Fused thia-heterocycles via isothiocyanates. Part II. A

Z. Naturforsch. 2018; 73(2)b: 109–113

Lana Z. El-Agha, Mustafa M. El-Abadelah*, Marwan R. Kamal, Salim S. Sabri, Randa M. Al-As’ad and Wolfgang Voelter*

Fused thia-heterocycles via isothiocyanates. Part II. A convenient synthesis of some new thieno[2,3-b]thiopyran-4-one derivatives https://doi.org/10.1515/znb-2017-0183 Received November 7, 2017; accepted December 7, 2017

Abstract: A facile synthesis of some methyl 5-(arylamino)4-oxothieno[2,3-b]thiopyran-6-carboxylates 6a–g is achieved via direct reaction of deprotonated methyl 3-(2,5-dichlorothien-3-yl)-3-oxopropanoate with aryl isothiocyanates in anhydrous tetrahydrofuran (THF) under reflux. Upon saponification of 6a,b, the ester group is eliminated, most logically via decarboxylation of the presumably-formed carboxyl group. Structures of the new compounds 6a–g and 7a,b are supported by micro analytical and spectral [NMR, MS electron impact (EI) and HRMS] data. Keywords: aryl isothiocyanates; ester decarboxylation; methyl 3-(2,5-dichlorothien-3-yl)-3-oxopropanoate; 4-oxothieno[2,3-b]thiopyran-6-carboxylates.

1 Introduction A number of substituted 5,6-dihydro-4-oxo-thieno[2,3-b] thiopyran-2-sulfonamides 1 (Fig. 1), prepared by a multistep route, were reported as useful bactericides, fungicides and in the treatment of allergies [1]. The related 5,6-dihydrothieno[2,3-b]thiopyrans 2a [2–4], 2b and 2c [5, 6] (Fig. 1) were reported as carbonic anhydrase inhibitors and are used therapeutically for the treatment of glaucoma.

*Corresponding authors: Mustafa M. El-Abadelah, Chemistry Department, Faculty of Science, The University of Jordan, Amman 11942, Jordan, e-mail: [email protected]; and Wolfgang Voelter, Interfakultäres Institut für Biochemie, Universität Tübingen, Hoppe-Seyler Straße 4, 72076 Tübingen, Germany, e-mail: [email protected] Lana Z. El-Agha, Marwan R. Kamal and Salim S. Sabri: Chemistry Department, Faculty of Science, The University of Jordan, Amman 11942, Jordan Randa M. Al-As’ad: Department of Chemistry, Faculty of Science, Al-Hussein Bin Talal University, Ma’an 71111, Jordan

Recently we have reported on a one-pot synthesis of 4H-benzothiopyran-4-ones (thiochromones) 3 utilising 3-(o-chlorophenyl)-3-oxopropanoate and various isothiocyanates as the source of the sulfur atom [7]. By adopting this methodology we sought it is worthwhile to prepare 2-chlorothieno[2,3-b]thiopyran-4-ones 6, bioisosteres of thiochromones 3, for comparative study (Fig. 1). Accordingly, the present work aims at the synthesis and characterisation of a selected set of the former compounds 6a–g as illustrated in Scheme 1.

2 Results and discussion 2.1 Chemistry The synthesis of the targeted thieno[2,3-b]thiopyran4-ones 6a–g commenced with the preparation of 3-(2,5-dichlorothien-3yl)-3-oxopropanoate (4) by the reaction of 3-acetyl-2,5-dichlorothiophene, accessible from acetylation of dichlorothiophene [8], with dimethyl carbonate following a reported procedure [9]. In a following step, direct interaction between deprotonated β-keto ester 4 and the appropriate aryl isothiocyanates 5a–g led to the construction of the 4-oxothiopyran ring across the 2, 3- bond of thiophene and consequent production of the corresponding bicyclic heterocycles 6a–g (Scheme 1). A proposed mechanism, showing the sequential transient intermediates in the production of 6, is postulated in Scheme 2, and is essentially similar to that previously suggested along with the synthesis of benzothiopyran-5-ones utilising an β-(o-chloroaryl)-β-keto ester and isothio cyanates [7]. Saponification of the ester group of compounds 6a,b using 0.4 N ethanolic sodium hydroxide solution at the refluxing temperature, delivered the respective derivatives 7a,b unsubstituted at the C-3 locus (Scheme 3). This result implies that the presumably formed 3-carboxy group underwent decaboxylation under the prevailing experimental conditions.

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110 L.Z. El-Agha et al.: Fused thia-heterocycles via isothiocyanates

R

O CONHR

R1

3

O H2 N

S

S S

O

R2

S 1

S O2

2a (R = OH) 2b (R = NHCH 2CHMe2) 2c (R = NHCH2CH3)

1

R = H, alkyl, halogen R2 = H, alkyl, aryl R3 = aryl, heterocyclyl

O

O CO 2 Me

Cl

CO 2 Me Cl

Cl

S

S

NHR

S

3 (R = aryl, cycloalkyl)

NHAr

6

Fig. 1: Examples of bioactive dihydrothieno[2,3-b]thiopyran-4-ones (1–2c) and thiochromones 3.

The newly synthesised compounds 6a–g and 7a,b were characterised by elemental analysis, IR, MS, 1H and 13C NMR spectral data. These data, detailed in the experimental section, are consistent with the suggested structures. Thus, the mass spectra display the correct molecular peaks for which the measured high resolution (HRMS) data are in good agreement with the evaluated values. Dept. and 2D (correlation spectroscopy, hetero nuclear multiple-quantum coherence, hetero nuclear multiple-bond correlation) experiments showed

correlations that helped in the 1H and 13C signal assignments to the different carbons and their attached and/or neighbouring hydrogen atoms.

2.2 Mass spectra Under electron impact (EI), compounds 6a–g undergo characteristic fragmentation modes whereby ions [A]–[D] are formed as the major fragments (Scheme 4). In essence,

O

O

3

CO 2 Me Cl

Ar S

Cl

4

N

C

S

Cl

(i)

4

5

7a

S

6

1

5a−g

CO 2 Me

2

S

(ii)

3a

Ar N H

7

6a−g

Compounds 5 and 6 entry

a

1'

1'

Ar 4'

c

b

4'

F

d 1'

4'

Cl

1'

4'

Me

e

g

f 1'

4'

OMe

1'

1'

4'

NO 2

4'

8'

5'

Scheme 1: Synthetic route for compounds 6a–g. (i) THF, NaH/r.t., 15–20 min.; (ii) reflux, 10 h.


L.Z. El-Agha et al.: Fused thia-heterocycles via isothiocyanates 111

O

O CO 2 Me

CO 2 Me

DMF

Cl S

Cl

NaH, r.t.

Cl

S

Cl

C

NR

O

O CO 2 Me H

Cl S

S

Cl

NR

Cl S

O

CO 2 Me S

NHR

Cl S

O

CO 2 Me H

CO 2 Me

Cl

Cl S

NR

S

S

S

NHR

Scheme 2: A plausible mechanism for the formation of compounds 6a–g.

the thiopyranone moiety in 6 follows a fragmentation pattern similar to that previously reported [7] for the related benzothiopyranones 3. Thus, the main fragmentation pathway of the molecular ions starts with ejection of MeOH from [M]+˙ to give ion [A], M–32 as the base peak in all cases. The latter ion [A] then suffers expulsion of CO to form ion [B], M–60 or loss of R-NC to produce ion [C] (m/z = 216) which in turn loses [:C=C=O] to deliver ion [D] (m/z = 176). Compound 6c, which incorporates two chlorine atoms, shows the corresponding isotopic clusters [M]+˙, [M+2]+˙ and [M+4]+˙ in the relative ratio of 9:6:1; corresponding isotopic clusters are also observed for the fragment ions [A] and [B]. On the other hand, the molecular ion region for compounds 6a,b and 6d–g (which incorporate one chlorine atom) displays the corresponding isotopic peaks [M]+˙ and [M+2]+˙ in the relative ratio of 3:1; corresponding isotopic peaks are likewise observed for the fragment ions [A]–[D].

O R

(6a, b)

(i) (ii)

3 Conclusion

Cl S

S

N H

7a (R = H) 7b (R = F) Scheme 3: Synthetic route for compounds 7a,b. (i) 0.4 n alcoholic NaOH/reflux, 24 h; (ii) 1 n aq. HCl.

O CO 2 Me

In this study, direct annulation of a disubstituted thiopyran-4-one moiety onto a thiophene ring has been successfully achieved utilising β-(o-chlorothienyl)-β-keto ester and aryl isothiocyanates. This new route paves the way towards one-pot synthesis of several substituted thieno[2,3-b]thiopyran-4-ones, bioisosteres of thiochromones and evaluation of their biological properties.

− MeOH Cl S

S

NHR

4 Experimental section

[M]

O C Cl S

O

O

− RNC

Cl

C S

NR

S

O

S

[C] m / z = 216

[A] − CO

− C C O

O C

C NR

Cl S

S

[B]

O

Cl S

S

[D] m / z = 176

Scheme 4: Principle fragmentation pathways of compounds 6a–g under electron impact (EI).

The aryl isothiocyanates used in this study were purchased from Sigma Aldrich (St. Louis, MO, USA). 2,5-dichlorothiophene, anhydrous aluminium trichloride, anhydrous tetrahydrofuran (THF), dimethyl carbonate, dry carbon disulfide and sodium hydride (80% dispersion in mineral oil) were purchased from Acros (Geel, Belgium). Melting points (uncorrected) were measured on a Gallenkamp electrothermal melting temperature apparatus (London, UK). 1H, 13C, and 2D NMR spectra were recorded on a Bruker-DPX 300 MHz spectrometer (Karlsruhe, Germany), using CDCl3 or [D6]DMSO as solvent and TMS as internal reference. Mass spectra (EI) were obtained using a Finnigan MAT TSQ-70 spectrometer (IL, USA) at 70 eV; ion source T = 200°C. Elemental analysis were performed at the Microanalytical Lab., Tuebingen University, Germany. Brought to you by | University of Jordan Authenticated | [email protected] author's copy Download Date | 2/11/18 10:53 AM

112 L.Z. El-Agha et al.: Fused thia-heterocycles via isothiocyanates

4.1 M ethyl 3-(2,5-dichlorothien-3-yl)-3-oxopropanoate (4) This compound was prepared according to a reported procedure [9] utilising 3-acetyl-2,5-dichlorothiophene which, in turn, was made accessible by Friedel-Crafts acetylation of 2,5-dichlorothiophene [8].

4.2 G eneral procedure for the preparation of methyl 6-arylamino-2-chloro-4oxothieno[2,3-b] thiopyran-5carboxylates (6a–g) To a stirred solution of the β-keto ester (4) (4.0 g, 16 mmol) in dry THF (60 mL) was portionwise added sodium hydride (0.35 g, 80% dispersion in oil; 18 mmol) whereby a precipitate of the respective enolate salt was formed during 15–20 min. A solution of the appropriate aryl isothio cyanate (16 mmol) in dry THF (3 mL) was then introduced into the reaction vessel, and the resulting mixture was refluxed for 8–10 h under anhydrous conditions. There after, THF was evaporated from the resulting brown reaction solution, and the residual crude product was purified by preparative TLC chromatography on silica gel plates using chloroform as the developing solvent.

4.2.1 Methyl 2-chloro-4-oxo-6-(phenylamino)-4Hthieno[2,3-b]thiopyran-5-carboxylate (6a) Yield: 58%; m. p. 136–137°C. – 1H NMR (300 MHz, CDCl3): δ = 3.95 (s, 3H, CO2CH3), 7.33 (dd, J = 8.2 Hz, 1.8 Hz, 2H, 2′-H/6′-H), 7.42 (m, 1H, 4′-H), 7.45 (pseudo t, 2H, 3′-H/5′H), 7.47 (s, 1H, 3-H), 11.52 (s, 1H, N–H, exchangeable with D2O) ppm. − 13C NMR (75 MHz, CDCl3): δ = 52.4 (CO2CH3), 101.4 (C-5), 125.5 (C-3), 127.8 (C-2′/C-6′), 128.9 (C-4′), 129.6 (C-3a), 130.0 (C-3′/C-5′), 131.6 (C-2), 136.1 (C-1′), 136.5 (C-7a), 166.1 (C-6), 170.2 (CO2Me), 173.2 (C-4) ppm. – C15H10ClNO3S2 (351.83): calcd. C 51.21, H 2.86, N 3.98, S 18.23; found C 51.03, H 2.82, N 3.91, S 18.04.

4.2.2 Methyl 2-chloro-6-(4-fluorophenylamino)-4-oxo4H-thieno[2,3-b]thiopyran-5-carboxylate (6b) Yield: 34%; m. p. 160–161°C. – 1H NMR (300 MHz, CDCl3): δ = 3.96 (s, 3H, CO2CH3), 7.19 (dd, JH−H = 8.5 Hz, JH−F = 11 Hz, 2H, 3′-H/5′-H), 7.33 (dd, JH−H = 8.5 Hz, JH−F = 3.0 Hz, 2H, 2′-H/6′-H), 7.45 (s, 1H, 3-H), 11.44 (s, 1H, N–H,

exchangeable with D2O) ppm. − 13C NMR (75 MHz, CDCl3): δ = 52.8 (CO2CH3), 101.4 (C-5), 117.1 (d, 2JC−F = 22.9 Hz, C-3′/C5′), 125.5 (C-3), 129.5 (d, 3JC−F = 8.8 Hz, C-2′/C-6′), 129.8 (C-3a), 131.5 (C-2), 132.3 (d, 4JC−F = 3.2 Hz, C-1′), 136.1 (C-7a), 162.4 (d, 1JC−F = 248 Hz, C-4′), 166.7 (C-6), 170.2 (CO2Me), 173.2 (C-4) ppm. – C15H9ClFNO3S2 (369.82): calcd. C 48.72, H 2.45, N 3.79, S 17.34; found C 48.55, H 2.44, N 3.70, S 17.02.

4.2.3 Methyl 2-chloro-6-(4-chlorophenylamino)-4-oxo4H-thieno[2,3-b]thiopyran-5-carboxylate (6c) Yield: 38%; m. p. 180–181°C. – 1H NMR (300 MHz, [D6] DMSO): δ = 3.73 (s, 3H, CO2CH3), 7.46 (s, 1H, 3-H), 7.49 (d, J = 8.7 Hz, 2′-H/6′-H), 7.52 (d, J = 8.7 Hz, 3′-H/5′-H), 9.80 (s, 1H, N–H, exchangeable with D2O) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ = 52.1 (CO2CH3), 109.2 (C-5), 124.0 (C-3), 127.7 (C-2′/C-6′), 128.5 (C-3a), 129.5 (C-3′/C-5′), 131.2 (C-2), 134.9 (C-4′), 136.9 (C-7a), 137.1 (C-1′), 158.2 (C-6), 166.2 (CO2Me), 171.9 (C-4) ppm. – C15H9Cl2NO3S2 (386.27): calcd. C 46.64, H 2.35, N 3.63, S 16.60; found C 46.48, H 2.29, N 3.54, S 16.42.

4.2.4 Methyl 2-chloro-4-oxo-6-(p-tolylamino)-4Hthieno[2,3-b]thiopyran-5-carboxylate (6d) Yield: 54%; m. p. 115–116°C. – 1H NMR (300 MHz, CDCl3): δ = 2.42 (s, 3H, 4′-CH3), 3.96 (s, 3H, CO2CH3), 7.27 (d, J = 8.3 Hz, 2H, 2′-H/6′-H), 7.30 (d, J = 8.3 Hz, 2H, 3′-H/5′H), 7.47 (s, 1H, 3-H), 11.47 (s, 1H, N–H, exchangeable with D2O) ppm. − 13C NMR (75 MHz, CDCl3): δ = 21.2 (4′-CH3), 52.3 (CO2CH3), 101.2 (C-5), 125.6 (C-3), 127.2 (C-2′/C-6′), 129.6 (C-3a), 130.7 (C-3′/C-5′), 131.8 (C-2), 133.9 (C-7a), 136.2 (C-4′), 139.3 (C-1′), 166.7 (C-6), 170.3 (CO2Me), 173.3 (C-4) ppm. – C16H12ClNO3S2 (365.85): calcd. C 52.53, H 3.31, N 3.83, S 17.53; found C 52.34, H 3.18, N 3.75, S 17.28.

4.2.5 Methyl 2-chloro-6-(4-methoxyphenylamino)-4oxo-4H-thieno[2,3-b]thiopyran-5-carboxylate (6e) Yield: 42%; m. p. 177–178°C. – 1H NMR (300 MHz, [D6] DMSO): δ = 3.74 (s, 3H, CO2CH3), 3.89 (s, 3H, 4′-OCH3), 6.90 (d, J = 8.8 Hz, 2′-H/6′-H), 7.30 (d, J = 8.8 Hz, 2H, 3′-H/5′-H), 7.41 (s, 1H, 3-H), 9.44 (s, 1H, N-H, exchangeable with D2O) ppm. − 13C NMR (75 MHz, [D6]DMSO): δ = 52.0 (CO2CH3), 55.1 (CH3O-4′), 105.6 (C-5), 113.5 (C-3′/C-5′), 124.1 (C-3), 126.0 (C-2′/C-6′), 129.7 (C-3a), 132.2 (C-2), 134.0 (C-7a), 134.7 (C-1′), 160.9 (C-4′), 166.8 (C-6), 171.6 (CO2Me), 180.1 (C-4) ppm. – C16H12ClNO4S2 (381.85): calcd. C 50.33, H 3.17, N 3.67, S 16.79; found C 50.14, H 3.12, N 3.53, S 16.61. Brought to you by | University of Jordan Authenticated | [email protected] author's copy Download Date | 2/11/18 10:53 AM

L.Z. El-Agha et al.: Fused thia-heterocycles via isothiocyanates 113

4.2.6 M ethyl 2-chloro-6-(4-nitrophenylamino)-4-oxo4H-thieno[2,3-b]thiopyran-5-carboxylate (6f) Yield: 56%; m. p. 195–196°C. – 1H NMR (300 MHz, [D6]DMSO): δ = 3.66 (s, 3H, CO2CH3), 7.30 (d, J = 8.9 Hz, 2H, 2′-H/6′-H), 7.60 (s, 1H, 3-H), 8.22 (d, J = 8.9 Hz, 2H, 3′-H/5′-H), 10.08 (s, 1H, N–H, exchangeable with D2O) ppm. − 13C NMR (75 MHz, [D6] DMSO): δ = 52.4 (CO2CH3), 119.0 (C-2′/C-6′), 122.1 (C-5), 124.1 (C-3′/C-5′), 125.3 (C-3), 125.4 (C-3a), 131.0 (C-2), 135.7 (C-7a), 138.5 (C-1′), 148.0 (C-4′), 152.7 (C-6), 164.8 (CO2Me), 172.7 (C-4) ppm. – C15H9ClN2O5S2 (396.83): calcd. C 45.40, H 2.29, N 7.06, S 16.16; found C 40.16, H 2.23, N 7.10, S 15.94.

4.2.7 Methyl 2-chloro-6-(naphthalene-1-ylamino)-4-oxo4H-thieno[2,3-b]thiopyran-5-carboxylate (6g) Yield: 52%; m. p. 189–190°C. – 1H NMR (300 MHz, CDCl3): δ = 4.01 (s, 3H, CO2CH3), 7.47 (s, 1H, 3-H), 7.56 (m, 4H, 2-H, 4′-H, 6′-H, 8′-H), 7.94 (m, 3H, 3′-H, 5′-H, 7′-H), 11.84 (s, 1H, N–H, exchangeable with D2O) ppm. − 13C NMR (75 MHz, CDCl3): δ = 52.4 (CO2CH3), 101.4 (C-5), 122.0 (C-2′), 125.4 (C-4′), 126.1 (C-3), 127.2 (C-6′), 127.9 (C-7′), 128.7 (C-8′), 129.8 (C-5′), 131.9 (C-3a), 132.2 (C-8′a), 132.7 (C-2), 134.5 (C-1′), 134.7 (C-4′a), 136.2 (C-7a), 167.7 (C-6), 170.5 (CO2Me), 173.2 (C-4) ppm. – C19H12ClNO3S2 (401.89): calcd. C 56.78, H 3.01, N 3.49, S 15.96; found C 56.61, H 3.04, N 3.42, S 15.75.

4.3 G eneral procedure for the formation of 6-arylamino-4H-2-chloro-4-thieno[2,3-b] thiopyran-4-ones (7a,b) The appropriate methyl ester (6a/6b) (0.2 g) was dissolved in ethanolic sodium hydroxide solution (0.4 n, 15 mL). The resulting alkaline solution was stirred at room temperature for 2 h and then refluxed for 12 h. Thereafter, the reaction mixture was cooled to room temperature and neutralised with aqueous hydrochloric acid (1.0 n). The precipitated title product was collected by suction filtration, washed with water and dried.

4.3.1 2-Chloro-6-(phenylamino)-4H-thieno[2,3-b] thiopyran-4-one (7a) Yield: 85%; m.p. 234–235°C. – 1H NMR (300 MHz, [D6] DMSO): δ = 6.13 (s, 1H, 5-H), 7.21 (t, J = 8.0 Hz, 1H, 4′-H), 7.29 (d, J = 8.1 Hz, 2H, 2′-H/6′-H), 7.42 (dd, J = 8.0, 8.1 Hz, 2H, 3′-H/5′-H), 7.46 (s, 1H, N–H, exchangeable with D2O) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ = 101.3 (C-5), 123.0

(C-2′/C-6′), 123.9 (C-3), 125.2 (C-4′), 128.6 (C-3a), 129.0 (C-2), 129.6 (C-3′/C-5′), 135.7 (C-7a), 138.8 (C-1′), 156.4 (C-6), 174.5 (C-4) ppm. – C13H8NOS2 (293.79): calcd. C 53.15, H 2.74, N 4.77, S 21.83; found C 53.03, H 2.80, N 4.65, S 21.64. 4.3.2 2-Chloro-6-(4-fluorophenylamino)-4Hthieno[2,3-b]thiopyran-4-one (7b) Yield: 83%; m.p. 245–246°C. – 1H NMR (300 MHz, [D6] DMSO): δ = 5.74 (s, 1H, 5-H), 7.15 (dd, JH−H = 8.3 Hz, 2 JH−F = 10.0 Hz, 2H, 3′-H/5′-H), 7.18 (dd, JH−H = 8.3 Hz, 3 JH−F = 10.0 Hz, 2H, 2′-H/6′-H), 7.34 (s, 1H, 3-H), 9.70 (s, 1H, N–H, exchangeable with D2O) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ = 107.0 (C-5), 115.9 (d, 2JC−F = 22.5 Hz, C-3′/C5′), 124.6 (d, 3JC-F = 12.0 Hz, C-2′/C-6′), 124.8 (C-3), 126.8 (C-3a), 134.9 (C-2), 135.9 (d, 4JC−F = 3.2 Hz, C-1′), 139.7 (C-7a), 157.9 (C-6), 158.6 (d, 1JC−F = 239.2 Hz, C-4′), 173.2 (C-4) ppm. – C13H7ClFNOS2 (311.78): calcd. C 50.08, H 2.26, N 4.49, S 20.57; found C 50.14, H 2.34, N 4.38, S 20.42.

5 Supporting information Copies of the 1H NMR, 13C NMR and mass spectra of compounds 6a, 6b, 6d, 6f and 7a are given as Supporting Information available online (DOI: 10.1515/znb-2017-0183).

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