Iron-mediated site-selective oxidative C–H/C–H cross-coupling of aryl radicals with quinones: synthesis of β-secretase-1 inhibitor B and related arylated quinones

Abstract

Phenoxy radicals generated from substituted arenes were converted into para site selective C-aryl radicals and coupled with quinones, using an inexpensive FeCl₃–K₂S₂O₈ system, to obtain several arylated quinones, in good to moderate yields, under operationally simple and mild conditions. This method is useful for one pot synthesis of β-secretase-1 (BACE) inhibitor B. The arylated quinones were used as intermediates in the synthesis of phenothiazin-5-ones. Theoretical studies on the pharmacokinetic properties of phenothiazin-5-ones showed high lipophilicity (log p = 3.69), poor water solubility (log s = −6.42), and high gastrointestinal absorption (GI).

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Introduction

Quinone scaffolds gained importance owing to their roles in biological systems,¹ and utilization as intermediates in biosynthesis,² bioinspired synthetic products,^1,3 and materials.⁴ Arylated quinones, especially phenol derivatives (Fig. 1), show inhibition of Alzheimer's disease (AD), through halting the production of β-amyloid peptides generated by β-secretase (BACE),^5a and other interesting biological activities.^1,5

Fig. 1 Selected biologically active quinone derivatives.

Owing to their importance, several approaches, involving different types of starting materials, were described for the synthesis of C-arylated quinones. Utilisation of pre-functionalised quinones and pre-functionalised arenes requires the presence of expensive transition metal reagents. Examples include Pd₂(dba)₃-CuI-Ph₃As^6a catalyzed coupling of a stannylquinone with an arylhalide, Pd(PPh₃)₄-CuI^5b/Pd(PPh₃)₄-CuBr^6b catalyzed coupling of a haloquinone with a styrylstannane, and Pd(dppf)Cl₂ ^6c/Pd(PPh₃)₄^6d,e/Pd(OAc)₂ ^6f catalyzed Suzuki coupling.

Heck type arylation often comprises unfunctionalized quinones and pre-functionalized arenes. Commercially available, stable, but costly arylboronic acids (Fig. 2) were used in such transformations, in the presence of expensive reagents made of metals such as Ag,^7a Rh,^7b Ir,^7c and Pd,^7d,e and also inexpensive Fe salts.^7f–h Furthermore, in addition to metal free arylation of quinones, using arylboronic acid-K₂S₂O₈, developed by us,^8a several metal free methods using diazonium salt-hydrazine catalysts,^8b diazonium salts,^8c,d diaryliodonium salts,^8e and arylhydrazine salts^8f have also been described. Recently, metal mediated C–H arylation of quinones using triazenes (Ag),^9a aryl halides (Pd),^9b and aniline (CuO-NPs/Gr^9c or PANI-g-C₃N₄-TiO₂)^9d has been demonstrated.

Fig. 2 Methods of synthesis of arylated naphthoquinones.

On the other hand, oxidative C–H/C–H coupling, a fast developing and straightforward concept, used in assembling a diverse variety of complex aromatic systems,^10a was also employed in coupling of quinones and arenes. Such C–H/C–H coupling was yet again carried out in the presence of costly metal salts such as highly electrophilic Pd(OAc)₂,^10b,c Rh salts,^10d and Pd(TFA)₂.^10e Conversely, ligand like binding of quinones with metal catalysts, the redox nature, the need for use of expensive reagents, and lengthy steps involved made such procedures disadvantageous.

In essence, this literature background shows that arene–quinone coupling was demonstrated using either organometallic reagents generated from prefunctionalised/unfunctionalised starting materials and expensive transition metals or aryl radicals obtained from prefunctionalised arenes. Consequently, there is no report known on the use of aryl radicals generated through oxidative C–H bond cleavage. This impelled us to make use of aryl radicals generated via oxidative C–H cleavage of electron rich arenes.

Aromatic compounds containing multiple C–H bonds make site-selective arylation a challenging task. Over the past few years meta¹¹ or ortho¹² or para¹³ site selective C–H activation of aromatics has been successfully demonstrated using steric, electronic, catalytic and directing group effects. Phenol directed C–H functionalization reactions involve a number of different mechanistic forms.¹⁴ Oxidative cross-coupling of phenols often leads to the formation of homocoupling by-products, higher-molecular weight polymers, and C,O-connected phenols, depending upon the oxidant and the experimental conditions. Thus controlling such reactions remains a challenge.^12b Recently, ortho C–H annulation of 4-methoxy phenol radicals (generated via oxidative C–H cleavage) with olefins has been described.^12b However, when we examined the coupling of 4-OMe-Ar-OH (2r) with 1,4-naphthoquinone (1a) in the presence of different concentrations of a FeCl₃ (0.1 or 1 or 3 equiv.) and DDQ (1.2 equiv.) system,^12b no product was obtained. Furthermore, in deviation from the literature methods, and in continuation of our interest in developing new methods for arylation of quinones,^8a,b we have identified para site selective coupling of an arene with a quinone in the presence of an inexpensive FeCl₃–K₂S₂O₈ system. Herein, we disclose the details.

Results and discussion

Since a para substituted phenol (4-OMe-Ar-OH (2r)) failed to elicit the desired product, alternatively, coupling of a mono-substituted phenol (2a, 1.0 mmol.) with napthoquinone (1a, 0.5 mmol.) was examined employing the FeCl₃ (0.1 equiv.)–DDQ (1.2 equiv.) system.^12b This reaction at rt lead to no product (Table 1, entry 1). Interestingly, increasing FeCl₃ to a stoichiometric amount (1.0 equiv.) led to the formation of product 3a, para site selectively, only in 30% yield. Given that, it would be an immoderate approach to use a quinone derivative (DDQ) in more than a stoichiometric quantity for arylation of another quinone, and the product is obtained only in low yield, we examined alternative oxidants. Notably, when a cheap and readily available oxidant K₂S₂O₈ was employed, at rt, in incremental concentrations (1.0/2.0/3.0 equiv., entry 2) with FeCl₃ (1.0 equiv.) product 3a was obtained in 39%, 51% and 60% yields, respectively. However, when the quantity of the mediator (FeCl₃) was increased to 2.0 (entry 3) and 3.0 equiv. (entry 4) the yield of the product 3a increased substantially and the highest yield (86%) was obtained when 3.0 equiv. of FeCl₃ and 3 equiv. of K₂S₂O₈ were used. Furthermore, when the temperature was raised to 60° C the yield decreased to 82% (entry 4), indicating that higher temperature would be detrimental to the reaction.

Table 1 Optimization of reaction conditions^a

Entry	Mediator	Oxidant/additive	Solvent	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol); 2a (1.0 mmol); solvent (4 mL); mediator and/or oxidant time 12 h, rt. b Isolated yield; NR = no reaction. c Quantity of the mediator or oxidant: 0.1 equiv. d Quantity of the mediator or oxidant: 1.2 equiv. e Quantity of the mediator or oxidant: 1.0 equiv. f Quantity of the mediator or oxidant: 2.0 equiv. g Quantity of the mediator or oxidant: 3.0 equiv. h Quantity of the mediator or oxidant: 4.0 equiv. i Quantity of the mediator or oxidant: 5.0 equiv. j Quantity of the mediator or oxidant: 0.5 equiv. k Temperature – 60 °C; oxidant: 3.0 equiv. l X = Zn, Sn, Cu and Ni.
1	FeCl3^c^,^e	DDQ^d	Tol	NR^c/30^e
2	FeCl3^e	K2S2O8^e^,^f^,^g	DCM	39^e/51^f/60^g
3	FeCl3^f	K2S2O8^e^,^f^,^g	DCM	56 ^e/64^f/68^g
4	FeCl3^g	K2S2O8^e^,^f^,^g	DCM	68^e/72^f/86^g/82^k
5	FeCl3^h^,^i	K2S2O8^g	DCM	82^h/55^i
6	FeCl3^g	(NH4)2S2O8^e	DCM	44^f
7	FeCl3^g	Oxone^e	DCM	62^f
8	FeCl3^g	H2O2^e	DCM	20^f
9	FeCl3^g	TBHP^e	DCM	40^f
10	FeCl3^g	K2S2O8^e	ACN/THF	Trace
11	FeCl3^g	K2S2O8^e	MeOH/H2O	NR
12	FeCl3^e^,^g/FeSO4^e	—	DCM	33^e/57^g/Trace
13	XCl2^l^,^e/AlCl3^e	—	DCM	NR
14	TFA^g/MsOH^g/pTSA^g	—	DCM	NR/Trace/15
15	FeCl3^e	pTSA^g	DCM	35
16	—	K2S2O8^g	DCM	Mixture

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With an additional increase in the quantity of FeCl₃ (4.0 equiv. and 5.0 equiv., entry 5) the yield of product 3a decreased to 82% and 55%, respectively. Furthermore, reactions carried out with 3.0 equiv. FeCl₃ and 1.0 equiv. of various oxidants such as (NH₄)₂S₂O₈, oxone, H₂O₂ and TBHP (entries 5–8), did not lead to an improvement in yield compared to that with K₂S₂O₈. Subsequently, examination of different solvents and their combination revealed that DCM is the best solvent (entries 10 and 11) to provide good yield.

Furthermore, we examined the possibility of Friedel–Crafts type arylation using different types of acids. When Lewis acid FeCl₃ (1.0 equiv. or 3.0 equiv., entry 12) alone was used, the desired product 3a was obtained in 33% and 57% yields, respectively. Other Lewis acid promoters such as FeSO₄, ZnCl₂, SnCl₂, CuCl₂·2H₂O, NiCl₂·6H₂O and AlCl₃ were found to be ineffective (entries 12 and 13). Among the Brønsted acids only PTSA elicited product 3a albeit in low yield (entry 15, 15%). A combination of Lewis acid FeCl₃ and Brønsted acid PTSA provided a low yield of the product 3a (35%). No desired product 3a was found when K₂S₂O₈ (3.0 equiv.) alone was employed. Since product 3a was obtained, via the Friedel–Crafts pathway, only in moderate yield (57%) using FeCl₃ (3.0 equiv.) as the mediator, this approach was abandoned. Finally, FeCl₃ (3.0 equiv.) and K₂S₂O₈ (3.0 equiv.) in DCM, which helps in generation of the aryl radical at rt, was identified as the optimal reaction condition for further study. FeCl₃ is expected to help in conversion of the phenoxy radical generated into a C radical and help moderate the reaction.

Interestingly, compound 3a, an effective β-secretase-1 inhibitor B (IC₅₀ = 8.22 μm),^5a has been reported to be synthesised in two steps involving deprotection using BBr₃ in moderate overall yield.^7c,9b,e However, our synthetic approach provides the same compound 3a in high yield (86%), in one pot, and is found to be simple and efficient. Next, we examined the scope and limitations of the FeCl₃ mediated C–H arylation of quinones using electron-rich arenes (phenol/anisole, Table 2). While o-cresol provided the desired product 3b in high yield (89%), sterically hindered m-cresol provided product 3c in moderate yield (47%). 2,5-Dimethyl phenol (2d) underwent the reaction smoothly to afford the product 3d in moderate yield (43%). Electron rich but sterically less hindered compounds 2-allylphenol (2e) and 2,6-di-tert-butylphenol (2f) provided the corresponding products 3e (82%) and 3f (92%) in excellent yields. Highly reactive catechol (2g) and 3-methylcatechol (2h) provided products 3g and 3h in low (34%) and moderate (60%) yields. Unremarkably, phenols 2i (2-NO₂), 2j (2-CHO), 2k (2-COMe), 2l (2-Cl), 2m (2-Br), and 2n (2-I) substituted with either strong or moderately deactivating substituents provided no corresponding products 3i–n even after 24 h.

Table 2 Scope of different phenols/anisole with 1,4-quinone^a

a Reaction conditions: 1 (1.0 equiv.), 2 (2.0 equiv.), FeCl₃ (3.0 equiv.), K₂S₂O₈ (3.0 equiv.), DCM (4 mL), rt, time: 6–24 h. Isolated yield. Gram scale 1a (1.0 g).

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Interestingly, the methylether of phenol 2o, underwent reaction with 1a to afford product 3o in good (77%) yield. Nevertheless, 2-bromo-naphthoquinone (1b), containing inductively electron withdrawing substituent 2-Br, offered product 3p in low yield (38%), after 24 h. However, naphthoquinone 1c–e containing 2-OH, 2-NH₂, and 2-CH₃ substituents underwent no reaction with anisole (2o), which indicates that electron donating substituents at a conjugating position to the reactive site do not favour the coupling reaction. Furthermore, on treating 1a separately with dimethoxy benzene (2p) and trimethoxy benzene (2q), the desired products 3t and 3u, respectively, were obtained in good yields. Subsequently, when the reaction of anisole (2i) was examined on 1,4-benzoquinone a mixture of products was obtained. However, the reaction of anisole (2i) with mono- or di-substituted 1,4-benzoquinone such as tert-butyl 1f and 1g afforded the desired products 3v (43%) and 3w (35%), respectively, in moderate yields. The reaction of 1a with benzene, as well as alkyl arenes such as xylene or toluene, provided no product. These results shows that the presence of an –OH or –OMe group is important for this oxidative coupling reaction. Moreover, gram-scale experiments carried out between 1a (1.0 g) and 2a or 2o provided products 3a and 3o in 70% and 66%, respectively. This shows that the present method can be easily adopted for large scale preparation.

Having examined the reaction of 1,4-quinones, next, the scope of the arylation was studied on 1,2-napthoquinone (4a) and the results are summarized in Table 3. In this case also we observed C–C bond formation but not C–O bond formation as reported by Lumb et al.¹⁵ Products corresponding to phenol (5a, 32%), o-cresol (5b, 54%), allyl phenol (5c, 52%) and 2,6-di-tert-butylphenol (5d, 33%) were obtained in moderate yields. However, similar to our previous observation (Scheme 2, entry 3q), 2-bromophenol (2m) failed to deliver the desired product 5e. Furthermore, anisole (2o) and veratrole (2p) underwent the reaction to deliver desired products 5f (58%) and 5g (50%) in good yields. In another reaction, highly sterically hindered 3,5-di-tert-butyl-1,2-benzoquinone (4d) unsuccessfully reacted with anisole (2o) and the desired product 5h could not be obtained.

Table 3 Scope of different phenols/anisole with 1,2-quinone^a

a Reaction conditions: 4 (1.0 equiv.), 2 (2.0 equiv.), FeCl₃ (3.0 equiv.), K₂S₂O₈ (3.0 equiv.), DCM (4 mL), RT, time 7–12 h. Isolated yield.

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The tert-butyl group plays an important role in chemistry and biology.¹⁶ Interestingly, when TBHP (1.0 equiv.) was used as an oxidant instead of K₂S₂O₈, two reactions, namely, tert-butyl group insertion at the C-2 position and oxidative C–H/C–H cross-coupling, took place to form unexpected product 3x (34%) along with the expected product 3a (40%). Furthermore, when tert-butyl hydroquinone (6a) was treated with 2o, a chain of oxidation reactions, namely, hydroquinone to quinone conversion and oxidative C–H/C–H cross-coupling took place in one pot to form product 3v in moderate yield (Scheme 1).

Scheme 1 Synthesis of aryl quinones with a tert-butyl group.

Additionally, the product 3p was further used as a starting material for the synthesis of a new phenothiazin-5-one derivative (Scheme 2).^17a Accordingly, compound 3p was treated with 2-amino-4-chlorobenzenethiol at 100 °C in the presence of a base to obtain the corresponding phenothiazin-5-one 8a in excellent yield.

Scheme 2 Synthesis of a phenothiazin-5-one derivative.

Furthermore, the pharmacokinetic properties of 8a, via absorption, distribution, metabolism and excretion (ADME), were predicted using DataWarrior software (Version 4.7.2),^17b and the SwissADME web tool^17c (see the ESI†). An in silico study revealed that 8a showed high lipophilicity (log p = 3.69), poor water solubility (log s = −6.42), and high gastrointestinal absorption (GI). These results show that compound 8a, falling in Class II described by Amidon et al.,^17d does not have blood brain barrier (BBB) permeability and has less skin permeation (log Kp = −4.73 cm s⁻¹). Compound 8a was found to show inhibition of four cytochrome P450 isomers (CYP1A2, CYP2C19, CYP2C9 and CYP3A4), exhibit a good bioavailability value (0.55) and meet the criteria of drug-likeness assessment based on Lipinski and Veber rules. Further in vitro studies of this compound are in progress in our laboratory.

Next, to gain understanding about the mechanism, control experiments were carried out (Scheme 3). In the absence of FeCl₃ and only with K₂S₂O₈ the reaction between compounds 1a and 2a failed (equiv. 1). As shown earlier (Table 1, entries 12 and 15) in the presence of FeCl₃ alone or FeCl₃ and pTSA product 3a is formed, and it must have been formed through the Friedel Crafts arylation mechanism. Furthermore, when 2.0 equiv. of a radical trapping agent such as TEMPO were added, the reaction between compounds 1a and 2a or 2o failed to deliver the desired product 3a or 3o (equiv. 2), which clearly indicated that, in the presence of an excess of the oxidant (K₂S₂O₈), the reaction takes place via the radical pathway and not through Friedel–Crafts arylation (Scheme 4, path C).

Scheme 3 Verification experiments for the mechanism.

Scheme 4 Plausible mechanism.

When the para position of the phenol was blocked as in the case of p-methyl phenol (2r) and p-methoxy phenol (2s) (equiv. 3), no ortho C–H functionalised products 3y and 3z or any other product was obtained, indicating that this reaction is para selective. All the forgoing discussion indicates that this FeCl₃ and K₂S₂O₈ mediated oxidative C–H/C–H coupling reaction is a radical process and para site-selective in C–H functionalization of phenol.

Based on these results and previous literature background, a plausible reaction mechanism is proposed (Scheme 4). Initially, in the presence of FeCl₃, K₂S₂O₈ dissociates to generate a highly reactive sulfate radical (SO4˙⁻K⁺),^7f–h which in turn reacts with phenol (2a) to form a phenoxy radical (A). Furthermore, FeCl₃ promotes the conversion of the O-radical to C-radical (B)^12b and helps its stabilization through resonance. The C-radical (B) on reaction with 1a gives rise to intermediate (C), which reoxidises to attain a more stable quinone form and afford arylated product 3a (Scheme 4, path A). In a similar manner, –OMe substituted aromatics are expected to undergo reaction through the radical cation mechanism (Scheme 4, path B).^10a

Conclusion

An efficient method for para site-selective C–H/C–H coupling of electron rich arenes with quinones under mild conditions is demonstrated. A series of arylated quinones were obtained in moderate to excellent yields at rt and they are also scalable to the gram scale level. To the best of our knowledge this is the first example in which an aryl radical generated through C–H cleavage is used in arylation of quinones. FeCl₃ promotes the O-radical to C-radical conversion and C-arylation reaction. This eco-friendly, inexpensive FeCl₃–K₂S₂O₈ mediator system tolerates a range of functional groups. Furthermore, β-secretase-1 inhibitor-B and related analogs were obtained in one pot. Arylated quinones were used as starting materials for the synthesis of phenothiazin-5-one derivatives which meets the requirement of drug-likeness which was studied using DataWarrior software and the SwissADME web tool. Further study on making use of this protocol is in progress in our research.

Experimental

General information

All the reagents were purchased commercially and used without further purification. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) were recorded with a Bruker 400 MHz spectrometer in CDCl₃ with tetramethylsilane (TMS) as the internal standard. Multiplicities are reported using the following abbreviations: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, sep = septet, br = broad resonance. All the NMR spectra were acquired at ambient temperature. Analytical thin layer chromatography (TLC) was performed using Silica Gel 60 Å F₂₅₄ pre-coated plates (0.25 mm thickness). Visualization was accomplished by irradiation with a UV lamp and staining with I₂ on silica gel. High resolution mass spectra (HRMS) were recorded on a Thermo Executive Plus spectrometer.

General method A. To a slowly stirred solution of napthoquinone (1.0 equiv.) and a suitable phenol or anisole (2.0 equiv.) in DCM (4 mL), K₂S₂O₈ (3.0 equiv.) and subsequently FeCl₃ (3.0 equiv.) were added portion-wise at room temperature. The progress of the reaction was monitored by TLC. Upon the complete consumption of starting materials the reaction mixture was diluted with ethyl acetate and water. The organic phase was separated, extracted two or more times with ethyl acetate, dried over Na₂SO₄, filtered and concentrated. The crude product was purified by silica gel column chromatography using hexane/ethyl acetate as the eluent to get the product.

General method B. To a slowly stirred solution of napthoquinone (1.0 equiv.) and a suitable phenol or anisole (2.0 equiv.) in DCM (4 mL), TBHP (1.0 equiv.) and subsequently FeCl₃ (3.0 equiv.) were added portion-wise at room temperature. The progress of the reaction was monitored by TLC. Upon the complete consumption of starting materials the reaction mixture was diluted with ethyl acetate and water. The organic phase was separated, extracted two or more times with ethyl acetate, dried over Na₂SO₄, filtered and concentrated. The crude product was purified by silica gel column chromatography using hexane/ethyl acetate as the eluent to get the product.

General method C. To a slowly stirred solution of 2-bromo-3-(4-methoxyphenyl) naphthalene-1,4-dione (1.0 equiv.) and 2-amino-4-chlorobenzenethiol (1.0 equiv.) in EtOH (2 mL) Na₂CO₃ (1.0 equiv.) was added at 100 °C. The progress of the reaction was monitored by TLC. Upon the complete consumption of starting materials the reaction mixture was diluted with DCM and water. The organic phase was separated, extracted two or more times with DCM, dried over Na₂SO₄, filtered and concentrated. The crude product was purified by silica gel column chromatography using DCM as the eluent to get the product.
2-(4-Hydroxyphenyl)naphthalene-1,4-dione (3a). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange solid, 136 mg, 86.0% yield, mp 172–174 °C); ¹H NMR (400 MHz, CDCl₃): δ 9.00 (s, 1H), 8.13–8.11 (m, 1H), 8.06–8.04 (m, 1H), 7.73–7.70 (m, 2H), 7.46 (d, J = 8.0, 2H), 6.98 (s, 1H), 6.90 (d, J = 8.0, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 185.3, 185.0, 159.5, 147.7, 133.70, 133.68, 133.1, 132.7, 132.1, 131.2, 127.0, 125.8, 124.4, 115.8. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₀O₃: 250.0630; found: 250.0631.
2-(4-Hydroxy-3-methylphenyl)naphthalene-1,4-dione (3b). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 148.33 mg, 89.0% yield, mp 170–172 °C); ¹H NMR (400 MHz, CDCl₃ and one drop of DMSO-D₆): δ 9.00 (s, 1H), 8.02–8.00 (m, 2H), 7.64–7.62 (m, 2H), 7.20 (d, J = 8.0 Hz, 2H), 6.90 (s, 1H), 6.81 (d, J = 8.0 Hz, 1H), 2.16 (s, 3H); ¹³C NMR (100 MHz, CDCl₃ and one drop of DMSO-D₆): δ = 185.1, 185.0, 157.8, 147.7, 133.6, 133.5, 132.8, 132.6, 132.1, 132.0, 128.5, 126.8, 125.6, 124.9, 124.1, 114.9, 16.2 HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₂O₃: 264.0786; found: 264.0776.
2-(4-Hydroxy-2-methylphenyl)naphthalene-1,4-dione (3c). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10, (78.64 mg, 47.0% yield, mp 130–132 °C); ¹H NMR (400 MHz, DMSO-D₆): δ = 8.00–7.95 (m, 2H), 7.85–7.82 (t, 2H), 7.01 (d, J = 8.0 Hz, 2H), 6.82 (s, 1H), 6.68–6.63 (m, 2H), 2.03 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 185.5, 184.6, 156.9, 150.4, 138.3, 136.8, 134.0, 132.3, 132.1, 130.9, 127.1, 126.2, 125.9, 117.5, 112.9, 20.7; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₂O₃: 264.0786; found: 264.0776.
2-(4-Hydroxy-2,5-dimethylphenyl)naphthalene-1,4-dione (3d). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 75 mg, 43.0% yield, mp 126–127 °C); ¹H NMR (400 MHz, CDCl₃): δ 8.16–8.12 (m, 2H), 7.79–7.77 (m, 2H), 6.93 (d, J = 12 Hz, 2H), 6.66 (s, 1H), 5.88 (s, 1H), 2.21 (s, 3H), 2.13 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 185.5, 184.8, 155.1, 150.4, 136.7, 135.4, 133.9, 132.3, 132.1, 127.1, 126.1, 125.7, 121.5, 117.1, 20.2, 15.3, HRMS (ESI): m/z [M − H]⁻ calcd for C₁₈H₁₄O₃: 278.0943; found: 278.0944.
2-(3-Allyl-4-hydroxyphenyl)naphthalene-1,4-dione (3e). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange solid, 150.55 mg, 82.0% yield, mp 164–165 °C); ¹H NMR (400 MHz, DMSO-D₆): δ 8.00–7.81 (m, 2H), 7.74–7.72 (m, 2H), 7.22 (d, J = 8.0, 2H), 6.82–6.79 (t, 2H), 5.94–5.83 (s, 1H), 4.97–4.95 (m, 2H), 3.22 (d, J = 6.0, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 185.3, 185.0, 159.5, 147.7, 133.7, 133.6, 133.1, 132.6, 132.1, 131.2, 126.9, 125.8, 124.4, 115.8. HRMS (ESI): m/z [M − H]⁻ calcd for C₁₉H₁₄O₃: 290.0943; found: 290.0946.
2-(3,5-Di-tert-butyl-4-hydroxyphenyl)naphthalene-1,4-dione (3f). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange red solid, 210.9 mg 92% yield, mp 152–151 °C); ¹H NMR (400 MHz, CDCl₃₎: δ = 8.16–8.14 (m, 1H), 8.08–8.06 (m, 1H), 7.72–7.70 (m, 2H), 7.47 (s, 2H), 7.04 (s, 1H), 5.62 (d, 1H), 1.5 (s, 18H). ¹³C NMR (100 MHz,): δ = 185.3, 185.1, 156.1, 148.5, 136.0, 133.7, 133.4, 132.7, 132.2, 127.1, 126.9, 125.9, 124.6, 34.5, 30.3. HRMS (ESI): m/z [M − H]⁻ calcd for C₂₄H₂₆O₃: 362.1882; found: 362.1884.
2-(3,4-Dihydroxyphenyl)naphthalene-1,4-dione (3g). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (brown solid, 57.25 mg, 34.0% yield, mp 240–241 °C); ¹H NMR (400 MHz, DMSO-D₆): δ = 8.08–8.06 (m, 1H), 8.01–7.99 (m, 1H), 7.90–7.87 (m, 2H), 7.11 (d, J = 4.0 Hz 1H), 7.02–7.0 (m, 2H), 6.85 (d, J = 8.0 Hz, 1H). ¹³C NMR (100 MHz,): δ = 184.5, 184.4, 147.6, 147.0, 144.8, 134.0, 132.4, 132.3, 131.5, 126.5, 125.2, 124.2, 121.6, 117.0, 115.4. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₀O₄: 266.0579; found: 266.0581.
2-(3,4-Dihydroxy-2-methylphenyl)naphthalene-1,4-dione (3h). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 106.34 mg, 60.0% yield, mp 182–184 °C); ¹H NMR (400 MHz, DMSO-D₆): δ = 8.08–8.06 (m, 1H), 8.00–7.98 (m, 1H), 7.90–7.87 (m, 2H), 6.99–6.98 (t, 2H), 6.93 (s, 1H), 2.16 (s, 3H). ¹³C NMR (100 MHz): δ = 189.8, 189.8, 152.4, 151.1, 149.5, 139.2, 137.6, 136.8, 131.7, 130.5, 129.6, 128.6, 128.3, 119.7, 21.2. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₂O₃: 280.0736; found: 280.0739.
2-(4-Methoxyphenyl)naphthalene-1,4-dione (3o). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange yellow solid, 128.80 mg, 77.08% yield, mp 134–135 °C); the product was obtained as an orange yellow solid after being subjected to short silica gel column chromatography (hexane/ethyl acetate, 90 : 10). ¹H NMR (400 MHz, CDCl₃): δ 8.16–8.14 (m, 2H), 8.10–8.07 (m, 2H), 7.80–7.74 (m, 2H), 7.57 (d, J = 8.0 Hz, 2H), 7.27 (s, 1H), 7.00 (d, J = 8.0 Hz, 2H), 3.85 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 185.2, 184.8, 161.3, 147.4, 133.8, 133.7, 132.6, 132.1, 131.1, 127.0, 125.9, 125.7, 114.0, 55.4; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₂O₃: 264.0786; found: 264.0406.
2-Bromo-3-(4-methoxyphenyl)naphthalene-1,4-dione (3p). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange solid, 55.30 mg, 38.0% yield, mp 126–127 °C); ¹H NMR (400 MHz, CDCl₃): δ 8.22–8.2 (m, 1H), 8.15–8.13 (m, 1H), 7.79–7.77 (m, 2H), 7.33 (d, J = 8.0 Hz, 2H), 7.01 (d, J = 8.0 Hz, 2H), 3.87 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 181.8, 178.4, 160.5, 149.3, 138.5, 134.4, 134.1, 131.7, 131.6, 131.3, 131.2, 127.5, 127.4, 126.0, 113.5, 55.4.
2-(3,4-Dimethoxyphenyl)naphthalene-1,4-dione (3t). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 130.30 mg, 70.0% yield, mp 132–134 °C); ¹H NMR (400 MHz, CDCl₃): δ = 8.14–8.12(m, 1H), 8.08–8.05 (m, 1H), 7.75–7.72 (m, Hz, 2H), 7.22–7.20 (dd, J₁ = 4.0, J₂ = 4.0 Hz, 1H), 7.14–7.13 (d, J = 4.0 Hz 1H), 7.02 (s, 1H), 6.94–6.92 (d, J = 8.0 Hz, 1H), 3.91–3.91(s, 6H); ¹³C NMR (100 MHz, CDCl₃): δ = 185.1, 184.7, 150.9, 148.7, 147.3, 133.9, 133.8, 138.1, 127.0, 125.9, 125.8, 122.9, 112.4, 110.9, 56.0, 56.0; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₁₄O₃: 294.0892; found: 294.0898.
2-(2,3,4-Trimethoxyphenyl)naphthalene-1,4-dione (3u). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (yellow solid, 78.64 mg, 47.0% yield, mp 85–87 °C); ¹H NMR (400 MHz, CDCl₃): δ = 8.16–8.09 (m, 2H), 7.76–7.74 (m, 2H), 6.99 (s, 1H), 6.97 (d, J = 4.0 Hz, 1H), 6.75 (d, J = 8.0 Hz, 1H), 3.91 (s, 3H), 3.89 (s, 3H), 3.86 (s, 3H). ¹³C NMR (100 MHz,): δ = 185.1, 184.0, 155.4, 151.9, 147.9, 142.0, 132.6, 133.8, 133.6, 132.6, 132.2, 126.9, 126.0, 124.9, 120.9, 107.0, 61.1, 60.8, 56.1.
6-(tert-Butyl)-4′-methoxy-[1,1′-biphenyl]-2,5-dione (3v). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange yellow solid, 70.82 mg, 43.0% yield, mp 242–243 °C); ¹H NMR (400 MHz, CDCl₃): δ = 7.64 (d, J = 8.0 Hz, 2H), 6.88 (d, J = 8.0 Hz, 2H), 6.66 (d, J = 4.0 Hz, 1H), 6.65 (d, J = 4.0 Hz 1H), 3.76 (s, 3H), 1.25 (s, 9H). ¹³C NMR (100 MHz,): δ = 188.4, 187.2, 161.1, 156.3, 147.3, 131.5, 130.5, 130.0, 128.8, 125.9, 113.9, 55.4, 35.6, 29.4. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₈O₃: 270.1256; found: 270.1258.
3,6-Dichloro-4′-methoxy-[1,1′-biphenyl]-2,5-dione (3w). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (orange red solid, 56.11 mg, 35.0% yield mp 135–136 °C); ¹H NMR (400 MHz, CDCl₃): δ = 7.28 (d, J = 8.0 Hz, 2H), 7.21 (s, 1H), 7.00 (d, J = 8.0 Hz, 2H), 3.86 (s, 3H). ¹³C NMR (100 MHz,): δ = 177.8, 177.4, 160.9, 144.4, 143.2, 140.0, 133.0, 131.7, 122.7, 113.7, 55.4.
2-(3-(tert-Butyl)-4-hydroxyphenyl)naphthalene-1,4-dione (3x). The reaction was carried out according to general method B. Eluent, hexane/ethyl acetate = 90 : 10 (orange red semisolid, 65 mg, 34.0% yield); ¹H NMR (400 MHz, CDCl₃₎: δ = 8.19–8.17 (m, 1H), 8.12–8.10 (m, 1H), 7.78–7.75 (m, 2H), 7.53 (d, J = 4.0 Hz, 1H), 7.38–7.35 (m, 1H), 7.05 (s, 1H), 6.79–6.77 (d, J = 8.0 Hz, 1H), 5.68 (s, 1H), 1.5 (s, 9H); ¹³C NMR (100 MHz, CDCl₃): δ = 185.5, 185.1, 156.6, 148.1, 136.5, 133.8, 133.5, 132.7, 132.2, 128.9, 128.7, 127.0, 125.9, 125.4, 116.7, 34.7, 29.5; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₀O₃: 306.1256; found: 306.1250.
4-(4-Hydroxyphenyl)naphthalene-1,2-dione (5a). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 25.31 mg, 32.0% yield, mp 167–168 °C); ¹H NMR (400 MHz, DMSO-D₆): δ = 7.97 (d, J = 7.2 Hz, 1H), 7.66–7.53 (m, 2H), 7.31 (s, 1H), 7.30 (d, J = 8.0 Hz 2H), 6.90 (d, J = 8.0 Hz, 2H), 6.22 (s, 1H); ¹³C NMR (100 MHz, CDCl₃ and one drop of DMSO-D₆): δ = 180.4, 179.9, 159.4, 157.5, 135.2, 135.0, 131.7, 130.7, 130.3, 129.9, 129.8, 126.6, 115.9; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₀O₃: 250.0630; found: 250.0629.
4-(4-Hydroxy-3-methylphenyl)naphthalene-1,2-dione (5b). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 45.11 mg, 54.0% yield mp 192–193 °C); ¹H NMR (400 MHz, DMSO-D₆): δ = 7.99 (d, J = 8.0 Hz, 1H), 7.67–7.55 (m, 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.19 (s, 1H), 7.15–7.13 (m, 1H), 6.89 (d, J = 8.0 Hz, 1H), 6.22 (s, 1H), 2.14 (s, 3H); ¹³C NMR (100 MHz, CDCl₃ with one drop of DMSO-D₆): δ = 180.5, 180.1, 157.7, 157.4, 135.4, 135.0, 131.7, 130.9, 130.6, 130.3, 129.9, 127.3, 127.2, 126.6, 125.3, 115.1, 16.2.
4-(3-Allyl-4-hydroxyphenyl)naphthalene-1,2-dione (5c). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 48 mg, 52.0% yield, mp-); ¹H NMR (400 MHz, DMSO-D₆): δ = 7.99 (d, J = 8.0 Hz, 1H), 7.66–7.62 (m, 1H), 7.57–7.53 (m, 1H), 7.30 (d, J = 8.0 Hz, 1H), 7.18–7.14 (m, 2H), 6.92 (d, J = 8.0 Hz, 1H), 6.20 (s, 1H), 5.96–5.89 (m, 1H), 5.03–4.98 (m, 2H), 3.29 (d, 2H); ¹³C NMR (100 MHz, CDCl₃ and one drop of DMSO-D₆): δ = 180.4, 180.1, 157.7, 157.1, 136.3, 135.4, 135.0, 131.7, 130.6, 130.3, 130.1, 129.8, 127.6, 127.3, 126.6, 115.9, 115.4, 96.1, 34.1; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₀O₃: 290.0943; found: 290.0947.
4-(3,5-Di-tert-butyl-4-hydroxyphenyl)naphthalene-1,2-dione (5d). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (reddish brown solid, 38.0 mg 33.0% yield, mp 214–215 °C); ¹H NMR (400 MHz, CDCl₃): δ = 8.22–8.20 (m, 1H), 7.63–7.60 (m, 1H), 7.56–7.52 (m, 1H), 7.45 (d, J = 8.0 Hz, 1H), 6.45 (s, 1H), 5.56 (s, 1H), 1.5 (s, 18H). ¹³C NMR (100 MHz,): δ = 180.6, 180.1, 158.2, 155.6, 136.4, 135.5, 135.0, 132.0, 130.6, 129.8, 127.6, 127.0, 125.4, 34.5, 30.3. HRMS (ESI): m/z [M − H]⁻ calcd for C₂₄H₂₆O₃: 362.1882; found: 362.1887.
4-(4-Methoxyphenyl)naphthalene-1,2-dione (5f). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (yellow solid, 48.45 mg, 58.0% yield, mp 128–129 °C); ¹H NMR (400 MHz, CDCl₃): δ = 8.18 (d, J = 8.0 Hz, 1H), 7.62–7.58 (m, 1H), 7.55–7.51 (m, 1H), 7.40 (d, J = 8.8 Hz, 2H), 7.38 (s, 1H), 7.04 (d, J = 8.8 Hz, 2H), 6.40 (s, 1H), 3.89 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 180.5, 179.8, 161.0, 156.9, 135.2, 135.1, 131.7, 130.8, 130.5, 129.8, 129.7, 128.7, 127.1, 114.3, 55.5; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₀O₃: 264.0786; found: 264.0775.
4-(3,4-Dimethoxyphenyl)naphthalene-1,2-dione (5g). The reaction was carried out according to general method A. Eluent, hexane/ethyl acetate = 90 : 10 (yellow solid, 96 mg, 50.0% yield, mp 164–165 °C); ¹H NMR (400 MHz, CDCl₃): δ = 8.20 (d, J = 8.0 Hz, 1H), 7.62–7.58 (m, 1H), 7.55–7.52 (t, 1H), 7.43–7.41 (d, 1H), 7.06–7.04 (q, 1H), 7.01–7.00 (d, 1H), 6.96–6.95 (d, 1H), 6.43 (s, 1H), 3.97 (s, 3H), 3.92 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 180.5, 179.7, 157.0, 150.5, 149.1, 135.2, 135.1, 131.7, 130.8, 130.6, 129.6, 129.0, 127.1, 121.2, 111.3, 111.2, 56.1, 56.0.
10-Chloro-6-(4-methoxyphenyl)-5H-benzo[a]phenothiazin-5-one (8a). The reaction was carried out according to general method C. Eluent, DCM (red powder, 54.13 mg, 92.0% yield, mp 220–223 °C); ¹H NMR (400 MHz, CDCl₃): δ = 8.90 (d, J = 7.2 Hz, 1H), 8.35 (d, J = 6.8 Hz, 1H), 7.94 (d, J = 2.0 Hz, 1H), 7.83–7.76 (m, 2H), 7.34–7.24 (m, 4H), 7.08 (d, J = 8.4 Hz, 2H), 3.89 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ = 178.9, 160.0, 146.2, 139.3, 135.1, 134.3, 132.9, 132.3, 132.1, 131.8, 131.7, 130.6, 129.7, 126.4, 126.2, 125.7, 125.6, 122.3, 114.7, 55.3. HRMS (ESI): m/z [M + H]⁺ calcd for C₂₃H₁₄ClNO₂S: 403.0434; found: 403.0464.

Conflicts of interest

Authors declare no conflict of interest.

Acknowledgements

A. K. T. P. and S. P thank the UGC-RFSMS, New Delhi for the award of the fellowship. We thank DST-FIST and DST-PURSE New Delhi, India for NMR and HRMS facilities at the School of Chemistry, Bharathidasan University, Tiruchirappalli, and India.

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Footnote

† Electronic supplementary information (ESI) available: Experimental, spectral data and copies of spectra. See DOI: 10.1039/c9qo00623k

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