Tandem addition-cyclization of o-ethynylphenyl isothiocyanates with N nucleophiles; Difference of cyclization mode between primary and secondary amines

Article abstract of DOI:10.1016/j.tetlet.2011.11.133

Silver triflate promotes the 6-exo-dig mode cyclization of the N-(2-ethynylphenyl)thioureas, which were easily obtained from the o-ethynylphenyl isothiocyanates and the primary amines, to provide the 2-imino-4-methylidene-1H-benzo[d][1,3]thiazines as the

Full text of DOI:10.1016/j.tetlet.2011.11.133

Tetrahedron Letters 53 (2012) 748–751
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Tandem addition–cyclization of o-ethynylphenyl isothiocyanates with N
nucleophiles; difference of cyclization mode between primary and
secondary amines q
⇑
Mamoru Kaname, Haruki Sashida
Faculty of Pharmaceutical Sciences, Hokuriku University, Kanagawa-machi, Kanazawa 920-1181, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Silver triflate promotes the 6-exo-dig mode cyclization of the N-(2-ethynylphenyl)thioureas, which were
easily obtained from the o-ethynylphenyl isothiocyanates and the primary amines, to provide the 2-
imino-4-methylidene-1H-benzo[d][1,3]thiazines as the sole product in excellent yields. The secondary
amines reacted with the o-ethynylphenyl isothiocyanates to give both the 6-exo and 5-endo-dig mode
cyclization products under the same conditions.
Received 12 October 2011
Revised 24 November 2011
Accepted 28 November 2011
Available online 9 December 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
o-Ethynylphenyl isothiocyanate
Tandem addition–cyclization
Silver catalyst
Benzo-1,3-thiazine
Indole
R1
1,3-Thiazines and benzo-1,3-thiazines, which are six-membered
heterocyclic compounds containing two atoms, nitrogen, and sulfur,
have attracted significant attention not only because of their inter-
esting syntheses,2 chemical properties, and reactivities,3 but also be-
cause of their pharmaceutical applications.4 The most efficient
routes of all the practicals protocol of these benzo-1,3-thiazines
are the tandem 6-exo-dig mode addition–cyclization reactions of
the 2-ethynylaniline derivatives with the isothiocyanates5 and re-
lated compounds6; they are promoted by a silver catalyst5c,6 or silica
gel5d (Scheme 1, Eq. 1). On the contrary, palladium(II) chloride cata-
lyzed reaction the of 2-ethynylanilines with the oxygen analogue,
isocyanates produced the 1,2-disubstituted indoles via the 5-endo-
dig mode cyclization (Eq. 2).7 Very recently, we reported the one-
pot synthesis of the selenium analogue, the 1,3-benzoselenazines,8
by the practical solvent- and catalyst-free tandem reaction of the
2-ethynylanilines with the isoselenocyanates (Eq. 3); the reaction
of the secondary aromatic amines with the isoselenocyanates gave
no 1,3-benzoselenazines. However, to the best our knowledge, the
reactions of the phenyl isothiocyanates having an ethynyl group at
the ortho-position with N nucleophiles still remains much less
explored.
NH2
X = S
R1
S
Lewis acid
or silica gel
R2 NCX
N
H
N
R2
R1
X = O
PdCl2
R1
X
N
R2
N
C
R2
H
N
H
N
H
O
R1
X = Se
Se
catalyst-,solvent-free
N
H
N
R2
As a part of the continuing research in our laboratory toward
the development of the chemistry of novel chalcogen-containing
Scheme 1.
heterocycles,9 we have previously disclosed the preparations of
selenium- and tellurium-containing heterocycles using the intra-
molecular cyclization of selenols and tellurols, which were gener-
q
See Ref. 1.
⇑
Corresponding author. Tel.: +81 76 229 6211; fax: +81 76 229 2781.
E-mail address: h-sashida@hokuriku-u.ac.jp (H. Sashida).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.133

M. Kaname, H. Sashida / Tetrahedron Letters 53 (2012) 748–751
749
Table 1
produce the corresponding benzo[d][1,3]thiazine 3Ab, 3Ac, and
3Ad in 90%, 92%, and 93% yields in one-pot, respectively (entries
3–5). Although the reaction of 1A with the aromatic amine, aniline
2e, required a slightly long reaction time under the standard reac-
tion conditions (AgOTf, 10 mol %), 3Ae was obtained in a 95% yield
by the use of 25 mol % of AgOTf (entry 6).
In order to demonstrate the efficiency of this tandem addition–
cyclization, the generality was explored by extending the sub-
strates to the o-ethynylphenyl isothiocyanates having an alkyl or
phenyl group at the sp-carbon atom of the triple bond with tert-
butylamine 2a. The treatment of o-propynylphenyl isothiocyanate
1B and the tert-butyl derivative 1C with tert-butylamine 2a affor-
ded the corresponding thiazine 3Ba and 3Ca in 85% and 93% yields,
respectively (entries 7 and 8). The 4-benzylidene-1,3-benzothi-
azine 3Da was also obtained from the corresponding isothiocya-
nate 1D and tert-butylamine 2a in a 96 yield (entry 9). However,
The TMS derivative 3Ea was too unstable to isolate; gradually
decomposed during the heating and purification with silica gel
chromatography (entry 10).
Reaction of 2-ethynylphenyl isothiocyanates 1 with primary amines 2a
R1
R1
R2 NH2
2
S
N
H
N
R2
N
C
S
AgOTf
3
1
n-Bu
Entry 2
S
N
H
C
HN t-Bu
4
Entry
Isothiocyanate
Amine
Product
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
1A: R1 = n-Bu
1A
1A
1A
1A
2a: R2 = t-Bu
3Aa
4
89
97c
90
92
93
95d
85
93
96
0e
2a
2b: R2 = n-Bu
3Ab
3Ac
3Ad
3Ae
3Ba
3Ca
3Da
3Ea
2c: R2 = cyclohexyl
2d: R2 = PhCH2
1A
2e: R2 = Ph
On the contrary, when the tandem addition–cyclization of o-
ethynylphenyl isothiocyanates 1 with the acyclic secondary amine
5 was performed under similar conditions, both the 6-exo-dig and
5-endo-dig mode cyclization occurred. o-Hexynylphenyl isothiocy-
anate 1A was similarly treated with diethylamine 5a to give the 5-
endo-dig mode cyclization product, the 2-butylindole derivative
1B: R1 = Me
1C: R1 = t-Bu
1D: R1 = Ph
1E: R1 = TMS
2a
2a
2a
2a
a
Standard reaction conditions: 1 (1 mmol), 2 (1.1 mmol), AgOTf (10 mol %), dry
THF (1.5 mL), refluxed, 4–12 h.
b
Isolated yield.
The reaction was carried out at room temperature.
AgOTf (25 mol %) was used.
Decomposed during the isolation.
6Aa, in
a 65% yield as the major product together with
c
benzo[d][1,3]thiazine 7Aa in a 19% yield (Table 2, entry 1).14 Treat-
ment of the isothiocyanates 1A with the cyclic amine, piperidine
5b, and pyrrolidine 5c, also gave the indoles 6Ab and 6Ac, together
with the benzo[d][1,3]thiazines 7Ab and 7Ac in the yield shown in
Table 2 (entries 2 and 3), respectively.
d
e
ated by the reaction of aryl (alkyl) halides with sodium hydrogen
selenide (NaHSe) and telluride (NaHTe) or aryllithium and elemen-
tal selenium and tellurium, with a triple bond as the synthetic
strategy.10 In this Letter, we now describe the tandem addition–
cyclization of the o-ethnylphenyl isothiocyanates with the primary
and secondary amines, and their difference of the cyclization
mode.
The starting o-ethynylphenyl isothiocyanates 1 were easily pre-
pared from commercially available 2-iodoaniline via Sonogashira
Pd coupling,11 followed by the treatment with thiophosgene12 in
two steps in good yields. The tandem addition–cyclization of the
o-ethynylphenyl isothiocyanate 1 with the primary amine 2 was
efficiently performed to give the (4Z)-2-imino-4-methylidene-
1H-benzo[d] [1,3]thiazine 3 under the silver catalyst conditions.
A preliminary survey to optimize the reaction conditions was car-
ried out. Thus, the following points for these reaction conditions
were found (1) use of silver triflate (AgOTf) as a catalyst, (2) reflux-
ing in anhydrous THF, and (3) inorganic base- and ligand-free
conditions. The extension of this tandem addition–cyclization of
the o-ethynylphenyl isothiocyanates 1 having some substituents
at the ethynyl moiety with primary amines 2 was carried out
and the results are summarized in Table 1.
The treatment of o-hexynylphenyl isothiocyanate 1A with
1.1 equiv of the primary aliphatic amine, tert-butylamine 2a in
the presence of AgOTf (10 mol %) in refluxing THF resulted in direct
ring-closure to give the desired benzo[d][1,3]thiazine 3Aa through
the 6-exo-dig mode cyclization in an 89% yield as the sole product
(entry 1).13 When the isothiocyanate 1A was treated with tert-
butylamine 2a in THF at room temperature, the adduct, N-tert-bu-
tyl-N’-[2-(hexyn-1-yl)phenyl]thiourea 4 was isolated in a 97% yield
(entry 2), and converted into the thiazine 3Aa using AgOTf in
refluxing THF in a 90% yield. Similarly, the isothiocyanate 1A was
treated with the aliphatic amines; n-butylamine 2b, cyclohexyl-
amine 2c and benzylamine 2d under the cyclization conditions to
Interestingly, for the reaction with morpholine 5d, the ratio of
products of the indole 6 and the benzo[d][1,3]thiazine 7 was
mostly reversed; the thiazine 7Ad was obtained in a 78% yield,
and the indole 6Ad was produced in only a 3% yield (entry 4). How-
ever, the reason why the ratio of products was reversed is not clear
at the present time. The examination of this tandem addition–
cyclization of the isothiocyanates 1 with the secondary amine
including the oxygen atom is now in progress.
Next, the aromatic secondary amine, such as N-methylaniline,
5e and imidazole 5f were found to selectively proceed affording
the corresponding indoles 6Ae, 6Af and thiazines 7Ae, 7Af in 19,
30 and 66, 44% yields, respectively (entries 5 and 6).
When the o-ethynylphenyl isothiocyanates 1B–D were re-
acted with diethylamine 5a under the same conditions, both
the indoles 6 and thiazines 7 were obtained in the yields as
shown in Table 2. (entries 7–9). In the case of the reaction of
the TMS derivative 1E with diethylamine 5a, both expected
products indole 6Ea and the thiazine 7Ea were unstable, 6Ea
was isolated in only a 5% yield after silica gel chromatography
(entry 10).
The configuration of the olefin moiety of the benzo[d][1,3]thia-
zines 3 and 7 was determined by its NOE, which was observed be-
tween the 10-H and the phenyl 5-H in the 500 MHz 1H NMR
spectra of 3Aa and 7Aa. Thus, the olefin moiety was determined
to have the (Z)-configuration. The stereospecific intramolecular
trans-addition of thiols to a triple bond, forming (Z)-vinylsulfides,
is known.9e Furthermore, the structure of 3Ae was directly identi-
cal to that of authentic sample.5c In both cases of the primary and
secondary amines, no 7-endo-dig mode cyclization products were
obtained.
In summary, the one-pot AgOTf catalyzed tandem addition–
cyclization of 2-ethynyphenyl isothiocyanates with the primary
and secondary amines smoothly occurred; the former gave the
benzo[d][1,3]thiazines via the 6-exo-dig mode cyclization, and the

750
M. Kaname, H. Sashida / Tetrahedron Letters 53 (2012) 748–751
Table 2
Reaction of o-ethynyphenyl isothiocyanates 1 with secondary amines 5
Entry
Isothiocyanate
Amine
Producta (%)
n-Bu
S
n-Bu
NEt2
NHEt2
5a
N
S
6Aa
7Aa
(19%)
(65%)
1
1A: R1 = n-Bu
N
N
NEt2
n-Bu
HN
n-Bu
N
N
S
6Ab (84%)
6Ac (51%)
6Ad (3%)
S
7Ab (8%)
2
3
4
5
6
1A
1A
1A
1A
1A
5b
N
n-Bu
HN
n-Bu
N
N
S
S
7Ac (40%)
5c
N
N
N
n-Bu
HN
O
n-Bu
N
N
S
S
7Ad (78%)
5d
O
N
O
n-Bu
n-Bu
NHMePH
5e
N
S
6Ae (19%)
6Af (30%)
S
7Ae (66%)
Ph
N
Ph
Me
N
N
Me
N
n-Bu
HN
n-Bu
N
S
S
7Af (44%)
5f
N
N
N
N
N
Me
Me
1B: R1 = Me
1C: R1 = t-Bu
1d: R1 = Ph
1E: R1 = TMS
5a
N
S
6Ba (92%)
6Ca (36%)
7Ba (2%)
7
8
9
S
NEt2
N
N
N
N
NEt2
t-Bu
t-Bu
NEt2
N
S
7Ca (54%)
5a
S
NEt2
Ph
S
Ph
N
S
6Da (42%)
6Ea (5%)b
7Da (53%)
7Ea (0%)b
5a
NEt2
NEt2
TMS
TMS
NEt2
N
S
10
5a
S
NEt2
a
Isolated yield.
Decomposed during the isolation.
b
latter produced both the 5-endo-dig mode cyclization product, 2-
References and notes
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13.
A
typical experimental procedure for tandem addition–cyclization of 2-
ethynylphenyl isothiocyanate with the primary amine is as follows:
A
mixture of 2-hexynylphenyl isothiocyanate 1A (215 mg, 1 mmol), tert-butyl
amine 2a (80 mg, 1.1 mmol) and AgOTf (26 mg, 0.1 mmol) in dry THF (2.5 mL)
was refluxed for 10–12 h under argon atmosphere. The mixture was diluted
with benzene (20 mL), and organic layer was washed with water (20 mL ꢀ 2),
dried over anhydrous Na2SO4, and evaporated in vacuo. The obtained residue
was chromatographed on silica gel using hexane–benzene (1:1) as an eluent to
give pure (4Z)-2-tert-butylimino-4-pentylidene-1H-benzo[d][1,3]thiazine 3Aa.
Yield: 256 mg (89%). Pale yellow oil. IR (KBr, neat): 3398 (NH), 1623
(C@N) cmꢁ1 1H NMR (500 MHz, CDCl3) d = 0.93, 1.32–1.42, 1.42–1.53, 2.24
.
(3H, t, J = 7.2 Hz, 2H, m, 2H, m, 2H, dt, J = 7.3, 7.4 Hz, n-Bu), 1.48 (9H, s, t-Bu),
4.20–4.38 (1H, br, NH), 6.07 (1H, t, J = 7.3 Hz, C@CH), 7.02, 7.15, 7.23, 7.34 (1H,
ddd, J = 7.7, 7.3, 1.2 Hz, 1H, d, J = 7.7 Hz, 1H, dd, J = 7.8, 7.3 Hz, 1H, dd, J = 7.8,
1.2 Hz, Ph-H). 13C NMR (125 MHz, CDCl3) d = 14.0 (q), 22.4 (t), 28.9 (t), 29.4 (q),
31.2 (t), 53.7 (s), 121.1 (s), 123.6 (d), 123.9 (d), 126.2 (s), 126.3 (d), 127.2 (d),
128.8 (d), 144.6 (s), 149.0 (s). MS (EI): m/z (relative intensity,%) 288 (M+, 100),
245 (52), 232 (54), 189 (90), 155 (26), 130 (18). HRMS (EI) m/z M+ calcd for
C17H24N2S: 288.1660; found: 288.1663.
14.
A typical experimental procedure for tandem addition–cyclization of 2-
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4412.
ethynylphenyl isothiocyanate with the secondary amine is as follows: A mixture
of 2-hexynylphenyl isothiocyanate 1A (215 mg, 1 mmol), diethylamine 5a (80 mg,
1.1 mmol) and AgOTf (26 mg, 0.1 mmol) in dry THF (2.5 mL) was refluxed for 2–
22 h under argon atmosphere, and worked up to give 6Aa and 7Aa
8. Sashida, H.; Pan, C.; Kaname, M.; Minoura, M. Synthesis 2010, 3091–3096.
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1352; (b) Sashida, H.; Nakabayashi, S.; Kaname, M.; Minoura, M. Heterocycles
2010, 80, 1339–1352; (c) Sashida, H.; Satoh, H.; Ohyanagi, K.; Kaname, M.
Molecules 2010, 15, 1466–1472; (d) Sashida, H.; Kaname, M.; Ohyanagi, K.
Heterocycles 2010, 82, 441–447; (e) Sashida, H.; Kaname, M.; Nakayama, A.;
Suzuki, H.; Minoura, M. Tetrahedron 2010, 66, 5149–5157; (f) Sashida, H.;
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51, 5395–5398; (g) Kaname, M.; Minoura, M.; Sashida, H. Tetrahedron Lett.
2011, 52, 505–508; (h) Kaname, M.; Sashida, H. Tetrahedron Lett. 2011, 52,
3279–3282.
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11. (a) Sonogashira, K.; Tohda, Y.; Hagiwara, N. Tetrahedron Lett. 1975, 4467–4470;
(b) Sonogashira, K. In Metal-catalyzed cross-coupling reaction; Diederich, F.,
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Sonogashira, K. J. Organomet. Chem. 2002, 653, 46–49.
Aa: Yield: 187 mg (65%). Colorless oil. 1H NMR (500 MHz, CDCl3) d = 0.95, 1.00,
1.39–1.50, 1.64–1.81, 2.63–2.71, 2.79–2.88, 3.13–3.29, 4.06–4.18 (3H, t,
J = 6.5 Hz, 3H, t, J = 7.2 Hz, 5H, m, 2H, m, 1H, m, 1H, m, 2H, m, 2H, m, n-Bu,
Et ꢀ 2), 6.36 (1H, s, 3-H), 7.06–7.17, 7.49 (3H, m, 1H, d, J = 5.7 Hz, Ph-H). 13C NMR
(125 MHz, CDCl3) d = 11.1 (q), 13.8 (q), 13.9 (q), 22.5 (t), 26.7 (t), 30.2 (t), 46.6 (t),
47.2 (t), 102.1 (d), 110.0 (d), 120.0 (d), 120.7 (d), 122.0 (d), 128.4 (s), 135.6 (s),
140.2 (s), 181.0 (s). MS (EI): m/z (relative intensity, %) 288 (M+, 100), 140 (31), 116
(84), 88 (53). HRMS (EI) m/z M+ calcd for C17H24N2S: 288.1660; found: 288.1662.
Aa: Yield: 55 mg (19%). Yellow oil. 1H NMR (500 MHz, CDCl3) d = 0.93, 1.30–1.43,
1.43–1.50, 2.25–2.32 (3H, t, J = 7.3 Hz, 2H, m, 2H, m, 2H, m, n-Bu), 1.21, 3.58 (6H,
t, J = 7.0 Hz, 4H, q, J = 7.0 Hz, Et ꢀ 2), 6.04 (1H, t, J = 7.3 Hz, C@CH), 6.96, 7.08,
7.20, 7.32 (1H, ddd, J = 7.7, 7.4, 1.5 Hz, 1H, dd, J = 7.9, 1.5 Hz, 1H, ddd, J = 7.9, 7.4,
1.5 Hz, 1H, dd, J = 7.7, 1.5 Hz, Ph-H). 13C NMR (125 MHz, CDCl3) d = 13.8 (q), 13.9
(q), 22.4 (t), 28.7 (t), 31.3 (t), 43.1 (t), 120.3 (s), 122.8 (d), 123.5 (d), 125.8 (s),
125.8 (d), 126.8 (d), 128.8 (d), 145.1 (s), 152.4 (s). MS (EI): m/z (relative
intensity,%) 288 (M+, 100), 259 (57), 245 (87), 219 (33). HRMS (EI) m/z M+ calcd
for C17H24N2S: 288.1660; found: 288.1670.