Construction of Unusual Indole-Based Heterocycles from Tetrahydro-1H-pyridazino[3,4-b]indoles

Herein, we report the successful syntheses of scarcely represented indole-based heterocycles which have a structural connection with biologically active natural-like molecules. The selective oxidation of indoline nucleus to indole, hydrolysis of ester and carbamoyl residues followed by decarboxylation with concomitant aromatization of the pyridazine ring starting from tetrahydro-1H-pyridazino[3,4-b]indole derivatives lead to fused indole-pyridazine compounds. On the other hand, non-fused indole-pyrazol-5-one scaffolds are easily prepared by subjecting the same C2,C3-fused indoline tetrahydropyridazines to treatment with trifluoroacetic acid (TFA). These methods feature mild conditions, easy operation, high yields in most cases avoiding the chromatographic purification, and broad substrate scope. Interestingly, the formation of indole linked pyrazol-5-one system serves as a good example of the application of the umpolung strategy in the synthesis of C3-alkylated indoles.

Following our sustained efforts toward the construction of novel azaheterocycles, especially around privileged structures [31], we have recently disclosed a zinc-catalyzed synthesis of polycylic fused indoline scaffolds through a substrate-guided reactivity switch [32]. With this as a background, we herein describe a successful procedure providing the fused indole-pyridazine scaffold of type c (Figure 1)through oxidation and hydrolysis processes from previously synthesized tetrahydro-1H-pyridazino [3,4-b]indole compounds (1). Surprised to observe a ring-opening/ring-closing pathway during the C2-C3 indole oxidation step, a facile and robust access to unusual non-fused derivative indole-pyrazol-5-ones 4 starting from the same cycloadducts (1) has been also achieved in excellent yields.

Results and Discussion
In a preliminary experiment, the conversion of tetrahydro-1H-pyridazino [3,4-b]indole 1b to fully aromatic 9H-pyridazino [3,4-b]indole 3b was attempted by a two-step process consisting of decarboxylation of the ester motif with removal of carbamoyl residue followed by oxidation of pyridazine ring under basic conditions. However, although removal of the ester with alcoholic KOH efficiently took place affording a mixture of CH/NH tautomers, unexpectedly pyridazine ring aromatization did not occur. With this failure in hand, we first turned our attention to the crucial C2-C3 indole oxidation step. A variety of oxidizing agents (e.g., DDQ [33], BQ, NBS/TBPB, MnO 2 [34][35][36], Na 2 Cr 2 O 7 , I 2 , PCC [37], Pd/C [38], CAN [39] and CuCl 2 ·2H 2 O [40]) was investigated (Table 1). Common reagents such as BQ, Na 2 Cr 2 O 7 , PCC, Pd/C, I 2 , and NBS in combination with TBPB, although frequently used for oxidation of the indoline ring, proved unsuccessful. With stoichiometric DDQ in dioxane, only trace of the aromatic indole-pyridazine 2b was formed (Table 1, entry 1). The dehydrogenations reaction yield could be further improved by using toluene and CH 2 Cl 2 as solvents (Table 1, entries 2 and 3). Manganese dioxide was reported in the literature to be suitable for the oxidation of indolines to indoles [34][35][36]. To our surprise, when compound 1b was subjected to treatment with MnO 2 in benzene at 70 • C, the sole demethylated indole derivative 2b' was isolated in 55% yield [41] (see Supplementary Materials). Reducing the amount of MnO 2 displayed an incomplete reaction, decreasing the reaction yield (40% yield) of the obtained product (Table 1, entries 6 and 7).
On the other hand, the use of oxidizing systems such as ceric ammonium nitrate (CAN) and catalytic copper (II) chloride dihydrate in DMSO did not afford the expected product 2b, providing instead the non-fused indole-pyrazole 4c (Table 1, entries 12 and 13).
With these results in hand, also tetracylic compounds incorporating 6 and 8 membered fused ring such as 1a and 1c were subjected to optimal oxidation conditions to obtain the relative compounds 2a/2a' and 2c' respectively (Scheme 1). Intrigued by the possibility of obtaining fully aromatic derivatives, the treatment of representative 6, 7, and 8 membered tetrahydro-1H-pyridazino [3,4-b]indole 2a, 2b', 2c' with alcoholic KOH was also carried out. To our delight, the formation of cyclo-fused pyridazino [3,4-b]indoles 3a, 3b' and 3c' (via hydrolysis of ester and carbamoyl residues followed by decarboxylation with concomitant aromatization of the pyridazine ring) was registered with success (43-68% yields) (Scheme 1).  With these results in hand, also tetracylic compounds incorporating 6 and 8 membered fused ring such as 1a and 1c were subjected to optimal oxidation conditions to obtain the relative compounds 2a/2a' and 2c' respectively (Scheme 1). Intrigued by the possibility of obtaining fully aromatic derivatives, the treatment of representative 6, 7, and 8 membered tetrahydro-1H-pyridazino [3,4-b]indole 2a, 2b', 2c' with alcoholic KOH was also carried out. To our delight, the formation of cyclo-fused pyridazino [3,4-b]indoles 3a, 3b' and 3c' (via hydrolysis of ester and carbamoyl residues followed by decarboxylation with concomitant aromatization of the pyridazine ring) was registered with success (43-68% yields) (Scheme 1). Encouraged by the unexpected formation of non-fused indole-pyrazol-5-one 4c during the investigation for indoline oxidation to indole, we next focused our attention to exploring this transformation. We identified that the use of trifluoroacetic acid (TFA) as common Brønsted acid induced the ring-opening of the cycloadduct 1b to give non-fused N-polyheterocyclic compound 4c in excellent yield (95%). It was found that all the tetracyclic compounds embedding 6, 7 and 8 membered-ring well tolerated the acidic environment (Scheme 2). Changing the substituents (alkyl, benzyl) on the N-indole ring, the reaction proceeded satisfactorily (4f, 4g, and 4i). Also, the free NH-indole was proven to be a good candidate for this reaction furnishing the relative products 4h, 4j, and 4p in very excellent yields. As expected, conversion to NH-pyrazol-5-one 4e obtained by removal of the t-butoxycarbonyl (Boc) protective group was observed under such conditions. The wide functional groups tolerance of this procedure was validated from the introduction of electron-donor (4i, and 4j) or electron-attractor (4k, 4l, 4m, 4n) substituents on the aromatic component which led to the corresponding indole linked pyrazol-5-one systems in almost quantitative yields. Interesting to note that no purification of the obtained products by flash chromatography column was necessary because of their precipitation from the reaction medium. A plausible mechanism could involve TFA-induced ring-opening of the tetrahydropyridazine, followed by sequential intramolecular nucleophilic acyl substitution (ring-closing) to form the final product accompanied by elimination of an alcohol molecule. This pathway reflects the well-established tendency of polycyclic fused indoline structures without 3-substituents to rearomatization [42]. Also, the formation of NH tautomeric form of II previously described by our group [32] supports this. Encouraged by the unexpected formation of non-fused indole-pyrazol-5-one 4c during the investigation for indoline oxidation to indole, we next focused our attention to exploring this transformation. We identified that the use of trifluoroacetic acid (TFA) as common Brønsted acid induced the ring-opening of the cycloadduct 1b to give non-fused N-polyheterocyclic compound 4c in excellent yield (95%). It was found that all the tetracyclic compounds embedding 6, 7 and 8 membered-ring well tolerated the acidic environment (Scheme 2). Changing the substituents (alkyl, benzyl) on the N-indole ring, the reaction proceeded satisfactorily (4f, 4g, and 4i). Also, the free NH-indole was proven to be a good candidate for this reaction furnishing the relative products 4h, 4j, and 4p in very excellent yields. As expected, conversion to NH-pyrazol-5-one 4e obtained by removal of the t-butoxycarbonyl (Boc) protective group was observed under such conditions. The wide functional groups tolerance of this procedure was validated from the introduction of electron-donor (4i, and 4j) or electron-attractor (4k, 4l, 4m, 4n) substituents on the aromatic component which led to the corresponding indole linked pyrazol-5-one systems in almost quantitative yields. Interesting to note that no purification of the obtained products by flash chromatography column was necessary because of their precipitation from the reaction medium. A plausible mechanism could involve TFA-induced ring-opening of the tetrahydropyridazine, followed by sequential intramolecular nucleophilic acyl substitution (ring-closing) to form the final product accompanied by elimination of an alcohol molecule. This pathway reflects the well-established tendency of polycyclic fused indoline structures without 3-substituents to rearomatization [42]. Also, the formation of NH tautomeric form of II previously described by our group [32]  To illustrate the synthetic potential of this transformation, a one-pot reaction involving the in situ formation of [4+2] cycloadduct 1b from N-methyl indole A1 and cyclic azoalkene B1 [32] was also performed. The reaction proceeded very well, and desired product 4c was obtained in 93% overall yield (Scheme 3). To illustrate the synthetic potential of this transformation, a one-pot reaction involving the in situ formation of [4+2] cycloadduct 1b from N-methyl indole A1 and cyclic azoalkene B1 [32] was also performed. The reaction proceeded very well, and desired product 4c was obtained in 93% overall yield (Scheme 3). To date, only one example of the synthesis of compound of type 4 was documented in the literature [43]. In their work, Shi and co-workers realized an umpolung of C3 indole reactivity, using 2-indolylmethanols (C) as an electrophile and pyrazol-5-ones (D) as a nucleophile to obtain, under the cooperative catalysis of Pd(0) and a chiral phosphoric acid, the strategic C-C bond formation.
Differently from Shi's work, in our case the same C-C bond formation is realized exploiting the reversal of polarity of the azoalkene (B) system ("umpoled" of carbonyl compounds) [44][45][46][47]. Taking advantage of the intrinsic reactivity of the pertinent substrates, a new and complementary synthetic approach toward these unusual N-polyheterocyclic structures can be successfully achieved (Scheme 4). We believe that the present method is of operational interest in terms of forming otherwise challenging-to-prepare C−C bonds.

General
All the commercially available reagents and solvents were used without further purification. Tetrahydro-1H-pyridazino[3,4-b]indoles 1 were prepared via a formal [4+2] cycloaddition reactions of 2,3-unsubstituted indoles with cyclic azoalkenes by known procedures [32]. Chromatographic purification of compounds was carried out on silica gel (60-200 μm). Thin-layer chromatography (TLC) analysis was performed on pre-loaded (0.25 mm) glass supported silica gel plates (Silica gel 60, F254, Merck; Darmstadt, Germany); compounds were visualized by exposure to UV light. To date, only one example of the synthesis of compound of type 4 was documented in the literature [43]. In their work, Shi and co-workers realized an umpolung of C3 indole reactivity, using 2-indolylmethanols (C) as an electrophile and pyrazol-5-ones (D) as a nucleophile to obtain, under the cooperative catalysis of Pd(0) and a chiral phosphoric acid, the strategic C-C bond formation.
Differently from Shi's work, in our case the same C-C bond formation is realized exploiting the reversal of polarity of the azoalkene (B) system ("umpoled" of carbonyl compounds) [44][45][46][47]. Taking advantage of the intrinsic reactivity of the pertinent substrates, a new and complementary synthetic approach toward these unusual N-polyheterocyclic structures can be successfully achieved (Scheme 4). We believe that the present method is of operational interest in terms of forming otherwise challenging-to-prepare C−C bonds. To date, only one example of the synthesis of compound of type 4 was documented in the literature [43]. In their work, Shi and co-workers realized an umpolung of C3 indole reactivity, using 2-indolylmethanols (C) as an electrophile and pyrazol-5-ones (D) as a nucleophile to obtain, under the cooperative catalysis of Pd(0) and a chiral phosphoric acid, the strategic C-C bond formation.
Differently from Shi's work, in our case the same C-C bond formation is realized exploiting the reversal of polarity of the azoalkene (B) system ("umpoled" of carbonyl compounds) [44][45][46][47]. Taking advantage of the intrinsic reactivity of the pertinent substrates, a new and complementary synthetic approach toward these unusual N-polyheterocyclic structures can be successfully achieved (Scheme 4). We believe that the present method is of operational interest in terms of forming otherwise challenging-to-prepare C−C bonds.
All 1 H-NMR and 13 C-NMR spectra were recorded at 400 and 100 MHz, respectively at 25 • C on a Bruker Ultrashield 400 spectrometer (Bruker, Billerica, MA, USA). Proton and carbon spectra were referenced internally to residual solvent signals as follows: δ = 2.50 ppm for proton (middle peak) and δ = 39.50 ppm for carbon (middle peak) in DMSO-d 6 and δ = 7.27 ppm for proton and δ = 77.00 ppm for carbon (middle peak) in CDCl 3 . The following abbreviations are used to describe peak patterns where appropriate: s = singlet, d = doublet, t = triplet q = quartet, m = multiplet and br = broad signal. All coupling constants (J) are given in Hz. FT-IR spectra were measured as Nujol mulls using a Nicolet Impact 400 (Thermo Scientific, Madison, WI, USA). Low-resolution mass spectra (LRMS) was performed on a Waters Micromass Q-ToF instrument (Waters, Milford, MA, USA) using an ESI source. Elemental analyses were within ± 0.4 of the theoretical values (C, H, N).

Two-
Step Procedure for the Synthesis of 3

Procedure for the Oxidation of Indolines 1 to Indoles 2
To a stirred solution of compound 1 (0.5 mmol) in benzene (1 mL), MnO 2 (25 eq) was added and the reaction was subsequently warmed up to 70 • C (oil bath). The reaction mixture was kept at this temperature until the reagent had been completely consumed as monitored by TLC (20 h). The crude mixture was then filtered and purified by column chromatography on silica gel using hexane-ethyl acetate as the eluent to afford product 2 (or 2').

Procedure for the Preparation of Fused Indole-Pyridazine 3 from 2
To a stirred solution of KOH (10 eq.) in alcohol (3 mL), the compound 2 (or 2') (0.2 mmol) was added and the mixture was refluxed until the disappearance of the reagent (TLC check, 7 h). The crude mixture was then filtered and purified by column chromatography on silica gel to afford product 3(or 3'). To a stirred solution of compound 1 (0.2 mmol) in methylene chloride (2 mL), TFA (2.5 eq) was added at room temperature. The reaction was refluxed until TLC indicated the disappearance of the reagent (TLC check, 2 h). Removed the solvent under reduced pressure, the crude mixture was diluted with water and extracted with EtOAc (2 × 10 mL). The organic phase was dried with Na 2 SO 4 and solvent was evaporated in vacuo. The compound 4 was collected by precipitation as a white solid, filtered, and washed with ethyl ether.    (2 × 10 mL). The organic phase was dried with Na 2 SO 4 and solvent was evaporated in vacuo. The crude mixture was purified by column chromatography on silica gel to afford product 4c in 93% yield.

Conclusions
In conclusion, successful routes providing indole-based heterocycles such as fused indole-pyridazines and non-fused indole-pyrazol-5-ones have been accomplished. Mild and practical reaction conditions, wide substrate scope in conjunction with functional group tolerance make these protocols particularly attractive. Moreover, crude products are usually obtained in high purity and high yield by simple precipitation from the reaction medium. We expect that these methodologies and the chemistry described here would be a new addition to the indole chemistry and would find wide usage in both organic and medicinal chemistry.