From Quinoxaline, Pyrido[2,3-b]pyrazine and Pyrido[3,4-b]pyrazine to Pyrazino-Fused Carbazoles and Carbolines

2,3-Diphenylated quinoxaline, pyrido[2,3-b]pyrazine and 8-bromopyrido[3,4-b]pyrazine were halogenated in deprotometalation-trapping reactions using mixed 2,2,6,6-tetramethyl piperidino-based lithium-zinc combinations in tetrahydrofuran. The 2,3-diphenylated 5-iodo- quinoxaline, 8-iodopyrido[2,3-b]pyrazine and 8-bromo-7-iodopyrido[3,4-b]pyrazine thus obtained were subjected to palladium-catalyzed couplings with arylboronic acids or anilines, and possible subsequent cyclizations to afford the corresponding pyrazino[2,3-a]carbazole, pyrazino[2′,3′:5,6] pyrido[4,3-b]indole and pyrazino[2′,3′:4,5]pyrido[2,3-d]indole, respectively. 8-Iodopyrido[2,3-b] pyrazine was subjected either to a copper-catalyzed C-N bond formation with azoles, or to direct substitution to introduce alkylamino, benzylamino, hydrazine and aryloxy groups at the 8 position. The 8-hydrazino product was converted into aryl hydrazones. Most of the compounds were evaluated for their biological properties (antiproliferative activity in A2058 melanoma cells and disease-relevant kinase inhibition).

The lithium-zinc base of Method A is prepared from ZnCl 2 ·TMEDA (TMEDA = N,N,N ,Ntetramethylethylenediamine) and LiTMP in a 1:3 ratio. Previous studies have suggested that it is a 1:1 LiTMP-Zn(TMP) 2 combination. While LiTMP deprotonates the substrate, Zn(TMP) 2 intercepts the generated aryllithium [18,19,22]. A recent computer study on anisole showed that the reactive species is solvated LiTMP. The effectiveness of the reaction derives from the stabilizing effect of the transmetalation step [21].
It is possible to replace Zn(TMP) 2 by ZnCl 2 provided that there is no contact between LiTMP and ZnCl 2 in the absence of the aromatic compound [23,24]. Thus, Method B is limited to activated substrates for which deprotonation is favored over reaction between LiTMP and ZnCl 2 .
Whereas Method A should provide a lithium arylzincate, Method B should rather generate an arylzinc. Nevertheless, both species are known to react with iodine by aryl transfer.
To evaluate Method B, 1a and 2a were mixed with ZnCl 2 ·TMEDA before addition of LiTMP at −20 • C and stirring for 0.5 h (Method B, entries 2 and 4). After subsequent interception with iodine, 1b and 2b-I were isolated in 70 and 62% yield, respectively (entries 2 and 4).
We explored the use of other electrophiles to intercept the heteroarylzinc chloride prepared from 2a by using Method B. Conversion to the corresponding bromide 2b-Br (60% yield, entry 5) and chloride 2b-Cl (62% yield, entry 6) was performed using bromine and trichloroisocyanuric acid, respectively, as the electrophile. the generated aryllithium [18,19,22]. A recent computer study on anisole showed that the reactive species is solvated LiTMP. The effectiveness of the reaction derives from the stabilizing effect of the transmetalation step [21]. It is possible to replace Zn(TMP)2 by ZnCl2 provided that there is no contact between LiTMP and ZnCl2 in the absence of the aromatic compound [23,24]. Thus, Method B is limited to activated substrates for which deprotonation is favored over reaction between LiTMP and ZnCl2.
Whereas Method A should provide a lithium arylzincate, Method B should rather generate an arylzinc. Nevertheless, both species are known to react with iodine by aryl transfer.
To evaluate Method B, 1a and 2a were mixed with ZnCl2·TMEDA before addition of LiTMP at −20 °C and stirring for 0.5 h (Method B, entries 2 and 4). After subsequent interception with iodine, 1b and 2b-I were isolated in 70 and 62% yield, respectively (entries 2 and 4).
We decided to combine both steps in an auto-tandem process under microwave irradiation (Table 3). Using (Pd 2 (dba) 3 ), we selected Xantphos for its higher efficiency in comparison with tri-tert-butylphosphine. From 2b, best results were obtained with three equivalents of DBU as base (entries 1 and 2). In addition, a longer reaction time was required to ensure complete conversion and this afforded carboline 2g in 70% yield (entry 3).
By testing a profile to maximize the microwave power, we noticed that an increase of the applied power favored the formation of 2f over 2g (entry 4). By carrying out one third of the reaction time under microwave irradiation and the rest by classical heating at the same temperature, a small microwave effect was evidenced (entry 5). While 2g was not formed without catalyst, C-N bond formation giving 2f could take place (entry 6; see Figure 1). However, increasing the catalyst amount had no impact on the conversion to 2g (entry 7). Finally, we intentionally chose a short reaction time (5 min) in order to compare the palladium-catalyzed reactions under microwave irradiation from 2b-I (entry 7), 2b-Br (entry 7) and 2b-Cl (entry 10). The results clearly showed decreasing reactivity from 2b-I to 2b-Cl, and thus, we selected iodo as halogeno group to pursue our investigations.  2 Evaluated from the NMR spectra of the crudes. 3 Yield after purification. 4 Microwave profile of irradiation: The sequence 'Maximum microwave power applied: 150-200 W to reach 180 • C then 2 min at 180 • C before cooling to 100 • C' was repeated every 4 min. 5 Then classical heating at 180 • C for 40 min. 6 Without catalyst. 7 By using 12 mol% Pd 2 (dba) 3 and 30 mol% Xantphos. 8 The rest was unreacted 2b-I (32%). 9 The rest was unreacted 2b-Br (38%) and 2a (28%). 10 The rest was unreacted 2b-Cl (78%).
By testing a profile to maximize the microwave power, we noticed that an increase of the applied power favored the formation of 2f over 2g (entry 4). By carrying out one third of the reaction time under microwave irradiation and the rest by classical heating at the same temperature, a small microwave effect was evidenced (entry 5). While 2g was not formed without catalyst, C-N bond formation giving 2f could take place (entry 6; see Figure 1). However, increasing the catalyst amount had no impact on the conversion to 2g (entry 7). Finally, we intentionally chose a short reaction time (5 min) in order to compare the palladium-catalyzed reactions under microwave irradiation from 2b-I (entry 7), 2b-Br (entry 7) and 2b-Cl (entry 10). The results clearly showed decreasing reactivity from 2b-I to 2b-Cl, and thus, we selected iodo as halogeno group to pursue our investigations.
We applied the optimized procedure to the synthesis of the pyrazino-fused α-carboline 3g from the bromoiodo substrate 3b and aniline. No trace of the expected product 3g was detected but the formation of 3g due to competitive debromination was noted, showing a less obvious intramolecular C-H arylation (Scheme 3, left). Consequently, we moved to the synthesis of the pyrazino-fused δ-carboline 3h. Upon treatment of 3b by 2-aminophenylboronic acid under standard conditions [29], coupling and subsequent cyclization occurred, providing 3h in 65% yield (Scheme 3, right).
As previously mentioned [22], such reactions work far less efficiently when performed on diiodides. Indeed, reacting the diiodide 1b with pyrazole only gave the monofunctionalized derivative 1k , regardless of the amount of azole employed (Scheme 4). As previously mentioned [22], such reactions work far less efficiently when performed on diiodides. Indeed, reacting the diiodide 1b' with pyrazole only gave the monofunctionalized derivative 1k', regardless of the amount of azole employed (Scheme 4).  Different amines and hydrazine reacted with 2b-I without recourse to catalyst (Table 5), affording the corresponding secondary amines 2n-p (entries 1-3) and arylhydrazine 2q (entry 4) in good yields. The latter was converted into the hydrazones 2r-u in the presence of aromatic aldehydes chosen for their ability to potentially interact with binding sites of biological interest [43] (Scheme 5). Finally, reaction of 2b-I with a phenol also proved possible without catalyst, giving the diaryl ether 2v in 64% yield (Scheme 6). Table 4. Copper-catalyzed N-arylation of 8-iodo-2,3-diphenylpyrido[2,3-b]pyrazine (2b-I) using azoles.
To take advantage of the iodo group on 2b-I, C-N bond formation with azoles was attempted under copper catalysis as reported previously [41,42] (Table 4). Thus, by treating 2b-I with pyrrole (entry 1; see Figure 1), indole (entry 2), pyrazole (entry 3), imidazole (entry 4) or 1,2,4-triazole (entry 5), in the presence of catalytic copper(I) oxide, cesium carbonate, and dimethylsulfoxide (DMSO) at 110 °C for 24 h, the expected N-arylated azoles were obtained in 51 to 79% yields. 1 Pyrrole (entry 1; see Figure 1), indole (entry 2), pyrazole (entry 3), imidazole (entry 4) or 1,2,4-triazole (entry 5), in the presence of catalytic copper(I) oxide, cesium carbonate, and dimethylsulfoxide (DMSO) at 110 °C for 24 h, the expected N-arylated azoles were obtained in 51 to 79% yields. As previously mentioned [22], such reactions work far less efficiently when performed on diiodides. Indeed, reacting the diiodide 1b' with pyrazole only gave the monofunctionalized derivative 1k', regardless of the amount of azole employed (Scheme 4).  As previously mentioned [22], such reactions work far less efficiently when performed on diiodides. Indeed, reacting the diiodide 1b' with pyrazole only gave the monofunctionalized derivative 1k', regardless of the amount of azole employed (Scheme 4).  As previously mentioned [22], such reactions work far less efficiently when performed on diiodides. Indeed, reacting the diiodide 1b' with pyrazole only gave the monofunctionalized derivative 1k', regardless of the amount of azole employed (Scheme 4).  As previously mentioned [22], such reactions work far less efficiently when performed on diiodides. Indeed, reacting the diiodide 1b' with pyrazole only gave the monofunctionalized derivative 1k', regardless of the amount of azole employed (Scheme 4).  Different amines and hydrazine reacted with 2b-I without recourse to catalyst (Table 5), affording the corresponding secondary amines 2n-p (entries 1-3) and arylhydrazine 2q (entry 4) in good yields. The latter was converted into the hydrazones 2r-u in the presence of aromatic aldehydes chosen for their ability to potentially interact with binding sites of biological interest [43] (Scheme 5). Finally, reaction of 2b-I with a phenol also proved possible without catalyst, giving the diaryl ether 2v in 64% yield (Scheme 6).      Different amines and hydrazine reacted with 2b-I without recourse to catalyst (Table 5), affording the corresponding secondary amines 2n-p (entries 1-3) and arylhydrazine 2q (entry 4) in good yields. The latter was converted into the hydrazones 2r-u in the presence of aromatic aldehydes chosen for their ability to potentially interact with binding sites of biological interest [43] (Scheme 5). Finally, reaction of 2b-I with a phenol also proved possible without catalyst, giving the diaryl ether 2v in 64% yield (Scheme 6). Different amines and hydrazine reacted with 2b-I without recourse to catalyst (Table 5), affording the corresponding secondary amines 2n-p (entries 1-3) and arylhydrazine 2q (entry 4) in good yields. The latter was converted into the hydrazones 2r-u in the presence of aromatic aldehydes chosen for their ability to potentially interact with binding sites of biological interest [43] (Scheme 5). Finally, reaction of 2b-I with a phenol also proved possible without catalyst, giving the diaryl ether 2v in 64% yield (Scheme 6).  Different amines and hydrazine reacted with 2b-I without recourse to catalyst (Table 5), affording the corresponding secondary amines 2n-p (entries 1-3) and arylhydrazine 2q (entry 4) in good yields. The latter was converted into the hydrazones 2r-u in the presence of aromatic aldehydes chosen for their ability to potentially interact with binding sites of biological interest [43] (Scheme 5). Finally, reaction of 2b-I with a phenol also proved possible without catalyst, giving the diaryl ether 2v in 64% yield (Scheme 6).  Different amines and hydrazine reacted with 2b-I without recourse to catalyst (Table 5), affording the corresponding secondary amines 2n-p (entries 1-3) and arylhydrazine 2q (entry 4) in good yields. The latter was converted into the hydrazones 2r-u in the presence of aromatic aldehydes chosen for their ability to potentially interact with binding sites of biological interest [43] (Scheme 5). Finally, reaction of 2b-I with a phenol also proved possible without catalyst, giving the diaryl ether 2v in 64% yield (Scheme 6).

Biological Activity
Some of the synthesized compounds were tested [44] for their antiproliferative activity in A2058 melanoma cells and proved to exert a modest to good activity ( Figure 2). The best results were Scheme 6. Conversion of 8-iodo-2,3-diphenylpyrido[2,3-b]pyrazine (2b-I) into ether 2v.

Biological Activity
Some of the synthesized compounds were tested [44] for their antiproliferative activity in A2058 melanoma cells and proved to exert a modest to good activity ( Figure 2). The best results were obtained with the 4-(trifluoromethyl)benzaldehyde hydrazone 2u and the 8-benzylamino pyrido[2,3-b]pyrazine 2o which induced~64% growth inhibition at 10 −5 M.

General Information
All the reactions were performed under a dry argon atmosphere. THF was distilled over sodium/benzophenone. Column chromatography separations were achieved on silica gel (40-63 μm). Melting points were measured on a Kofler apparatus. IR spectra were taken on an ATR Spectrum 100 spectrometer (Perkin-Elmer). 1 H-and 13 C-Nuclear Magnetic Resonance (NMR) spectra were recorded either on an Avance III spectrometer (291 K) at 300 MHz and 75 MHz, respectively, or on an Avance III HD spectrometer (298 K) at 500 MHz and 126 MHz, respectively (Bruker, Billevica, Massachussets, USA). 1 H chemical shifts (δ) are given in ppm relative to the solvent residual peak and 13 C chemical shifts are relative to the central peak of the solvent signal [46]. [25,26] and 7-bromo-2,3-diphenylpyrido[2,3-b] pyrazine (4a) [6] were prepared as reported previously. The biological activity assays were performed as reported previously [44].

Crystallography.
The X-ray diffraction data were collected either using an APEXII Bruker-AXS diffractometer (graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å)) for the compounds 1b' and 2i, or using a D8 VENTURE Bruker AXS diffractometer (multilayer monochromatized Mo-Kα radiation (λ = 0.71073 Å )) equipped with a (CMOS) PHOTON 100 detector for 2f, 2p, 3h and 2d, at the temperature given in the crystal data. For 1b' and 2i, the structure was solved by direct methods using SIR97 [47]. For 2f, 2p, 3h and 2d, they were solved by dual-space algorithm using the SHELXT program [48]. Structural refinements were performed with full-matrix least-square methods based on F 2 (SHELXL) [49]. In the case of 2f and 3h, the contribution of the disordered solvents to the calculated structure factors was estimated following the BYPASS algorithm [50], implemented as the SQUEEZE option in PLATON [51]; a new data set, free of solvent contribution, was then used in the final refinement. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. Except nitrogen linked hydrogen atom that was introduced in the structural model through Fourier difference maps analysis (2f, 2p, 3h), H atoms were finally included in their

General Information
All the reactions were performed under a dry argon atmosphere. THF was distilled over sodium/benzophenone. Column chromatography separations were achieved on silica gel (40-63 µm). Melting points were measured on a Kofler apparatus. IR spectra were taken on an ATR Spectrum 100 spectrometer (Perkin-Elmer). 1 H-and 13 C-Nuclear Magnetic Resonance (NMR) spectra were recorded either on an Avance III spectrometer (291 K) at 300 MHz and 75 MHz, respectively, or on an Avance III HD spectrometer (298 K) at 500 MHz and 126 MHz, respectively (Bruker, Billevica, Massachussets, USA). 1 H chemical shifts (δ) are given in ppm relative to the solvent residual peak and 13 C chemical shifts are relative to the central peak of the solvent signal [46] [25,26] and 7-bromo-2,3-diphenylpyrido [2,3-b] pyrazine (4a) [6] were prepared as reported previously. The biological activity assays were performed as reported previously [44].

Crystallography
The X-ray diffraction data were collected either using an APEXII Bruker-AXS diffractometer (graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å)) for the compounds 1b and 2i, or using a D8 VENTURE Bruker AXS diffractometer (multilayer monochromatized Mo-Kα radiation (λ = 0.71073 Å)) equipped with a (CMOS) PHOTON 100 detector for 2f, 2p, 3h and 2d, at the temperature given in the crystal data. For 1b and 2i, the structure was solved by direct methods using SIR97 [47]. For 2f, 2p, 3h and 2d, they were solved by dual-space algorithm using the SHELXT program [48]. Structural refinements were performed with full-matrix least-square methods based on F 2 (SHELXL) [49]. In the case of 2f and 3h, the contribution of the disordered solvents to the calculated structure factors was estimated following the BYPASS algorithm [50], implemented as the SQUEEZE option in PLATON [51]; a new data set, free of solvent contribution, was then used in the final refinement. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. Except nitrogen linked hydrogen atom that was introduced in the structural model through Fourier difference maps analysis (2f, 2p, 3h), H atoms were finally included in their calculated positions and treated as riding on their parent atom with constrained thermal parameters. The molecular diagrams were generated by ORTEP-3 (version 2.02) [52].

General Procedure 1
To a solution of 2,2,6,6-tetramethylpiperidine (0.51 mL, 3.0 mmol) in THF (3 mL) at 0 • C were successively added BuLi (about 1.6 M hexanes solution, 3.0 mmol) and, 15 min later, ZnCl 2 ·TMEDA [53] (0.25 g, 1.0 mmol). After 15 min at 0 • C, the pyrazine (2.0 mmol) was introduced, and the mixture was stirred for 2 h at rt before addition of I 2 (0.76 g, 3.0 mmol) in THF (3 mL) at 0 • C. The mixture was stirred at this temperature for 1 h before addition of an aqueous saturated solution of Na 2 S 2 O 3 (10 mL) and extraction with EtOAc (3 × 20 mL). The combined organic layers were dried over MgSO 4 , filtered and concentrated under reduced pressure. The crude product was purified by chromatography over silica gel (the eluent is given in the product description).
The crude product was purified by chromatography over silica gel (the eluent is given in the product description).