One-pot synthesis of sulfonyl benzotropones

Meng-Yang Chang*^abc and Chin-Huey Ho^a
aDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan. E-mail: mychang@kmu.edu.tw
^bDepartment of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung 807, Taiwan
^cNPUST College of Professional Studies, National Pingtung University of Science and Technology, Pingtung 912, Taiwan

First published on 11th August 2025

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Introduction

The bicyclic benzotropone core structure (i.e., benzotropolone or benzofused cycloheptatrienone) offers diverse synthetic applications,¹ and a variety of pharmacologically unique activities² enabling the development of novel chemical frameworks with a broad range of multifunctional properties.³ Among these, purpurogallin represents a biologically useful natural pigment that is biogenetically produced by oxidation of pyrogallol, as shown in Fig. 1.⁴ Theaflavin is formed by the enzymatic oxidation of black tea and exhibits antipathogenic and anticancer activities.⁵ Aurantricholone is a bright orange-red compound with a calcium ion-chelated benzotropone structure.⁶ With benzotropone analogs exhibiting potential biological activities, the development of a one-pot synthetic approach is of considerable interest.

Traditionally, benzotropones have commonly been synthesized via the Mukaiyama-aldol reaction of 1,3-bis-silyl enol ethers with o-phthalaldehydes (Scheme 1 and path a).⁷ Simple access to core benzotropone was achieved through the Dieckmann cyclization of the o-benzodipropionate arm and subsequent unsaturation of the resulting benzosuberone (path b).⁸ Next, the benzotropone skeleton was synthesized by Diels–Alder cycloaddition of p-tropoquinone and 1-acetoxy-1,3-butadiene, followed by aromatization (path c).⁹ The benzotropone scaffold was obtained through nanoparticle Pd-catalyzed Heck coupling of 2-bromobenzaldehyde with methyl vinyl ketone and then intramolecular aldol condensation (path d).¹⁰ However, upon exhaustive literature search and to the best of our knowledge, there is no report on the one-pot formation of bis-sulfonyl-conjugated benzotropones.

Based on our previous experience with tandem annulation of 1,3-bis-sulfonylacetones (such as the synthesis of tris-sulfonyl 3-arylphenols and bis-sulfonyl 5-aryl-4-arylidene-2-cyclohexenones),¹¹ and considering the synthetic applications¹² as well as the potential biological activity of versatile sulfonyl-containing molecules,¹³ in this study, we developed a one-pot high-yielding method for SeO₂-mediated simultaneous synthesis of bis-sulfonyl benzotropones 4. This process involves double Riley benzylic oxidation of o-dimethylarenes 1¹⁴ and subsequent Knoevenagel condensation of 1,3-bis-sulfonylacetones 3 with in situ generated o-phthalaldehydes 2 under open-flask atmospheric conditions (Scheme 2).¹⁵ Compared with previous research, we believe that this one-pot synthesis of functionalized bis-sulfonyl benzotropones 4 is still highly desirable due to reduced reaction steps and enhanced efficiency of synthesis.

Results and discussion

First, the starting 1,3-bis-sulfonylacetones 3 were synthesized in good to excellent yields through double nucleophilic alpha-substitution of commercially available 1,3-dichloroacetone with an excess of sulfinic acid sodium salt (RSO₂Na, 2.2 equiv.) in a cosolvent of dioxane and water (v/v = 1/1) at reflux, following a facile and easy-to-operate procedure based on the previous report.^11a The other starting material, commercially available o-dimethylarene 1, was utilized directly in the following reaction without further purification. The initial study commenced with SeO₂ (2.2 equiv.)-mediated treatment of 1,3-bis-phenylsulfonylacetone 3a (R = Ph, 169 mg, 0.5 mmol) and o-xylene 1a (Ar = Ph, 53 mg, 0.55 mmol) in dioxane (10 mL) at 25 °C for 20 h. However, no reaction occurred and only 3a was recovered as the major component (Table 1, entry 1). When the reaction temperature was elevated from room temperature to reflux (101 °C), 4a was obtained in a 47% yield (entry 2). Considering this encouraging result, stoichiometric amounts of SeO₂ were investigated. In entries 3–5, we found that 3.3, 4.4, and 5.5 equiv. of SeO₂ afforded 4a in 65%, 92%, and 89% yields, respectively. According to these data, 4.4 equivalents were considered a suitable amount to produce a high yield of 4a. Using 4.4 equiv. of SeO₂, 20 h and reflux, three kinds of solvents were screened. However, using CH₂Cl₂, the desired 4a could not be obtained, and a 34% yield of 3a was recovered (entry 6). Moreover, the use of benzene as the solvent resulted in only a 55% yield, and 3a was recovered in an 11% yield (entry 7). When DMF was used as the solvent, only complex and unidentified products were isolated (entry 8).

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Using DME (dimethoxyethane, an acyclic ether), 4a was obtained in a 70% yield (entry 9). The results revealed dioxane as an optimal solvent for improving the yield of 4a. As shown in entries 10–12, three reaction durations (15, 25 and 30 h) provided 85%, 82% and 77% yields of 4a, respectively. Furthermore, the shorter and longer reaction times did not enhance the yield of 4a. Based on the above results, the reaction concentration was studied next. After increasing and diminishing the volume of dioxane from 10 mL to 15 mL and 5 mL, respectively, slightly lower yields (88% and 85%) were obtained (entries 13 and 14). Subsequently, other oxidants were examined. When MnO₂ and TEMPO were used as oxidants, no desired 4a was observed (entries 15 and 16), and only 3a was recovered. When the reaction was run with PDC and DDQ (2,3-dichloro-5,6-dicyanobenzoquinone), 4a was not detected and an unidentified mixture was obtained as the major product (entries 17 and 18). Thus, we could conclude that entry 4 provided the optimal conditions for the formation of 4a through the stepwise (4 + 3) annulation of 1a with 3a.

A plausible mechanism for the formation of 4a, based on the optimal reaction conditions (Table 1, entry 1), is illustrated in Scheme 3. Initially, SeO₂ mediated the double benzylic oxidation of two o-methyl groups on 1a, yielding the dialdehyde 2a as an intermediate. The step was monitored by TLC until 1a was consumed completely within 2 h. Then, the α-position on enolized 3a could attack the in situ formed formyl group of 2a, leading to the removal of 2 equivalents of H₂O, precipitation of black selenium powder, and formation of 4a under the Knoevenagel condensation conditions. In this SeO₂-mediated sequential cyclocondensation process, 4a acquired a skeleton of bis-sulfonyl benzotropone via the formation of two carbon–carbon bonds.

The scope and limitations of this synthetic route were studied. A number of bis-sulfonyl benzotropones 4a–4af were obtained in 84%–94% yields via SeO₂-mediated stepwise (4 + 3) cyclocondensation of different o-dimethylarenes 1a–1d and diverse 1,3-bis-sulfonylacetones 3a–3ac based on the optimal conditions established in this study (Table 1, entry 1). As shown in Table 2, by controlling 1a as the starting material (Ar = Ph), symmetrical 3a–3q with the same substituent (R = R′) provided 4a–4q in good to excellent yields (84%–94%, entries 1–17) and included electron-neutral aliphatic aryl (4b, 4c, and 4h–4k), mono-halo-containing aryl (4d–4f), electron-donating oxygenated aryl (4g), electron-withdrawing trifluoromethyl aryl (4l–4m), biphenyl (4n), di-halo-containing aryl (4o), bicyclic naphthyl (4p) and heterocyclic 2-thienyl (4q) substituents. The molecular structures of 4k and 4m were determined by single-crystal X-ray diffraction.¹⁶

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By fixing R as the phenyl (Ph) group, the R′ group with different aromatic groups (e.g., alkylaryl, haloaryl, methoxyaryl, trifluoromethylphenyl, biphenyl, naphthyl, and thienyl; entries 18–29) on the sulfonyl moiety of 3r–3ac produced asymmetrical 4r–4ac in similar yields. These results show that different R and R′ groups bearing electron-withdrawing, electron-neutral, and electron-donating aryl groups were well tolerated and provided good yields of diverse bis-sulfonyl benzotropones 4. By adjusting R = R′ = Ph (for 3), the 4,5-dichlorophenyl (4,5-Cl₂C₆H₂) group on 1b formed 4ad in a 92% yield (entry 30). However, SeO₂-mediated oxidation of 1,2,4-trimethylbenzene (1c) and sequential condensation afforded the desired 4ae in a 36% yield and a major complex mixture could also be isolated (entry 31). The possible reason for this outcome could be the over-oxidation of 1c, resulting in its low yield. According to the resulting data, different Ar, R, and R′ groups having different electronic natures of the aryl groups were appropriate for yielding various products 4.

Furthermore, by controlling 3a as the mode substrate, different Ar groups (for 1e–1i) were examined (Scheme 4, eqn (1)). One-pot SeO₂-mediated cyclocondensation of 3a with 1d provided 4af with a benzofused nine-membered ring system in an 80% yield. On the basis of the protocol, 4ag (80%), with a ten-membered ring, was obtained via SeO₂-mediated treatment of 3a with 1e. However, no detection of the benzophenone skeleton was observed. The molecular structure of 4ag was determined through single-crystal X-ray analysis.¹⁶ However, when 3a was treated with o-diethylbenzene (1f), only 3a was recovered because SeO₂ could not trigger the initial oxidation such that 4ah was not isolated. In the next stage, we switched the starting 1,3-bis-carbonyl substrate from bis-sulfonyl to diesters 3ad–3ae (R = OMe, OEt) and tri-ketone 3af (R = Ph) and then examined the one-pot cyclocondensation (eqn (2)). Using the optimal conditions, the SeO₂-mediated reaction of 1a with 3ad–3af provided 5a–5c in 84%–90% yields. Thus, we realized that different 1,3-bis-carbonyl synthons (e.g., sulfonyl, ester, and ketone) could construct the benzotropone skeleton.

Next, as an extension of one-pot cyclocondensation, the double Michael addition of 4a with 1,3-dicarbonyls was explored (Scheme 5, eqn (3)). By combining K₂CO₃, acyclic β-diketones 7a and 7b and cyclohexa-1,3-dione 7c, model substrate 4a could be converted into 6a–6c bearing the benzofused bicyclo[3.2.1]core skeleton in 86%–89% yields. The bridged structures of 6a, 6b, and 6c were determined by single-crystal X-ray diffraction.¹⁶ Treatment of 4b with 7b produced 6d in an 86% yield. In particular, 6a–6d led to the formation of a six-membered ring system having the sulfonyl-containing enol conformation (a hydrogen bond formation, δ = 10.4 ppm) via the one-pot double conjugation pathway.

The reduction of 4a with a double conjugated system skeleton was examined (Scheme 6). Interestingly, catalytic hydrogenation of 4a afforded symmetric 8a with an all cis-configuration in an 84% yield (eqn (4)) in the presence of a 10% Pd/C catalyst under 1 atm. On the other hand, the reaction of 4a with NaBH₄ produced 8b at an 89% yield via the tandem double 1,4-reduction followed by a 1,2-reduction process (eqn (5)). The stereochemical structure of 8b with a trans–cis configuration between bis-sulfonyl and hydroxyl groups, respectively, was determined by single-crystal X-ray diffraction analysis.¹⁶ These results suggest that different reduction conditions could switch three relative chiral centers of 8a and 8b stereoselectively.

Next, the Grignard reagent-mediated addition of 4a was examined (Scheme 7). When 4a was treated with excess MeMgBr, only one trans-stereoisomer 9a was generated in an 86% yield through the Michael addition reaction (eqn (6)). Next, the desulfonylative reaction of 4a was examined. Using excess magnesium, however, only one sulfonyl group was removed to achieve the formation of 10a under refluxing MeOH conditions (eqn (7)). The molecular structure of 10a was determined by single-crystal X-ray diffraction analysis.¹⁶

The above-mentioned results and the potential applications of this protocol in the synthesis of the benzofused seven-membered skeleton 4 are highly encouraging, and scaling up the conversion would improve the significance of these results. Thus, the development of a gram-scale route was highly sought after. As shown in Scheme 8, the SeO₂-mediated reaction of 1a (350 mg, 3.3 mmol) with 3a (1.01 g, 3.0 mmol) produced 4a in a 78% yield (1.02 g) under dioxane reflux for 20 h. Compared with the 0.5 mmol scale of 1a (Table 2, entry 1, 92%), 3.0 mmol provided a slightly lower yield (78%); nonetheless, the gram-scale synthetic route was well-established.

Conclusion

To conclude, we developed SeO₂-mediated one-pot tandem cyclocondensation of 1,3-bis-sulfonylacetones and o-dimethylarenes under dioxane reflux with good yields under operationally simple reaction conditions. This single-step route provides a variety of diverse bis-sulfonyl benzotropones through stepwise (4 + 3) annulation. Large-scale synthesis was also explored and the plausible reaction mechanism was discussed. In due course, the synthetic applications of 1,3-bis-sulfonylacetones will be further investigated and published.

Experimental

General

All reagents and solvents were obtained from commercial sources and used without further purification. Reactions were routinely carried out under a dry nitrogen atmosphere with magnetic stirring. Products in organic solvents were dried with anhydrous magnesium sulfate before concentration in vacuo. Melting points were determined with a SMP3 melting apparatus. ¹H and ¹³C NMR spectra were recorded on a Varian INOVA-400 spectrometer operating at 400/600 and at 100/150 MHz, respectively. Chemical shifts (δ) are reported in parts per million (ppm) and the coupling constants (J) are given in Hertz. High resolution mass spectra (HRMS) were recorded with a mass spectrometer Finnigan/Thermo Quest MAT 95XL. X-ray crystal structures were obtained with an Enraf-Nonius FR-590 diffractometer (CAD4, Kappa CCD).

Representative synthetic procedure of compounds 4a–4ag

SeO₂ (244 mg, 2.2 mmol) was added to a solution of 1a–1f (0.55 mmol) in dioxane (5 mL) at 25 °C. The reaction mixture was stirred at reflux for 2 h and then cooled to 25 °C. The reaction was monitored using a TLC plate until dialdehydes were generated. Without further purification, 3a–3ac (0.5 mmol) in dioxane (5 mL) were added to the reaction mixture at 25 °C. The reaction mixture was stirred at reflux for 18 h. The reaction mixture was cooled to 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford the crude product under reduced pressure. Purification on silica gel (hexanes/EtOAc = 30/1–1/1) afforded 4a–4ag.

6,8-Bis(phenylsulfonyl)-7H-benzo[7]annulen-7-one (4a)

Yield = 92% (201 mg); white solid; mp > 250 °C (recrystallized from hexanes and EtOAc); HRMS (ESI-TOF) m/z: [M + H]⁺ calcd for C₂₃H₁₇O₅S₂ 437.0518, found 437.0522; ¹H NMR (400 MHz, CDCl₃): δ 8.89 (s, 2H), 8.03 (dt, J = 3.2, 5.6 Hz, 2H), 7.95–7.92 (m, 4H), 7.88 (dd, J = 3.2, 6.0 Hz, 2H), 7.56–7.53 (m, 2H), 7.45–7.41 (m, 4H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 175.6, 144.7 (2×), 143.7 (2×), 139.3 (2×), 136.9 (2×), 133.9 (2×), 133.6 (2×), 133.3 (2×), 129.0 (4×), 128.6 (4×).

Gram-scale synthesis of compound 4a

SeO₂ (1.47 g, 13.2 mmol) was added to a solution of 1a (350 mg, 3.3 mmol) in dioxane (30 mL) at 25 °C. The reaction mixture was stirred at reflux for 2 h and then cooled to 25 °C. The reaction was monitored using a TLC plate until 2a was generated. Without further purification, 3a (1.01 g, 3.0 mmol) in dioxane (30 mL) was added to the reaction mixture at 25 °C. The reaction mixture was stirred at reflux for 18 h. The reaction mixture was cooled to 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford the crude product under reduced pressure. Purification on silica gel (hexanes/EtOAc = 30/1–1/1) afforded 4a (78%, 1.02 g).

Author contributions

Meng-Yang Chang: writing – review & editing and supervision. Chen-Huey Ho: writing – original draft, conceptualization, and methodology.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Scanned photocopies of ¹H- and ¹³C-NMR spectral data for all compounds and X-ray analysis data of 4k, 4m, 4ag, 6a–6c, 8b and 10a. See DOI: https://doi.org/10.1039/d5ob01123j.

CCDC 2244547–2244554 contain the supplementary crystallographic data for this paper.^16a–h

Acknowledgements

The authors would like to thank the National Science and Technology Council of the Republic of China (Taiwan) for financial support (NSTC 112-2113-M-037-016-MY3).

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