PBr₃-mediated [5 + 1] annulation of α-alkenoyl-α-carbamoyl ketene-S,S-acetals: access to substituted pyridine-2,6(1H,3H)-diones

Abstract

A facile and efficient synthesis of substituted pyridine-2,6(1H,3H)-diones via an intramolecular [5 + 1] annulation of readily available α-alkenoyl-α-carbamoyl ketene-S,S-acetals mediated by phosphorus bromide (PBr₃) under very mild conditions is described.

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Introduction

Cyclic imide and its derivatives have attracted considerable research interest since they are distributed in numerous natural products along with diverse useful bioactivities.^1–3 For example, actiketal (RK-441S) was isolated from Streptomyces pulveraceus as a new antibiotic,⁴ and AG-1 is an agent widely used for the treatment of breast cancer for postmenopausal patients (Fig. 1).⁵ In addition, the functionalized cyclic imides have been utilized as versatile intermediates in the synthesis of a wide variety of six-membered aza-heterocycles, and applied as disperse dyes in dyestuff industry as well.⁶ To date, a variety of synthetic approaches have been well established to access to such cyclic imides and their analogues. The notable approaches involve the cyclization of dinitriles,⁷ cyclization of monoamides with acids,⁸ condensation of diacids with amines,⁹ or [3 + 3] cycloaddition of α,β-unsaturated esters with acetamides.¹⁰ Nevertheless, the development of efficient and convenient synthetic methods for such aza-heterocycles under milder conditions is still desirable.

Fig. 1 Structures of selected natural products.

During the course of our studies on the chemistry of α-oxo ketene-S,S-acetals, we successfully developed novel strategies for the synthesis of highly valuable six-membered carbocycles¹¹ and heterocycles,¹² relying upon the utilization of α-alkenoyl ketene-S,S-acetals as a five carbon 1,5-dielectrophilic species in the formal [5 + 1] annulation with various nucleophiles. Also, we achieved facile and efficient synthesis of substituted pyridine-2,4(1H,3H)-diones via an intramolecular [5 + 1] annulations of α-aminopropenoyl ketene-S,S-acetals (Scheme 1).¹³ In connection with our previous work and our continuing interest in the synthesis of functionalized heterocycles, we synthesized a series of α-dimethyl aminopropenoyl-α-carbamoyl ketene-S,S-acetals, and examined their reaction behaviours under different conditions. As a result, we developed a facile one-pot synthesis of substituted pyridine-2,6(1H,3H)-diones from readily available α-alkenoyl-α-carbamoyl ketene-S,S-acetals mediated by phosphorus tribromide (PBr₃) in dichloromethane (DCM). Herein, we wish to report our results and proposed a mechanism involved in the reactions.

Scheme 1 Synthesis of substituted pyridine-2,4(1H,3H)-diones.¹³

Results and discussion

The substrates, α-aminopropenoyl-α-carbamoyl ketene-S,S-acetals 1 were prepared from α-acyl-α-carbamoyl ketene-S,S-acetals with N,N-dimethyl formamide dimethylacetal (DMFDMA) in high yields according to our published procedure.¹³ Recently, we investigated the reaction behaviors of 2-arylamino-3-acetyl-5,6-dihydro-4H-pyrans and 1-carbamyl-1-oximyl cyclopropanes toward Vilsmeier reagent, e.g. POCl₃/DMF, and POCl₃ in DCM, respectively.¹⁴ These obtained results suggested that POCl₃ showed different reaction behaviour when employed with or without DMF. Thus, in the present work, the reaction of 2-[bis(methylthio)methylene]-5-(dimethylamino)-3-oxo-N-phenylpent-4-enamide 1a and POCl₃ (1.0 equiv.) was first attempted in DCM at room temperature. The reaction could proceed as indicated by TLC result, and furnished a product after workup and purification by silica column chromatography, which was characterized as 3-[bis(methylthio)methylene]-1-phenylpyridine-2,6(1H,3H)-dione 2a on the basis of its spectral and analytical data (Table 1, entry 1). It was observed that the variation of addition amount of POCl₃ had significant influence on the reaction time and yield of 2a (Table 1, entries 2 and 3). When the reaction of 1a was performed with PBr₃ (1.2 equiv.) in DCM, 2a could be obtained in 75% yield. The results suggested that PBr₃ was more effective than the previously investigated POCl₃ for the transformation of 1a to 2a (Table 1, entry 4). However, no reaction occurred when the reaction temperature was decreased to 0 °C (Table 1, entry 5). By increasing the reaction temperature to 45 °C, the reaction could be complete within 3.5 h as indicated by TLC along with 67% yield of 2a (Table 1, entry 6). Subjecting 1a and PBr₃ (1.2 equiv.) to toluene at room temperature, a complex mixture was formed, in which no main product could be isolated (Table 1, entry 7). It is should be mentioned that no reaction was observed when 1a was treated with PBr₃ (1.2 equiv.) in DMF at room temperature (Table 1, entry 8), whereas 4-bromo-pyridin-2(1H)-ones were obtained in Chen's work by subjecting α-acyl ketene-S,S-acetals to Vilsmeier conditions at 100 °C (Scheme 2a).¹⁵ It should also be noted that 4-halo-pyridin-2(1H)-ones could be obtained by the reaction of 1-aminopropenoyl-1-carbamoyl cyclopropanes with POCl₃ or PBr₃ at room temperature (Scheme 2b).¹⁶

Table 1 Screening of reaction conditions^a

Entry	Reagent (equiv.)	Solvent	Temp. (°C)	Time (h)	Yield^b (%)
a Reagents and conditions: 1a (1.0 mmol), solvent (15 mL).b Isolated yield.c No reaction.
1	POCl3 (1.0)	DCM	rt	9.0	48
2	POCl3 (1.2)	DCM	rt	7.0	52
3	POCl3 (2.0)	DCM	rt	5.0	53
4	PBr3 (1.2)	DCM	rt	6.0	75
5	PBr3 (1.2)	DCM	0	6.0	nr^c
6	PBr3 (1.2)	DCM	45	3.5	67
7	PBr3 (1.2)	Toluene	rt	6.0	Mixture
8	PBr3 (1.2)	DMF	rt	6.0	nr

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Scheme 2 Synthesis of 4-halo-pyridin-2(1H)-ones.^15,16

Under the conditions as for 2a in entry 4, Table 1, a series of reactions of α-alkenoyl ketene-S,S-acetals 1 were carried out, and some of the results are listed in Table 2. It was found that the α-aminopropenoyl-α-carbamoyl ketene-S,S-acetals 1b–f bearing varied aryl groups or benzyl group R¹ could proceed efficiently to afford the corresponding pyridine-2,6(1H,3H)-diones 2b–f in good to high yields (Table 2, entries 2–6). The versatility of this pyridine-2,6(1H,3H)-dione synthesis was further evaluated by performing 1g–l bearing varied aryl groups or alkyl group R¹ and ethyl or benzyl groups R² under the identical conditions (entries 7–13). The structure of 2i was further confirmed by X-ray single crystal analysis and its spectral and analytical data (Fig. 2). The results shown above demonstrate the efficiency and synthetic value of the cyclization reaction of a variety of α-alkenoyl ketene-S,S-acetals 1. It should be noted that the richness of the functionality of substituted pyridine-2,6(1H,3H)-diones 2 may render them versatile as synthons in further synthetic transformations, e.g. selective reduction of C–C double bond or carbonyl groups,¹⁷ Michael addition,¹⁸ and nucleophilic vinylic substitution (S_NV) reactions.¹⁹

Table 2 Synthesis of substituted pyridine-2,6(1H,3H)-diones 2^a

Entry	1	R^1	R^2	2	Yield^b (%)
a Reagents and conditions: 1a (1.0 mmol), PBr3 (1.2 equiv.), CH2Cl2 (15 mL), rt, 5.0–7.0 h.b Isolated yield.
1	1a	Ph	Me	2a	75
2	1b	4-MeC6H4	Me	2b	83
3	1c	4-ClC6H4	Me	2c	80
4	1d	4-MeOC6H4	Me	2d	87
5	1e	2-MeOC6H4	Me	2e	76
6	1f	Bn	Me	2f	82
7	1g	Ph	Et	2g	86
8	1h	4-MeC6H4	Et	2h	85
9	1i	4-ClC6H4	Et	2i	79
10	1j	4-MeOC6H4	Et	2j	76
11	1k	Ph	Bn	2k	78
12	1l	4-MeC6H4	Bn	2l	84
13	1m	Me	Et	2m	77

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Fig. 2 ORTEP drawing of 2i.

On the basis of the above experimental results together with some literatures, a mechanism for the synthesis of pyridine-2,6(1H,3H)-dione 2 is proposed as depicted in Scheme 3. In the presence of PBr₃, α-aminopropenoyl-α-carbamoyl ketene-S,S-acetal 1 is transformed into iminium ion intermediate A, which undergoes an intramolecular cyclization to afford intermediate B and its tautomer B′.^20,21 The protonation of the C–C double bond of B′ gives carbocation C,¹⁵ followed by elimination of HOPBr₂ to form carbocation D and its tautomer iminium ion D′. The latter is hydrolyzed to the final product pyridine-2,6(1H,3H)-dione 2 during the workup process under acidic conditions.^14b,21

Scheme 3 Plausible mechanism for the reaction of α-alkenoyl-α-carbamoyl ketene-S,S-acetal 1 with PBr₃.

Conclusions

In summary, an efficient synthesis of substituted pyridine-2,6(1H,3H)-diones via an intramolecular [5 + 1] annulation of readily available α-alkenoyl-α-carbamoyl ketene-S,S-acetals 1 mediated by phosphorus bromide (PBr₃) is developed. This protocol is associated with readily available starting materials, mild conditions, high yields, a wide range of substrate scope, and rich functionalities and important synthetic potential of the products.

Experimental

General

All reagents were purchased from commercial sources and used without treatment, unless otherwise indicated. The products were purified by column chromatography over silica gel. ¹H NMR spectra and ¹³C NMR spectra were obtained at 25 °C at 300 MHz (or 400 MHz) and 100 MHz, respectively, on a Bruker AV300 (or AV 400) spectrometer using CDCl₃ (otherwise indicated) as solvent and TMS as internal standard. Mass spectra were recorded on a Bruker autoflex III (smartbeam MALDI-TOF) mass spectrometer. IR spectra (KBr) were recorded on a Shimadzu FTIR-8400S spectrophotometer in the range of 400–4000 cm⁻¹.

Typical procedure for the synthesis of 2 (2a as an examples)

To a solution of 1a (336 mg, 1.0 mmol) in CH₂Cl₂ (15 mL) was added PBr₃ (325 mg, 1.2 mmol) at 0 °C. Then the reaction mixture was allowed to warm to room temperature and stirred for 6.0 h. After the reaction was completed, the resulting mixture was poured into saturated aqueous NaCl (100 mL), which was extracted with dichloromethane (3 × 30 mL). The organic extracts were washed with water, dried over MgSO₄, filtered and concentrated in vacuo. Purification was carried out by flash silica gel chromatography using petroleum ether : ethyl acetate (9 : 1, v/v) as eluent to give product 2a (218 mg, 75%).

Analytical data of 2

3-[Bis(methylthio)methylene]-1-phenylpyridine-2,6(1H,3H)-dione (2a). Yellow solid; mp 149–150 °C. ¹H NMR (300 MHz, CDCl₃): δ 2.54 (s, 3H), 2.67 (s, 3H), 6.24 (d, J = 10.0 Hz, 1H), 7.18–7.21 (m, 2H), 7.47–7.49 (m, 3H), 8.07 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 174.1, 164.6, 162.9, 138.6, 135.2, 129.2, 128.6, 128.4, 123.2, 116.3, 21.6, 19.8. IR (KBr, cm⁻¹) 3446, 2918, 1677, 1639, 1592, 1491, 1452, 1411, 1178, 782. Anal. calcd for C₁₄H₁₃NO₂S₂: C, 57.71; H, 4.50; N, 4.81. Found: C, 57.48; H, 4.43; N, 4.87.

3-[Bis(methylthio)methylene]-1-p-tolylpyridine-2,6(1H,3H)-dione (2b). Yellow solid; mp 142–143 °C. ¹H NMR (300 MHz, CDCl₃): δ 2.39 (s, 3H), 2.52 (s, 3H), 2.66 (s, 3H), 6.24 (d, J = 10.0 Hz, 1H), 7.06 (d, J = 8.0 Hz, 2H), 7.28 (d, J = 8.0 Hz, 2H), 8.05 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 174.0, 164.7, 163.0, 138.5, 138.3, 132.4, 130.0, 128.2, 123.1, 116.3, 21.6, 21.2, 19.8. IR (KBr, cm⁻¹) 3446, 2919, 1686, 1637, 1513, 1449, 1408, 1187, 841. Calcd for C₁₅H₁₅NO₂S₂: C, 58.99; H, 4.95; N, 4.59. Found: C, 58.68; H, 5.06; N, 4.50.

3-[Bis(methylthio)methylene]-1-(4-chlorophenyl)pyridine-2,6(1H,3H)-dione (2c). Yellow solid; mp 137–139 °C. ¹H NMR (300 MHz, CDCl₃): δ 2.54 (s, 3H), 2.68 (s, 3H), 6.22 (d, J = 10.2 Hz, 1H), 7.12–7.15 (m, 2H), 7.43–7.53 (m, 2H), 8.06 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 175.0, 164.5, 162.7, 138.9, 134.4, 133.7, 130.0, 129.5, 122.8, 116.0, 21.7, 19.9. IR (KBr, cm⁻¹) 3421, 2956, 1685, 1645, 1603, 1505, 1469, 1453, 1186, 839. Anal. calcd for C₁₄H₁₂ClNO₂S₂: C, 51.61; H, 3.71; N, 4.30. Found: C, 51.81; H, 3.79; N, 4.18.

3-[Bis(methylthio)methylene]-1-(4-methoxyphenyl)pyridine-2,6(1H,3H)-dione (2d). Yellow solid; mp 166–168 °C. ¹H NMR (300 MHz, CDCl₃): δ 2.53 (s, 3H), 2.67 (s, 3H), 3.83 (s, 3H), 6.23 (d, J = 10.2 Hz, 1H), 7.00 (d, J = 8.7, 2H), 7.11 (d, J = 8.7, 2H), 8.05 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 174.0, 164.8, 163.1, 159.3, 138.5, 129.4, 127.6, 123.2, 116.3, 114.6, 55.4, 21.6, 19.8. Anal. calcd for C₁₅H₁₅NO₃S₂: C, 56.05; H, 4.70; N, 4.36. Found: C, 56.33; H, 4.82; N, 4.45.

3-[Bis(methylthio)methylene]-1-(2-methoxyphenyl)pyridine-2,6(1H,3H)-dione (2e). Yellow solid; mp 142–143 °C. ¹H NMR (300 MHz, CDCl₃): δ 2.53 (s, 3H), 2.65 (s, 3H), 3.76 (s, 3H), 6.22 (d, J = 10.2 Hz, 1H), 7.01–7.07 (m, 2H), 7.12–7.15 (m, 1H), 7.37–7.42 (m, 1H), 8.06 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 173.3, 164.3, 162.5, 154.9, 138.6, 130.0, 129.9, 124.0, 123.3, 120.9, 116.4, 111.9, 55.7, 21.5, 19.8. IR (KBr, cm⁻¹) 3446, 1683, 1642, 1600, 1500, 1451, 1410, 1192, 759. Anal. calcd for C₁₅H₁₅NO₃S₂: C, 56.05; H, 4.70; N, 4.36. Found: C, 56.31; H, 4.56; N, 4.29.

1-Benzyl-3-[bis(methylthio)methylene]pyridine-2,6(1H,3H)-dione (2f). Yellow solid: mp 98–100 °C; ¹H NMR (400 MHz, CDCl₃): δ 2.53 (s, 3H), 2.72 (s, 3H), 5.21 (s, 2H), 6.21 (d, J = 10.4 Hz, 1H), 7.28–7.30 (m, 1H), 7.32–7.36 (m, 2H), 7.52 (d, J = 7.6 Hz, 2H), 8.01 (d, J = 10.4 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 172.2, 163.6, 161.6, 137.0, 136.3, 127.9, 127.2, 126.2, 122.3, 115.2, 41.9, 20.6, 18.7; anal. calcd for C₁₅H₁₅NO₂S₂: C, 58.99; H, 4.95; N, 4.59; found: C, 58.76; H, 4.99; N, 4.72. MS (MALDI): calcd for C₁₅H₁₆NO₂S₂ [M + H]⁺ 306.1, found 306.1.

3-[Bis(ethylthio)methylene]-1-phenylpyridine-2,6(1H,3H)-dione (2g). Yellow solid; mp 119–120 °C. ¹H NMR (300 MHz, CDCl₃): δ 1.32–1.39 (m, 6H), 2.96–3.03 (m, 2H), 3.18–3.26 (m, 2H), 6.23 (d, J = 10.2 Hz, 1H), 7.18–7.21 (m, 1H), 7.38–7.51 (m, 4H), 8.09 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 171.8, 164.6, 163.0, 138.8, 135.2, 129.2, 128.6, 126.3, 123.9, 116.3, 34.1, 30.8, 14.8, 13.9. IR (KBr, cm⁻¹) 3442, 2924, 1686, 1641, 1593, 1493, 1453, 1413, 1190, 698. Anal. calcd for C₁₆H₁₇NO₂S₂: C, 60.16; H, 5.36; N, 4.38. Found: C, 60.35; H, 5.28; N, 4.50.

3-[Bis(ethylthio)methylene]-1-p-tolylpyridine-2,6(1H,3H)-dione (2h). Yellow solid; mp 115–117 °C. ¹H NMR (300 MHz, CDCl₃): δ 1.27–1.37 (m, 6H), 2.39 (s, 3H), 2.96–2.99 (m, 2H), 3.19–3.22 (m, 2H), 6.22 (d, J = 10.0 Hz, 1H), 7.06–7.08 (m, 2H), 7.26–7.29 (m, 2H), 8.08 (d, J = 10.0 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 171.7, 164.7, 163.0, 138.7, 138.2, 132.4, 129.9, 128.2, 123.8, 116.2, 34.0, 30.7, 21.2, 14.7, 13.8. Anal. calcd for C₁₇H₁₉NO₂S₂: C, 61.23; H, 5.74; N, 4.20. Found: C, 61.48; H, 5.88; N, 4.31.

3-[Bis(ethylthio)methylene]-1-(4-chlorophenyl)pyridine-2,6(1H,3H)-dione (2i). Yellow solid; mp 122–123 °C. ¹H NMR (300 MHz, CDCl₃): δ 1.32–1.39 (m, 6H), 2.96–3.04 (m, 2H), 3.19–3.24 (m, 2H), 6.21 (d, J = 10.2 Hz, 1H), 7.13 (d, J = 8.4 Hz, 2H), 7.43 (J = 8.4 Hz, 2H), 8.09 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 172.7, 164.4, 162.8, 139.0, 134.3, 133.7, 130.1, 129.4, 123.5, 115.9, 34.1, 30.9, 14.8, 13.9. IR (KBr, cm⁻¹) 3443, 1679, 1647, 1598, 1493, 1454, 1407, 1190, 783, 695. Anal. calcd for C₁₆H₁₆ClNO₂S₂: C, 54.30; H, 4.56; N, 3.96. Found: C, 53.94; H, 4.45; N, 4.09.
Crystal data for 2i. C₁₆H₁₆ClNO₂S₂, colourless crystal, M = 352.9, monoclinic, P2(1)/n, a = 12.739(5) Å, b = 5.382(2) Å, c = 24.848(10) Å, α = 90.00°, β = 103.362(6)°, γ = 90.00°, V = 1657.3(11) ų, Z = 4, T = 273(2), F000 = 736.0, R₁ = 0.0636, wR₂ = 0.1423. CCDC deposition number: 1058034.†

3-[Bis(ethylthio)methylene]-1-(4-methoxyphenyl)pyridine-2,6(1H,3H)-dione (2j). Yellow solid; mp 118–120 °C. ¹H NMR (300 MHz, CDCl₃): δ 1.31–1.38 (m, 6H), 2.95–3.02 (m, 2H), 3.18–3.25 (m, 2H), 3.83 (s, 3H), 6.22 (d, J = 10.2 Hz, 1H), 6.98 (d, J = 9.0 Hz, 2H), 7.09 (d, J = 9.0 Hz, 2H), 8.07 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 171.6, 164.9, 163.2, 159.3, 138.7, 129.5, 127.7, 123.9, 116.3, 114.6, 55.4, 34.0, 30.7, 14.8, 13.9. IR (KBr, cm⁻¹) 3441, 1693, 1647, 1454, 1412, 1190, 674. Anal. calcd for C₁₇H₁₉NO₃S₂: C, 58.43; H, 5.48; N, 4.01. Found: C, 58.77; H, 5.57; N, 4.10.

3-[Bis(benzylthio)methylene]-1-phenylpyridine-2,6(1H,3H)-dione (2k). Yellow solid; mp 165–166 °C. ¹H NMR (300 MHz, CDCl₃): δ 4.05 (s, 2H), 4.29 (s, 2H), 6.09 (d, J = 10.2 Hz, 1H), 7.15–7.49 (m, 15H), 7.81 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 169.0, 164.4, 163.0, 138.5, 135.6, 135.1, 135.0, 129.4, 129.2, 128.9, 128.8, 128.7, 128.6, 128.5, 128.3, 127.7, 124.6, 116.6, 43.9, 41.4. IR (KBr, cm⁻¹): 3445, 2923, 2853, 1658, 1557, 1540, 1487, 1350, 1260, 748. Anal. calcd for C₂₆H₂₁NO₂S₂: C, 70.40; H, 4.77; N, 3.16. Found: C, 70.69; H, 4.70; N, 3.07.

3-[Bis(benzylthio)methylene]-1-(p-tolyl)pyridine-2,6(1H,3H)-dione (2l). Yellow solid; mp 166–168 °C. ¹H NMR (300 MHz, CDCl₃): δ 2.37 (s, 3H), 4.04 (s, 2H), 4.29 (s, 2H), 6.09 (d, J = 10.2 Hz, 1H), 7.03 (d, J = 7.8 Hz, 2H), 7.22–7.32 (m, 12H), 7.79 (d, J = 10.2 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 168.9, 164.5, 163.1, 138.4, 135.7, 135.2, 132.3, 130.0, 129.4, 128.9, 128.8, 128.7, 128.3, 128.2, 127.7, 124.6, 116.6, 43.9, 41.1, 21.2. IR (KBr, cm⁻¹) 3446, 2935, 1692, 1644, 1600, 1511, 1493, 1454, 1191, 820. Anal. calcd for C₂₇H₂₃NO₂S₂: C, 70.87; H, 5.07; N, 3.06. Found: C, 70.66; H, 5.13; N, 2.98.

3-[Bis(ethylthio)methylene]-1-methylpyridine-2,6(1H,3H)-dione (2m). Yellow oil. ¹H NMR (300 MHz, CDCl₃): δ 1.31 (t, J = 7.5 Hz, 3H), 1.37 (t, J = 7.5 Hz, 3H), 2.93 (q, J = 7.5 Hz, 2H), 3.22 (q, J = 7.5 Hz, 2H), 3.33 (s, 3H), 6.12 (d, J = 10.2 Hz, 1H), 7.97 (d, J = 10.2 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 14.0, 14.6, 26.2, 30.6, 33.9, 116.0, 124.0, 137.8, 163.1, 164.8, 170.5; anal. calcd for C₁₁H₁₅NO₂S₂: C, 51.33; H, 5.87; N, 5.44. Found: C, 51.60; H, 5.72; N, 5.37. MS (MALDI): calcd for C₁₁H₁₆NO₂S₂ [M + H]⁺ 258.1, found 258.1.

Acknowledgements

Financial support of this research by the National Natural Science Foundation of China (21172211) and Jilin Provincial Science and Technology Development (201205027) is greatly acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data in CIF for 2i, and copies of NMR spectra for compounds 2. CCDC 1058034. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13498f

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