One-pot synthesis of indenobenzofurans via tandem Michael addition-elimination and palladium-catalysed C-H activation

A tandem, one-pot reaction of 2,3-dibromoindenone with phenols is reported, which provides a straightforward approach to accomplish structurally diverse indenobenzofurans in excellent yields. Besides, the control experiments revealed that reaction proceeds through a two-step sequence via an intermolecular Michael addition-elimination reaction, which enables the C − O bond formation, followed by palladium-catalyzed intramolecular C − C bond formation through C − H activation. Significantly, this protocol offers a convenient approach to synthesize the diverse tetracyclic indenobenzofurans


Introduction
Indenone core structure is omnipresent in various important natural products and pharmaceuticals. 1,2For instance, natural products such as onychnine, 3 euplectin, 4 neo-lignin, 5 and indotecan (LMP400) 6 contain the indanone core moiety (Figure 1).The synthetic molecules endowed with an indenone framework have a wide range of biological activities such as anti-HIV, 7 cytotoxicity, 8 phosphodiesterase inhibition, 9 antiinflammatory, 10 and adenosine A2A receptor antagonists. 11Besides, indenone fused heterocyclic substructures are attractive pharmacomodulations where O-containing heterocycles such as furans and benzofurans play an important role in the architecture of polycyclic heterocyclic frames, such as the tricyclic indenofurans, which are key substructures for natural product solanacol, 12 ramelteon, 13 and (-)galiellalactone. 14][21][22][23][24][25][26][27][28] These indenone fused heterocycles drew our attention because of their importance as privileged scaffolds in drug discovery.Therefore, we have become fascinated by their extensive properties, as a result, we have reported the synthesis and biological applications of indanones, [29][30][31][32] which encouraged us to further develop in the indenone core skeletoned heterocycles.The significance of palladium-catalyzed C−H activation has been demonstrated in many classical organic transformations in the formation of C−C bonds. 33To the best of our knowledge, threre are no reports which involve the synthesis of indenobenzofurans via the palladium catalyzed C−H activation as a key sequence.5][36][37][38][39][40][41][42] In this context, herein, we have developed a convenient method to furnish benzofuran fused indenone heterocycles using 2,3-dibromoindenones and phenols.
Our synthetic strategy involved Michael addition-elimination followed by C−H activation in a one pot protocol.In general, 2,3-dibromoindenone is a good Michael acceptor wherein the β-position is electrondeficient to facilitate nucleophilic addition.Therefore, initially a base promoted Michael addition-elimination between 2,3-dibromoindenone and phenol is anticipated.Subsequently, the palladium-catalyzed ortho-C−H activation of the phenolic ring accompanied by cyclization to get indenobenzofurans is planned in a one-pot fashion.

Results and Discussion
We initiated our preliminary investigation with 2,3-dibromoindenone 1a (0.1736 m.mol), and phenol 2a (0.1736 m.mol) as model substrates and carried out the reaction in the presence of K2CO3 (1 equiv.),Pd(OAc)2 (10 mol%), PPh3 (20 mol%), in DMF at room temperature (entry 1, Table 1).However, the desired product 3a was not obtained; instead, we ended up isolating the intermediate 4a in 87% yield.Next, the reaction is carried out at a higher temperature of 80 °C with K2CO3 and Cs2CO3 (entries 2-3, Table 1) for 24 h.Pleasingly, the expected product indenobenzofuran 3a was isolated, though in a poor yield of 23% and 19%, respectively.
The formation of 3a indicated that the reaction was proceeding via Michael addition-elimination followed by an intramolecular C−C bond formation via C−H activation of phenolic ortho position, therefore, we have screened various reaction conditions to get this effective transformation in better yield.
Next, the reaction is performed at an elevated temperature of 120 °C, in different solvents such as DMF, DMA, and toluene by taking K2CO3 as a base (entries 4-6, Table 1).It was found that toluene furnished the expected product 3a in a moderate yield of 60%, therefore, further optimization studies were carried out in toluene as solvent (entry 6, Table 1).In addition, altering the quantity of base from 1 to 3 equivalents has further improved the yield of the tetracyclic product 3a up to 72% (entry 7, Table 1).Subsequently, we have screened various bases such as KOAc, DBU, NaH, K3PO4 and Cs2CO3 (entries 8-12, Table 1).Delightfully, the reaction with 3 equivalents of Cs2CO3 as a base at 120 °C has provided the highest yield of the requisite product 3a in 90%, under the short reaction time of 3 h.Besides, the reaction was checked with 1.5 equivalents of Cs2CO3 (entry 13, Table 1), where the yield of 3a was isolated in 65%.Next, the reaction was attempted at the elevated temperature 140 °C in toluene and xylene as solvents.However, the yield is not improved and has furnished the final product 3a in 25 and 10% (entries 14-15, Table 1).Further decreasing the catalyst loading Pd(OAc)2 to 5 mol% has caused the reaction to be sluggish, as a result, the reaction time is increased from 3 h to 16 h to complete the reaction and has offered the tetracyclic product 3a in 86% of yield (entry 16, Table 1).a Reaction conditions: 1a (0.17 mmol), 2a (0.17 mmol), Pd(OAc)2 (10 mol%), PPh3 (20 mol%), base (equiv), solvent (1 mL) under N2 atmosphere.b Pd(OAc)2 (5 mol%), PPh3 (10 mol%).c Isolated yields.d Yields in the parenthesis represent the intermediate 4a.
After having an optimized reaction condition, the scope of the reaction was explored through which various indenobenzofurans have been synthesized by treating 2,3-dibromoindenone with various phenols (Scheme 1).All the synthesized indenobenzofurans were obtained in good to excellent yields.Besides, the unambiguous confirmation of 3i was achieved by using single crystal X-ray data analysis.

Scheme 2. Synthesis of indenobenzofurans 3aa-3ae.
As anticipated, strong electron-withdrawing substrates like 4-nitrophenol and 2-nitrophenol could not yield the expected tetracyclic moiety but rather afforded an addition product.However, phenolic substrates containing electron-withdrawing groups like -CHO, -COCH3, F, and Cl gave good yields.Also, the substrate scope was checked using different substituted 2,3-dibromoindenones which also gave good yields (Scheme 2).
Interestingly, the compounds 3c, 3i, 3m, 3n, and 3r were obtained as a single regioisomer, although there is a possibility of formation of one more regioisomers in each of these cases.The structural assignment for these compounds was done based on the internal consistency in the 1 H NMR as depicted in Figure 2. Appearence of a diagnostic singlet at 7.24 ppm (s, H A ) in 3c and two singlets in 3i at 7.25 ppm (s H A ) and 7.46 ppm (s, H B ) strongly supportive of their structures which was further corroborated by the unambiguous structural assignment by single crystal x-ray data of 3i.The same diagnostic pattern was observed for 3m, showing two singlets at 7.03 ppm (s, H A ) and 7.13 ppm (s, H B ).In the case of 3n, two singlets were detected at 7.01 ppm (s, H A ) and 7.11 ppm (s, H B ). Similarly, for the compound 3r two singlets were observed at 7.93 ppm (s, H A ) and 8.19 ppm (s, H B ). Generally, 2-naphthol is know to react at C1 and hence there is a possibility of the formation of the other isomer 3r'.However, we found only a single regioisomer 3r, which is further correlated with two singlets in the parent naphtho[2,3-b]furan 5a 44 , whereas in the regioisomeric naphtho[2,1-b]furan 5b 45 two doublets were observed (Figure 2).To gain insight into the reaction mechanism, a few control experiments were performed to identify the pathway of the reaction mechanism.Initially, in presence of base phenoxide nucleophile attacks at the βposition of the 2,3-dibromoindenone which acts as Michael acceptor where the β-position is found to be the electron-deficient center and in a later stage the bromine gets eliminated as a leaving group.By this addition elimination reaction, we isolated compound 4a in good yield (Scheme 3), which was further treated under the standard condition which involves intramolecular C ̶ H activation to form C ̶ C bond to get the cyclized product, which was obtained with an excellent yield of 99% (Scheme 3).

Scheme 3. Control experiments.
This confirms that the reaction involving Michael addition elimination reaction is the first step.Next, the palladium would play a key role in the subsequent intramolecular C−C bond formation (Scheme 4).Initially, substrate 4 would undergo oxidative addition to deliver the intermediate (I), which then forms the sixmembered palladacycle (II) by C−H activation at the ortho position of the phenolic ring.In the final step, the desired compound 3a was obtained through reductive elimination.

Conclusions
An efficient route to synthesize indenobenzofurans has been developed using easily accessible starting materials which involve C−O bond formation via Michael addition-elimination reaction and C−C bond formation by intramolecular C−H activation strategy, in a one-pot fashion.Also, a plausible mechanism is proposed and supported based on the control experiments.

Experimental Section
General.IR spectra were recorded on an FTIR spectrophotometer. 1 H NMR spectra were recorded on a 400 MHz Bruker spectrometer in CDCl3; chemical shifts (δ ppm) and coupling constants (Hz) are reported in a standard fashion about either internal standard tetramethylsilane (TMS) (δH 0.00 ppm) or CHCl3 (δH 7.26 ppm). 13C NMR spectra were recorded on a 100 MHz spectrometer in CDCl3; chemical shifts (δ ppm) are reported relative to CHCl3 [δC 77.00 ppm (central line of triplet)].In the 1 H NMR, the following abbreviations were used throughout: s = singlet, d = doublet, t = triplet, q = quartet, qui = quintet, dd = doublet of doublets, m = multiplet and br.s.= broad singlet.The assignment of signals was confirmed by 1 H, 13 C, and DEPT spectra.High-resolution mass spectra (HR-MS) were recorded using electron spray ionization (ESI); Melting points were determined on an electrothermal melting point apparatus.Phenols were purchased from commercially, 2,3dibromoindenone was prepared using known protocaol 29,43 .All reactions were performed in oven-dried apparatus under an N2 atmosphere.Commercial grade solvents were distilled before use.all reactions were monitored by thin-layer chromatography (TLC) on microscopic slides coated with silica gel and visualization of spots was accomplished by exposure to UV radiation or iodine vapor.Compounds were purified using flash chromatography.

Table 1 .
Optimization of reaction conditions a