), catalyst
(0.04mmol), acid (0.4 mmol ) in solvent (2mL) for 20 h. bIsolated yield based
on o-phenylethynylphenyl isothiocyanate 1a. c The reaction time was 12 h. d
The reaction time was 24 h.
o
HCl (2.0 equiv.) in THF (2mL) at 80 C for 20 h. To our delight,
the desired product 3a was successfully isolated in 43% yield
(Table 1, entry 1). Other metal salts used as catalysts in the
reaction were then screened (Table 1, entries 1-5). Silver
sources were found to be more effective than copper salts and
CF3CO2Ag gave the best results (Table 1, entry 5).
Subsequently, we examined the acid effect on the reaction. It
is evident that CH3CO2H was the best choice, leading to the
desired product 3a in 79% yield (Table 1, entry 9). Lower yields
were observed when other protonic acid such as HNO3, TsOH,
PivOH, NaHCO3 were employed, whereas H2SO4 failed to
promote the reaction (Table 1, entries 6-11). It is worth noting
that only 15% yield of product was obtained in the absence of
acid (Table 1, entry 12). We next examined the solvent effect
(Table 1, entries 13−18). When MeCN was employed as the
solvent, the highest yield of 84% was obtained. Finally, we
examined the effect of temperature on the reaction. When the
reaction temperature was reduced to 60 °C, the reaction was
completed with a yield of 40% (Table 1, entry 19). Rising the
reaction temperature to 100 oC and 120 oC resulted in a
diminished yield (Table 1, entries 20-21).
To further demonstrate the substrate scope and flexibility of
the present optimized conditions, different o-alkynyl phenyl
isothiocyanates were then explored, the results are
summarized in Table 2. All reactions proceeded smoothly,
leading to the desired 5H-benzo[d]tetrazolo[5,1-b][1,3]thia-
zines in moderate to good yields. For example, the
substituents on the R2 position of o-alkynylphenyl
isothiocyanates showed obvious electronic effects on the
reaction. When R2 group in the substrates 1 was an electron-
deficient aryl, such as p-FC6H4, p-ClC6H4, p-BrC6H4, the
reactions with
2 proceeded smoothly to afford the
corresponding products 3b-3d in lower yields compared with
the reactions of substrates 1 with an electron-rich aryl group
such as p-MeOC6H4, p-MeC6H4 at the R2 position, which
afforded the corresponding products 3e and 3f in 85% and
87% yields, respectively. Surprisingly, no desired products
were obtained when the R2 group in the substrate o-
alkynylphenyl isothiocyanates 1 was an alkyl group, such as n-
butyl, t-butyl, n-hexyl. This may be due to the fact that the
alkyl group cannot conjugate with anacetylenic bond and
activate it. However, when the R2 group in the substrate o-
alkynylphenyl isothiocyanates 1 was the cyclopropyl group，
the desired 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazine was not
obtained, and an unexpected seven-member-ring compound
4a was isolated instead. The structure of 4a was further
confirmed using X-ray diffraction analysis (see Fig. 2 in the
ESI†). The other two substitution products 4b, 4c also were
obtained successfully under standard conditions. In addition,
the reactions of o-alkynylphenyl isothiocyanates bearing
various substituents such as fluoro, chloro, bromo,
trifluoromethyl and methyl groups on the aryl rings, regardless
of their electronic properties and substitution positions, gave
the desired products 3g-3l in moderate to good yields.
Table 1 Initial studies for the tandem reaction of o-phenyl
ethynylphenyl isothiocyanate 1a with sodium azide 2aa
Catalyst,additive
Solvent, T.
S
S
+
NaN3
2
C
N
N
N
N
N
1a
3a
Entry
1
Catalyst
CuI
Additive
Solvent
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
Temp.
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
80 oC
Yield (%) b
43
HCl
HCl
2
Pd(OAc)2
Ag2CO3
Trace
50
3
HCl
4
AgSO3F3
HCl
48
5
AgO2CCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
HCl
60
6
H2SO4
HNO3
TsOH
CH3CO2H
PivOH
NaHCO3
/
NR
Similarly,
benzo[d]tetrazolo[5,1-b][1,3] thiazine 3m and (Z)-5-(4-
fluorobenzylidene)-7-(trifluoromethyl) -5H-
(Z)-7-fluoro-5-(4-fluorobenzylidene)-5H-
7
45
8
48
9
79
benzo[d]tetrazolo[5,1-b][1,3]thiazine 3n could also be
conventiently obtained in good yields by the silver-catalyzed
cascade bicyclization reaction under the same reaction
conditions. The structure of 3n was further confirmed using X-
ray diffraction analysis (see Fig. 2 in the ESI†). All products
were uniformly formed as the Z-isomer, which might be due to
a kinetic effect according to Baldwin’s rules and smaller steric
effect compared to the E-isomer.17
10
11
12
72
75
15
13
14
AgOOCCF3
AgOOCCF3
CH3CO2H
CH3CO2H
MeCN
DMF
80 oC
80 oC
84
52
15
16
17
18
19
20
21
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
AgOOCCF3
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
DMSO
Toluene
DCM
80 oC
80 oC
80 oC
80 oC
60 oC
100oC
120oC
62
NR
trace
NR
40
DCE
Table 2 Synthesis of 5H-benzo[d]imidazo[5,1-b][1,3]thiazines
via the silver(I)-promoted tandem reaction of o-alkynylphenyl
MeCN
MeCN
MeCN
65
isothiocyanatesa
R2
R2
60
CF3CO2Ag (20 mol%)
CH3COOH (2.0 equv.)
S
R1
R1
S
+
NaN3
C
N
N
N
MeCN, 80 o
C
N
N
2 | J. Name., 2012, 00, 1-3
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Scheme 2 Reaction of -2-isothiocyanatopyridines with 2
F
Cl
Afterwards, we also examined the reaction of methyl (E)-3-(2-
isothiocyanatophenyl)acrylate 1a’ with 2 under the standard
conditions (Scheme 3). Unsuspensefully, the more
activated acrylate fragment can also be carried into the
reaction and the corresponding product methyl (R)-2-(5H-
S
S
S
N
N
N
N
N
N
N
N
N
N
N
N
3b 69% yield
3c 71% yield
3a 84% yield
OCH3
Br
benzo[d]tetrazolo[5,1-b][1,3]thiazin-5-yl)acetate
3a’
was
S
S
S
obtained in 55% yield.
N
N
N
N
N
N
N
N
N
N
N
N
O
S
3f 87% yield
3e 85% yield
3d 67% yield
OCH3
O
CF3CO2Ag (10 mol%)
OCH3
S
+
NaN3
F
C
CH3COOH (2.0 equv.)
MeCN, 80 o
N
N
N
S
N
C
N
N
S
N
2
3a'
1a'
N
N
N
N
55% yield
N
N
4b 75% yield
Scheme 3 Reaction methyl (E)-3-(2-isothiocyanatophenyl)
acrylate with 2
4a
70%yield
F
We next focus our attention on the different cyclization modes
of 1, and a possible mechanism was proposed in Scheme 4.
Firstly, the formal [3+2] cycloaddition of azido anion 2 to an
isothiocyanate moiety in compound 1 would occur first to
produce the intermediate A. Intermediate A could then
undergo isomerization to afford intermediate B. Then, it
appears that R is critical in determining the regioselectivity.
When R was an aryl group, 6-exo-dig mode is favourable. The
sulfur anion would attack the triple bond which is activated by
coordination of the alkynyl moiety of B to the silver salt to
produce the Intermediate C. However, when R was the
cyclopropyl group, the electronic bias on the triple bond
favours 7-endo-dig cyclization to generate Intermediate D.
Finally, the protonation of intermediate C and D would happen
to give the target products 3 and 4.
S
S
N
S
N
N
N
N
N
N
N
N
N
N
N
4c 60% yield
3g 87% yield
3h 75% yield
Br
Cl
S
S
N
S
N
N
N
N
N
Cl
N
N
N
N
N
N
3k 74% yield
3j 75% yield
3i 76% yield
F
F
F3C
F
F3C
S
S
S
N
N
N
N
N
N
N
N
N
N
N
R
N
R
R
3n 67% yield
3l 66% yield
3m 70% yield
CF3COOAg, CH3COOH
CH3CN, T.
S
+
S
S
+
NaN3
aReaction was performed with o-alkynylphenyl isothiocyanate
mmol), sodium azide 2 (0.4 mmol), CH3COOH (0.4 mmol), AgOOCCF3 (0.04
1
(0.2
C
N
N
N
N
N
N
N
N
N
N
N
N
mmol) in MeCN (2 mL) under 80 0C for 20 h.
1
3
2
4
CH3COO-
H+
+
CH3CO2H
In order to further expand the substrate scope, we examined
the reaction of 2-isothiocyanato-3-(phenylethynyl)pyridine and
[Ag]
N
R
[3+2] cycloaddition
R= aryl
S
N
3-((4-chlorophenyl)ethynyl)-2-isothiocyanatopyridine with
2
H+
6-exo-dig
N
under the standard conditions (Scheme 2), the corresponding
R
S
R
N
C
[Ag]
products
[1,3]thiazine
(Z)-5-benzylidene-5H-pyrido[2,3-d]tetrazolo[5,1-b]
(3o) and (Z)-5-(4-chlorobenzylidene)-5H-
S
N
[Ag]
+
N
N
N
R
N
N
N
N
A
pyrido[2,3-d]tetrazolo[5,1-b][1,3]thiazine (3p) were obtained
B
R = cyclopropyl
S
N
in 78% and 71% yields, respectively.
N
N
7-endo-dig
N
Scheme 4 Proposed mechanistic pathwDay
AgOOCCF3, CH3CO2H
MeCN, 80 oC, 20 h
S
S
+
NaN3
2
C
Conclusions
N
N
N
N
N
N
N
In summary, we have developed an efficient method for the
synthesis of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines via the
silver(I)-catalyzed tandem bicyclization of o-alkynylphenyl
isothiocyanates and sodium azide. Different kinds of 5H-
benzo[d]tetrazolo[5,1-b][1,3]thiazines could be synthesized in
1o
3o 78% yield
Cl
Cl
AgOOCCF3, CH3CO2H
MeCN, 80 oC, 20 h
S
S
+
C
NaN3
N
N
N
N
N
N
N
1p
3p 71% yield
2
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Journal Name
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Ponnuswamy, New J. Chem., 2013, 37, 488.
good yields. In this reaction, a [3+2] cycloaddition reaction
mechanism was involved and two new rings were formed in
one-pot. This present cascade bicyclization strategy
represented an effective way to construct small molecular N,
S-heterocycles.
9
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support from the Natural Science Foundation of China
(NSFC) (grant number 21762023), the Natural Science
Foundation of Jiangxi Province of China (2018BAB203006) and
the Scientific Research Fund of Jiangxi Provincial Education
Department (grant number GJJ160285) are gratefully
acknowledged.
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Page 5 of 5
Organic & Biomolecular Chemistry
The Silver(I)-catalyzed Cascade Bicyclization Strategy for Synthesis of 5H-benzo[d]tetrazolo[5,1-b]
[1,3] thiazines
Jiankang Miao, Yahui Zhang, Xiaoyan Sang and Wenyan Hao*
College of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People’s Republic of China,
18 examples;
55%~87% yield ;
X = C, N;
An efficient method for the synthesis of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines via the
silver(I)-catalyzed tandem bicyclization of o-alkynylphenyl isothiocyanates and sodium azide.