Exploring the Reactivity of Electrophilic Organic Carbonates and Thiocarbonates as Vehicles to Convert Hydrosulfide into COS and CS₂

Abstract

Hydrogen sulfide (H₂S) and other reactive sulfur species are important small molecules with biological significance. In addition to common reactive sulfur species like H₂S, polysulfides, and persulfides, both carbonyl sulfide (COS) and carbon disulfide (CS₂) have been postulated to be potential sources of reduced sulfur. To better understand this possible connection, we demonstrate that H₂S can be converted to COS and CS₂ by reaction with simple organic carbonate and thiocarbonate electrophiles, respectively.

Graphical Abstract

Reactive sulfur species (RSS) play important roles in synthetic chemistry and also in chemical biology, with common RSS oxidation states spanning from −2 to +6 in most organisms.¹ The most common fully reduced RSS are thiols, hydrogen sulfide (H₂S), and sulfides found in FeS clusters. Other small molecules like carbonyl sulfide (COS) and carbon disulfide (CS₂) have been proposed to be potentially important biological molecules and also contain sulfur atoms in the −2 oxidation state. Focusing on COS, it has been previously proposed as a potential biomarker for various disease states including cystic fibrosis,² acute rejection of organs,³ and liver disease.⁴ In addition, COS and CS₂ may also play roles at the interface of H₂S and NO chemistry by reacting with NO₂⁻ to generate different combinations of S/N hybrid species, NO, and sulfane sulfur motifs.^5–6 These and other factors led to the proposition that COS may be involved in potential crosstalk through the gasotransmitters CO and H₂S.⁷ In addition, Zn-based carbonic anhydrase (CA), which normally converts CO₂ into bicarbonate, can also hydrolyze COS to H₂S.⁸ We and others have leveraged this conversion to develop small molecule H₂S donors that initially release COS as a precursor to H₂S, which offers significant advantages over direct addition of sulfide salts like NaSH or Na₂S.^9–10

Common approaches to release COS include activatable organic thiocarbonates¹¹ and thiocarbamates,⁹ both of which have been incorporated into responsive donors that respond to different triggers, including H₂O₂,¹² enzyme activation,¹³ or reduction¹⁴ to undergo self-immolative cascade reactions to release COS, which is then converted to H₂S by CA (Figure 1a). In addition, other COS-releasing platforms, including N-thiocarboxyanhydrides and perthiocarbonates have also been developed.^15–16 Based on the utility of COS to H₂S conversion, we were curious whether organic platforms could be designed to carry out the reverse reaction and convert H₂S into COS (Figure 1b). To approach this question, we envisioned that addition of nucleophilic hydrosulfide (HS⁻),¹⁷ which is the most prevalent form of H₂S at physiological pH (7.4),¹⁸ to carbonyl electrophiles could result in COS generation.

Figure 1.

(a) Example of prior work to develop distally activated COS/H₂S donors. (b) General approach from this work to convert H₂S to COS using activated carbonyl electrophiles.

To test this general approach, we treated 4-nitrophenyl chloroformate with 1 equiv. of NaSH in CD₃OD, which resulted in immediate formation of a yellow solution corresponding to p-nitrophenol generation. This product identity was confirmed by ¹H NMR spectroscopy (57% conversion), and the conversion could be improved to 96% conversion by using 2 equiv. of NaSH (Scheme 1, See SI for additional details). Under these unbuffered conditions, NaSH is likely acting as both nucleophile and a base based on the similar pK_a values of H₂S (7.0)¹⁸ and 4-nitrophenol (7.15),¹⁹ which is consistent with the increased conversion when excess NaSH is used.

Scheme 1.

Reaction of 4-nitrophenyl chloroformate with NaSH in CD₃OD.

Next we investigated this reaction in pH 7.4 buffer to both determine the water compatibility of this reaction and to simplify the proton inventory. We monitored the initial rate of p-nitrophenol formation from 4-nitrophenyl chloroformate and observed rapid hydrolysis (rate ~ 7×10⁻⁵ M/s, t_1/2 < 10 sec).²⁰ To improve the stability of these compounds, we modified the electrophilicity of the carbonyl motif with the goal of improving water compatibility while maintaining reactivity toward HS⁻. We prepared an electronically varied series of aryl and alkyl carbonate derivates while maintaining the p-nitrophenol motif as a convenient UV-vis handle (p-nitrophenolate: λ_max = 401 nm, ε = 14,400 ± 500 M⁻¹ in pH 7.4 PBS).

To measure the reactivity of these electrophiles toward HS⁻, we treated 25 μM of each compound with 40 equiv. of NaSH in pH 7.4 PBS, measured initial rates of p-nitrophenolate formation, and compared reaction rates with NaSH with the background rate of hydrolysis. Using 4-nitrophenyl methyl carbonate (1a), we found the initial reaction rate with HS⁻ was only 1.8 times faster than background hydrolysis.²¹ We found that CF₃CH₂ functionalized 1b reacted significantly faster NaSH (initial rate 9.05(8) × 10⁻⁹ M/s), which matched our expectation based on the pK_a of the leaving group (MeOH = 15.54, CF₃CH₂OH = 12.46) (Figure 2). More importantly, the selectivity of 1b for NaSH over background hydrolysis was improved to 4.22:1. We also used this compound to definitively confirm COS release by GC after treating 25 μM of 1b with 40 equiv. of NaSH for 10 minutes and analyzing the headspace (see S21–S22).

Figure 2.

(a) Reaction scheme for conversion of 1b to COS by NaSH. (b) UV-vis spectra at various timepoints, and (c) initial rate data for the reaction. Condition: 25 μM 1b with 40 equiv. of NaSH in pH 7.4 PBS at 25 °C.

We further expanded the scope to include diaryl carbonates with electron withdrawing, neutral, or donating groups in bis(4-nitrophenyl) carbonate (1c), 4-nitrophenyl phenyl carbonate (1d), and 4-nitrophenyl 4′-methoxyphenyl carbonate (1e), respectively. Compounds 1c-1e followed the same general reactivity trends, with electron withdrawing groups facilitating with reaction with NaSH, with a change in initial rates of ~1 order of magnitude difference for each compound. Matching the trend observed for alkyl carbonates, we also observed improved selectivity for reaction with NaSH over background hydrolysis for electron withdrawn leaving groups, with 1c, 1d, and 1e showing selectivity ratios of 3.7:1, 3.0:1, and 1.4:1, respectively. Although the pK_a values of the leaving group for 1c-1e were all lower than for CF₃CH₂OH (4-nitrophenol = 7.15, PhOH = 9.98, and 4-methoxyphenol = 10.21), only 1c reacted faster than 1b, which we attribute to steric influences in the accessibility of the carbonyl electrophile. Based on the pK_a of H₂S (7.0), about 72% is in the anionic HS⁻ form at pH = 7.4 (Table 1). Our data show that organic carbonates 1a-e are significantly more reactive toward HS⁻ (720 μM based on speciation) than water (~ 55 M), although clear differentiation from HO⁻ (0.25 μM at pH = 7.4) is more difficult.²²

Table 1.

Reaction parameters of the reaction of organic carbonates with NaSH

Substrate	1a	1b	1c	1d	1e
R group	CH3	CH2CF3	4-NO2-Ph	Ph	4-OMe-Ph
pKa (ROH)	15.54	12.46	7.15	9.98	10.21
NaSHa	4.8(4)	90.5(8)	230(50)	49(8)	6(2)
k (M−1s−1)b	0.018(2)	0.362(3)	0.9(2)	0.21(3)	0.023(7)
Hydrolysisc	2.7(1)	21.5(1)	60(20)	16(2)	4(1)
Ratio	1.8:1	4.22:1	3.7:1	3.0:1	1.4:1

Initial rates (10⁻¹⁰ × M/s). Reaction monitored by UV-vis kinetics at 401 nm.

^aConditions: 25 μM 1, 1000 μM NaSH, pH 7.4 PBS

^bCalculated from concentrations of 1 and NaSH.

^cConditions: 25 μM 1, pH 7.4 PBS.

Building from HS⁻-mediated COS release, we next investigated whether thiocarbonate analogues could undergo a similar reaction to generate CS₂. In general, small molecules that release CS₂ are much less common than those that release COS.²³ Ford and co-workers demonstrated acid-mediated CS₂ release from hindered dithiocarbamates.²⁴ In contrast, general strategies used for triggered COS release do not appear to work for CS₂ release, with caged dithiocarbamates releasing H₂S and an isothiocyanate rather than CS₂.²⁵ To determine whether electron poor thiocarbonates could release CS₂, we treated 4-nitrophenyl chlorothioformate with 2 equiv. of NBu₄SH in CD₃CN (for better solubility than NaSH and to better monitor HS⁻ concentration through NBu₄⁺ integration) and observed quantitative formation of 4-nitrophenolate ion by ¹H NMR spectroscopy.²⁶ In addition, the ¹³C{¹H} NMR spectrum showed a new peak at 193.6 ppm that matched the chemical shift of authentic CS₂ (Figure 3). CS₂ was not quantified directly by ¹³C NMR spectroscopy, but no other C=S containing resonances were observed.

Figure 3.

(a) Reaction for CS₂ release from chlorothioformates. (b) Stacked ¹³C{H} NMR spectra in CD₃CN of CS₂ formation from chlorothioformate treated with 2 equiv. of NBu₄SH, and the parent 4-nitrophenyl chlorothioformate.

Much like the oxygen congener, the parent chlorothioformate showed limited stability in water (Figure S23), but we prepared thiocarbonate analogues 2a-2e to both improve water stability and also allow for more direct comparisons between carbonates and thiocarbonates. Electron deficient 4-nitrophenyl methyl thiocarbonate (2a) showed significantly higher selectivity for HS⁻ than 1a (see Table 2), with a 60-fold selectivity for reaction with NaSH over background hydrolysis. Similar improved selectivity was observed for trifluoroethyl derivative 2b. The most electrophilic thiocarbonate, bis(4-nitrophenyl) thiocarbonate (2c), not only the reacted faster than any of the carbonates or thiocarbonates, but also showed the highest selectivity over hydrolysis (690:1). The diaryl thiocarbonates with neutral or donating groups (2d and 2e) reacted with NaSH much more slowly than the corresponding carbonates 2a-c, with 4-nitrophenyl phenyl carbonate (2d) showing the slowest overall reactivity. Overall, with the exceptions of 2a and 2d, the series of thiocarbonates followed the same reactivity and selectivity trends as 1a-e.

Table 2.

Initial Rates of Organic Thiocarbonates with NaSH

Substrate	2a	2b	2c	2d	2e
R identity	CH3	CH2CF3	4-NO2-Ph	Ph	4-OMe-Ph
pKa (ROH)	15.54	12.46	7.15	9.98	10.21
NaSHa	124(6)	500(30)	820(40)	3.7(0.4)	16.5(0.4)
k (M−1s−1)b	0.51(2)	2.1(1)	3.3(2)	0.015(2)	0.07(2)
Hydrolysisc	2.06(4)	15.9(7)	1.2(3)	0.8(3)	9.4(5)
Ratio	59.8:1	32:1	690	5:1	1.8:1

Initial rates (10⁻¹⁰ × M/s). Reaction monitored by UV-vis kinetics at 401 nm.

^aConditions: 25 μM 2, 1000 μM NaSH, pH 7.4 PBS

^bCalculated from concentrations of 2 and NaSH.

^bConditions: 25 μM 2, pH 7.4 PBS.

One key observation in comparing the carbonate and thiocarbonate derivatives is the higher selectivity of the thiocarbonates for attack by HS⁻ over background hydrolysis, which is particularly apparent for 2a and 1a. Thiocarbonate 2a exhibits higher water stability than 1a, yet in stark contrast, the rate of 4-nitrophenolate release in the presence of NaSH for 2a is 26 times faster than for 1a. Considering that nucleophilic addition of HS⁻ is faster with electron withdrawing and sterically unhindered substrates, we hypothesize that formation of the initial tetrahedral intermediate is likely the rate limiting step of the reaction with HS⁻. It is possible that there are changes in subsequent steps of the mechanism between the HS⁻ and hydrolysis reactions, particularly between differently electronically-substituted compounds where the degree of resonance donation from the alkoxy group may impact the electrophilicity of the thiocarbonyl.

In conclusion, we have demonstrated that electrophilic organic carbonates and thiocarbonates can react with HS⁻ to produce COS and CS₂, respectively. In general, the thiocarbonates react faster with hydrosulfide than the respective carbonates and are also more stable in water, which we attribute to differences in electrophilic character on the carbonyl / thiocarbonyl carbon. We found that the selectivity for COS and CS₂ formation over background hydrolysis can be modified by electronic substitution of the (thio)carbonate precursor. In general, this approach for interconversion between HS⁻ and COS/CS₂ may open new pathways for developing constructs for shuttling between different RSS while maintaining fully-reduced sulfur incorporation. In addition, if carried out under conditions to limit the hydrolysis of COS, this method could likely be adapted as a simple approach to deliver COS on demand from organic precursors for synthetic applications.

Experimental Section

General.

The reagents 4-nitrophenyl chloroformate, p-substituted phenols, thiophosgene, 1c, and anhydrous NaSH were purchased from TCI, Thermo Fischer Scientific, Oakwood, and Strem. DMSO, CDCl₃, and CD₃CN were purchased from Sigma-Aldrich and Cambridge Isotope Laboratories. CD₃CN was dried over CaH₂, distilled, and freeze-pump-thawed. NBu₄SH,²⁷ 4-nitrophenyl chlorothioformate,²⁸ and 1a²⁹ were prepared as previously reported. PBS buffer (pH 7.4) was prepared with 140 mM NaCl, 3 mM KCl, and 10 mM phosphate with PBS tablets in milliQ water. UV-Vis experiments were performed using an Agilent Cary 60 spectrophotometer. NMR spectra were acquired using a Bruker 500 MHz instrument and processed with MestRe Nova software version 14.0.0. GC measurements were made on an Agilent 8890A GC with a PFPD and SPME headspace autosampler. Note: carbonyl sulfide, carbon disulfide, and hydrosulfide salts are acutely toxic. These reagents were used inside a fume hood or glovebox in order to minimize exposure. Air-tight screw-cap cuvettes were used for UV-vis experiments. Finally, all reactions in which hydrosulfide salts were utilized were quenched using an aqueous solution of zinc acetate, sodium citrate, and sodium hydroxide. This quenching procedure converts all hydrosulfide salts into zinc sulfide which is insoluble and substantially less toxic than the precursor salts.

General Procedure for UV-vis Kinetic Studies.

In an O₂-free glovebox, screw-cap UV-Vis cuvettes were filled with 3.00 mL of 7.4 pH PBS buffer. A 2.5 mM solution of the desired carbonate or thiocarbonate were prepared in DMSO. A vial containing NaSH (3.4 mg, 0.061 mmol) was evacuated and backfilled with N₂ on the Schlenk line, after which 500 μL of degassed 7.4 pH PBS was added to generate a 0.12 M solution. To measure the background hydrolysis reactions, 30 μL of a 2.5 mM carbonate or thiocarbonate solution was injected into the cuvette, which was then inverted several times prior to analysis by UV-vis spectroscopy. For reactions with NaSH, 25 μL of 0.12 M NaSH solution (40 equiv.) was injected within 2 min of carbonate or thiocarbonate addition to provide kinetics that were readily measurable under the reaction conditions while staying within the buffering capacity of the solution. The reaction was monitored at 401 nm by UV-vis spectroscopy at specific time intervals (~1 min) depending on the rate of the reaction.

Synthesis of 1b. 4-Nitrophenyl chloroformate (2.017 g, 10.00 mmol) was dissolved in 40 mL of CH₂Cl₂ in a round bottom flask with a stir bar. 2,2,2-Trifluoroethanol (755 μL, 10.0 mmol) and NEt₃ were dissolved in 10 mL of CH₂Cl₂ in a 20 mL scintillation vial. Both solutions were cooled to 0 °C and the trifluoroethoxide solution was slowly added to the stirring solution of 4-nitrophenyl chloroformate. The reaction mixture was stirred at 0 °C for 20 minutes and then allowed to warm to room temperature. After 30 minutes, an aliquot of the reaction mixture was concentrated under vacuum and analyzed by ¹H and ¹⁹F NMR spectroscopy to confirm conversion. The reaction mixture was then filtered through celite and the celite was washed with CH₂Cl₂ (3 × 10 mL). The crude product was concentrated onto 9 g of silica and dry loaded onto a column and purified by column chromatograph (2%–20% EtOAc in hexanes) to afford the desired product as a clear liquid (1.722 g, 65% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.31 (d, J = 9.2 Hz, 2H), 7.42 (d, J = 9.2 Hz, 2H), 4.65 (q, J_H-F = 8.0 Hz, 2H). ¹³C{¹H} NMR (126 MHz, CDCl₃) δ: 155.1, 151.7, 146.0, 125.6, 122.4 (q, J_C-F = 277 Hz), 121.8, 64.4 (q, J_C-F = 37.6 Hz). ¹⁹F NMR (471 MHz, CD₃Cl) δ: −74.1 (t, J_F-H = 7.9 Hz, 3F). HRMS-ESI m/z [M]+ calcd for [C₉H₆O₅NF₃], 265.0198; found, 265.0190.

Synthesis of 1d. Phenyl chloroformate (112.7 mg, 0.7189 mmol) was dissolved in 2 mL of CH₂Cl₂ in a 20 mL scintillation vial with a stir bar and stirred at 0 °C. 4-Nitrophenol (100 mg, 0.719 mmol) was dissolved in 2 mL of CH₂Cl₂. NEt₃ (100 μL, 0.717 mmol) was added to the 4-nitrophenol solution, which turned a bright yellow. The 4-nitrophenolate solution was added slowly over 2 minutes to the phenyl chloroformate solution. The solution was allowed to stir at 0 °C for 15 minutes and was then warmed to room temperature over the course of 30 minutes. An aliquot of the reaction was concentrated under vacuum and analyzed by ¹H NMR spectroscopy. The crude reaction mixture was purified by recrystallization by layering hexanes at −30 °C to afford the product as a white solid (60.8 mg, 33% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.32 (d, J = 9.2 Hz, 2H), 7.49 (d, J = 9.2 Hz, 2H), 7.47–7.43 (m, 2H), 7.32 (t, J = 7.4 Hz, 1H), 7.28 (d, J = 7.6 Hz, 2H). ¹³C{¹H} NMR (126 MHz, CDCl₃) δ: 155.4, 151.2, 150.8, 145.7, 129.9, 126.9, 125.6, 121.9, 120.9. The NMR data matched prior reports of this compound.³⁰

Synthesis of 1e. 4-Methoxyphenol (124.2 mg, 1.001 mmol) and NEt₃ (139.4 μL, 1.000 mmol) were combined in 4 mL of CH₂Cl₂ in a scintillation vial. 4-Nitrophenyl chloroformate (201.8 mg, 1.001 mmol) was dissolved in 4 mL of CH₂Cl₂ with a stir bar in a scintillation vial. Both solutions were allowed to cool to 0 °C, and the 4-methoxy phenolate solution was slowly added to the stirring solution of 4-chloroformate. The reaction mixture was stirred at 0 °C for 15 minutes, warmed to room temperature, and stirred for an additional 30 minutes. The CH₂Cl₂ was removed under vacuum and the residue was reconstituted in ethyl acetate. Unreacted 4-methoxyphenol and 4-nitrophenol byproduct were removed by aqueous washing with 5% NaOH (aq) (3 × 10 mL). During these washings, the aqueous layer was initially a bright yellow color and changed to nearly clear. The organic layer was further washed with 3 M HCl (3 × 10 mL) to remove residual NEt₃. The product was purified by recrystallization at –30 °C (ethyl acetate and hexanes layering) to provide the white crystalline product (113.5 mg, 39% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.31 (d, J = 9.2 Hz, 2H), 7.48 (d, J = 9.2 Hz, 2H), 7.19 (d, J = 9.1 Hz, 2H), 6.93 (d, J = 9.1 Hz, 2H), 3.82 (s, 3H). The NMR data matched prior reports of the same compound.³¹

Synthesis of 2a. 4-Nitrophenyl chlorothioformate (93.9 mg, 0.431 mmol) was dissolved in 2.2 mL of CH₂Cl₂ in a scintillation vial, after which methanol (17.4 μL, 0.431 mmol), and then NEt₃ (60.1 μL, 0.431 mmol) were added. Monitoring the reaction by TLC showed that most starting material was consumed after 10 min. The crude reaction mixture was purified by column chromatography (2–20% ethyl acetate in hexanes) to afford the desired product as a yellow oil (13.5 mg, 15% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.32 (d, J = 9.1 Hz, 2H), 7.29 (d, J = 9.1 Hz, 2H), 4.19 (s, 3H), which matched the previously reported spectrum.³²

Synthesis of 2b. 4-Nitrophenyl chlorothioformate (108.4 mg, 0.4981 mmol) was dissolved in 2.5 mL of CH₂Cl₂ in scintillation vial, after which 2,2,2-trifluoroethanol (38 μL, 0.50 mmol) and then NEt₃ (70 μL, 0.50 mmol) were added. After 5 min, TLC showed complete consumption of starting material. The crude reaction mixture was purified by column chromatography (2–20% ethyl acetate in hexanes) to afford the desired product as a clear liquid (81.2 mg, 58% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.34 (d, J = 9.1 Hz, 2H), 7.33 (d, J = 9.1 Hz, 2H), 4.89 (q, J_H-F = 8.0 Hz, 2H). ¹³C{¹H} NMR (126 MHz, CDCl₃) δ: 192.5, 157.4, 146.5, 125.7, 123.2, 122.3 (q, J_C-F = 278 Hz), 68.7 (q, J_C-F = 37.2 Hz). ¹⁹F NMR (471 MHz, CD₃Cl) δ: −73.2 (t, J_F-H = 7.8 Hz, 3F). HRMS-ESI m/z [M]+ calcd for [C₉H₆O₄NSF₃], 280.9970; found, 280.9957.

Synthesis of 2c. Prepared by modifying an existing procedure.³³ 4-Nitrophenol (500.8 mg, 3.600 mmol) was dissolved in 20 mL of dry THF in a 100 mL round bottom flask and cooled to 0 °C. Triethyl amine (251 μL, 1.80 mmol) was added to the flask, after which thiophosgene (85%, 162 μL, 1.80 mmol) was added dropwise. The resultant reaction mixture stirred at 0 °C for 25 minutes, during which time a precipitate formed. The precipitate was filtered, washed with THF (3 × 5 mL), and dried to afford the pure product as an orange solid (514 mg, 89% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.37 (d, J = 9.1 Hz, 4H), 7.41 (d, J = 9.2 Hz, 4H), which matched the previously reported spectrum.

Synthesis of 2d. 4-Nitrophenyl chlorothioformate (54.5 mg, 0.250 mmol) was dissolved in 1 mL of CH₂Cl₂ in a 20 mL scintillation with a stir-bar. Phenol (23.5 mg, 0.250 mmol) and NEt₃ (38 μL, 0.27 mmol) were dissolved in 1 mL of CH₂Cl₂, and the phenolate solution was slowly added to the stirring solution of 4-nitrophenyl chlorothioformate over 1 minute. After 5.5 h, the crude reaction mixture was directly loaded onto a SiO₂ column and purified by flash chromatography (30% ethyl acetate in hexanes) to afford a white solid (43.1 mg, 63% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.35 (d, J = 9.1 Hz, 2H), 7.50–7.45 (m, 2H), 7.41 (d, J = 9.1 Hz, 2H), 7.38–7.34 (m, 1H), 7.24–7.20 (m, 2H). ¹³C{¹H} NMR (126 MHz, CDCl₃) δ: 193.5, 157.7, 153.5, 146.3, 130.0, 127.3, 125.6, 123.4, 121.8. The NMR spectra matched prior reports of the compound.³⁴

Synthesis of 2e. Compound 2c (100 mg, 0.312 mmol) was dissolved in 2 mL of CH₂Cl₂ in a 20 mL scintillation vial with a stir-bar. 4-Methoxyphenol (37.2 mg, 0.300 mmol) was dissolved in 2 mL of CH₂Cl₂. The solutions were cooled to 0 °C, and NEt₃ (42 μL, 0.30 mmol) was added to the 4-methoxyphenol solution. The 4-methoxyphenolate solution was added slowly over about 2 min to the stirring solution of the 2c. The reaction mixture was stirred at 0 °C for 15 min, then allowed to warm to room temperature over the course of 15 minutes. The reaction mixture was washed with H₂O (2 × 20 mL), concentrated, and recrystallized twice by layering with hexanes at −30 °C. Remaining 4-methoxylphenol was removed by washing with 5% NaOH to afford the pure product as a white solid (45.4 mg, 50% yield). ¹H NMR (500 MHz, CDCl₃) δ: 8.35 (d, J = 9.1 Hz, 2H), 7.40 (d, J = 9.1 Hz, 2H), 7.13 (d, J = 9.0 Hz, 2H), 6.96 (d, J = 9.2 Hz, 2H), 3.83 (s, 3H). ¹³C{¹H} NMR (126 MHz, CDCl₃) δ: 194.1, 158.3, 157.7, 147.2, 146.3, 125.6, 123.4, 122.5, 114.9, 55.8. HRMS-ESI m/z [M]+ calcd for [C₁₄H₁₁O₅NS], 305.0358; found, 305.0357.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.