Synthesis of isothiocoumarin derivatives

Article abstract of DOI:10.1007/s10593-010-0484-3

A method was developed for the synthesis of 1-oxo-1H-isothiochromenes from 2-benzofuran-1(3H)-one (phthalide). 3-Bromo-6-chloro- and 3,6-dibromo-2- benzofuran-1(3H)-ones were prepared by the bromination of 6-chloro- and 6-bromo-2-benzofuran-1(3H)-ones and were converted by hydrolysis into 5-chloro- or 5-bromo-2-formylbenzoic acids. The condensation of these acids with rhodanine followed by recyclization gave 7-chloro- and 7-bromo-1-oxo-1H-isothiochromene- 3-carboxylic acids.

Full text of DOI:10.1007/s10593-010-0484-3

Chemistry of Heterocyclic Compounds, Vol. 46, No. 2, 2010
SYNTHESIS OF ISOTHIOCOUMARIN
DERIVATIVES
N. T. Pokhodylo1, V. S. Matiychuk1, and M. D. Obushak1*
A method was developed for the synthesis of 1-oxo-1H-isothiochromenes from 2-benzofuran-1(3H)-one
(phthalide). 3-Bromo-6-chloro- and 3,6-dibromo-2-benzofuran-1(3H)-ones were prepared by the
bromination of 6-chloro- and 6-bromo-2-benzofuran-1(3H)-ones and were converted by hydrolysis into
5-chloro- or 5-bromo-2-formylbenzoic acids. The condensation of these acids with rhodanine followed
by recyclization gave 7-chloro- and 7-bromo-1-oxo-1H-isothiochromene-3-carboxylic acids.
Keywords: 2-benzofuran-1(3H)-one, 1-oxo-1H-isothiochromenone, 2-formylbenzoic acid, heterocycli-
zation, recyclization.
Many derivatives of 1H-2-benzothiopyran (1H-isothiochromenes) exhibit biological activity [1-5].
However, due to the limited number of methods available for their synthesis the properties of these compounds
have been insufficiently investigated [6]. 1H-2-Benzothiopyrans are produced by the reactions of 2-(aroyl-
methyl)benzoates with P2S5 [7], 2-cyanomethylbenzoates with carbon disulfide [8], and lithium N,N-diethyl-
o-toluamide with thioethers [9]. Isothiochromene-1-thione was obtained with a yield of 12% by the reaction of
N-cyanomethyl-N-methyl-2-chloro-5-nitrobenzamide with NaH and carbon disulfide in DMSO [10]. A more
convenient method for the synthesis of isothiocoumarins is the reaction of ortho-formylbenzoic acids with
rhodanine (2-thioxo-4-thiazolidinone) followed by recyclization of the obtained 5-arylidenerhodanine in a basic
medium [11, 12] although the initial ortho-formylbenzoic acids are difficult to obtain. The possibilities of
applying this method to the creation of combinatorial libraries of heterocyclic compounds with an
isothiocoumarin fragment were demonstrated in [13]. The possibilities of the method are limited by the fact that
the available starting compounds are formylbenzoic and opianic acids. During the production of substituted
2-formylbenzoic acids organometallic compounds are often used for formylation or carboxylation [14-16]. We
note that the significant lability of the isothiocoumarin ring complicates the direct introduction of substituents
by, for example, electrophilic substitution.
In the present work a convenient method is proposed for the synthesis of substituted isothiocoumarins of
type I suitable for further transformations and for use in combinatorial chemistry. For this purpose derivatives of
phthalide III, obtained by the reduction of phthalic anhydride, were used as precursors of 2-formylbenzoic acids
II.
_______
* To whom correspondence should be addressed, e-mail: obushak@in.lviv.ua.
1Ivan Franko National University of Lviv, Lviv 79005, Ukraine.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 173-179, February, 2010. Original
article submitted February 24, 2009.
140
0009-3122/10/4602-0140©2010 Springer Science+Business Media, Inc.

O
O
O
N
N
N
O2N
O
H2N
a
c
f
N
O
O
O
O
1
b
3
2
6
d
ROOC
N
Me
e
O
R1
O
O
R1
N
O
g
O
N
N3
O
h
O
O
Br
7 a, b
8 a, b
4 a, b
5
Me
S
H
i
N
S
NH
O
COOH
k
l
O
O
j
S
S
R1
R1
R1
COOH
R1
COOH
10 a, b
O
O
9 a, b
11 a, b
12 a, b
4, 7, 9-12 a R1 = Сl, b R1= Br; 8 a R = H, b R = Me
Conditions and reagents: a KNO3 in H2SO4; b 1.2 equiv. Br2, 0.12 equiv. Fe, nitrotoluene, 135°C, 4 h;
c Fe, EtOH–water, boiling 6 h (75%); d 1. NaNO2, HCl/HBr. 2. CuCl/CuBr (74/71%); e 1. NaNO2,
HCl. 2. NaN3 (78%); f 4.6 equiv. CH(OEt)3, 1.2 equiv. NaN3, AcOH, 100°C, 3 h (86%); g NBS, CCl4,
boiling 2 h; h For compound 8a, acetoacetic ester, Na, MeOH (79%), (compounds 8a→8b). 1. SOCl2.
2. MeOH, 1 h (93%); i H2O, boiling 3 h; j Rhodanine, AcOH, Et3N (94/89%); k NaOH (64/61%);
l l. SOCl2, dioxane; 2. p-toluidine and Et3N.
O
COOH
O
OH
O
S
O
O
II
III
I
Attempts at the bromination of the phthalide 1 in the aromatic ring with bromine in the presence of
FeBr3 were unsuccessful; the reaction mixture resinifies under these conditions. We note that of electrophilic
substitution reactions in the aromatic ring of phthalide only nitration has been described [17]. We reduced the
nitrophthalide 2 formed in this reaction, and we converted the aminophthalide 3 into chloro(bromo)phthalides
4a,b by the Sandmeyer reaction, into the azide 5 by diazotization and the subsequent action of sodium azide,
and into the tetrazolylphthalide 6 by a three-component reaction with ethyl orthoformate and sodium azide by
the procedure in [18].
Reactions involving bromination of the phthalides 2 and 4-6 at position 3 with N-bromosuccinimide
were studied (Scheme). It was established that the reaction only takes place successfully in the case of
compounds 4a,b, resulting in the production of the phthalides 7a,b. Bromination under these conditions is
complicated by the poor solubility of compounds 2 and 6 in CCl4 and oxidation of the azido group (compound 5).
Triazolecarboxylic acid 8a and also the ester 8b were obtained from the azide 5 by reaction with acetoacetic
ester. Compounds 8a,b are not brominated at position 3 by N-bromosuccinimide.
141

Formylbenzoic acids 9a,b are formed during the hydrolysis of the phthalides 7a,b. They are converted
into isothiocoumarin-3-carboxylic acids 11a,b in two stages by condensation with rhodanine and recyclization
of the 5-arylidenerhodanines 10a,b. In the case of the amides 12a,b it was shown that the acids 11a,b can be
used in the synthesis of compounds with an isothiocoumarin fragment by acylation of the corresponding acid
chlorides.
Thus, a method was developed for the synthesis of 1-oxo-1H-isothiochromene-3-carboxylic acids from
substituted phthalides, and the limitations of this method in the case of certain substituents were revealed.
EXPERIMENTAL
1
The H NMR spectra were recorded in DMSO-d6 on a Varian Unity +400 instrument (400 MHz) with
TMS as standard. The mass spectra were obtained on an Agilent 1100 LC/MSD chromato-mass spectrometer
with chemical ionization.
6-Amino-2-benzofuran-1(3H)-one (3). We heated 51.8 g of iron powder in 311 ml of a 3:1 mixture of
ethanol and water with stirring. We added conc. HCl (6.2 ml), and slowly added the nitro derivative 2 (35.8 g,
200 mmol). The mixture was heated for 8 h, and the hot solution was quickly filtered. The amine released when
the filtrate had cooled was filtered off and recrystallized. Yield 22.35 g (75%); mp 182°C (ethanol–water).
Synthesis of 6-Halo-2-benzofuran-1(3H)-ones 4a,b. The amine 3 (37.2 g, 250 mmol) was dissolved in
conc. HCl (100 ml) or conc. HBr (190 ml). The mixture was cooled to 0°C, and a solution of NaNO2 (17.2 g,
250 mmol) in the minimum amount of water was added dropwise with cooling. The temperature range of
diazotization was 0-5°C. When necessary, the obtained solution of the diazonium salt was filtered. It was then
added with stirring at 0°C to a previously prepared solution of copper(I) chloride (24.8 g, 250 mmol) in conc.
HCl (50 ml) or copper(I) bromide (35.9 g, 250 mmol) in conc. HBr (90 ml). The reaction mixture was heated on
a water bath. When the release of nitrogen had stopped, the solid product was filtered off and purified by
recrystallization.
6-Chloro-2-benzofuran-1(3H)-one (4a). Yield 74%; mp 107-108°C (ethanol–water). 1H NMR
3
3
4
spectrum, δ, ppm (J, Hz): 5.36 (2Н, s, СН2); 7.67 (1Н, d, J = 8.8, Н-4); 7.73 (1Н, dd, J = 8.8, J = 2.0, Н-5);
7.79 (1Н, d, J = 2.0, Н-7). Mass spectrum, m/z: 169 [М + Н]+. Found, %: C 56.81; H 2.86. C8H5ClO2.
4
Calculated, %: C 57.00; H 2.99.
6-Bromo-2-benzofuran-1-(3H)-one (4b). Yield 71%; mp 114-115°C (ethanol–water). 1H NMR
3
3
4
spectrum, δ, ppm (J, Hz): 5.35 (2Н, s, СН2); 7.63 (1Н, d, J = 8.0, Н-4); 7.70 (1Н, dd, J = 8.0, J = 1.6, Н-5);
7.78 (1Н, d, J = 1.6, Н-7). Mass spectrum, m/z: 214 [М + Н]+. Found, %: C 45.22; H 2.19. C8H5BrO2.
4
Calculated, %: C 45.10; H 2.37.
6-Azido-2-benzofuran-1(3H)-one (5). The amine 3 (7.4 g, 50 mmol) was dissolved in conc. HCl
(19 ml), the solution was cooled to 0°C, and a cooled solution of NaNO2 (3.45 g, 50 mmol) in the minimum
amount of water was added dropwise with stirring, while keeping the temperature below 5°C. When necessary,
the obtained solution of 3-oxo-1,3-dihydroisobenzofuran-5-diazonium chloride was filtered and cooled to -5°C.
A solution of NaN3 (3.25 g, 50 mmol) in water (10 ml) was slowly added to it drop by drop with vigorous
stirring while the temperature was kept below 7°C. After the addition of NaN3, the mixture was kept at room
temperature for 1 h. The azide 5 was filtered off, washed with a large volume of iced water to a neutral reaction,
and dried in a dark cool place. Yield 6.8 g (78%); decomp. p. 127-128°C (ethanol–water). Mass spectrum, m/z:
176 [М + Н]+. Found, %: C 54.62; H 2.61; N 23.74. C8H5N3O2. Calculated, %: C 54.86; H 2.88; N 23.99.
6-(1H-Tetrazol-1-yl)-2-benzofuran-1(3H)-one (6). AcOH (40 ml) was added with stirring to a
suspension of 5-aminophthalide 3 (0.74 g, 5 mmol) and NaN3 (0.39 g, 6 mmol) in ethyl orthoformate (3.79 ml,
23 mmol). The mixture was heated at 95-100°C for 4 h. After cooling, 0.7 ml of conc. HCl was added, and the
mixture was filtered. The filtrate was evaporated under vacuum, and the residue was recrystallized from ethanol.
142

1
Yield 1 g (86%); mp 216-218°C (ethanol). H NMR spectrum, δ, ppm (J, Hz): 5.49 (2Н, s, СН2); 7.93 (1Н, d,
3J = 8.8, Н-4); 8.35 (1Н, d, 3J = 8.8, Н-5); 8.46 (1Н, s, Н-7); 10.22 (1Н, s, tetrazole). Mass spectrum, m/z: 203 [М +
Н]+. Found, %: C 53.58; H 2.82; N 27.57. C9H6N4O2. Calculated, %: C 53.47; H 2.99; N 27.71.
Synthesis of 3-Bromo-6-halo-2-benzofuran-1(3H)-ones 7a,b. Phthalide 4 (10 mmol) was dissolved in
CCl4 (10 ml) that had been dried over P2O5. N-bromosuccinimide (1.78 g, 10 mmol) and then benzoyl peroxide
(0.02 g) was added. The mixture was cautiously heated, and after the end of the reaction (the formation of
succinimide on the surface), it was boiled for a further 10 min. After cooling, the succinimide was filtered off
and washed with a small amount of CCl4. The solvent was distilled from the combined filtrates under a weak
vacuum on a water bath. The crystals were filtered off and recrystallized.
3-Bromo-6-chloro-2-benzofuran-1-(3H)-one (7a). Yield 1.9 g (77%); mp 57-58°C (hexane). Mass
spectrum, m/z: 248 [М + Н]+. Found, %: C 38.74; H 1.48. C8H4BrClO2. Calculated, %: C 38.83; H 1.63.
3,6-Dibromo-2-benzofuran-1(3H)-one (7b). Yield 2.36 g (81%); mp 64-65°C (hexane). Mass
spectrum m/z: 293 [М + Н]+. Found, %: C 32.79; H 1.14. C8H4Br2O2. Calculated, %: C 32.91; H 1.38.
5-Methyl-1-(3-oxo-1,3-dihydro-2-benzofuran-5-yl)-1H-1,2,3-triazole-4-carboxylic
Acid
(8a).
Sodium (0.3 g, 13 mmol) was dissolved in absolute ethanol (15 ml). To the cooled solution of sodium ethoxide,
acetoacetic ester (1.75 g, 10 mmol) was added, and then azide 5 (1.75 g, 10 mmol) was slowly added (with
cooling in iced water). The mixture was kept in an ice bath for 30 min and was then slowly heated to boiling and
boiled for 1 h. A precipitate separated. Hot water was added to dissolve the precipitate (15-20 ml), and the
mixture was boiled for a further hour. The hot solution was poured into 10 ml of conc. HCl and left to
crystallize. The product was filtered off, washed on the filter with a small amount of water, and recrystallized.
Yeld 2.05 g (79%); mp 207-208°C (ethanol). 1H NMR spectrum, δ, ppm (J, Hz): 2.58 (3Н, s, СН3); 5.51 (2Н, s,
СН2); 7.93 (1Н, d, J = 7.8, Н-7); 7.98 (1Н, d, J = 7.8, Н-6); 8.06 (1Н, s, Н-4); 12.94 (1Н, br. s, СООН). Mass
spectrum, m/z: 260 [М + Н]+. Found, %: C 55.42; H 3.43; N 16.38. C12H9N3O4. Calculated, %: C 55.60; H 3.50;
N 16.21.
Methyl 5-Methyl-1-(3-oxo-1,3-dihydro-2-benzofuran-5-yl)-1H-1,2,3-triazole-4-carboxylate (8b). A
mixture of the acid 8a (6.48 g, 25 mmol) and thionyl chloride (1.85 ml, 25 mmol) in dioxane (25 ml) was boiled
for 1 h. The mixture was cooled, and the precipitated acid chloride was filtered off, washed with hexane, and
added to methanol (15 ml). The reaction mixture was boiled for 1 h. The precipitate was filtered off, washed
1
several times with water, and recrystallized from methanol. Yield 6.3 g (93%); mp 193-194°C. H NMR
spectrum, δ, ppm (J, Hz): 2.58 (3Н, s, СН3); 3.92 (3Н, s, СН3O); 5.52 (2Н, s, СН2); 7.93 (1Н, d, J = 7.8, Н-7);
7.98 (1Н, d, J = 7.8, Н-6); 8.08 (1Н, s, Н-4). Mass spectrum, m/z: 274 [М + Н]+. Found, %: C 57.42; H 3.92;
N 15.24. C13H11N3O4. Calculated, %: C 57.14; H 4.06; N 15.38.
Synthesis of 2-Formyl-5-halobenzoic Acids 9a,b. A suspension of 3-bromobenzofuran 7a,b (10 mmol)
in water (5 ml) was heated for 3 h. The mixture was left in the refrigerator overnight, and the product was
filtered off , washed with a small amount of cold ethanol, and dried in air.
5-Chloro-2-formylbenzoic Acid (9a). Yield 1.73 g (94%); mp 137-138°C (ethanol) [15].
5-Bromo 2-formylbenzoic Acid (9b), Yield 2.06 g (90%); mp 130-131°C (ethanol). Mass spectrum,
m/z: 230 [М + Н]+. Found, %: C 42.07; H 2.09. C8H5BrO3. Calculated, %: C 41.95; H 2.20.
Synthesis of 5-Halo-2-[(4-oxo-2-thioxo-1,3-thiazolidin-5-ylidene)methyl]benzoic Acids 10a,b. A
mixture of aldehyde 9a,b (10 mmol), rhodanine (1.3 g, 10 mmol), and triethylamine (2 ml) in acetic acid (25 ml)
was boiled for 2 h. The product that formed on cooling was filtered off and was recrystallized after drying in air.
5-Chloro-2-[(4-oxo-2-thioxo-1,3-thiazolidin-5-ylidene)methyl]benzoic Acid (10a). Yield 2.8 g
1
3
(94%); mp 261-262°C (AcOH). H NMR spectrum, δ, ppm (J, Hz): 7.55 (1Н, d, J = 8.8, Н-3); 7.69 (1Н, d,
3J = 8.8, J = 2.0, Н-4); 8.01 (1Н, d, J = 2.0, Н-6); 8.15 (1Н, s, CН=). Mass spectrum, m/z: 300 [М + Н]+.
4
4
Found, %: C 44.01; H 2.16; N 4.42. C11H6ClNO3S2. Calculated, %: C 44.08; H 2.02; N 4.67.
5-Bromo-2-[(4-oxo-2-thioxo-1,3-thiazolidin-5-ylidene)methyl]benzoic Acid (10b). Yield 3.06 g
1
3
(89%); mp 274-275°C (AcOH). H NMR spectrum, δ, ppm (J, Hz): 7.46 (1Н, d, J = 8.8, Н-3); 7.83 (1Н, dd,
143

3J = 8.8, 4J = 2.0, Н-4); 8.11 (1Н, d, 4J = 2.0, Н-6), 8.17 (1Н, s, CН=). Mass spectrum, m/z: 345 [М+Н]+. Found,
%: C 38.24; H 1.51; N 4.01. C11H6BrNO3S2. Calculated, %: C 38.38; H 1.76; N 4.07.
Synthesis of 7-Halo-1-oxo-1H-isothiochromene-3-carboxylic acids 11a,b. Compound 10 (3.3 g,
10 mmol) was added to a solution of KOH (2.24 g, 40 mmol) in water (50 ml) and the mixture was boiled for
3 h. The reaction mixture was poured into conc. HCl (15 ml) and ice (75 g). The precipitate was filtered off and
purified by recrystallization.
7-Chloro-1-oxo-1H-isothiochromene-3-carboxylic Acid (11a). Yield 1.54 g (64%); mp 273-274°C
(ethanol–water). 1H NMR spectrum, δ, ppm (J, Hz): 7.91 (1Н, dd, 3J = 7.8, 4J = 2.0, Н-6); 8.08 (1Н, d, 3J = 7.8,
4
Н-5); 8.16 (1Н, s, Н-4); 8.25 (1Н, d, J = 2.0, Н-8). Mass spectrum, m/z: 241 [М + Н]+. Found, %: C 50.03; H
1.94. C10H5ClO3S. Calculated, %: C 49.91; H 2.09.
7-Bromo-1-oxo-1H-isothiochromene-3-carboxylic Acid (11b). Yield 1.74 g (61%); mp 281-282°C
(ethanol–water). 1H NMR spectrum, δ, ppm (J, Hz): 7.97 (1Н, d, 3J = 7.8, Н-5); 8.02 (1Н, dd, 3J = 7.8, 4J = 2.0,
4
Н-6); 8.22 (1Н, s, Н-4); 8.27 (1Н, d, J = 2.0, Н-8). Mass spectrum, m/z: 286 [М + Н]+. Found, %: C 42.29;
H 1.54. C10H5BrO3S. Calculated, %: C 42.13; H 1.77.
Synthesis of Amides 12a,b (General Method). A mixture of the acid 11a,b (50 mmol) and thionyl
chloride (3.7 ml, 50 mmol) in dioxane (50 ml) was boiled for 1 h. The mixture was cooled, and the precipitated
acid chloride was filtered off and washed with hexane. To a solution of toluidine (0.59 g, 5.5 mmol) in dioxane,
triethylamine (0.6 g, 5.5 mmol) and acid chloride 11a or compound 11b (5.5 mmol) was added. The reaction
mixture was kept at room temperature for 2 h and was then heated nearly to boiling point. After cooling, it was
diluted with water, and the precipitate was filtered off, washed several times with water, and recrystallized.
N-(4-Methylphenyl)-7-chloro-1-oxo-1H-isothiochromene-3-carboxamide (12a). Yield 1.72 (95%);
mp 270-271°C (ethanol). 1H NMR spectrum, δ, ppm (J, Hz): 2.33 (3Н, s, СН3); 7.13 (2Н, d, 3J = 7.8, Н-3,5 Ar);
7.57 (2Н, d, 3J = 7.8, Н-2,6 Ar); 7.90 (1Н, dd, 3J = 8.8, 4J = 2.0, Н-6); 7.94 (1Н, d, 3J = 8.8, Н-5); 8.15 (1Н, d,
4J = 2.0, Н-8); 8.27 (1Н, s, Н-4); 10.43 (3Н, s, NH). Mass spectrum, m/z: 330 [М + Н]+. Found, %: C 61.80;
H 3.84; N 4.11. C17H12ClNO2S. Calculated, %: C 61.91; H 3.67; N 4.25.
N-(4-Methylphenyl)-7-bromo-1-oxo-1H-isothiochromene-3-carboxamide (12b). Yield 2 g (97%);
mp 277-278°C (ethanol). 1H NMR spectrum, δ, ppm (J, Hz): 2.33 (3Н, s, СН3); 7.13 (2Н, d, 3J = 7.8, Н-3,5 Ar);
7.57 (2Н, d, 3J = 7.8, Н-2,6 Ar); 7.85 (1Н, d, 3J = 8.8, Н-5); 8.04 (1Н, dd, 3J = 8.8, 4J = 2.0, Н-6); 8.25 (1Н, s,
4
Н-4); 8.29 (1Н, d, J = 2.0, Н-8); 10.43 (3Н, s, NH) . Mass spectrum, m/z: 375 [М + Н]+. Found, %: C 54.70;
H 3.06; N 3.89. C17H12BrNO2S. Calculated, %: C 54.56; H 3.23; N 3.74.
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