Synthesis of 2-((2-(4-(Benzyloxy)-4-Oxobutanamido)ethyl)dimethylammonio)propane-1-Sulfonate and 2-((2-(5-(Benzyloxy)-5-Oxopentanamido)ethyl)dimethylammonio)propane-1-Sulfonate

1. Introduction

Sulfobetaine-type zwitterions represent a highly versatile class of compounds characterized by the presence of both a positively charged quaternary ammonium group and a negatively charged sulfonate group, typically linked by a short alkyl spacer. [1] This unique headgroup architecture imparts remarkable physicochemical properties, enabling their widespread use as surfactants in diverse fields such as cosmetics, [2] analytical separation, [3] enhanced oil recovery, [4] nanoscience, [5] and wastewater treatment via clay intercalation. [6] Beyond their role as surfactants, polymeric sulfobetaines have attracted significant attention for their exceptional antifouling behavior and biotolerance, [7] applications in ultrafiltration membranes, [8] blood-contacting devices, [9] and drug delivery systems. [10] Notably, sulfobetaines that do not function as classical surfactants also exhibit intriguing features, including thermoresponsive behavior, chemically tunable structures, and the potential to serve as smart materials with lower critical solution temperature (LCST) properties. [11] These attributes highlight the interest of presenting here the two never previously described sulfobeteines presented on Figure 1: 2-((2-(4-(benzyloxy)-4-oxobutanamido)ethyl)dimethylammonio)propane-1-sulfonate (1) and 2-((2-(5-(benzyloxy)-5-oxopentanamido)ethyl)dimethylammonio)ethane-1-sulfonate (2).

2. Results and Discussion

The synthesizes of compounds 1 and 2 was achieved with moderate yields starting from benzylic alcohol and the appropriate anhydride - succinic anhydride for compound 1 and glutaric anhydride for compound 2. The proposed synthetic methodology involves a sequence of three steps: initial esterification, followed by amidation, and finally a ring-opening reaction of 1,3-propane sultone by the resulting tertiary amine, as illustrated in Scheme 1.

The primary difference between compounds 1 and 2 is the number of carbon atoms in the diacid-derived aliphatic chain. Anhydrides were used to introduce this chain and succinic anhydride, proved to be more moisture-sensitive than glutaric anhydride; therefore, succinic anhydride was recrystallized from acetic anhydride prior to the synthesis of 4. Initial attempts to prepare 4 and 5 in diethyl ether were unsuccessful. In contrast, using dichloromethane enabled the isolation of both compounds. Even under analogous conditions, the isolated yield of 4 remained substantially lower than that of 5, which was obtained in quantitative yield. Reaction progress was readily monitored by TLC (ethyl acetate as the mobile phase). Visualization with bromocresol green [12] gave a yellow spot (indicating the presence of free carboxylic acid groups) on a blue background.

Compounds 4 and 5, previously reported in the literature, [13] were converted to the corresponding amides 6 and 7 by first activating the carboxylic acids with coupling reagents, followed by addition of N,N-dimethylpropane-1,3-diamine. [14] For amide 6, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC·HCl) and N-hydroxysuccinimide (NHS) were used, whilst for amide 7 a carbonyldiimidazole (CDI)/N,N-dimethylaminopyridine (DMAP)/triethylamine system proved more effective.

Both zwitterionic compounds 1 and 2 were successfully synthesised by direct reaction of compounds 6 and 7 in dichloromethane (DCM) with 1,3-propanesultone without any chromatography purification step in the synthesis of products or intermediates. This allowed the avoidance of excessive use of silica and solvents, thereby reducing the waste generated by this process.

3. Materials and Methods

All used reagents were commercially acquired and used without further purification except for succinic anhydride that was recrystallized from acetic anhydride. Thin-layer chromatography was performed on Merck Kieselgel GF 254 0.2 mm plates supported on aluminum, which were visualized under UV light (254 nm) and stained with ninhydrin, bromocresol green, phosphomolybdic acid or Dragendorff solutions. [12] Infrared spectra were recorded using Perkin Elmer Spectrum Two spectrometer in ATR mode. ¹H and ¹³C NMR spectra were recorded using Bruker Avance III 400 spectrometer and measured at 400 and 101 MHz, respectively. NMR signals are described with chemical shift (δ, in ppm), number of protons and multiplicity with respective coupling constant (J) given in Hertz (Hz). Multiplicity is described as singlet (s), doublet (d), triplet (t), quartet (q), quintet (quint) and multiplet (m). High-resolution mass spectra (HRMS) were obtained at the University of Salamanca (Spain), Elemental Analysis, Chromatography and Mass Spectrometry Service (NUCLEUS), using a Thermo Scientific™ Q Exactive Focus mass spectrometer.

Synthesis of 4-(benzyloxy)-4-oxobutanoic acid (4)

The product was synthesised based on literature [15] with some modifications. In a flask, 16 mL of DCM and 0.96 mL of benzyl alcohol (9.25 mmol) were added. Next, 1.29 mL of triethylamine (9.25 mmol) was added followed by 1.39 g of succinic anhydride (13.9 mmol), dissolved in 5 mL of DCM. After 21 hours at room temperature, the reaction was complete as confirmed by TLC. The mixture was washed with a small volume of HCl 1M, extracted three times with water and washed with brine. The organic phases were then dried with Na₂SO₄, filtered, and evaporated to dryness, affording 1.76 g (9.04 mmol, 91%) of the final product as a white solid.

¹H NMR (400 MHz, DMSO-D6) δ 12.29 (1H, br, OH), 7.39 – 7.31 (5H, m, ArH), 5.11 (2H, s, ArCH₂), 2.50-2.53 (4H, m, COCH₂).

Synthesis of 5-(benzyloxy)-5-oxopentanoic acid (5)

The product was synthesised based on literature [13] with some modifications. In a flask, 8 mL of DCM and 0.96 mL of benzyl alcohol (9.25 mmol) were added. Next, 1.29 mL of triethylamine (9.25 mmol) was added followed by 1.58 g of glutaric anhydride (13.8 mmol), dissolved in 5 mL of DCM. After 1 hour and 30 minutes at RT, the reaction was complete (confirmed by TLC). The mixture was extracted with water (3x10 mL), and the organic phase was dried with Na₂SO₄, filtered, and evaporated to dryness. Once the product (oil) was thoroughly dried (vacuum, 60 ºC), the final product mass was determined to be 2.01 g (9.04 mmol, 98%).

¹H NMR (400 MHz, CDCl₃) δ 7.36 (5H, m, ArH), 5.13 (2H, s, ArCH₂), 2.48-2.42 (4H, m, COCH₂), 1.98 (2H, quint, J = 7.3Hz, CH₂).

Synthesis of benzyl 4-((3-(dimethylamino)propyl)amino)-4-oxobutanoate (6)

In a round-bottomed flask, 800.2mg of 4-(benzyloxy)-4-oxobutanoic acid (3.84 mmol) was dissolved in 3 mL of DCM and the solution was cooled in an ice-bath. Then, 1.1 g of EDC·HCl (5.76 mmol, in 6 mL of DMC) and 663.3 mg of NHS (5.76 mmol, in 3 mL of DMC) were added dropwise, and the solution was warmed to room temperature. After stirring for 1 hour, 785.2 mg of 3-(dimethylamino)-1-propylamine (7.68 mmol) were added. After 21 hours, the reaction was complete as confirmed by TLC. The crude reaction was washed once with a saturated NaHCO₃ solution, five times with water and once with brine. The organic phases were combined, dried with anhydrous Na₂SO₄, filtered, and concentrated in vacuo. After drying, 599.6 mg (2.05 mmol, 53%) was obtained with the appearance of a clear oil.

IR (ATR) ν_max (cm^-1): 3350, 1695, 1604, 1526. ¹H NMR (400 MHz, CDCl₃) δ 7.33 (5H, m, ArH), 6.98 (bl, 1H, NH) 5.11 (s, 2H, ArCH₂), 3.28 (m, 2H, CH₂NCO), 2.69 (t, J = 6.6 Hz, 2H, CH₂N(CH₃)₂), 2.44 (t, J = 7.0 Hz, 2H, CH₂COO), 2.32 (t, J = 6.5 Hz, 2H, CH₂CON), 2.19 (s, 6H, CH₃N), 1.61 (p, J = 6.5 Hz, 2H, CH₂). ¹³C NMR (101 MHz, CDCl₃) δ 172.85, 171.16, 135.86, 128.55, 128.23, 128.17, 66.46, 58.33, 45.39, 39.18, 31.04, 29.65, 26.23. ESI-HRMS: m/z calculated for [M+H]⁺ 293.1859, found 293.1852;

Synthesis of benzyl 5-((3-(dimethylamino)propyl)amino)-5-oxopentanoate (7)

In a flask, 1.59 g of 5-(benzyloxy)-5-oxopentanoic acid (7.15 mmol) was reacted in 6 mL of DCM with 2.32 g of CDI (14.31 mmol, 7 mL of DCM) and 87.46 mg of DMAP (7.15 mmol, 1 mL of DCM). After 30 minutes, the formation of the intermediate was achieved and 1.81 mL of 3-(dimethylamino)-1-propylamine (14.31 mmol) was added. The reaction was left to proceed over 48 h at RT. Four extractions were carried out with 25 mL of water each, with collection of the organic phase. The organic phase was dried with anhydrous Na₂SO₄, filtered and evaporated. After drying, the mass of the final product (oil) was 1.46 g (4.76 mmol, 67%).

IR (ATR) ν_max (cm^-1): 3278, 1717, 1644, 1531, ¹H-NMR (400 MHz, CDCl₃), δ (ppm): 7.37-7.29 (5H, m, ArH), 6.97 (1H, s, NH), 5.11 (2H, s, ArCH₂), 3.34 (2H, dd, J = 12.0 and 4.0Hz, CH₂NCO), 2.41 (2H, t, J = 7.28 Hz, CH₂N(CH₃)₂, 2.34 (2H, d, J = 12.8 Hz, CH₂COO), 2.20 (6H, s, CH₃N), 2.18 (2H, m, CH₂), 1.65 (2H, quint, J = 8 Hz, CH₂). ¹³C NMR (JMOD) (101 MHz, D₂O) δ 174.98, 174.69, 135.38, 128.81, 128.48, 127.82, 66.88, 62.20, 50.57, 47.15, 30.15, 29.33, 22.16, 18.07. ESI-HRMS: m/z calculated for [M+H]⁺ 307.216, found 307.2008;

Synthesis of 3-((3-(4-(benzyloxy)-4-oxobutanamido)propyl)dimethylammonio)propane-1-sulfonate (1)

300 mg of benzyl 4-((3-(dimethylamino)propyl)amino)-4-oxobutanoate (1.03 mmol, in 3 mL of DCM) was reacted with 376 mg of 1,3-propanesultone (1.03 mmol, in 2 mL of DCM). A white precipitate formed following 3 hours of reaction. The product was extracted with water 3x. The aqueous phase was freeze-dried obtaining 309.9 mg (0.75 mmol, 73%) with the appearance of a white foam.

IR (ATR) ν_max (cm^-1): 3438, 1711, 1626, 1174. ¹H NMR (400 MHz, D₂O) δ 7.46 – 7.43 (5H, m), 5.19 (2H, s, ArCH₂), 3.43 – 3.38i (2H, m, CH₂SO₃^-), 3.30 – 3.22 (4H, m, CH₂N⁺), 3.02 (6H, s, CH₃N⁺), 2.96 (2H, t, J = 7.2 Hz, CH₂NCO), 2.75 (2H, t, J = 7.4 Hz, CH₂COO), 2.57 (2H, d, J = 7.4, CH₂CON), 2.20 – 2.16 (2H, m, CH₂), 1.93 (2H, m, CH₂). ¹³C NMR (101 MHz, D₂O) δ 174.99, 174.69, 135.74, 128.81, 128.48, 127.82, 66.88, 62.20, 61.76, 50.57, 47.15, 35.85, 30.15, 29.33, 22.16, 18.08. ESI-HRMS: m/z calculated for [M+H]⁺ 415.1898, found 415.1890; 7.45 (dd, J = 9.1, 3.7 Hz, 5H), 5.19 (s, 2H), 3.44 – 3.37 (m, 2H), 3.31 – 3.25 (m, 2H), 3.23 (t, J = 6.5 Hz, 2H), 3.02 (s, 6H), 2.96 (t, J = 7.2 Hz, 2H), 2.75 (dd, J = 7.4, 5.6 Hz, 2H), 2.57 (dd, J = 7.4, 5.6 Hz, 2H), 2.23 – 2.14 (m, 2H), 1.93 (q, J = 8.6, 7.5 Hz, 2H).

Synthesis of 3-((3-(5-(benzyloxy)-5-oxopentanamido)propyl)dimethylammonio)propane-1-sulfonate (2)

430 mg of 1,3-propanesultone (3.5 mmol, in 2 mL of DCM) was added to 538.5 mg of benzyl 5-((3-(dimethylamino)propyl)amino)-5-oxopentanoate (1.76 mmol, in 6 mL of DCM). The reaction proceeded for 15 h at RT. The organic phase was extracted with water (3x20 mL). The presence of the product was confirmed in the aqueous phase which was freeze-dried. The product has the appearance of a white foam with a final mass of 623.7 mg (1.46 mmol, 83%).

IR (ATR) ν_max (cm^-1): 3431, 1711, 1652, 1539, 1168, 1032, 701, 697, 602, 520. ¹H NMR (400 MHz, D₂O) δ 7.59 (5H, m, ArH), 5.19 (2H, s, ArCH₂), 3.46 – 3.42 (2H, m, CH₂SO₃^-), 3.34 – 3.25 (4H, m, CH₂N⁺), 3.07 (6H, s, CH₃N⁺), 2.97 (2H, t, J = 7.1 Hz, CH₂NCO), 2.46 (2H, t, J = 7.2 Hz, CH₂COO), 2.30 (2H, t, J = 7.4 Hz, CH₂CON), 2.21 – 2.15 (2H, m, CH₂) 2.00 – 1.91 (4H, m, CH₂).¹³C NMR (101 MHz, CDCl₃) δ 175.72, 175.48, 135.65, 128.85, 128.64, 128.30, 66.97, 62.25, 61.80, 50.67, 47.16, 35.91, 34.66, 33.02, 22.18, 20.54, 18.07. ESI-HRMS: m/z calculated for [M+H]⁺ 429.2054, found 429.2050: