A Novel Method to Construct 2-Aminobenzofurans via [4 + 1] Cycloaddition Reaction of In Situ Generated Ortho-Quinone Methides with Isocyanides

A new approach for the synthesis of 2-aminobenzofurans has been described via Sc(OTf)3 mediated formal cycloaddition of isocyanides with the in situ generated ortho-quinone methides (o-QMs) from o-hydroxybenzhydryl alcohol. Notably, as a class of readily available and highly active intermediates, o-QMs were first used in the construction of benzofurans. This [4 + 1] cycloaddition reaction provides a straightforward and efficient methodology for the construction of 2-aminobenzofurans scaffold in good yield (up to 93% yield) under mild conditions.


Introduction
Benzofuran core, an important class of structural fragments, is widely distributed in natural products and biologically active compounds [1][2][3][4]. The benzofuran subunit is also present in a host of medicines, such as amiodarone, methoxypsoralen, dronedarone, etc [5]. Therefore, various methods for the preparation of benzofurans have been developed. As a special kind of functionalized benzofurans, 2-aminobenzofurans are of considerable interest and feature profound bioactivities, such as antifungal, P-glycoprotein inhibitors, anticancer activities, and tubulin polymerization inhibitors [6][7][8][9].
Although such structures are important, only limited methods have been reported for accessing 2-aminobenzofurans and the structural diversity of the products is insufficient. For example, in 2005, Ishikawa's group reported the synthesis of 2-aminobenzofurans from 1-aryl-2-nitroethylenes and cyclohexane-1,3-diones via a one-pot multistep strategy, but only moderate yield can be obtained. Moreover, unsymmetrical cyclohexane-1,3-diones have poor regiochemistry (Scheme 1a, Equation (1)) [10]. Soon after, Ohe and co-workers provided a new method to obtain 2-aminobenzofurans through palladium-catalyzed intramolecular cycloisomerization of 2-(cyanomethyl) phenyl ester; however, the substrate range is relatively limited (Scheme 1a, Equation (2)) [11]. In addition, Maurya's group also demonstrated the synthesis of very similar products (3-acyl-2-aminobenzofurans) via visible light-triggered intramolecular cyclization of α-azidochalcones (Scheme 1a, Equation (3)) [12]. In 2013, Cao's group developed a method for the synthesis of 3-alkyl-or 3-allenyl-2-amidobenzofurans by carbocation-induced electrophilic cyclization of o-anisolesubstituted ynamides (Scheme 1a, Equation (4)) [13]. In this method, the substituent on the benzene ring is fixed at the 5 position, and at least one electron withdrawing substituent is required on nitrogen. Finally, Kumar et al. reported strong base (tBuOK) mediated synthesis of 3-phenylbenzofuran-2-amines (one example) (Scheme 1a, Equation (5)) [14]. While those methods allow 2-aminobenzofurans to be obtained in an efficient way, new methods that can access a variety of structural skeletons under mild reaction conditions and from simple starting materials are still highly desired (Scheme 1b). efficient way, new methods that can access a variety of structural skeletons under mild reaction conditions and from simple starting materials are still highly desired (Scheme 1b). Scheme 1. Strategies for the diversified synthesis of 2-aminobenzofurans.
The initial experiment was conducted in CH 2 Cl 2 in the presence of various Brønsted acids, such as benzoic acid, TsOH and TfOH, at room temperature. It was observed that, except for benzoic acid, which only offered a trace amount of the desired product, both TsOH and TfOH provided the cycloaddition product 3a in roughly the same yield, even though the yield was relatively low (entries 1-3). Considering that isocyanide could be hydrolyzed under fairly strong acidic conditions [47], we replaced Brønsted acids with Lewis acids to further optimize reaction conditions. A range of Lewis acids, such as BF 3 ·Et 2 O, InCl 3 , and Sc(OTf) 3 , were then screened (entries 4-6). Among them, the desired cycloaddition product 3a could be obtained with 53% isolated yield when 0.5 equiv. of Sc(OTf) 3 was employed. To further improve the yield, different solvents, including THF, MeCN, and toluene, were also examined (entries 7−10). Toluene proved to be the best solvent for this transformation.  The initial experiment was conducted in CH2Cl2 in the presence of various Brønsted acids, such as benzoic acid, TsOH and TfOH, at room temperature. It was observed that, except for benzoic acid, which only offered a trace amount of the desired product, both TsOH and TfOH provided the cycloaddition product 3a in roughly the same yield, even though the yield was relatively low (entries 1-3). Considering that isocyanide could be hydrolyzed under fairly strong acidic conditions [47], we replaced Brønsted acids with Lewis acids to further optimize reaction conditions. A range of Lewis acids, such as BF3·Et2O, InCl3, and Sc(OTf)3, were then screened (entries 4-6). Among them, the desired cycloaddition product 3a could be obtained with 53% isolated yield when 0.5 equiv. of Sc(OTf)3 was employed. To further improve the yield, different solvents, including THF, MeCN, and toluene, were also examined (entries 7−10). Toluene proved to be the best solvent for this transformation.
Encouraged by these results, we investigated the effect of the loading of Sc(OTf)3. It was found that, when we increased the loading of Sc(OTf)3 from 0.5 equiv. to 1.0 equiv., the yield of 3a was improved to 75% (entry 11). However, when 1.2 equiv. of Sc(OTf)3 was used, the yield was reduced slightly (entry 12). It is noteworthy that the cycloaddition product 3a could be improved to 81% yield (entry 13) when the reaction was performed at 0 °C, but further cooling the temperature to −10 °C led to the yield's reduction to 69% (entry 14). Lastly, the addition a small number of 4 Å MS could increase the yield of 3a to 87% (entry 15).
With the optimized conditions in hand, a number of 2-aminobenzofurans were successfully obtained in moderate to excellent yields within 30  Encouraged by these results, we investigated the effect of the loading of Sc(OTf) 3 . It was found that, when we increased the loading of Sc(OTf) 3 from 0.5 equiv. to 1.0 equiv., the yield of 3a was improved to 75% (entry 11). However, when 1.2 equiv. of Sc(OTf) 3 was used, the yield was reduced slightly (entry 12). It is noteworthy that the cycloaddition product 3a could be improved to 81% yield (entry 13) when the reaction was performed at 0 • C, but further cooling the temperature to −10 • C led to the yield's reduction to 69% (entry 14). Lastly, the addition a small number of 4 Å MS could increase the yield of 3a to 87% (entry 15).
With the optimized conditions in hand, a number of 2-aminobenzofurans were successfully obtained in moderate to excellent yields within 30 min through the formal [4 + 1] cycloaddition of o-hydroxybenzhydryl alcohol (1a-1s) and p-nitrophenyl isocyanide 2a (Scheme 2). As shown in Scheme 2, both electron-donating substituents (3ba-3fa) and electron-deficient substituents (3ga-3ia) on the phenol were well tolerated in this formal [4 + 1] cycloaddition reaction and afforded the desired products in 70% to 84% yields. Obviously, the position of the substituents on phenol moiety had little influence on the reaction (3ba and 3ca). The structure of products was unambiguously confirmed by single-crystal X-ray analysis of 3ia (please see Supplementary Materials). Next, different substitutions on the benzyl phenol moiety were examined. We found that methyl substitution at the ortho-, metaand paraof benzyl alcohol moiety can afford the corresponding products (3ja, 3ka, and 3la) good to excellent yields. Strong electron-donating substituent (methoxy) in a different position was also converted smoothly into the desired 2-aminobenzofurans (3na and 3oa). Notably, the benzyl alcohol with high steric hindrance substitutions at ortho-position also efficiently underwent the formal [4 + 1] addition to provide corresponding products (3ma and 3pa) in 73% and 58% yields, respectively. Electron-withdrawing substituents, including F, Cl, and CF 3 , were also suitable for this transformation, providing the 2-aminobenzofurans with good results (3qa-3sa). From the above results, it can be concluded that both strong electron-donating substituents and electron-withdrawing substituents at the benzyl alcohol slightly reduce the yield; the yield of the product decreases slightly when high steric hindrance substitutions at ortho-position of the benzyl alcohol take place.
[4 + 1] cycloaddition reaction and afforded the desired products in 70% to 84% yields. Obviously, the position of the substituents on phenol moiety had little influence on the reaction (3ba and 3ca). The structure of products was unambiguously confirmed by single-crystal X-ray analysis of 3ia (please see Supplementary Materials). Next, different substitutions on the benzyl phenol moiety were examined. We found that methyl substitution at the ortho-, meta-and para-of benzyl alcohol moiety can afford the corresponding products (3ja, 3ka, and 3la) good to excellent yields. Strong electron-donating substituent (methoxy) in a different position was also converted smoothly into the desired 2-aminobenzofurans (3na and 3oa). Notably, the benzyl alcohol with high steric hindrance substitutions at ortho-position also efficiently underwent the formal [4 + 1] addition to provide corresponding products (3ma and 3pa) in 73% and 58% yields, respectively. Electronwithdrawing substituents, including F, Cl, and CF3, were also suitable for this transformation, providing the 2-aminobenzofurans with good results (3qa-3sa). From the above results, it can be concluded that both strong electron-donating substituents and electronwithdrawing substituents at the benzyl alcohol slightly reduce the yield; the yield of the product decreases slightly when high steric hindrance substitutions at ortho-position of the benzyl alcohol take place. We further evaluated the substrate scope of isocyanides (Scheme 3). A series of phenylisocyanides, with electron-withdrawing substituents at paraand meta-positions of the benzene ring, were smoothly converted to the corresponding products (4ab-4ad). However, for methyl substituted phenyl isocyanides, the yield decreased (4ae). Therefore, it can be inferred that electron-withdrawing substituents on phenyl isocyanides are beneficial to the formation of the product. β-Naphthyl isocyanide were employed in the transformation, offering the corresponding 2-aminobenzofurans in 83% yield (4af). Notably, alkyl isocyanides, including ethyl isocyanoacetate and tert-butyl isocyanide, could also smoothly transform to the desired cycloaddition products in 46% and 85% yields, respectively (4ag and 4ah), which enriched the diversity of structural skeletons.
We further evaluated the substrate scope of isocyanides (Scheme 3). A series of phenylisocyanides, with electron-withdrawing substituents at para-and meta-positions of the benzene ring, were smoothly converted to the corresponding products (4ab-4ad). However, for methyl substituted phenyl isocyanides, the yield decreased (4ae). Therefore, it can be inferred that electron-withdrawing substituents on phenyl isocyanides are beneficial to the formation of the product. β-Naphthyl isocyanide were employed in the transformation, offering the corresponding 2-aminobenzofurans in 83% yield (4af). Notably, alkyl isocyanides, including ethyl isocyanoacetate and tert-butyl isocyanide, could also smoothly transform to the desired cycloaddition products in 46% and 85% yields, respectively (4ag and 4ah), which enriched the diversity of structural skeletons.  We further evaluated the substrate scope of isocyanides (Scheme 3). A series of phenylisocyanides, with electron-withdrawing substituents at para-and meta-positions of the benzene ring, were smoothly converted to the corresponding products (4ab-4ad). However, for methyl substituted phenyl isocyanides, the yield decreased (4ae). Therefore, it can be inferred that electron-withdrawing substituents on phenyl isocyanides are beneficial to the formation of the product. β-Naphthyl isocyanide were employed in the transformation, offering the corresponding 2-aminobenzofurans in 83% yield (4af). Notably, alkyl isocyanides, including ethyl isocyanoacetate and tert-butyl isocyanide, could also smoothly transform to the desired cycloaddition products in 46% and 85% yields, respectively (4ag and 4ah), which enriched the diversity of structural skeletons.  Given that 2-aminobenzofurans have been proven to have a variety of biological activities, we decide to conduct in silico researches of the synthesized 2-aminobenzofurans to evaluate their drug-likeness, which were carried out using the SwissADME platform [52]. Satisfyingly, except for compounds 3ea, 3ma, 3pa, and 4ab-4af, other compounds were found to have good obedience (100%) with two drug-likeness filters (Lipinski [53] and Veber [54]) ( Table 2). In addition, some substituted (Cl, F, Br, CH 3 , OCH 3 ) 2-aminobenzofurans' pharmacokinetic properties were predicted through admetSAR [55], and it was found that these products showed a great range of average ADMET score [56,57] (0.68-0.74) with regard to human intestinal absorption, blood-brain barrier penetration, Caco-2 permeability, Ames mutagenicity, carcinogenicity, and acute oral toxicity class (Table 3). Finally, taking 4ae as an example, we predicted its possible molecular targets using SwissTargetPrediction [58,59]. The results show that it can act on multiple targets, such as nuclear receptor, family A-G protein-coupled receptor, etc., and the probability of prediction is around 10%.  In summary, we have developed a novel and efficient method for the acquisition of 2-aminobenzofuran derivatives via Sc(OTf) 3 -promoted [4 + 1] cycloaddition reaction of isocyanides with the in situ generated ortho-quinone methides (o-QMs) under mild conditions. In addition, o-QMs were first successfully used in this transformation and its advantage of this transformation is the simplicity of the reaction and the increased variety of 2-aminobenzofurans. Further exploration of the construction of other heterocyclics from o-QMs and applications of this product is in progress.

General Procedures
Unless otherwise noted, reagents were commercially available and were used without further purification. A 4 Å molecular sieve was pre-dried in an oven at 200 • C for 3 h Thin-layer chromatography (TLC) was performed using silica gel GF254 precoated plates (0.20 mm thickness). Visualization on TLC was achieved by UV light (254 nm). Column chromatography was performed on silica gel 90, 200-300 mesh. 1 H NMR and 13 C NMR spectra were recorded at 25 • C on a Bruker Avance 400 spectrometer ( 1 H: 400 MHz and 13 C: 101 MHz). 1 H NMR chemical shifts are reported in ppm (δ) relative to tetramethylsilane (TMS) with the solvent resonance employed as the internal standard (CDCl 3 , δ 7.26 ppm; DMSO-d 6 , δ 2.5 ppm). 13 C NMR chemical shifts were determined relative to the signal of the solvent: CDCl 3 at δ 77.00 ppm, DMSO-d 6 at δ 39.5 ppm. Data for 1H and 13 C NMR were recorded as follows: chemical shift (δ, ppm), multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet, q = quartet, dd = doublet of doublets, dt = doublet of triplets, td = triplet of doublets), coupling constants (Hz), and integration. ESI-HRMS spectra were recorded on a BioTOF Q instrument. Infrared (IR) spectra are obtained by the use of Spectrum One and expressed in wave number (cm −1 ). o-hydroxybenzhydryl alcohols 1a-1s [60] and isocyanides 2a-2f [51] were synthesized according to the previously reported.

Typical Procedure for Synthesis of 3aa
To a solution of p-nitrophenyl isocyanide 2a (0.2 mmol, 30 mg) in toluene (0.5 mL), we immediately added the o-hydroxybenzhydryl alcohols 1a (0.1 mmol, 20 mg), Sc(OTf) 3 (0.1 mmol, 49 mg) in toluene (0.5 mL) under N 2 in a Schlenck tube. The reaction mixture was stirred at 0 • C for 30 min. Upon completion, the reaction mixture was quenched with water, and then extracted with EtOAc and washed with brine. The combined organic phase was dried over anhydrous Na 2 SO 4 and the solvent was evaporated under vacuum. The crude product was purified using flash chromatography column eluting with (petroleum ether:ethyl acetate = 15:1) to obtain the product 3aa.

X-ray Crystallographic Data of 3ia
The crystal of 3ia for XRD analysis was prepared by recrystallization from the DMSO (see the supporting information for details). CCDC 1914402 containing the supplementary crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 5 July 2019). (remarks: The unit cell contains several 3ia and DMSO, which are weakly clustered together, but this does not affect the structural characterization of compound 3ia.) Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27238538/s1, Figure S1: X-ray molecular structure of 3ia; Table S1: Crystal data and structure refinement for 3ia; Figures