Investigation of the Reaction between 3-Benzisothiazolones, an Isoindole Isoster and Activated Acetylenes: Synthesis of Heterocyclic Backbones for Building Bioactive Molecules

Article abstract of DOI:10.1002/jhet.2197

Investigation of the reaction between benzo[d]isothiazol-3-one, 2-aminobenzo[d]isothiazol-3-one, its isoster 2-aminoisoindolin-1-one, and activated acetylenes, in the presence of triphenylphosphine, led us to synthesize novel heterocyclic compounds that c

Full text of DOI:10.1002/jhet.2197

Month 2014 Investigation of the Reaction between 3-Benzisothiazolones, an Isoindole
Isoster and Activated Acetylenes: Synthesis of Heterocyclic Backbones for
Building Bioactive Molecules
*
Matteo Incerti and Paola Vicini
Dipartimento di Farmacia, Università degli Studi di Parma, Viale delle Scienze 27/A, Parma 43124, Italy
*
E-mail: paola.vicini@unipr.it
Received January 20, 2014
DOI 10.1002/jhet.2197
Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com).
Investigation of the reaction between benzo[d]isothiazol-3-one, 2-aminobenzo[d]isothiazol-3-one, its
isoster 2-aminoisoindolin-1-one, and activated acetylenes, in the presence of triphenylphosphine, led us to
synthesize novel heterocyclic compounds that could be attractive for the building of biologically active
molecules.
A
one-pot PPh3-promoted tandem reaction, with acetylene dicarboxylates and
dibenzoylacetylene, afforded new tricyclic pyrazolo-fused benzisothiazoles. The PPh3-promoted reaction
between benzisothiazolones and methyl propiolate afforded 1,4-benzothiazepine-5-one derivatives, via an
isothiazole ring expansion. These studies are providing additional insights in benzisothiazolone chemistry
and describe simple and original synthetic accesses to novel derivatives.
J. Heterocyclic Chem., 00, 00 (2014).
INTRODUCTION
a 1,2-methyl ester migration to give 3-H-benzo[d]pyrazolo
[1,5-b]isothiazole-2,3a-dicarboxylic acid dimethyl ester (2),
a novel functionalized pyrazolo-fused benzisothiazole, in
satisfactory (65%) yield [1]. The reaction, here described,
of 1 with diethyl and di-tert-butyl acetylenedicarboxylate
in refluxing toluene for 24h resulted in complete consump-
tion of the starting materials and formation of 3 (40% yield)
and 4 (54% yield), respectively, along with the recovery of
triphenylphosphine oxide (Fig. 1). The mechanism for the
whole reaction process is outlined in Figure 2.
A first Michael addition of triphenylphosphine to the
acetylenic esters gives a 1:1 adduct that undergoes proton-
ation by the NH2-group of 2-aminobenzo[d]isothiazol-3-
one. Then, a second addition of the NH anion to the
positively charged Michael acceptor provides phosphorane,
which spontaneously undergoes an intramolecular Wittig-
type reaction to produce, through the intermediacy of
betaine, a tricyclic heterocycle and triphenylphosphine
oxide. At this point, 1,2-hydride migration provides iminium
compound, upon which a 1,2-methyl ester migration occurs
via cyclopropane intermediate, furnishing the reaction
products 2–4.
In the search for chemical modification of heterocyclic
compounds that give access to molecular diversity,
recently we have focused our attention on PPh3-promoted
tandem reaction between 2-aminobenzo[d]isothiazol-3-one
(1) and activated acetylenic reagents such as dimethyl
acetylenedicarboxylate (DMAD) and alkyl propiolate
[1,2]. Following our work on exploiting PPh3-promoted
reactions to obtain novel benzisothiazole derivatives of
interest in drug discovery, which have been the object of many
years of our studies [3–12], herein we wish to report the reac-
tion of 1 with further dialkyl acetylenedicarboxylates, with
dibenzoylacetylene (DBA), and with phenylmethylpropiolate.
In order to explore the applicability of the reactions over dif-
ferent benzisothiazolones and to have deeper understanding
of the structural requirements and of the processes involved,
benzo[d]isothiazol-3-one (8) and 2-aminoisoindolin-1-one
(10), an isoindole isoster of 1, were also subjected to identical
reaction conditions. The results are summarized in Table 1.
The structures of the obtained compounds were elucidated
mainly by means of 1H-NMR and 13C-NMR techniques.
Afterward, 2-aminobenzo[d]isothiazol-3-one (1) was
reacted with DBA, as activated alkyne. A new tricyclic
pyrazolo-fused benzisothiazole, namely 2,3-dibenzoyl-2-H-
benzo[d]pyrazolo[1,5-b]isothiazole(5), was obtained (Fig. 1).
Contrary to the use of dialkyl acetylenedicarboxylates
(refluxed in toluene), the reaction with DBA turned out to
take place in 5 h, at room temperature, in CH2Cl2 solution.
RESULTS AND DISCUSSION
In our initial investigations, we examined the reaction be-
tween 2-aminobenzo[d]isothiazol-3-one (1) and DMAD in
the presence of an equimolar amount of triphenylphosphine.
The one-pot reaction proceeded smoothly as a tandem
sequence of a phospha-Michael, an aza-Michael addition,
and a final intramolecular Wittig-type reaction followed by
1
Moreover, remarkably, the H-NMR analysis of 5 pointed
out that no benzoyl group migration occurred, as deduced
© 2014 HeteroCorporation

M. Incerti and P. Vicini
Vol 000
Table 1
A successful nucleophilic attack of the dipolar interme-
diate, formed by a Michael addition of triphenylphosphine
to alkyl propiolate, on the nitrogen atom of the isothiazole
ring of 1, was recently reported by us to afford, through an
isothiazole ring expansion, in 45% yield, the noteworthy
functionalized 1,4-benzothiazepine-5-ones 6 and 7 (Fig. 1) [2].
Given the interest of the unprecedented reactions discov-
ered between 2-aminobenzo[d]isothiazol-3-one (1) and
activated acetylenic reagents, we tried under the same
conditions (equimolar amounts of the reagents, in the pres-
ence of triphenylphosphine) the reaction between 1 and
methylphenylpropiolate. As shown in Figure 1, despite
few trials with different solvents as toluene, CH2Cl2, and
THF, the reaction did not succeed. At room temperature,
the reagents failed to react and were recovered unchanged,
while, by heating in different solvents, an unsolvable
mixture was formed. This led us to suppose that the steric
bulk and the electronic feature of the phenyl ring directly
linked to the alkyne prevents the first Michael addition of
triphenylphosphine, at room temperature, to the
methylphenylpropiolate; refluxed in different solvents, the
reactants underwent decomposition.
In order to expand the scope of the approach described
in Figure 1, benzo[d]isothiazol-3-one (8) was subjected to
the same reaction conditions applied for 1. The results in
Table 1 show that the only successful reaction was the
one with the activated acetylene methyl propiolate (repre-
sentative of the alkyl propiolates) that afforded, in 46%
yield, through an isothiazole ring expansion, methyl
5-oxo-4,5-dihydrobenzo[f][1,4]thiazepine-3-carboxylate (9).
A plausible reaction mechanism for the formation of com-
pound 9, in full agreement with the sequence proposed
for the tandem reaction that led to methyl 4-amino-5-
oxo-4,5-tetrahydrobenzo[f][1,4]thiazepine-3-carboxylate 6 [2],
is depicted in Figure 3.
Reactions of benzisothiazolones 1 and 8 and isoindolone 10 with various
acetylenic reagents in the presence of triphenylphosphine.
Substrate
R1
R2
Product
1 (X = S; Y = NH2)
1
1
1
1
1
1
8 (X = S; Y = H)
8
8
8
8
8
–COOMe
–COOEt
–COOt-Bu
–COPh
–H
–COOMe
–COOEt
–COOt-Bu
–COPh
–COOMe
–COOEt
–COOMe
–COOMe
–COOEt
–COOt-Bu
–COPh
–COOMe
–COOMe
–COOMe
–COOEt
–COOt-Bu
–COPh
2[1]
3
4
5
6[2]
7[2]
—
—
—
—
—
9
–H
–Ph
–COOMe
–COOEt
–COOt-Bu
–COPh
–H
–Ph
—
—
—
—
—
—
—
10 (X = CH2; Y = NH2)
–COOMe
–COOEt
–COOt-Bu
–COPh
–H
10
10
10
10
10
–COOMe
–COOMe
–Ph
by no signal of diastereotopic H-atoms as was, on the
contrary, previously observed in the 1H-NMR spectra of the
tricyclic products 2–4. The presence of signals at δ = 6.78
ppm, of a CH proton, and, downfield, at δ = 13.44 ppm, of a
typical H-atom bound to a heteroatom (D2O exchangeable)
further supported the formation of compound 5. It is thus
the nature of the ethyne substituents that does govern the last
step of compounds 2, 3, 4, and 5 formation. Mechanistically,
it is conceivable that the reaction process involved in the for-
mation of compound 5 proceeded as described earlier for 2, 3,
and 4 (Fig. 2); however, the reaction ended with the intramo-
lecular Wittig-type cyclization.
The nitrogen atom of the isothiazole ring of 8 is suscepti-
ble to nucleophilic attack as activated for the adjacent sulfur
and carbonyl group. The nucleophilic attack of the dipolar
intermediate, formed in situ by a Michael addition of
triphenylphosphine to methyl propiolate, is accompanied
Figure 1. PPh3-promoted reaction between 2-aminobenzo[d]isothiazol-3-one (1) and acetylenic reagents.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2014
Investigation of the Reaction between 3-Benzisothiazolones, an Isoindole Isoster and Ac-
tivated Acetylenes: Synthesis of Heterocyclic Backbones for Building Bioactive Molecules
Figure 2. Mechanistic pathway for the formation of compounds 2–4.
Figure 3. Mechanistic pathway for the formation of compound 9.
by a spontaneous cleavage of the N–S bond, reasonably
supported by the chemistry of isothiazolones that well
explains the liability of N–S bond [13–15]. The open-chain
undergoes a further nucleophilic attack of the sulfur to the
electrophilic centre C(2) of the betainic intermediate. Thus,
benzothiazepine 9 is formed by elimination of PPh3 and
intramolecular cyclocondensation with ring expansion to
the stable seven-membered benzofused ring. To explain the
complete lack of reactivity between benzo[d]isothiazol-3-
one (8) and dialkyl acetylenedicarboxylates or DBA, which
gave, on the contrary, products 2–5 in good yields starting
from 1, we want to point out the absence in compound 8 of
the NH2 nitrogen as the fifth term of a stable transition inter-
mediate in the Wittig reaction leading to cyclization, as what
occurred for compound 1. Despite the presence in 8 of the
isothiazole NH as proton source, whose importance has been
widely demonstrated in Yavari’s studies [16–19] the aza
Michael addition to the protonated 1:1 adduct did not
succeed. In fact the instable four terms cycle prevented the
cyclization step that led to tryciclic derivatives 2, 3, 4, and
5. The 1H-NMR monitoring of the reactions between 8 and
disubstituted acetylenes confirmed lack of reaction at
room temperature and, by heating, consumption of the
reactants along with formation of a complex mixture of
unidentified products.
To further extend the study of the previously reported
processes, a desulfurated isoster of 1, namely 2-
aminoisoindolin-1-one (10, Table 1), was subjected, under
similar conditions, to the reactions described between
benzisothiazolones 1 and 8 and activated acetylenes. As
expected, no significant reaction was observed, and the
lack of reaction led us to confirm the pivotal role, in
benzisothiazolones, of the S-atom, which is crucial for
the Michael addition of the NH group to the phosphorated
adduct that starts these tandem processes.
Synthetic methods, characterization data for compounds
1
3–5 and 9, including mp, H-NMR and 13C-NMR, MS
data, and the results of elemental analysis are reported in
the experimental section.
In conclusion, this work provides a direct route to the
synthesis of some novel biologically relevant heterocylic
compounds. In any case, all these studies are providing
few additional insights in benzisothiazolone chemistry as
well as simple and original synthetic accesses. When appli-
cable, the selectivity, good to high yields, simple work up
procedures, and mild conditions are advantages of these
processes. We hope in the future to further exploit, with
the present approach, the benzisothiazolone scaffold as a
source of highly functionalized heterocycles for the build-
ing of biologically active molecules.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

M. Incerti and P. Vicini
Vol 000
(CH), 129.30 (C), 130.27 (CH), 130.39 (C), 131.71 (CH),
EXPERIMENTAL
157.48 (C), 164.76 (C═O), 168.92 (C═O); IR (KBr) (νmax
/
cmÀ1): 3041 (CH Ar), 2977 and 2911 (CH3), 1720 and 1681
2-Aminobenzo[d]isothiazol-3-one (1) and benzo[d]isothiazol-
3-one one (8) were synthesized as reported earlier by us [12].
DBA was prepared according to [20], and 2-aminoisoindolin-
1-one (10) was synthesized following a modification of a de-
scribed procedure [21,22]. Unless otherwise noted, reagents were
obtained from commercial suppliers and were used without puri-
fication. Anhydrous toluene was obtained by distillation from Na,
and anhydrous CH2Cl2 was obtained by distillation from calcium
hydride. All reactions were carried out using flame-dried glass-
ware under a nitrogen atmosphere. Melting points were measured
on a Buechi 512 apparatus (Buechi Italia, Milano, Italia) and are
uncorrected. The progress of the reactions was monitored by
TLC with F254 silica-gel precoated sheets (Merck, Darmstadt,
Germany). UV light was used for detection. Flash chromatogra-
phy was performed using Merck silica gel 60 (Si 60, 40–63 μm,
230–400 mesh ASTM). Elemental analyses for C, H, and N were
performed using a ThermoQuest Flash 1112 elemental analyzer
(Termoquest Italia, Milano, Italy). IR spectra were recorded on
a Jasco FT-IR 300E spectrometer (Jasco Europe, Carpi (MO),
Italia). Mass spectra were recorded on an Applied Biosystem
API 150 EX LC-MS system spectrometer (AB/SCIEX, Toronto,
Canada). 1H-NMR and 13C-NMR spectra were recorded on a
Bruker Avance 300 spectrometer (Brucker Italia, Milano, Italia).
Spectra were acquired from samples in DMSO-d6 (25 mg mLÀ1).
1H and 13C spectra (at 300 and 75 MHz, respectively) were mea-
sured at 25°C in 5 mm o.d. tubes. Chemical shifts are reported as
δ (ppm); coupling constants (J) are expressed in Hz.
(C═O ester). MS (APCI) m/z: 376 (M + 1). Anal. Calcd. for
C
19H24N2O4S (376.47): C, 60.62; H, 6.43; N, 7.44. Found
C, 60.32; H, 6.40; N, 7.28%.
2,3-Dibenzoyl-2-H-benzo[d]pyrazolo[1,5-b]isothiazole (5).
To a magnetically stirred solution of triphenylphosphine (0.22 g,
0.84mmol) and compound 1 (0.14g, 0.84 mmol) in anhydrous
dichloromethane (10 mL), kept under nitrogen atmosphere was
added dropwise a solution of DBA (0.20 g, 0.84 mmol) in
anhydrous dichloromethane (1mL). After stirring for 5 h at room
temperature, the solvent was removed under reduced pressure.
The reddish solid residue was purified by silica gel flash
chromatography (dichloromethane/ethyl acetate 95:5 v/v) to
afford compound 5, which was recrystallized from ethanol/water.
Yield 45%; mp 176–177°C; 1H-NMR (300MHz DMSO-d6):
δ = 6.78 (d, 1H, J = 1.5 CH), 7.26–7.56 (m, 9H, Ar), 7.63–7.66
(d, 2H, J = 7.5 Ar), 7.67–7.70 (d, 2H, J = 7.5 Ar,), 7.95 (d, 1H,
J = 7.8 Ar), 13.44 (d, 1H, J = 1.5 NH); 13C-NMR (75MHz
DMSO-d6): δ = 102.51 (CH), 123.89 (C), 126.11 (CH), 126.22
(CH), 126.75 (CH), 128.06 (CH), 128.23 (CH), 128.48 (CH),
128.72 (CH), 129.08 (C), 131.74 (CH), 133.60 (CH), 136.54 (C),
139.14 (C), 145.89 (C), 150.59 (C═O), 163.31 (C═O); (IR (KBr)
(νmax/cmÀ1): 3242 (NH), 3061 (CH Ar), 2919 (CH), 1725 and
1703 (C═O). MS (APCI) m/z: 385 (M + 1). Anal. Calcd.
C23H16N2O2S (384.45): C, 71.85; H, 4.19; N, 7.29. Found. C,
71.90; H, 3.98; N, 7.08, %.
Methyl 5-oxo-4,5-dihydrobenzo[f][1,4]thiazepine-3-carboxylate
(9).
To a magnetically stirred mixture of 2-aminobenzo[d]
General procedure for the preparation of 3-H-benzo[d]
pyrazolo[1,5-b]isothiazole-2,3a-dicarboxylic acid dialkyl
esters. To a magnetically stirred mixture of 2-aminobenzo[d]
isothiazol-3-one (1) (0.5 g, 3.0 mmol) and triphenylphosphine
(0.8 g, 3.0 mmol) in anhydrous toluene (7 mL), a solution of
DMAD (0.43 g, 3.0 mmol) in anhydrous toluene (7 mL) was
added dropwise over 5 min keeping the temperature at À5°C.
The reaction mixture was allowed to warm to room temperature
and then refluxed for 24 h. The solvent was removed under
reduced pressure, and the oily residue was purified by silica gel
chromatography using CH2Cl2/ethyl acetate 90:10 (3) or 95/5 (4)
v/v mixture as eluent. The solid obtained was then recrystallized
from ethyl acetate.
isothiazol-3-one 1 (0.5 g, 3.0 mmol) and triphenylphosphine
(0.8g, 3.0 mmol) in anhydrous toluene (7mL), a solution of alkyl
propiolate (3.0 mmol) in anhydrous toluene (2 mL) was added.
The reaction mixture was stirred at room temperature for 5 h. The
solvent was removed under reduced pressure, and the oily residue
was purified by silica gel flash chromatography (CH3Cl/ethanol
98:2 v/v). The solid obtained was recrystallized from toluene
furnishing the product as a white solid. Yield 40%; mp 115°C;
1H-NMR (300MHz DMSO-d6): δ = 3.72 (s, 3H, CH3), 7.24
(s, 1H, CH), 7.41 (td, 1H, J = 1.2 J = 7.8 Ar), 7.45–7.53 (m, 2H,
Ar,), 7.66 (dd, 1H, J = 1.5J = 7.2 Ar), 9.80 (s, 1H, NH); 13C-NMR
(75 MHz DMSO-d6): δ = 52.74 (CH), 120.85 (CH), 128.38 (CH),
131.22 (CH), 132.15 (CH), 132.42 (CH), 133.36 (C), 135.77 (C),
136.46 (C), 161.79 (C═O), 167.95 (C═O); IR (KBr)
(νmax/cmÀ1): 3190 (NH), 3086 (CH Ar), 2957 (CH3), 1724 (C═O
ester) and 1653 (C═O). MS (APCI) m/z: 236 (M+ 1). Anal.
Calcd. for C11H9NO3S (235.26): C, 56.16; H, 3.86; N, 5.94.
Found C, 56.35; H, 3.87; N, 5.94%.
3-H-Benzo[d]pyrazolo[1,5-b]isothiazole-2,3a-dicarboxylic acid
diethyl ester (3).
Yield 40%; mp 146–147°C; 1H-NMR
(300 MHz DMSO-d6): δ =0.94 (t, 3H, J=7.2 CH3), 1.33 (t, 3H,
J=7.1 CH3), 3.64 (d, 1H, J= 17.70 CH), 3.82 (d, 1H, J= 17.70
CH), 4.02 (q, 2H, J=7.2 CH2), 4.29 (q, 2H, J=7.2 CH2) 7.37–7.52
(m,3H, Ar), 8.01 (dd,1H, J=1.5 J= 8.1 Ar); 13C-NMR (75 MHz
DMSO-d6): δ = 27.27 (CH3), 123.89 (C), 27.71 (CH3), 44.19
(CH2), 67.84 (CH2), 84.31 (C), 85.03 (C), 126.56 (CH), 126.94
(CH), 129.86 (C), 130.08 (CH), 130.92 (C), 131.43 (CH), 157.32
(C), 162.61 (C═O), 167.67 (C═O); IR (KBr) (νmax/cmÀ1): 3039
(CH Ar), 2960 and 2836 (CH3), 1727 and 1690 (C═O ester). MS
(APCI) m/z: 321 (M + 1). Anal. Calcd. for C15H16N2O4S (320.36):
C, 56.24; H, 5.03; N, 8.74. Found C, 55.97; H, 5.10; N, 8.54%.
3-H-Benzo[d]pyrazolo[1,5-b]isothiazole-2,3a-dicarboxylic acid
2-Aminoisoindolin-1-one (10).
A modification of a
described procedure [21] was used: to a magnetically stirred
solution of N-(t-butyloxycarbonyl)-2-aminoisoindolin-1-one [22]
(0.10 g, 0.40 mmol) in ethyl acetate (10 mL) cooled at 0°C was
added a solution of 3M HCl in ethyl acetate (5 mL). The
reaction mixture was then allowed to warm to room temperature
and stirred overnight. Evaporation of the solvent under reduce
pressure afforded a white solid. Yield 94%; mp 94–96.
1H-NMR (300 MHz DMSO-d6): δ = 4.28 (s, 2H, NH2), 4.80
(s, 2H, CH2), 7.37–7.44 (m, 2H, Ar), 7.50 (t, 1H, J = 6.9 Ar),
7.78 (d, 1H, J = 7.8 Ar); 13C-NMR (75 MHz DMSO-d6):
δ = 53.30 (CH2), 122.82(CH), 123.67 (CH), 128.25 (CH),
131.48 (CH), 132.34 (C), 140.31 (C), 166.09 (C═O); IR (KBr)
(νmax/cmÀ1): 3290 and 3185 (NH2), 3045 (CH Ar), 2823 (CH2),
1690 (C═O), MS (APCI) m/z: 185 (M + 1).
di-tert-butyl ester (4).
Yield 54%; mp 147–148°C; 1H-NMR
(300 MHz DMSO-d6): δ =0.94 (s, 9H, (CH3)3), 1.54 (s, 9H, (CH3)
3), 3.45 (d, 1H, J= 17.70 CH), 3.69 (d, 1H, J= 17.40 CH),
7.38–7.55 (m, 3H, Ar), 8.00 (dd, 1H, J=1.2 J= 8.1 Ar); 13C-NMR
(75 MHz DMSO-d6): δ = 13.52 (CH3), 14.27 (CH3), 42.91 (C),
126.22 (CH), 63.25 (C), 67.21 (C), 68.43 (C), 126.70 (CH), 127.19
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2014
Investigation of the Reaction between 3-Benzisothiazolones, an Isoindole Isoster and Ac-
tivated Acetylenes: Synthesis of Heterocyclic Backbones for Building Bioactive Molecules
[8] Geronikaki, A.; Eleftheriou, P.; Vicini, P.; Dixit, A.; Saxena,
A.; Alam, I. J Med Chem 2008, 51, 5221.
[9] Zani, F.; Vicini, P.; Incerti, M. Eur J Med Chem 2004, 39, 135.
[10] Vicini, P.; Zani, F.; Cozzini, P.; Doytchinova, I. Eur J Med
Chem 2002, 37, 553.
[11] Vicini, P.; Geronikaki, A.; Incerti, M.; Busonera, B.; Poni, C.;
Cabras, C. A.; La Colla, P. Bioorg Med Chem 2003, 11, 4785.
[12] Vicini, P.; Manotti, C.; Caretta, A.; Amoretti, L. Arzneim-
Forsch Drug Res 1997, 47, 1218.
Acknowledgments. The authors acknowledge the financial support
of the Ministero dell’Istruzione, dell’Università e della Ricerca
(MIUR), Italy, and are grateful to the Centro Interfacoltà Misure
(CIM), University of Parma, for the use of NMR instruments.
Dr. Domenico Acquotti is also acknowledged for the helpful
discussion on NMR.
[13] Siegemund, A.; Taubert, K.; Schulze, B. Sulfur Rep 2002, 23,
279.
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