2-(4-(Fluorosulfonyloxy)phenyl)benzoxazole

Abstract

New 2-(4-(fluorosulfonyloxy)phenyl)benzoxazole (2) was synthesized through the SuFEx click reaction in a two-chamber reactor. The effect of silylation on the yield of the target compound was investigated. The fluorescent properties of compound 2 were determined using experimental and computational methods.

1. Introduction

Benzoxazole scaffolds are found in different biologically active compounds with antitumor, antimicrobial, antiviral, antihistamine, antioxidant, anti-ulcer, anticonvulsant, antihelmintic, antidepressant, and analgesic effects [1]. The benzoxazole pharmacophore is contained in molecules of some known drugs, such as chlorzoxazone (muscle relaxant), benoxaprofen, and flunoxaprofen (anti-inflammatory drugs) (Figure 1) [2]. Numerous benzoxazoles possess fluorescent properties and can be used as fluorescent labels or materials for sensor technologies [3]. The photophysical properties of 2-(2-hydroxyphenyl)benzoxazole (Figure 1) were determined via excited-state intramolecular proton transfer (ESIPT) [4].

Click reactions of sulfur(VI) fluoride exchange (SuFEx) have successfully been used for the synthesis of small molecules and are a useful tool for late-stage functionalization (LSF) of bioactive molecules [5,6]. Wu and co-authors used this method for the conversion of a panel of NIH-approved anticancer drugs [7]. They showed that the fluorosulfate derivative of combretastatin A4 displayed a 70-fold increase in potency against the colon cancer cell line HT-29 compared to the parent compound. In addition, the fluorosulfonylated analogue of fulvestrant had a stronger binding affinity towards ERα (Figure 2).

Thus, benzoxazole derivatives with higher biological activity can be obtained using the SuFEx reaction. Moreover, the fluorosulfate group in a benzoxazole-containing molecule could be very useful for the immobilization of the compounds on material surfaces. The aim of the present work was to obtain a new fluorosulfate derivative of benzoxazole with fluorescent properties.

2. Results and Discussion

The SuFEx reaction is operationally simple and proceeds with high yields and high rates. This reaction is useful for the preparation of new fluorosulfate-containing benzoxazoles.

We synthesized 2-(4-(fluorosulfonyloxy)phenyl)benzoxazole (2) (Scheme 1) via the SuFEx reaction between compound 1 and SO₂F₂ in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). For this purpose, we used a two-chamber reactor (Figure S1). In one of the chambers, gaseous SO₂F₂ was generated upon the interaction of 1,1′-sulfonyldiimidazole (SDI) with KF and formic acid, while the click reaction proceeded in the second chamber.

Compound 2 is one of the first examples of 2-substituted benzoxazole with a fluorosulfate group. The presence of the -OSO₂F group in the para position of the benzene ring makes it possible to immobilize compound 2 in polymer chains or on material surfaces using the SuFEx click reaction. Previously, only an analog with the fluorosulfate group in the ortho position was known [8], which is less suitable for covalent immobilization due to steric hindrances.

The moderate yield of the target product (

Table 1. Influence of Si center on the yield of the target compound 2 of the SuFEx reaction.

No. *	Chamber B				Target Product Yield, % **
	Substrate	mmol	DBU, mmol	DCM, mL
1		0.5	1	3	59
2		0.5	1	3	17

* In both variants, the loading in chamber A was the same: 2.5 mmol SDI, 6.5 mmol KF, 1.5 mL HCOOH. ** The presented yields were obtained after chromatographic column purification.

) may be due to the formation of some amounts of bis(4-(benzoxazol-2-yl)phenyl)sulfate (3) as a by-product, analogously to the corresponding ortho-derivative [8]. It is known that the presence of the Si center improves the course of the reaction, due to the stabilization of the resulting fluoride ion through the formation of a strong Si-F bond [5,9]. In this regard, we investigated the effect of silylation on the yield of the target compound. We obtained a tert-butyldimethylsilyl derivative of compound 2 (Scheme 2).

The results of the SuFEx reaction with two different substrates (compound 1 and its silyl derivative 4) are shown in Table 1.

However, the yield of target product 2 was lower when we used silyl derivative 4. Such a result could be due to a higher reactivity of compound 4, which rapidly reacts with product 2 with the formation of compound 3.

The main characteristics of the target product 2 are as follows: colorless crystals, M.p. 131.8–132.4 °C, soluble in ethyl acetate, chloroform, acetonitrile.

Fluorescent Properties

We experimentally observed fluorescence of the title compound 2 in solution. The maximum fluorescence of compound 2 in acetonitrile was at 359 nm with an excitation wavelength of 330 nm (Figure 3). The absorption maximum of compound 2 in acetonitrile was at 300 nm; thus, the Stokes shifts was 59 nm (5478 cm⁻¹).

The fluorescence characteristics of compound 2 were also calculated through the density functional method (DFT and TDDFT) using the Gaussian 16 program (Revision A.03) with the high-level hybrid functional M06-2X, which is effective for studying the thermochemistry of chemical processes [10] and energies of excited states [11].

Comparing the experimental and calculated values of fluorescence wavelengths, one can notice that they are quite close (the difference was 8–9 nm). The theoretical absorption maximum at 313 nm was calculated through the TDDFT method; thus, the difference between the calculated and experimental absorption maxima was 13 nm. Therefore, the TDDFT method is applicable to predict the fluorescent properties of benzoxazole derivatives.

The pronounced fluorescence and significant Stokes shift make compound 2 promising for the creation of luminescent sensors for technology and medicine.

3. Materials and Methods

3.1. General Information and Compounds Synthesis

Gas chromatography–mass spectrometry (GC/MS) data were collected using a GC-MS system with an Agilent 5975C (Agilent Technologies, Inc., Santa Clara, CA, USA) mass detector and an Agilent 7890A (Agilent Technologies, Inc., Santa Clara, CA, USA) gas chromatograph. The ¹H, ¹³C, and ¹⁹F NMR spectra were recorded using a Bruker AVANCE III HD instrument (Bruker Corporation, Billerica, MA, USA) (operating frequencies: ¹H—400 MHz; ¹³C—100 MHz; ¹⁹F—376 MHz). For the registration of ¹⁹F NMR spectra, trichlorofluoromethane was used as an internal standard. Melting points of the synthesized compounds were determined with Melting Point Apparatus SMP30 (Cole-Parmer Instrument Company, Vernon Hills, IL, USA) at a heating rate of 2.5 °C/min. The reaction mixtures were monitored through thin-layer chromatography (TLC) on Merck plates, silica gel 60, F254 (Merck & Co., Inc., Rahway, NJ, USA). Elemental analysis was performed with a Carlo Erba instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA).

Compound 1 was prepared as described previously [12].

2-(4-(tert-butyldimethylsilyloxy)phenyl)benzoxazole(4)

To a stirred solution of hydroxyphenyl-substituted benzoxazole 1 (106 mg, 0.5 mmol) and imidazole (112 mg, 1 mmol) in dry dichloromethane (DCM, 2 mL), tert-butyldimethylsilyl chloride (TBSCl, 185 mg, 0.75 mmol) was added dropwise at 0 °C. After stirring for 3 h at room temperature (TLC monitoring, hexane–ethyl acetate, 8:2), the reaction mixture was poured into a saturated aqueous solution of sodium hydrocarbonate and extracted with CH₂Cl₂. The extract was dried with anhydrous sodium sulfate, filtered, and evaporated under reduced pressure. Brown oil. Yield 65%. ¹H NMR (DMSO-d₆), δ, ppm: 8.04 (d, J = 8.8 Hz, 2H), 7.74–7.71 (m, 2H), 7.37–7.35 (m, 2H), 6.97 (d, J = 8.8 Hz, 2H), 0.83 (s, 9H), –0.05 (s, 6H). ¹³C NMR (DMSO-d₆), δ, ppm: 163.2, 161.4, 150.5, 142.3, 129.8, 125.2, 125.1, 119.7, 117.6, 116.6, 111.1, 26.3, 18.3, –2.7. GC-MS m/z 325.2 (27.2%, M^{+ ●}).

2-(4-(Fluorosulfonyloxy)phenyl)benzoxazole (2)

For this synthesis, a small flame-dried two-chamber reactor (Figure S1) was used. 1,1′-Sulfonyldiimidazole (SDI, 495 mg, 2.5 mmol) and potassium fluoride (378 mg, 6.5 mmol) were added in chamber A. Next, chamber B was loaded with compound 1 (106 mg, 0.5 mmol) or compound 4 (163 mg, 0.5 mmol), DBU (0.3 mL, 2.0 mmol), and dichloromethane (DCM, 3 mL). Finally, formic acid (1.6 mL) was added via injection through the septum in chamber A to ensure the immediate formation of SO₂F₂ gas.

After 24 h stirring at room temperature, one of the two-chamber reactor caps was carefully opened to relieve the residual pressure and remove sulfuryl fluoride through the fume hood. The reaction mixture from chamber B was transferred to a 100 mL flask. Chamber B was rinsed with 4 mL of DCM, and this fraction was poured into the same flask. The solvent was removed from the flask under reduced pressure. The raw product was purified through column chromatography on silica gel (hexane–ethyl acetate, 1:1). The title compound 2 was isolated as colorless crystals. The yields of compound 2 were 59% or 17% when substrates 1 or 4 were used as starting materials, respectively.

M.p. 131.8–132.4 °C, ¹H NMR (CDCl₃), δ, ppm: 8.38 (d, J = 8.8 Hz, 2H, H meta to OSO₂F), 7.79–7.81 (m, 1H, H-4), 7.60–7.62 (m, 1H, H-7), 7.52 (d, J = 8.8 Hz, 2H, H ortho to OSO₂F), 7.38–7.43 (m, 2H, H-5, H-6). ¹³C NMR (CDCl₃), δ, ppm: 161.1, 151.7, 150.9, 141.8, 129.8, 127.8, 125.8, 125.0, 121.7, 120.4, 110.8, ¹⁹F NMR (CDCl₃), δ, ppm: 38.48.

Found, %: C 53.52, H 2.61, N 4.96. C₁₃H₈FNO₄S. Calculated, %: C 53.24, H 2.75, N 4.78.

GC-MS m/z 293.0 (75.7%, M^{+ ●}).

The GC-MS data, NMR, and UV spectra of compound 2 are shown in Figures S2–S6.

3.2. Fluoresent Properties

Photophysical properties were measured using a Fluorat-02 Panorama spectrofluorometer (St. Petersburg, Russia). The concentration of the studied solution was 0.001 M in a 1 cm cuvette. The absorption and steady-state fluorescence spectra in acetonitrile solution were recorded in the wavelength range of 210–600 nm. The whole operation was repeated at least three times.

The absorption spectrum was simulated using the ground-state optimized geometry at the time-dependent density functional theory (TDDFT) level with exchange-correlation functional M06-2X [10] and basis set aug-cc-pVDZ [13]. Solvent effects were treated implicitly, using the IEFPCM solvation model (acetonitrile solvent) [14]. The nature of the stationary point (energy minimum) in each case was confirmed by analyzing the frequencies of normal vibrations. The excitation energy and oscillator strength of the first six singlet and ten triplet vertically excited electronic states were computed using the same combination of functional and basis sets as used for the ground state optimization and vibrational frequencies calculations; the states’ nature was contextually characterized. The geometry of the first singlet excited state was subsequently optimized in acetonitrile (IEFPCM solvation model), and the harmonic frequencies and normal modes were calculated at the same level of theory and retained. Then, the de-excitation energy (fluorescence) and the oscillator strength were calculated. The bands of absorption and fluorescence spectra were simulated including the vibronic effects at the adiabatic Hessian level. All the calculations were performed using the GAUSSIAN 16 package (Revision A.03) [15].

4. Conclusions

In this work, we presented the synthesis of a new benzoxazole fluorosulfate derivative 2 and investigation of its photophysical properties through experimental and computational methods. The title compound is of great interest for further studies as a possible fluorescent probe that could be used for the analysis of cellular and tissue functions, for targeted drug delivery, for studies of drug action mechanisms, and for the development of luminescent materials useful in bioimaging, chemophysical sensing, and the creation of organic LEDs (OLEDs).