Ru(II)-Catalyzed Regiospecific C-H/O-H Oxidative Annulation to Access Isochromeno[8,1-ab]phenazines: Far-Red Fluorescence and Live Cancer Cell Imaging.

A facile ruthenium(II)-catalyzed regiospecific C-H/O-H oxidative annulation methodology was developed to construct isochromeno[8,1-ab]phenazines. This methodology delivers various advantages, such as scope for diverse substrates, tolerance to a range of functional groups, stability under air, and yields regioselective products. This methodology was successfully applied to synthesize far red (FR) fluorescent probes for live cancer cell imaging. The synthesized compounds displayed notable fluorescence properties in solution and thin-film. Their application in live cancer cell imaging was investigated using various cancer cell lines. The synthesized compound showed prominent FR fluorescence, with high quantum yield, and exhibited better cell-imaging properties, with excellent biocompatibility.

Introduction

Transition metal-catalyzed C–H bond functionalization has evolved rapidly because of its potential to create a diverse range of products efficiently without the requirement to prepare preactivated starting material.[1] The advantages of this methodology are that it is atom economical, has a reduced number of reaction steps, and produces minimal waste.[2] Transition metal catalysts are effective and efficient for annulation via chelation-assisted functionalization of C–H/N–H and/or C–H/O–H bonds.[3] Even though great advances in C–H annulation have been executed in recent years, the skill to selectively functionalize precise sp2 C–H bonds in the presence of contending C–H bonds persists as the critical and strategic feature for the success of this tandem process.[4]

Cancer is the primary cause of death in developed countries and the second leading cause of death in developing countries.[5] Improvement in the methodology for early detection of benign and propagated tumor cells in patients is extremely significant to the success of cancer therapy and improvement in the survival rates of patients.[6] Nowadays, optical imaging methodologies are evolving as promising noninvasive, real-time and high-resolution modalities for the diagnosis of cancer.[7] Because of better cell permeability and minimal perturbation to living systems, low molecular weight organic fluorophores are extremely appropriate for biological studies.[8] Among the organic fluorophores for in vivo bioanalysis and bioimaging, far red/near-infrared (FR/NIR) fluorophores (λ = 650–900 nm)[9] have been shown to be promising molecules due to their low-light scattering, deep-tissue penetration and marginal photodamage.[10] Hence, the FR/NIR fluorescent probes are likely to be appropriate for biological imaging.[11] However, the development of novel FR/NIR fluorophores with requisite properties for biological imaging is an appealing and significantly challenging task.

For proficient cancer cell fluorescence imaging, FR/NIR probes must exhibit greater chemical and photophysical properties, such as high quantum yield for intense fluorescence, large Stokes shift for least interference between absorption and emission spectra, adequate chemical and photostability in solvents, buffers, or biological conditions, less cytotoxicity, and suitable chemical functionality for bioconjugation with specific scaffolds for the purpose of targeting. For the construction of fluorescent probes, polycyclic heteroaromatic molecules (PHAs) with extended π-conjugated structures have been proven to be better chromophores in numerous reports. Significantly, PHAs are disclosed as NIR-active dyes, two-photon absorbers, and fluorescence sensors.[12]

We have already explored Rh(III)-catalyzed dehydrogenative C–H/N–H functionalization to synthesize cinnoline derivatives as fluorescent probes for live cancer cell imaging in our recent work.[13] In the course of our efforts on the development of novel transition metal-catalyzed C–H functionalization[14] and annulation reactions,[15] with the aim of design and synthesis of novel organic fluorophores,[16] we report here Ru(II)-catalyzed regiospecific C–H/O–H oxidative annulation reaction to synthesize isochromeno[8,1-ab]phenazine, which was successfully applied as an FR fluorescent probe for live cancer cell imaging.

Results and Discussion

Synthesis of Isochromeno[8,1-ab]phenazines

Initially, benzo[a]phenazin-5-ol 1a was treated with [RuCl2(p-cymene)]2 (5 mol %), AgSbF6 (20 mol %), Cu(OAc)2·H2O (1 equiv), and diphenyl acetylene 2a (1.5 equiv) in toluene (5 mL) at 110 °C under nitrogen for 12 h. After completion of the reaction evidenced by thin-layer chromatography (TLC), the product 3a was isolated by column chromatography, with the yield of 14% (Table , entry 1). The structure of the product was confirmed as a highly substituted isochromeno[8,1-ab]phenazine (3a) using various analytical techniques, such as high-resolution mass spectrometry (HRMS), 1H and 13C NMR, and single crystal X-ray diffraction (Supporting Information). When the reaction was performed under air, the yield was improved to 30% (Table , entry 2). Switching the solvent from toluene to chlorobenzene (PhCl) under identical reaction conditions provided the annulated product 3a in an improved yield of 40%. When other solvents, such as 1,4-dioxane, t-AmOH, CH3OH, dimethylformamide (DMF), and 1,2-dichloroethane (1,2-DCE) (Table , entries 3–7) were employed in this reaction, 1,2-DCE provided the best yield of 72% (Table , entry 7). Further, varying the oxidant from Cu(OAc)2·H2O to Cu(OAc)2, AgOAc, Ag2CO3, K2S2O8, and PhI(OAc)2 under identical conditions to that of Table , entry 7, did not improve the yield of product 3a (Table , entries 8–12). Reducing the amount of oxidant to 0.5 equiv decreased the yield, and without the oxidant, it failed to afford the desired compound 3a (Table , entries 13–14). Similarly, replacing AgSbF6 with other additives, such as KPF6 and AgBF4, afforded the product in reduced yields (Table , entries 15–16). Absence of the additive AgSbF6 also delivered very low yield (Table , entry 17). Moreover, replacing the catalyst [RuCl2(p-cymene)]2 with RuCl2·5H2O, [RhCp*Cl2]2 and Pd(OAc)2 did not improve the results (Table , entries 18–20).

Table 1

Optimization of the Reaction Condition of Benzo[a]phenazin-5-ol with Diphenyl Acetylenea

entry	catalyst	oxidant	solvent	yield (%)b
1	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	toluene	14c, 30
2	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	Cl-benzene	40
3	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	1,4-dioxane	10
4	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	t-AmOH	20
5	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	CH3OH	18
6	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	DMF	30
7	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	1,2-DCE	72
8	[RuCl2(p-cymene)]2	Cu(OAc)2	1,2-DCE	68
9	[RuCl2(p-cymene)]2	AgOAc	1,2-DCE	45
10	[RuCl2(p-cymene)]2	Ag2CO3	1,2-DCE	25
11	[RuCl2(p-cymene)]2	K2S2O8	1,2-DCE	18
12	[RuCl2(p-cymene)]2	PhI(OAc)2	1,2-DCE	trace
13d	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	1,2-DCE	28
14	[RuCl2(p-cymene)]2	−	1,2-DCE	0
15e	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	1,2-DCE	65
16f	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	1,2-DCE	50
17g	[RuCl2(p-cymene)]2	Cu(OAc)2·H2O	1,2-DCE	10
18	RuCl2·5H2O	Cu(OAc)2·H2O	1,2-DCE	trace
19	[RhCp*Cl2]2	Cu(OAc)2·H2O	1,2-DCE	60
20g	Pd(OAc)2	Cu(OAc)2	DMF	trace

Reaction conditions: 1a (0.3 mmol), 2a (0.3 mmol), catalyst (5.0 mol %), additive AgSbF6 (20 mol %), and oxidant (1 equiv) in the indicated solvent (2.0 mL) at reflux or 110 °C for 12 h under air.

Isolated yield.

Under nitrogen.

Oxidant (0.5 equiv).

KPF6 was employed instead of AgSbF6.

AgBF4 was employed instead of AgSbF6.

Without additive.

Under optimized reaction conditions, we first scrutinized the scope of versatility in the oxidative annulation of benzo[a]phenazin-5-ol 1a by employing diversely substituted alkynes (2a–m) (Table ). Symmetric diaryl acetylenes bearing various substitutions such as electron-donating methyl and methoxy at the para position coupled smoothly with 1a in 65 and 57% yields, respectively (3b–c). Diaryl acetylene bearing halogen groups at the para position of the benzene ring was generally well tolerated and afforded products 3d–f in moderate to good isolated yields of 50–64%. Substitution of the strong electron-withdrawing groups at the para position of the phenyl ring provided 3g in 39% yield. This reaction is also compatible with 1,2-di(naphthalene-diaryl acetylene) by which product 3h was isolated in 50% of the yield.

Table 2

Ru(II)-Catalyzed Regiospecific C–H/O–H Oxidative Annulation of Benzo[a]phenazin-5-ol with Alkynesa,b

Reaction conditions: 1 (0.3 mmol), 2 (0.3 mmol), [RuCl2(p-cymene)]2 (5.0 mol %), Cu(OAc)2·H2O (1 equiv), and AgSbF6 (20 mol %) in 1,2-DCE at reflux for 12 h under air.

Isolated yield.

To understand the regioselectivity of the present reaction, unsymmetrical alkyne was used as the substrate for the reaction with 1a. Thus, 1-phenyl-1-propyne acetylene 2i gave the single regioisomeric product 3i in 68% yield. The present catalytic reaction was successfully extended to the symmetrical aliphatic alkynes 2j–m, which offered moderate to good isolated yields (3j–m). We next explored the scope and limitations of benzo[a]phenazin-5-ol substrates (1b–h) in this coupling reaction with diphenyl acetylene 2a. Dibromo substitutions in benzo[a]phenazin-5-ol decreased yield of the product 3o (58%). Diphenyl substitutions in the ortho and para positions afforded 3p and 3q in 67 and 70% yield, respectively. The reaction underwent smoothly with highly substituted triphenylamine tagged benzo[a]phenazin-5-ol and yielded 3r in 56%. In addition, substitution of the electron-withdrawing groups, such as CN, delivered the product 3s in 67% yield. Interestingly, a very high yield of 3t (80%) was observed when the phenyl ring was replaced by the cyclohexyl ring.

To check intermolecular competitive reaction between the electron-deficient diphenyl acetylene (2a) and electron-rich 4-octyne (2l), we studied the annulation reaction, as indicated in Scheme . Benzo[a]phenazin-5-ol (1a) was reacted with an equimolar mixture of 2a and 2l under optimized conditions. Notably, the annulated product, 3a, originating from the electron-deficient alkyne (2a), was obtained in higher proportion (1.7/1.0) compared to that of product 3l, originating from the electron-rich alkyne (2l).

Scheme 1

Control Experiment

To achieve a preliminary understanding of the mechanism, deuterium exchange experiments were conducted. Heating the combination of 1a and CD3OD in 1,2-DCE at 100 °C provided 1aa, with one incorporated D atom (Scheme , eq 1). Under the standard procedure, heating the combination of 1a and CD3OD in 1,2-DCE at 100 °C provided 1ab in a significant D/H exchange in the peri-position (Scheme , eq 2), thereby providing evidence for the reversible C–H bond ruthenation step in this annulation reaction. In both reactions, no O–H deuteration was detected, which could be rationalized by the labile O–D/H exchange during the workup on silica gel chromatography.

Scheme 2

Preliminary Mechanism Study

On the basis of the experimental results and previous report,[17] the plausible mechanism for the formation of isochromeno[8,1-ab]phenazine (3) by the direct Ru(II)-catalyzed cyclisation of benzo[a]phenazin-5-ol (1) with alkynes (2) is shown in Scheme . The additive AgSbF6 likely initiates the catalytic reaction by dissociation of the dimeric form of the ruthenium complex [RuCl2(p-cymene)]2 and gives a reactive cationic ruthenium species. Then, the ruthenium complex gets attached with the directing group OH and forms the intermediate A. Then, peri-C–H metalation of the intermediate A provides a five-membered ruthenacycle intermediate B. Now, alkyne 2 attaches with the intermediate B giving rise to intermediate C. At this point, coordinative regioselective insertion of alkyne 2 into the Ru–C bond of intermediate C gives intermediate D. Finally, the ruthenium complex that is regenerated by Cu(OAc)2 forms the isochromeno[8,1-ab]phenazine (3).

Scheme 3

Plausible Mechanism for the Ru(II)-Catalyzed Regiospecific C–H/O–H Oxidative Annulation of Benzo[a]phenazin-5-ol with Alkynes

Photophysical Properties

After we had a diverse library of isochromeno[8,1-ab]phenazine in hand, their photophysical properties in solution and thin-film states, including absorptions, emissions, and quantum yields, were investigated (Supporting Information).[18] It is worth mentioning that synthesized compounds exhibit emissions (λmax,em: 532–632 nm in dichloromethane (DCM); 565–640 nm in thin-film) with large Stokes shifts (up to 4853 cm–1 in DCM; 5747 cm–1 in thin-film) and high fluorescence quantum yields (up to 86% in DCM) (Table ).

Table 3

Optical Properties of Compounds (3a–v)

	λmax,ab (nm)		λmax,em (nm)		Stokes shifte (cm–1)		quantum yield
entry	sola	TFb	sola,c	TFb,d	sola	TFb	(ΦF)f
3a	494	508	553	590	2160	2736	0.54
3b	498	511	559	585	2191	2475	0.43
3c	500	514	572	604	2517	2899	0.48
3d	490	504	544	619	2026	3686	0.35
3e	489	496	540	565	1931	2462	0.37
3f	488	501	539	565	1939	2261	0.39
3h	486	498	538	566	1989	2412	0.32
3i	495	497	554	566	2151	2453	0.45
3j	497	550	563	599	2359	1487	0.41
3k	501	527	563	579	2198	1704	0.47
3l	499	503	563	581	2278	2669	0.52
3m	503	510	567	588	2244	2601	0.65
3n	491	503	544	573	1984	2429	0.37
3o	518	529	580	616	2064	2670	0.12
3p	507	521	559	607	1835	2719	0.86
3q	503	509	551	582	1732	2464	0.23
3r	516	525	568	599	1774	2353	0.37
3s	501	512	594	630	3125	3658	0.17
3t	431	435	545	580	4853	5747	0.52
3u	526	532	609	640	2591	3172	0.28
3v	511	540	632	637	3747	2820	0.22

Sol—in DCM solvent at 5 μM.

TF—thin-film.

Excitation wavelength: 450–500 nm.

Excitation wavelength: 450–540 nm.

Stokes shift = λmax,ab – λmax,em (cm–1).

Determined by fluorescein (ΦF = 0.79), as a standard in 0.1 M ethanol solution.

Compound 3a displays emissions (λmax,em) of 553 and 590 nm with Stokes shifts of 2160 and 2736 cm–1 in DCM and thin film, respectively, and fluorescence quantum yield of 54% in DCM. We studied the impact of R1 and R2 in the aromatic alkynes by varying the substitution by 3b (R1 = R2 = CH3), 3c (R1 = R2 = OCH3), 3d (R1 = R2 = F), 3e (R1 = R2 = Cl), 3f (R1 = R2 = Br), and 3g (R1 = R2 = NO2). Interestingly, OCH3 (3c) and F (3d) substitutions increase the emission value, with larger Stokes shifts when compared to the results with compound 3a. However, compound 3c shows quantum yield of 48%, which is comparable to 3a (54%), whereas 3d shows lesser quantum yield of 35%. On the other hand, NO2 substitution quenched the fluorescence.

When a naphthyl ring (3h) was employed instead of a phenyl ring (3a), compound 3h showed a reduced emission wavelength and quantum yield. Subsequently, the influence of the R1 and R2 substituents in the aliphatic alkynes (3j–m) was examined. Compounds 3j (R1 = R2 = CH3), 3k (R1 = R2 = CH2CH3), 3l (R1 = R2 = (CH2)2CH3), and 3m (R1 = R2 = (CH2)3CH3) demonstrate similar absorptions (497, 501, 499, and 503 nm, respectively) and emission wavelengths (563, 563, 563, and 567 nm, respectively) in DCM. However, compound 3m exhibits higher quantum yield of 65% compared to that with 3j (41%).

Once variations in the alkynes were studied, we then explored the variations in benzo[a]phenazin-5-ol (1). When comparing the emission wavelengths of compounds 3a and 3n–r, a notable bathochromic shift was observed for the compounds 3o, 3p, and 3r. Even though 3o and 3r show lesser quantum yield of 12 and 37%, respectively, compound 3p, shows a much higher quantum yield of 86% compared to that of 3a (54%). When the phenyl ring in 3a was replaced by the cyano groups (3s), a significant bathochromic shift was observed from 590 to 630 nm, with a large Stokes shift change from 2736 to 3658 cm–1. The phenyl ring in 3a was replaced by the cyclohexyl ring (3t), whereas a much higher rise in Stokes shift from 2736 to 5747 cm–1 was observed, with only a slight variation of the emission wavelengths and quantum yield.

After we examined the physical properties of the diverse substituted compounds, we designed 3u and 3v by the combination of 3c, 3o, and 3s to obtain compounds with enhanced physical properties. Compounds 3u and 3v disclose a large bathochromic shift with wavelengths of 637 and 640 nm, respectively (Figures and 2).

Figure 1

Designed compounds to enhance the fluorescence in FR region.

Figure 2

(A) Normalized absorption (dash line) and fluorescence (solid line) spectra of 3p in DCM (blue) and thin-film (red). (B) Normalized absorption (dash line) and fluorescence (solid line) spectra of 3u in DCM (blue) and thin-film (red).

The fluorescence solvatochromism of 3p was studied in several solvents, and the results are presented in Figure . Compound 3p displays a large positive solvatochromism, with strong fluorescence. Compound 3p unveils solvatochromism with a bathochromic shift by an increase in the solvent polarity from toluene to dimethyl sulfoxide, the green, yellow, orange, and red regions of the visible spectrum covering them. Notably, the fluorescence of compound 3p in methanol shifted radically to the higher wavelength (λmax,em = 610 nm). The reason for this strong solvatochromism is owing to the combination of intramolecular charge transfer and dipole–solvent interactions in solution.[19]

Figure 3

Emission spectra of 3p in various solvents. Inset in panel: photo of 3p in various solvents taken under UV luminescence.

Density Functional Theory (DFT) Calculations

DFT calculation of compound 3p at B3LYP-6-311G level was estimated, and the results are shown in Figure . On the basis of the DFT calculation, the lowest unoccupied molecular orbital (LUMO) is mostly covered on the phenazine moiety, whereas the highest occupied molecular orbital (HOMO) is largely confined around the chromene moiety.

Figure 4

Pictorial presentation of the HOMO (A) and LUMO (B) diagrams of 3p.

Application for Live Cancer Cell Imaging

For live cancer cell imaging, 3p was investigated using various cancer cells, such as A549 (lung cancer cells), HepG2 (liver cancer cells), and U937 (lymphoma cells), due to its high quantum yield.[20] Compound 3p (10 μM) was added to the tested cell cultures and incubated at 37 °C for 30 min. Later, the cells were stained with nucleus staining dye 4′,6-diamidino-2-phenylindole (DAPI) (2 μM) for another 20 min. To our delight, 3p effectively entered the cell membranes of the tested cancer cells, labeled with an intense FR luminescence when compared to the fluorescence images of the cells before and after the treatment with 3p (Figure ). This showed the potential of compound 3p in intracellular imaging in the tested cancer cells. It is noteworthy that during the cell imaging experiment, the tested cells were healthy and unharmed and displayed an adherent morphology. From this study, we conclude that compound 3p could be used for live cancer cell imaging (Supporting Information).

Figure 5

Live cell imaging of compound 3p in A549, HepG2, and U937 cells. (A) Row 1 (a1–a3): A549 cells treated with 10 μM 3p for 30 min. (B) Row 2 (b1–b3): HepG2 cells treated with 10 μM 3p for 30 min. (C) Row 3 (c1–c3): U937 cells treated with 10 μM 3p for 30 min. For (A)–(C), column 1 (a1–c1): DAPI-stained blue fluorescence images; column 2 (a2–c2): red fluorescence images; column 3 (a3–c3): merging of the blue and red fluorescence images. Scale bar = 10 μm. Excitation and emission wavelength: 405 and 410–460 nm for blue fluorescence images; 561 and 570–680 nm for red fluorescence images.

Cell Viability Assay

We studied the cytotoxicity of the synthesized compound 3p, which is an important factor for cell-imaging applications (Figure ). The MTS assay method was applied to test the cytotoxicity of compound 3p by adding up to 200 μM of the compound in the tested cancer cell culture. The observed cytotoxicity is very nominal for compound 3p against that of the tested cancer cells, such as A549, HepG2, and U937. Because of the excellent biocompatibility, the synthesized compound 3p could be utilized as a fluorescent bioprobe in live cancer cell imaging.

Figure 6

Cell viability assay in A549, HepG2, and U937 cells using MTS reagent. Cells have been incubated with 3p (0–200 μM) for 48 h.

Conclusions

In summary, we developed a novel Ru(II)-catalyzed C–H/O–H oxidative annulation reaction to obtain isochromeno[8,1-ab]phenazines. This synthetic methodology offers various advantages, such as diverse substrates in a range of functional groups executed under air, and yields regioselective products. This methodology was successfully applied to design and synthesize FR fluorescent probes for live cancer cell imaging. Photophysical properties were examined both in solution and thin-film states. Notably, synthesized compounds displayed green to FR emissions with large Stokes shifts and high fluorescence quantum yields. Furthermore, 3p shows a large positive solvatochromism, with strong fluorescence. To study the photophysical properties, DFT calculation was done to look at the possible HOMO and LUMO properties. Among the synthesized compounds, 3p was investigated for live cancer cell imaging due to its high quantum yield. Compound 3p labeled the cancer cells efficiently with an intense FR luminescence by passing through the cell membranes. It is noteworthy that the tested cells were unharmed and displayed an adherent morphology during the cell-imaging experiments. We successfully demonstrated 3p as a fluorescent bioprobe in live cancer cell imaging, as a result of its prominent FR fluorescence, high quantum yield, and excellent biocompatibility.

Experimental Section

Materials and Methods

Reagents and solvents were purchased from commercial sources. [RuCl2(p-cymene)]2, Cu(OAc)2·H2O, and AgSbF6 were used as purchased, without further purification, unless otherwise noted. Column chromatography was performed on silica gel (100–200 mesh). Analytical TLC was performed on precoated aluminum sheets of silica gel 60F254 of 0.2 mm thickness. Melting points were determined in capillary tubes and remain uncorrected. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded in CDCl3 solution, with tetramethylsilane as internal standard. HRMS electrospray ionization (ESI) were recorded using HRMS. UV–visible absorption spectra were recorded using UV spectrophotometer. The steady state fluorescence measurements were recorded using fluorescence spectrophotometer. Cell imaging was done using laser confocal microscope instrument.

Synthetic Procedures

General Procedure for Synthesis of Isochromeno[8,1-ab]phenazines (3a–v)

To an oven-dried 25 mL round-bottom flask fitted with a reflux condenser were added benzo[a]phenazin-5-ol 1a (73.88 mg, 0.3 mmol, 1.0 equiv), diphenyl acetylene 2a (53.43 mg, 0.3 mmol, 1.0 equiv), [RuCl2(p-cymene)]2 (9.18 mg, 0.015 mmol, 0.05 equiv), AgSbF6 (20.62 mg, 0.06 mmol, 0.2 equiv), Cu(OAc)2·H2O (59.89 mg, 0.3 mmol, 1.0 equiv) in 3.0 mL of 1,2-DCE. The reaction mixture was refluxed for 12 h. After cooling to ambient temperature, the reaction mixture was diluted with CH2Cl2, filtered through celite, and the filtrate was concentrated under reduced pressure. The crude residue was purified through a silica gel column using hexane and ethyl acetate (85:15) as eluent. The desired product, an orange solid 3a (91.25 mg) was obtained in 72% yield.

4,5-Diphenylisochromeno[8,1-ab]phenazine (3a)

The compound is prepared according to the general procedure. Orange-colored solid; yield 72% (91.25 mg); mp 228–230 °C; FT-IR (cm–1) neat; 2933, 1654, 1437, 1385, 1231, 1159, 1093, 834, 757, 659; 1H NMR (400 MHz, CDCl3) δH 9.10 (d, J = 8.1 Hz, 1H), 8.28 (d, J = 8.3 Hz, 1H), 8.18 (d, J = 8.5 Hz, 1H), 7.84–7.80 (m, 1H), 7.78–7.74 (m, 1H), 7.64 (t, J = 7.9 Hz, 1H), 7.46–7.36 (m, 5H), 7.34–7.30 (m, 3H), 7.25 (d, J = 7.9 Hz, 3H), 7.12 (d, J = 7.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.5, 151.6, 149.6, 145.7, 143.5, 140.7, 140.1, 134.8, 133.5, 131.9, 131.6, 131.1, 130.1, 129.7, 129.1, 129.0, 128.8, 128.4, 128.3, 127.9, 127.8, 124.0, 123.4, 122.8, 116.4, 103.1 ppm; HRMS m/z (ESI): calcd for C30H19N2O [M + H]+ 423.1497; found 423.1494.

4,5-Di-p-tolylisochromeno[8,1-ab]phenazine (3b)

The compound is prepared according to the general procedure. Orange-colored solid; yield 65% (87.85 mg); mp 250–252 °C; FT-IR (cm–1) neat; 2975, 1701, 1609, 1538, 1490, 1441, 1365, 1307, 1287, 1234, 1154, 1117, 1059, 977, 910, 864, 811, 748, 683, 652; 1H NMR (400 MHz, CDCl3) δH 9.01 (d, J = 8.0 Hz, 1H), 8.23 (d, J = 7.8 Hz, 1H), 8.14 (d, J = 8.0 Hz, 1H), 7.80–7.69 (m, 2H), 7.57 (t, J = 7.9 Hz, 1H), 7.22 (dt, J = 20.5, 6.1 Hz, 7H), 7.05 (dd, J = 14.6, 7.8 Hz, 3H), 2.41 (s, 3H), 2.31 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.6, 149.5, 145.7, 143.5, 140.6, 140.1, 138.7, 137.5, 132.2, 131.9, 131.5, 130.9, 130.8, 130.0, 129.9, 129.6, 129.6, 128.9, 128.5, 128.3, 128.2, 123.7, 123.3, 122.7, 115.8, 102.9, 21.3, 21.3 ppm; HRMS m/z (ESI): calcd for C32H23N2O [M + H]+ 451.1810; found 451.1812.

4,5-Bis(4-methoxyphenyl)isochromeno[8,1-ab]phenazine (3c)

The compound is prepared according to the general procedure. Orange-colored solid; yield 57% (82.44 mg); mp 208–210 °C; FT-IR (cm–1) neat; 2983, 1694, 1641, 1551, 1481, 1451, 1363, 1308, 1240, 1153, 1066, 1018, 914, 871, 772, 730; 1H NMR (400 MHz, CDCl3) δH 9.05 (dd, J = 8.1, 1.0 Hz, 1H), 8.26 (dd J = 8.4, 1.6 Hz, 1H), 8.16 (dd, J = 8.5, 1.5 Hz, 1H), 7.81 (dd, J = 8.5, 6.7 Hz, 1H), 7.75 (dd, J = 8.2, 6.7 Hz, 1H), 7.63 (t, J = 7.9 Hz, 1H), 7.35–7.28 (m, 3H), 7.22 (d, J = 8.7 Hz, 2H), 7.12 (dd, J = 7.6, 1.0 Hz, 1H), 6.98 (d, J = 8.7 Hz, 2H), 6.78 (d, J = 9.0 Hz, 2H), 3.87 (s, 3H), 3.80 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 159.7, 159.1, 155.6, 149.4, 145.7, 143.5, 140.6, 140.1, 132.5, 132.2, 131.5, 130.4, 130.0, 129.6, 129.6, 128.3, 128.1, 127.2, 126.0, 123.6, 123.1, 122.6, 114.9, 114.7, 113.3, 102.8, 55.28, 55.2 ppm; HRMS m/z (ESI): calcd for C32H23N2O3 [M + H]+ 483.1709; found 483.1703.

4,5-Bis(4-fluorophenyl)isochromeno[8,1-ab]phenazine (3d)

The compound is prepared according to the general procedure. Orange-colored solid; yield 64% (88.02 mg); mp 208–210 °C; FT-IR (cm–1) neat; 2969, 1688, 1591, 1524, 1446, 1390, 1287, 1237, 1153, 1055, 861, 743, 625; 1H NMR (400 MHz, CDCl3) δH 9.07 (dd, J = 8.1, 1.0 Hz, 1H), 8.29–8.24 (m, 1H), 8.19–8.13 (m, 1H), 7.81–7.77 (m, 2H), 7.61 (t, J = 7.9 Hz, 1H), 7.35–7.30 (m, 2H), 7.29–7.25 (m, 3H), 7.17–7.12 (m, 2H), 7.04 (dd, J = 7.6, 1.1 Hz, 1H), 6.98–6.92 (m, 2H) ppm; 13C NMR (100 MHz, CDCl3) δC 163.9, 163.6, 161.4, 161.2, 155.1, 148.9, 145.5, 143.5, 140.8, 139.9, 133.4, 133.3, 132.8, 132.8, 131.6, 131.5, 131.0, 130.9, 130.6, 130.5, 130.3, 129.7, 129.7, 129.5, 129.4, 128.5, 128.4, 124.2, 123.1, 122.7, 116.5, 116.3, 115.7, 115.5, 115.4, 115.2, 115.0, 103.3 ppm. HRMS m/z (ESI): calcd for C30H17F2N2O [M + H]+ 459.1309; found 459.1321.

4,5-Bis(4-chlorophenyl)isochromeno[8,1-ab]phenazine (3e)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 52% (76.65 mg); mp 260–262 °C; FT-IR (cm–1) neat; 2967, 2361, 1660, 1592, 1524, 1448, 1385, 1289, 1239, 1155, 1056, 934, 863, 745, 626; 1H NMR (400 MHz, CDCl3) δH 9.10 (d, J = 8.1 Hz, 1H), 8.27 (d, J = 8.2 Hz, 1H), 8.17 (d, J = 8.3 Hz, 1H), 7.80 (dt, J = 14.9, 7.0 Hz, 2H), 7.63 (t, J = 7.9 Hz, 1H), 7.43 (d, J = 8.3 Hz, 2H), 7.27 (dd, J = 11.1, 6.2 Hz, 7H), 7.05 (d, J = 7.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.0, 148.7, 140.8, 139.9, 135.0, 134.1, 133.0, 132.4, 132.4, 131.7, 131.2, 130.4, 130.3, 130.3, 129.7, 129.7, 129.0, 128.5, 128.4, 128.3, 125.7, 124.4, 123.1, 122.7, 115.7, 103.4 ppm. HRMS m/z (ESI): calcd for C30H17Cl2N2O [M + H]+ 491.0718; found 491.0723.

4,5-Bis(4-bromophenyl)isochromeno[8,1-ab]phenazine (3f)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 50% (87 mg); mp 270–272 °C; FT-IR (cm–1) neat; 2969, 1682, 1538, 1512, 1480, 1433, 1375, 1327, 1288, 1231, 1178, 1148, 1068, 988, 915, 682; 1H NMR (400 MHz, CDCl3) δH 9.10 (d, J = 8.1 Hz, 1H), 8.27 (d, J = 8.5 Hz, 1H), 8.17 (d, J = 8.3 Hz, 1H), 7.79 (dt, J = 14.7, 7.1 Hz, 2H), 7.66–7.57 (m, 3H), 7.40 (d, J = 8.5 Hz, 2H), 7.30 (s, 1H), 7.20 (dd, J = 13.5, 8.4 Hz, 4H), 7.05 (d, J = 7.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.0, 148.7, 140.8, 139.9, 133.5, 132.7, 132.7, 132.1, 131.7, 131.3, 131.1, 130.5, 130.4, 130.3, 129.7, 129.7, 128.6, 128.4, 124.4, 123.4, 123.2, 123.0, 122.7, 122.3, 115.7, 103.5 ppm; HRMS m/z (ESI): calcd for C30H17Br2N2O [M + H]+ 578.9708; found 578.9705.

4,5-Bis(4-nitrophenyl)isochromeno[8,1-ab]phenazine (3g)

The compound is prepared according to the general procedure. Orange-colored solid; yield 39% (59 mg); mp 188–190 °C; FT-IR (cm–1) neat; 2927, 1654, 1551, 1438, 1384, 1241, 1159, 1061, 759; 1H NMR (400 MHz, CDCl3) δH 9.19 (d, J = 8.1 Hz, 1H), 8.34 (d, J = 8.5 Hz, 1H), 8.30 (d, J = 8.3 Hz, 1H), 8.20 (d, J = 8.4 Hz, 1H), 8.13 (d, J = 8.7 Hz, 2H), 7.83 (dt, J = 21.8, 7.1 Hz, 3H), 7.69 (d, J = 7.8 Hz, 1H), 7.52 (t, J = 8.8 Hz, 4H), 7.39 (s, 1H), 7.00 (d, J = 7.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 159.3, 154.1, 148.0, 141.2, 139.7, 138.9, 132.1, 131.2, 130.6, 129.9, 129.8, 129.83, 129.7, 129.0, 128.5, 125.4, 124.8, 123.4, 123.2, 122.8, 122.7, 117.5, 117.0, 104.4 ppm; HRMS m/z (ESI): calcd for C30H17N4O5 [M + H]+ 513.1199; found 513.1196.

4,5-Di(naphthalen-1-yl)isochromeno[8,1-ab]phenazine (3h)

The compound is prepared according to the general procedure. Orange-colored solid; yield 50% (78 mg); mp 188–190 °C; FT-IR (cm–1) neat; 2979, 2361, 1726, 1654, 1545, 1374, 1239, 1161, 1097, 1044, 660, 619; 1H NMR (400 MHz, CDCl3) δH 9.07 (d, J = 8.1 Hz, 1H), 8.28 (d, J = 8.4 Hz, 1H), 8.23 (d, J = 8.3 Hz, 1H), 8.11 (d, J = 8.2 Hz, 1H), 8.01 (d, J = 7.9 Hz, 1H), 7.77–7.70 (m, 4H), 7.66–7.60 (m, 2H), 7.50–7.44 (m, 3H), 7.39 (dd, J = 16.0, 7.5 Hz, 3H), 7.18 (s, 2H), 7.00 (t, J = 7.7 Hz, 1H), 6.87–6.79 (m, 1H), 6.66 (d, J = 7.5 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 151.4, 145.6, 133.6, 133.4, 132.7, 132.6, 132.0, 131.7, 131.7, 131.6, 130.9, 130.2, 129.8, 129.7, 128.5, 128.5, 128.48, 128.46, 128.44, 128.3, 127.9, 127.8, 127.2, 127.1, 126.72, 126.70, 126.4, 126.4, 126.09, 126.00, 125.8, 125.6, 124.7, 124.2, 124.0, 122.4, 120.8, 103.7 ppm; HRMS m/z (ESI): calcd for C38H23N2O [M + H]+ 523.1810; found 523.1808.

4-Methyl-5-phenylisochromeno[8,1-ab]phenazine (3i)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 68% (73 mg); mp 234–236 °C; FT-IR (cm–1) neat; 2970, 1690, 1592, 1524, 1448, 1288, 1237, 1153, 1056, 826, 743, 678; 1H NMR (400 MHz, CDCl3) δH 9.13 (d, J = 8.1 Hz, 1H), 8.26 (d, J = 8.3 Hz, 1H), 8.15 (d, J = 8.4 Hz, 1H), 7.83–7.72 (m, 3H), 7.65 (d, J = 7.0 Hz, 2H), 7.51 (d, J = 7.6 Hz, 4H), 7.23 (s, 1H), 2.19 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.5, 149.7, 145.6, 143.5, 140.5, 134.0, 131.7, 131.6, 131.5, 130.1, 129.8, 129.6, 129.4, 129.2, 128.39, 128.35, 128.2, 123.8, 123.0, 121.3, 108.4, 102.9, 13.7 ppm; HRMS m/z (ESI): calcd for C25H16N2O [M + H]+ 361.1341; found 361.1339.

4,5-Dimethylisochromeno[8,1-ab]phenazine (3j)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 39% (35 mg); mp 140–142 °C; FT-IR (cm–1) neat; 2935, 1654, 1386, 1240, 1096, 1044, 842, 749, 660; 1H NMR (400 MHz, CDCl3) δH 9.03 (d, J = 8.1 Hz, 1H), 8.24 (d, J = 8.5 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 7.80–7.70 (m, 3H), 7.35 (d, J = 7.5 Hz, 1H), 7.13 (s, 1H), 2.25 (s, 3H), 2.06 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 148.4, 143.4, 142.4, 140.5, 140.0, 133.8, 131.5, 130.0, 129.7, 129.6, 128.3, 128.2, 128.0, 122.9, 122.7, 120.2, 106.5, 102.4, 17.1, 12.4 ppm; HRMS m/z (ESI): calcd for C20H14N2O [M + H]+ 299.1184; found 299.1186.

4,5-Diethylisochromeno[8,1-ab]phenazine (3k)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 45% (44 mg); mp 150–152 °C; FT-IR (cm–1) neat; 2970, 2359, 1688, 1591, 1523, 1447, 1391, 1365, 1287, 1237, 1152, 1055, 942, 861, 816, 773, 747, 683, 660, 625; 1H NMR (400 MHz, CDCl3) δH 9.02 (d, J = 8.1 Hz, 1H), 8.23 (d, J = 8.2 Hz, 1H), 8.13 (d, J = 8.0 Hz, 1H), 7.80–7.69 (m, 3H), 7.42 (d, J = 7.6 Hz, 1H), 7.13 (s, 1H), 2.56 (q, J = 7.5 Hz, 4H), 1.32 (t, J = 7.5 Hz, 3H), 1.22 (t, J = 7.5 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.8, 153.3, 149.1, 140.4, 140.0, 131.8, 130.6, 129.9, 129.7, 129.6, 128.3, 127.9, 122.9, 120.8, 120.3, 111.9, 111.4, 102.3, 23.9, 19.4, 13.4, 12.4 ppm; HRMS m/z (ESI): calcd for C22H19N2O [M + H]+ 327.1497; found 327.1502.

4,5-Dipropylisochromeno[8,1-ab]phenazine (3l)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 50% (53 mg); mp 178–180 °C; FT-IR (cm–1) neat; 2952, 1701, 1652, 1517, 1434, 1390, 1366, 1290, 1238, 1163, 1101, 976, 843, 753, 682; 1H NMR (400 MHz, CDCl3) δH 9.02 (d, J = 8.1 Hz, 1H), 8.23 (d, J = 8.3 Hz, 1H), 8.13 (d, J = 8.4 Hz, 1H), 7.79–7.69 (m, 3H), 7.40 (d, J = 7.6 Hz, 1H), 7.13 (s, 1H), 2.52 (dd, J = 14.6, 6.6 Hz, 4H), 1.78 (dd, J = 15.0, 7.5 Hz, 2H), 1.63 (dd, J = 15.4, 7.6 Hz, 2H), 1.07 (t, J = 7.4 Hz, 6H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.7, 152.5, 145.7, 143.4, 140.4, 140.0, 131.8, 130.9, 129.9, 129.7, 129.6, 128.2, 127.9, 123.1, 122.9, 120.5, 111.1, 102.2, 32.5, 28.4, 21.8, 21.1, 14.2, 13.9 ppm; HRMS m/z (ESI): calcd for C24H23N2O [M + H]+ 355.1810; found 355.1811.

4,5-Dibutylisochromeno[8,1-ab]phenazine (3m)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 62% (71 mg); mp 196–198 °C; FT-IR (cm–1) neat; 2952, 1701, 1652, 1517, 1434, 1390, 1366, 1290, 1238, 1163, 1101, 976, 843, 753, 682; 1H NMR (400 MHz, CDCl3) δH 9.03 (d, J = 8.1 Hz, 1H), 8.24 (d, J = 8.3 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 7.82–7.67 (m, 3H), 7.41 (d, J = 7.6 Hz, 1H), 7.13 (s, 1H), 2.53 (dd, J = 14.9, 6.9 Hz, 4H), 1.76–1.65 (m, 4H), 1.49 (dd, J = 14.9, 7.6 Hz, 4H), 1.00 (t, J = 7.3 Hz, 6H); 13C NMR (100 MHz, CDCl3) δC 155.7, 152.6, 145.7, 143.4, 140.4, 140.0, 131.8, 130.9, 129.9, 129.7, 129.6, 128.2, 127.9, 123.1, 122.9, 120.5, 111.1, 102.2, 30.8, 30.3, 29.9, 26.1, 22.9, 22.5, 13.9, 13.9 ppm. HRMS m/z (ESI): calcd for C26H27N2O [M + H]+ 383.2123; found 383.213.

11-Methyl-4,5-diphenylisochromeno[8,1-ab]phenazine (3n)

The compound is prepared according to the general procedure. Yellow-colored solid; yield 69% (90 mg); mp: 182–184 °C; FT-IR (cm–1) neat; 2969, 1655, 1592, 1524, 1439, 1412, 1388, 1287, 1236, 1154, 1098, 1055, 942, 900, 844, 816, 744; 1H NMR (400 MHz, CDCl3) δH 9.06 (d, J = 8.1 Hz, 1H), 8.17–8.00 (m, 2H), 7.67–7.57 (m, 2H), 7.39 (ddd, J = 11.0, 9.6, 4.3 Hz, 5H), 7.32–7.30 (m, 2H), 7.27–7.20 (m, 4H), 7.08 (d, J = 7.6 Hz, 1H), 2.65 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.0, 149.5, 145.0, 140.8, 139.9, 138.8, 134.9, 133.6, 132.9, 131.8, 131.6, 131.1, 131.0, 129.6, 129.5, 129.1, 129.0, 128.7, 128.1, 127.8, 127.8, 123.8, 123.7, 123.1, 116.4, 103.2, 21.9 ppm; HRMS m/z (ESI): calcd for C31H21N2O [M + H]+ 437.1654; found 437.1648.

10,11-Dibromo-4,5-diphenylisochromeno[8,1-ab]phenazine (3o)

The compound is prepared according to the general procedure. Orange-colored solid; yield 58% (101 mg); mp 200–202 °C; FT-IR (cm–1) neat; 2965, 1684, 1548, 1523, 1495, 1448, 1433, 1385, 1338, 1273, 1246, 1193, 1163, 1068, 988; 1H NMR (400 MHz, CDCl3) δH 8.97 (d, J = 8.1 Hz, 1H), 8.53 (s, 1H), 8.45 (s, 1H), 7.52 (s, 3H), 7.36–7.32 (m, 5H), 7.17–7.09 (m, 4H), 6.70 (s, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 156.4, 147.1, 134.6, 133.3, 132.2, 131.09, 130.10, 129.2, 129.0, 128.9, 128.8, 128.6, 128.6, 128.5, 128.4, 128.2, 128.0, 127.9, 127.6, 124.7, 124.4, 124.2, 124.1, 123.9, 116.8, 102.9 ppm. HRMS m/z (ESI): calcd for C30H17Br2N2O [M + H]+ 578.9708; found 578.9711.

4,5,10,11-Tetraphenylisochromeno[8,1-ab]phenazine (3p)

The compound is prepared according to the general procedure. Red-colored solid; yield 67% (115 mg); mp 216–218 °C; FT-IR (cm–1) neat; 2967, 1648, 1541, 1413, 1385, 1239, 1157, 1069, 993, 865, 816, 754, 660; 1H NMR (400 MHz, CDCl3) δH 8.96 (d, J = 8.1 Hz, 1H), 8.22 (s, 1H), 8.13 (s, 1H), 7.52 (t, J = 7.9 Hz, 1H), 7.37–7.26 (m, 5H), 7.25–7.21 (m, 11H), 7.20–7.12 (m, 5H), 7.00 (d, J = 7.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.4, 149.5, 146.0, 143.7, 142.8, 141.9, 140.6, 140.5, 140.4, 140.0, 134.8, 133.5, 131.9, 131.6, 131.1, 130.6, 130.0, 129.7, 129.3, 129.3, 129.2, 129.0, 128.8, 128.0, 127.9, 127.8, 127.1, 127.0, 124.0, 123.3, 122.8, 117.8, 116.5, 103.2 ppm; HRMS m/z (ESI): calcd for C42H27N2O [M + H]+ 575.2123; found 575.2118.

4,5,9,12-Tetraphenylisochromeno[8,1-ab]phenazine (3q)

The compound is prepared according to the general procedure. Red-colored solid; yield 70% (121 mg); mp 202–204 °C; FT-IR (cm–1) neat; 2969, 2359, 1650, 1544, 1412, 1384, 1329, 1158, 1063, 992, 754, 619, 606; 1H NMR (400 MHz, CDCl3) δH 8.74 (d, J = 8.0 Hz, 1H), 7.92–7.81 (m, 6H), 7.49 (ddd, J = 20.4, 13.6, 7.3 Hz, 8H), 7.34 (d, J = 7.3 Hz, 2H), 7.28 (s, 2H), 7.15 (dd, J = 17.2, 9.6 Hz, 5H), 7.05 (d, J = 8.8 Hz, 1H), 7.00 (d, J = 7.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) δC 155.3, 151.8, 151.7, 149.4, 147.0, 144.8, 141.7, 140.1, 139.1, 138.9, 138.8, 138.5, 134.9, 133.6, 131.9, 131.8, 131.2, 131.1, 131.0, 130.8, 130.1, 129.6, 129.1, 128.9, 128.7, 128.3, 128.0, 127.9, 127.8, 127.5, 127.4, 124.3, 123.1, 119.1, 103.7 ppm. HRMS m/z (ESI): calcd for C42H27N2O [M + H]+ 575.2123; found 575.2131.

4,4′-(4,5-Diphenylisochromeno[8,1-ab]phenazine-10,11-diyl)bis(N,N-diphenylaniline) (3r)

The compound is prepared according to the general procedure. Orange-colored solid; yield 56% (158 mg); mp: 202–204 °C; FT-IR (cm–1) neat; 2923, 1651, 1592, 1544, 1489, 1439, 1413, 1386, 1235, 1159, 1092, 844, 759, 661, 619; 1H NMR (400 MHz, CDCl3) δH 8.98 (d, J = 8.1 Hz, 1H), 8.21 (s, 1H), 8.12 (s, 1H), 7.53 (t, J = 7.9 Hz, 1H), 7.36–7.27 (m, 5H), 7.24–7.14 (m, 14H), 7.10 (d, J = 8.5 Hz, 4H), 7.05 (d, J = 8.1 Hz, 8H), 7.01 (d, J = 7.6 Hz, 1H), 6.95 (dd, J = 12.1, 5.3 Hz, 8H) ppm; 13C NMR (100 MHz, CDCl3) δC 155.3, 149.6, 149.0, 147.6, 147.6, 147.0, 146.9, 145.8, 145.8, 143.7, 142.9, 142.9, 141.9, 140.3, 140.2, 140.1, 134.9, 134.7, 134.4, 133.5, 131.9, 131.6, 131.1, 130.8, 129.9, 129.7, 129.3, 129.2, 129.0, 128.8, 128.6, 127.9, 127.8, 124.6, 124.5, 123.9, 123.2, 123.0, 123.0, 122.8, 122.6, 120.8, 116.5, 111.4, 103.3 ppm. HRMS m/z (ESI): calcd for C66H45N4O [M + H]+ 909.3593; found 909.3583.

4,5-Diphenylisochromeno[8,1-fg]quinoxaline-9,10-dicarbonitrile (3s)

Yield: The compound is prepared according to the general procedure. Orange-colored solid; yield 67% (85 mg); mp 206–208 °C; FT-IR (cm–1) neat; 2925, 2203, 1655, 1543, 1437, 1387, 1235, 1159, 1088, 834, 616; 1H NMR (400 MHz, CDCl3) δH 8.78 (d, J = 8.1 Hz, 1H), 7.69 (t, J = 8.0 Hz, 1H), 7.41–7.33 (m, 3H), 7.28–7.18 (m, 9H) ppm; 13C NMR (100 MHz, CDCl3) δC 159.3, 150.2, 145.8, 138.6, 133.7, 132.5, 132.4, 131.8, 131.5, 130.7, 130.3, 129.5, 129.4, 129.0, 128.4, 128.0, 125.6, 124.7, 124.0, 123.9, 117.6, 114.3, 113.8, 103.2 ppm; HRMS m/z (ESI): calcd for C28H15N4O [M + H]+ 423.1246; found 423.1242.

4,5-Diphenyl-8a,9,10,11,12,12a-hexahydroisochromeno[8,1-ab]phenazine (3t)

The compound is prepared according to the general procedure. Green-colored solid; yield 80% (103 mg); mp 184–186 °C; FT-IR (cm–1) neat; 2966, 1614, 1551, 1438, 1384, 1360, 1324, 1243, 1161, 1128, 865, 755; 1H NMR (400 MHz, CDCl3) δH 8.68 (d, J = 8.2 Hz, 1H), 7.40 (t, J = 7.9 Hz, 1H), 7.35–7.26 (m, 5H), 7.23–7.19 (m, 2H), 7.14 (dd, J = 13.2, 5.7 Hz, 4H), 6.81 (d, J = 7.4 Hz, 1H), 3.09 (d, J = 15.0 Hz, 4H), 1.97 (t, J = 2.9 Hz, 4H), 1.59 (s, 2H) ppm; 13C NMR (100 MHz, CDCl3) δC 152.8, 152.4, 148.6, 148.3, 141.3, 134.4, 134.1, 132.8, 130.7, 130.4, 130.1, 128.1, 128.0, 127.9, 127.6, 127.5, 127.5, 126.7, 126.7, 123.7, 121.7, 120.8, 119.5, 115.5, 102.5, 31.8, 31.8, 28.6, 22.0, 21.9 ppm; HRMS m/z (ESI): calcd for C30H25N2O [M + H]+ 429.1967; found 429.1961.

10,11-Dibromo-4,5-bis(4-methoxyphenyl)isochromeno[8,1-ab]phenazine (3u)

The compound is prepared according to the general procedure. Red-colored solid; yield 52% (100 mg); mp 184–186 °C; FT-IR (cm–1) neat; 2970, 1675, 1544, 1528, 1495, 1448, 1433, 1385, 1338, 1273, 1246, 1193, 1163, 1068, 988; 1H NMR (400 MHz, CDCl3) δH 8.54 (s, 1H), 8.45 (s, 1H), 7.64 (dd, J = 13.3, 5.4 Hz, 2H), 7.33–7.29 (m, 4H), 7.20 (d, J = 1.3 Hz, 2H), 6.99 (s, 1H), 6.76 (s, 1H), 6.72 (d, J = 2.1 Hz, 1H), 6.31 (s, 1H), 3.87 (s, 3H), 3.80 (s, 3H); 13C NMR (100 MHz, CDCl3) δC 159.8, 159.2, 158.6, 156.5, 139.6, 133.4, 133.3, 132.2, 132.2, 130.4, 130.2, 130.0, 130.0, 126.9, 123.9, 118.7, 115.0, 114.7, 113.8, 113.6, 113.3, 109.8, 104.3, 102.6, 55.3, 55.2 ppm. HRMS m/z (ESI): calcd for C32H21Br2N2O3 [M + H]+ 638.9919; found 638.9916.

4,5-Bis(4-methoxyphenyl)isochromeno[8,1-fg]quinoxaline-9,10-dicarbonitrile (3v)

The compound is prepared according to the general procedure. Red-colored solid; yield 60% (87 mg); mp 214–216 °C; FT-IR (cm–1) neat; 2938, 2226, 1688, 1655, 1578, 1431, 1391, 1227, 1160, 1092, 1066, 834, 616; 1H NMR (400 MHz, CDCl3) δH 8.83 (d, J = 8.2 Hz, 1H), 7.95 (d, J = 8.8 Hz, 1H), 7.75 (t, J = 7.9 Hz, 1H), 7.31–7.28 (m, 2H), 7.20 (d, J = 8.5 Hz, 2H), 6.99 (dd, J = 12.4, 8.8 Hz, 3H), 6.78 (d, J = 8.8 Hz, 2H), 3.88 (s, 3H), 3.80 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δC 160.2, 159.5, 150.1, 145.9, 133.1, 132.3, 131.9, 131.5, 130.4, 126.3, 126.0, 125.4, 124.9, 124.4, 123.7, 123.6, 119.4, 117.0, 116.0, 115.0, 114.4, 114.3, 113.5, 103.0, 55.3, 55.3 ppm. HRMS m/z (ESI): calcd for C30H19N4O3 [M + H]+ 483.1457; found 483.1467.

Competitive Experiment

To an oven-dried 25 mL round-bottom flask fitted with a reflux condenser were added benzo[a]phenazin-5-ol 1a (73.88 mg, 0.3 mmol, 1.0 equiv), diphenyl acetylene 2a (53.43 mg, 0.3 mmol, 1.0 equiv), 2l (36.06 mg, 0.3 mmol, 1.0 equiv), [RuCl2(p-cymene)]2 (9.18 mg, 0.015 mmol, 0.05 equiv), AgSbF6 (20.62 mg, 0.06 mmol, 0.2 equiv), Cu(OAc)2·H2O (59.89 mg, 0.3 mmol, 1.0 equiv) in 3.0 mL of 1,2-DCE. The reaction mixture was refluxed for 12 h. After cooling to the ambient temperature, the reaction mixture was diluted with CH2Cl2 and filtered through celite and the filtrate was concentrated under reduced pressure. The crude residue was purified through a silica gel column using hexane and ethyl acetate (85:15) as eluent. The desired products 3a and 3l were obtained as orange solids. 1H NMR analysis confirmed that the annulated product originating from the electron-deficient alkyne, 3a, was obtained in higher proportion, 1.7:1.0, when compared to that obtained from 3l.

Regioselectivity of the Reaction

To an oven-dried 25 mL round-bottom flask fitted with a reflux condenser were added benzo[a]phenazin-5-ol 1a (73.88 mg, 0.3 mmol, 1.0 equiv), 1-phenyl-1-propyne 2i (34.84 mg, 0.3 mmol, 1.0 equiv), [RuCl2(p-cymene)]2 (5.0 mol %), Cu(OAc)2·H2O (1 equiv), and AgSbF6 (20 mol %) in 1,2-DCE. The reaction mixture was refluxed for 12 h. After cooling to the ambient temperature, the reaction mixture was diluted with CH2Cl2 and filtered through celite and the filtrate was concentrated under reduced pressure. The crude residue was purified through a silica gel column using hexane and ethyl acetate (85:15) as eluents. The desired product, orange solid 3i, 73.52 mg, was obtained in 68% yield. The regioselectivity of the annulated product was confirmed by the nuclear Overhauser effect experiment.

Preliminary Mechanism Study

(1) To an oven-dried 25 mL round-bottom flask fitted with a reflux condenser were added benzo[a]phenazin-5-ol 1 (0.3 mmol) and CD3OD (10 equiv) in 1,2-DCE. This mixture was refluxed for 12 h. After cooling to the ambient temperature, the solvent was evaporated under reduced pressure and the residue was purified by flash column chromatography on silica gel to give the mixture 1a/1aa. The mixture was analyzed using 1H NMR spectrometer. The deuterium incorporation was estimated to be 69%. (2) To an oven-dried 25 mL round-bottom flask fitted with a reflux condenser were added benzo[a]phenazin-5-ol 1 (0.3 mmol), acetylene 2 (0.3 mmol), [RuCl2(p-cymene)]2 (5.0 mol %), Cu(OAc)2·H2O (1 equiv), AgSbF6 (20 mol %), and CD3OD (10 equiv) in 1,2-DCE. This mixture was refluxed for 12 h. After cooling to the ambient temperature, the solvent was evaporated under reduced pressure, and the residue was purified by flash column chromatography on silica gel to give the mixture 1a/1ab. The mixture was analyzed using 1H NMR spectrometer. The deuterium incorporation was estimated to be 66 and 75%.

Gram Scale Synthesize of 4,5-Diphenylisochromeno[8,1-ab]phenazine

To an oven-dried 100 mL round-bottom flask fitted with a reflux condenser were added benzo[a]phenazin-5-ol 1a (1.03 g, 4.2 mmol, 1.0 equiv), diphenyl acetylene 2a (0.722 g, 4.2 mmol, 1.0 equiv), [RuCl2(p-cymene)]2 (128 mg, 5.0 mol %), Cu(OAc)2·H2O (0.8350 g, 1.0 equiv), and AgSbF6 (0.287 g, 0.2 equiv) in 50 mL of 1,2-DCE. The reaction mixture was refluxed for 12 h. After cooling to the ambient temperature, the reaction mixture was diluted with CH2Cl2 and filtered through celite and the filtrate was concentrated under reduced pressure. The crude residue was purified through a silica gel column using hexane and ethyl acetate as eluent (85:15) to afford 3a in 66% (1.17g) yield.

Computational Calculations

DFT calculation of compound 3p was carried out using the Gaussian 09 program package. Ground state geometries of the boron dyes were optimized using DFT-B3LYP-6-311G basis set. To get information about absorption properties, time-dependent density functional theory calculations are carried out using the optimized geometries.

Live Cancer Cell-Imaging Experiment

Fluorescence compound 3p was examined for intracellular imaging in A549, HepG2, and U937 cells. A549 and HepG2 cells were grown in Dulbecco’s modified Eagle’s medium (DMEM), and U937 cells were grown in RMPI media, with 10% fetal bovine serum, 1% penicillin/streptomycin at 37 °C under a 5% CO2 atmosphere for 24 h. The cells were incubated at 37 °C with 10 μM of compound 3p for 30 min. After thorough washing with phosphate-buffered saline (PBS), the cells were stained with nucleus staining dye DAPI (2 μM) for another 20 min. Thereafter, the staining solution was replaced with fresh PBS to remove the remaining dye. Finally, the blue and red fluorescence images of A549, HepG2, and U937 cells were captured using a confocal laser scanning microscope (LSM710; ZEISS). The excitation wavelength for blue: 405 nm; emission collection: 410–460 nm; excitation wavelength for red: 561 nm; emission collection: 570–680 nm.

Cytotoxic properties of the synthesized compound 3p were studied against A549, HepG2, and U937 cells. A549 and HepG2 cells were maintained in a complete tissue culture medium DMEM, and U937 cells were maintained in Roswell Park Memorial Institute medium, with 10% fetal bovine serum and 2 mM l-glutamine, along with antibiotics (about 100 IU mL–1 of penicillin, 100 μg mL–1 of streptomycin), with the pH adjusted to 7.2. A medium of 50 μL containing 5000 cells per well and different concentrations of compounds 3p was seeded in 96-well plates. The cells were cultivated at 37 °C, with 5% CO2 and 95% air, in 100% relative humidity. AQueous One Solution reagent of 20 μL was added to each well of CellTiter 96 according to the manufacturer’s guidelines and incubated at 37 °C for 1–4 h under a humidified, 5% CO2 atmosphere. The cytotoxicity against cells was determined by measuring the absorbance of the converted dye at 490 nm in an enzyme-linked immunosorbent (immunoadsorbent) assay reader. Cytotoxicity of each sample was expressed as an IC50 value.