HETEROCYCLES

Short Paper | Regular issue | Vol. 92, No. 4, 2016, pp. 708-719
Received, 30th December, 2015, Accepted, 29th January, 2016, Published online, 16th February, 2016.
DOI: 10.3987/COM-15-13402

■ A Novel Solid-Phase Synthesis of 2H-Chromenes

E Tang,* Yinjiao Zhao, Meng Zhang, Xin Dai, Weilin Wang, and Deshou Mao*

School of Chemical Science and Technology, Yunnan University, No. 2 Green Lake North Road, Kunming 650091, China

Abstract

A method for synthesizing substituted 2H-chromenes using TMSOTf-catalyzed polystyrene-supported succinimidyl selenide-induced intramolecular seleno-arylation of tethered alkenes as a key step has been developed. The catalytic process provides an efficient method for the stereoselective and regioselective synthesis of 3,4-dihydro-2H-chromenes possessing a seleno-functionality, followed by syn-elimination of selenoxides to provide 2H-chromenes in good yields and with high purities.

The regioselective and stereoselective solid-phase synthesis of heterocycles and its application to the generation of drug-like molecules has attracted widespread attention.1 2H-Chromenes display a broad spectrum of biological activities2 such as anticancer, antioxidant, anti-inflammatory, antitubercular, antiviral, antitumor, antibacterial/antimicrobial, antidiabetic, anticoagulant, antianaphylatic, diuretic, fungicidal, and anti-HIV activity; additionally, the 2H-chromene substructure is an important structural motif present in a variety of medicines, natural products,3 and materials showing unique photophysical properties.4 Hence, the structural importance of the benzopyran moiety has elicited a great deal of interest in the field of organic synthesis and chemical biology to develop new and improved synthesis of these molecular skeletons. The synthesis of 2H-chromenes is structured around the three main approaches applied in catalytic 2H-chromene synthesis: (i) catalysis with (transition) metals,5 (ii) metal-free Brønsted and Lewis acid/base catalysis, which includes examples of nonenantioselective organocatalysis,6 and (iii) enantioselective organo-catalysis.7 Despite such synthetic advances, the original method of Iwai and Ide8 remains as a very useful method as it can provide the simplest 2H-chromene ring from an easily accessible phenol in just two steps. However, the post reaction workup is quite tedious, and it produces significant amount of environmentally unfriendly waste besides the poor yield. A method for the solid-phase synthesis of 2H-chromenes in good yields has been developed by Nicolaou et al.9 This approach was based on an electrophilic cyclization reaction via forming a carbon-heteroatom bond and using the resin-bounded selenenyl bromide.9 However, to the best of our knowledge, the solid-phase synthesis of 2H-chromenes via the selenium-induced and a carbon-carbon bond forming cyclization reaction has not been reported. In recent years, we have been keen to study the solid-phase synthesis of heterocyclic compounds,10 using organoselenium as a linker and the reagent since organoselenium compounds can be utilized as synthetic intermediates11 and selenium-carbon bond can be easily broken by various methods.12 Herein, we report an efficient solid-phase synthesis of 2H-chromenes via Lewis acid-catalyzed, polymer-supported selenium-mediated, and intramolecular carbon-carbon bond forming reaction, as well as the subsequent oxidative cleavage of selenium resins. Advantages of this method are easy post reaction workup operations, odorlessness, easy preparation of the substrates, and good yield of the product.
Firstly, the solid-phase cyclization of 1-​methoxy-​4-​[[(2E)​-​3-​phenyl-​2-​propen-​1-​yl]​oxy]benzene (3a)13 with polystyrene-supported selenenyl bromide (1)14 (Br: 0.99 mmol/g) was explored at -78 °C to 40 °C in dry CH2Cl2 for 48 h. But the selenium resin-bound annular intermediate 4a was not produced, since no product 5a was obtained by treatment of the selenium resin-bound intermediate with 30% H2O2 at 0 °C to room temperature. Inspired by Hajra’s work of Lewis acid-catalyzed intramolecular halo-arylation of tethered alkenes,15 we had successfully developed convenient solid-phase syntheses of quinolines10f and coumarins10e via TMSOTf-catalyzed intramolecular seleno-arylation of alkenes, using polystyrene- supported succinimidyl selenide (PSSS) as a selenium source. PSSS resin was prepared from dark-red polystyrene-supported selenenyl bromide (1)14 (Br: 0.99 mmol/g) sequentially via reduction reaction with NaBH4, alkylation reaction with allyl bromide, and amidation reaction with N-chlorosuccinimide (NCS). The reactions were monitored by FTIR (Scheme 1). With the help of Lewis acid, PSSS could react with

3a to form seleniranium ion intermediate 6a which was subsequently attacked by the intramolecular aromatic carbon-centered nucleophile from the anti-side to form a new carbon-carbon bond, and offer the annular product 4a (Scheme 2). Considering PSSS is sensitive to moisture, a one-pot synthesis of polymer-supported annular product 4 was employed. After completion of the reaction of the resin 2 with NCS, PSSS was directly washed by dry CH2Cl2 under nitrogen atmosphere without being removed from the reaction flask. PSSS was then treated with 10 mol% TMSOTf and 3a in dry CH2Cl2 at -78 °C for 2 h. And then the reaction mixture was kept at -20 °C for 4 h to afford 3-polystyrene-supported seleno-6-methoxy-4-phenyl-3,4-dihydro-2H-chromene (4a). The treatment of 4a with H2O2 in THF at 0 °C to room temperature gave 6-methoxy-4-phenyl-2H-chromene (5a) in 86% isolated yield and with more than 90 % purity (Scheme 2).

Furthermore, a range of cyclization reaction conditions involving 3a and PSSS were explored. The results are depicted in Table 1. This reaction was rather sluggish in the absence of catalyst, even under reflux conditions (Table 1, entry 1). An investigation using a series of Lewis and Brønsted acids identified TMSOTf as an active catalyst. Addition of double dose of TMSOTf and a prolonged reaction time did not improve the yield of 5a (Table 1, entries 11 and 12 ); the yield and the purity of 5a decreased when 5 mol% of TMSOTf was employed (Table 1, entry 10); the employment of 10 mol% BF3·Et2O, AlCl3, FeCl3, ZnCl2, TsOH, TFA, and HOAc afforded the product in very low yield when the cyclization reaction was performed at -78 °C for 2 h and then -20 °C for 4 h in dry CH2Cl2 (Table 1, entries 5-8, 15-17); less than 5% yield of the annular product was obtained when the reaction was performed only at -78 °C or at -20 °C (Table 1, entries 13 and 14); no product was obtained when other Lewis acids such as TiCl4, Sm(OTf)3, and AgOTf were used (Table 1, entries 2-4).

Mechanistically, it appears that TMSOTf activates PSSS by chelating to the amide carbonyl group to facilitate the formation of the seleniranium ion intermediate 6a from alkenes; the subsequent reaction with arene led to the formation of annular products 4a and the regeneration of the TMSOTf catalyst (Scheme 3).

Then the polystyrene-supported selenium-mediated cyclization reactions of a series of olefins 3 in the one-pot procedure were studied. The products 5 were obtained in good yields and with high purities via intramolecular cyclization reaction and syn-elimination of selenoxides. The results are summarized in Table 2. It is quite obvious that when R1, R2, R3, and R4 were H, bromo, electron-donating substituents such as alkoxyl and alkyl, the carbon-based annulation reaction proceeded smoothly to give annular compounds (Table 2, entries 1-15); good results were also obtained when R was alkyl, phenyl, electron-donating group-substituted phenyl, and electron-withdrawing group-substituted phenyl (Table 2, entries 1-9, 11-15); no annular products were obtained when R1, R2, R3, and R4 were electron-withdrawing substituents such as formyl (Table 2, entry 16); besides the annular product 5k was obtained in the yield of 60%, an unidentified by-product was found when R was H (Table 2, entry 10); both 7-methoxy-4-phenyl-2H-chromene (5d) and 5-methoxy-4-phenyl-2H-chromene (5e) were obtained from (E)-1-(cinnamyloxy)-3-methoxybenzene (3d) in the yields of 46% and 41% respectively (Table 2, entry 4). It is interesting that the TMSOTf-catalyzed PSSS-mediated cyclization reaction of compounds 3 gave rise to the six-membered cyclic compounds 4 as a result of 6-endo-trig cyclization of compounds 3. The five-membered cyclic compounds were not obtained.

In conclusion, we have developed a highly regioselective selenium-mediated intramolecular Friedel-Crafts alkylation of substituted 1-(allyloxy)benzene 3 using polymer-supported organoselenium reagent as a selenium source. Among the catalysts investigated, TMSOTf was found to be the best one. The target products were obtained in good yields and with good purities by the cleavage of the selenium linker. Furthermore, the easy workup procedure and easily prepared the substrates provide an approach that is well-suited for building the parallel libraries upon the basis of further transformation of polymer-supported seleno-3,4-dihydro-2H-chromene 4. The further modifications of resin 4 are currently ongoing.

EXPERIMENTAL
Melting points were measured with an X-6 micro-melting apparatus and were uncorrected. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a Bruker Avance 300 spectrometer in CDCl3 with TMS as the internal standard; chemical shifts were quoted in ppm and J values were given in Hz. IR spectra were recorded on a Thermo Nicolet Avatar 360 spectrometer. HRMS were performed on an Agllent LC/Msd TOF instrument. HPLC were run on an Agilernt 1100 High performance liquid chromatograph with a tunable UV detector. Dry CH2Cl2 and DMF were distilled from CaH2. Dry THF was distilled from Na. Purities and yields of the products were determined by the crude products and NMR, HRMS, and FTIR were determined by the purified products (the crude products were subjected to TLC on silica gel with AcOEt and light petroleum as eluent to give the purified products. Allyl polystyrene-supported selenide 210f and the substrates 313 were prepared according the literatures.

General procedure for the preparation of 3-polystyrene-supported seleno-3,4-dihydro-2H- chromene (4): To a suspension of the swollen resin 2 (1.0 g) in dry CH2Cl2 (15 mL) was added NCS (0.668 g, 5.0 mmol) at 0 °C. The mixture was stirred for 0.5 h at 0 °C and 2 h at room temperature. After filtrating and washing with dry CH2Cl2 (15 mL×3), PSSS resin was suspended with dry CH2Cl2 (15 mL) and cooled to -78 °C. Trimethylsilyl trifluoromethanesulfonate (0.022 g, 0.10 mmol) was added into the reaction mixture. After stirring for 0.5 h at -78 °C, substituted 1-(allyloxy)benzene 3 (5.0 mmol) was added into the reaction mixture under nitrogen atmosphere. The suspension was stirred for another 2 h at -78 °C and then stored in a freezer at -20 °C for 4 h. Saturated NaHCO3 aqueous solution (5 mL) was poured into the flask to quench the reaction. The resin 4 was collected by filtration, washed with THF (20 mL×2), Et2O (20 mL×2), THF/H2O (V/V = 3:1, 20 mL×2), H2O (20 mL×2), THF (20 mL×2), MeOH (20 mL×2), and CH2Cl2 (20 mL×2), and dried under vacuum.
General procedure for the preparation of 2H-chromenes 5: To a suspension of the swollen resin 4 in THF (15 mL) was added 30% aqueous H2O2 (1.2 mL). After stirring for 1 h at 0 °C and for another 20 min at room temperature, the mixture was filtered and the resin was washed with CH2Cl2 (15 mL×2). The filtrate was washed with H2O (30 mL×2), dried over MgSO4 and filtered. After removal of the solvents under reduced pressure, the crude products 5 were obtained and then subjected to column chromatography on silica gel (n-hexane/AcOEt (V/V) =10:1) to give the purified products 5.
6-Methoxy-4-phenyl-2H-chromene (5a)16: Light yellow solid, mp 66.3-68.6 °C; 1H NMR (300 MHz, CDCl3): δ = 7.43-7.34 (m, 5H), 6.85 (d, J = 8.7 Hz, 1H), 6.72 (dd, J = 8.7 Hz, 3.0 Hz, 1H), 6.58 (d, J = 3.0 Hz, 1H), 5.85 (t, J = 3.9 Hz, 1H), 4.79 (d, J = 4.2 Hz, 2H), 3.67 (s, 3H); IR (KBr): υmax = 1655, 1618, 1507, 1484, 1254, 1202 cm–1; HRMS m/z: calcd for C16H14O2 [M]+ 238.0994; found 238.0999.
8-Methoxy-4-phenyl-2H-chromene (5b)17: Light yellow solid, mp 70.4-72.1 °C; 1H NMR (300 MHz, CDCl3): δ = 7.39-7.30 (m, 5H), 6.84-6.76 (m, 2H), 6.65-6.61 (m, 1H), 5.79 (t, J = 3.9 Hz, 1H), 4.89 (d, J = 3.9 Hz, 2H), 3.88 (s, 3H); IR (KBr): υmax = 1646, 1614, 1454, 1401, 1263, 1168 cm–1; HRMS m/z: calcd for C16H14O2 [M]+ 238.0994; found 238.0983.
6,7-Dimethoxy-4-phenyl-2H-chromene (5c): Brown-orange oil; 1H NMR (300 MHz, CDCl3): δ = 7.42-7.31 (m, 5H), 6.58 (s, 1H), 6.54 (s, 1H), 5.68 (t, J = 3.9Hz, 1H), 4.76 (d, J = 3.9 Hz, 2H), 3.86 (s, 3H), 3.67 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 149.8, 149.5, 143.3, 138.4, 137.2, 128.5, 128.4, 127.8, 117.3, 115.6, 109.6, 100.8, 65.3, 56.6, 56.0; IR (KBr): υmax = 1654, 1612, 1505, 1454, 1401, 1169 cm–1; HRMS m/z: calcd for C17H16O3 [M]+ 268.1099; found 268.10990.
7-Methoxy-4-phenyl-2H-chromene (5d)18: Colorless oil; 1H NMR (300 MHz, CDCl3): δ = 7.36-7.18 (m, 6H), 6.93 (s, 1H), 6.46 (s, 1H), 5.61 (t, J = 3.9Hz, 1H), 4.78 (d, J = 3.9 Hz, 2H), 3.81 (s, 3H); IR (KBr): υmax = 1655, 1613, 1596, 1479, 1444, 1381, 1319, 1266, 1140, 1032 cm–1; HRMS m/z: calcd for C16H14O2 [M]+ 238.0994; found 238.0993.
5-Methoxy-4-phenyl-2H-chromene (5e): Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.27-7.24 (m, 5H), 6.84 (d, J = 8.4 Hz, 1H), 6.40 (d, J = 2.4 Hz, 1H), 6.32 (dd, J = 2.4 Hz, 8.4 Hz, 1H), 5.56 (t, J = 3.9Hz, 1H), 4.74 (d, J = 3.9 Hz, 2H), 3.69 (s, 3H); 13C NMR (75 MHz, CDCl3): δ = 160.6, 156.1, 138.5, 137.0, 128.6, 128.4, 127.7, 126.7, 117.0, 106.9, 102.0, 65.6, 55.4; IR (KBr): υmax = 1655, 1614, 1498, 1446, 1270, 1198, 1160, 1116, 1032 cm–1; HRMS m/z: calcd for C16H14O2 [M]+ 238.0994; found 238.0982.
5,7,8-Trimethyl-4-phenyl-2H-chromene (5f): Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.35-7.19 (m, 5H), 6.57 (s, 1H), 5.88 (t, J = 4.8 Hz, 1H), 4.55 (d, J = 4.8 Hz, 2H), 2.24 (s, 3H), 2.17 (s, 3H), 1.68 (s, 3H); 13C NMR (75 MHz, CDCl3): δ = 154.3, 141.6, 139.1, 137.7, 132.3, 128.3, 127.6, 127.1, 125.9, 121.6, 121.5, 121.1, 64.4, 22.3, 20.0, 11.8; IR (KBr): υmax = 1664, 1601, 1449, 1112, 1061 cm–1; HRMS m/z: calcd for C18H18O [M]+ 250.1358; found 250.1368.
6-(tert-Butyl)-4-phenyl-2H-chromene (5g): Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.38-7.34 (m, 5H), 7.18 (d, J = 2.4 Hz, 8.4 Hz, 1H), 7.07 (d, J = 2.1 Hz, 1H), 6.84 (d, J = 8.4 Hz, 1H), 5.78 (t, J = 3.9Hz, 1H), 4.79 (d, J = 3.9 Hz, 2H), 1.20 (s, 3H); 13C NMR (75 MHz, CDCl3): δ = 152.7, 143.9, 138.5, 137.6, 128.7, 128.5, 127.9, 127.1, 126.1, 123.0, 120.0, 115.7, 65.3, 34.3, 31.5; IR (KBr): υmax =1656, 1612, 1504, 1451, 1399 cm–1; HRMS m/z: calcd for C19H20O [M]+ 264.1514; found 264.1510.
6,8-Di(tert-butyl)-4-phenyl-2H-chromene (5h): Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.36 (s, 5H), 7.23 (s, 1H), 6.92 (s, 1H), 5.87 (t, J = 3.9 Hz, 1H), 4.73 (d, J = 3.9 Hz, 2H), 1.42 (s, 9H), 1.21 (s, 9H); 13C NMR (75 MHz, CDCl3): δ = 151.0, 142.7, 139.1, 138.3, 137.1, 128.7, 128.2, 127.6, 124.0, 123.7, 121.1, 119.6, 64.0, 34.8, 34.4, 31.5, 30.0; IR (KBr): υmax = 1654, 1611, 1503, 1449, 1400 cm–1; HRMS m/z: calcd for C23H28O [M]+ 320.2140; found 320.2146.
8-(tert-Butyl)-6-methyl-4-phenyl-2H-chromene (5i): Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.33-7.31 (m, 5H), 6.99(d, J = 1.8 Hz, 1H), 6.69(d, J = 1.5 Hz, 1H), 5.81 (t, J = 3.9Hz, 1H), 4.67 (d, J = 3.9 Hz, 2H), 2.17 (s, 3H), 1.40 (s, 9H); 13C NMR (75 MHz, CDCl3): δ = 151.2, 139.1. 138.1, 137.9, 129.6, 128.8, 128.6, 128.4, 127.7, 127.5, 124.9, 124.5, 120.1, 64.1, 34.6, 30.0, 21.2; IR (KBr): υmax = 1653, 1612, 1452, 1401 cm–1; HRMS m/z: calcd for C20H22O [M]+ 278.1671; found 278.1679.
8-Isopropyl-5-methyl-4-phenyl-2H-chromene (5j): Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.30-7.20 (m, 5H), 7.05 (d, J = 7.8 Hz, 1H), 6.70 (d, J = 8.1 Hz, 1H), 5.93 (t, J = 4.5 Hz, 1H), 4.53 (d, J = 4.8 Hz, 2H), 3.36-3.23 (m, 1H), 1.71 (s, 3H), 1.25 (d, J = 6.9 Hz, 6H); 13C NMR (75 MHz, CDCl3): δ =153.6, 141.6, 139.1, 133.9, 133.1, 128.4, 127.6, 127.2, 125.8, 124.5, 123.6, 122.6, 64.2, 27.2, 22.9, 22.5; IR (KBr): υmax = 1654, 1613, 1453, 1401, 1168, 1083 cm–1; HRMS m/z: calcd for C19H20O [M]+ 264.1514; found 264.1505.
6-Methoxy-2H-chromene (5k)19: Colorless oil; 1H NMR (300 MHz, CDCl3): δ = 6.76-6.61 (m, 2H), 6.54 (d, J = 2.4 Hz, 1H), 6.40 (d, J = 9.3 Hz, 1H), 5.81 (dt, J = 9.3, 4.2 Hz, 1H), 4.76 (dd, J = 4.2, 2.4 Hz, 2H), 3.75 (s, 3H). IR (NaCl): υmax = 2981, 2826, 1474, 1265, 1210, 1150 cm–1; HRMS m/z: calcd for C10H10O2 [M]+ 162.0681; found 162.0670.
4-(3’,5’-Dimethoxyphenyl)-6-methyl-2H-chromene (5l)20: Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 6.85 (d, J = 8.2 Hz, 1H), 6.80 (s, 1H), 6.74 (d, J = 8.0 Hz, 1H), 6.41 (s, 3H), 5.71 (t, J = 3.9 Hz, 1H), 4.70 (d, J = 4.0 Hz, 2H), 3.71 (s, 6H), 2.11 (s, 3H); IR (neat): υmax = 1606, 1490, 1454, 1420, 1351, 1220, 1204, 1152 cm–1; HRMS (APCI) m/z: calcd for C18H19O3 [M+H]+ 283.1334; found 283.1325.
4-(4’-Methoxy-2’-nitrophenyl)-6-methyl-2H-chromene (5m)20: Brown oil; 1H NMR (300 MHz, CDCl3): δ = 7.50 (d, J = 2.7 Hz, 1H), 7.27 (d, J = 8.7 Hz, 1H), 7.16 (dd, J = 8.6 Hz, 2.7 Hz, 1H), 6.93 (d, J = 8.1 Hz, 1H), 6.76 (d, J = 8.1 Hz, 1H), 6.37 (s, 1H), 5.66 (t, J = 3.9 Hz, 1H), 4.81 (m, 2H), 3.90 (s, 3H), 2.11 (s, 3H); IR (neat): υmax = 1622, 1530, 1490, 1341, 1269, 1224, 1125 cm–1; HRMS (ESI) m/z: calcd for C17H15NNaO4 [M+Na]+ 320.0899; found 320.0889.
4-(4-Fluorophenyl)-6-methyl-2H-chromene (5n)20: Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.40-7.28 (m, 2H), 7.15-7.11 (t, J = 8.7 Hz, 2H), 7.03-6.70 (dd, J = 8.1, 1.8 Hz, 1H), 6.86-6.81 (d, J = 8.1 Hz, 1H), 6.76 (d, J = 1.8 Hz, 1H), 5.77 (t, J = 4.2 Hz, 1H), 4.80 (d, J = 4.2 Hz, 2H), 2.20 (s, 3H); IR (neat): υmax = 2836, 1731, 1509, 1481, 1346, 1297, 1173 cm–1; HRMS (APCI) m/z: calcd for C16H14FO [M+H]+ 241.1029; found 241.1021.
8-Bromo-7-methoxy-4-(p-tolyl)-2H-chromene (5o)20: Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 7.30 (m, 4H), 6.90 (d, J = 8.6 Hz, 1H), 6.40 (d, J = 8.6 Hz, 1H), 5.64 (t, J = 3.9 Hz, 1H), 4.96 (d, J = 3.9 Hz, 2H), 3.86 (s, 3H), 2.37 (s, 3H); IR (neat): υmax = 2960, 2843, 1601, 1480, 1456, 1348, 1272 cm–1; HRMS (APCI) m/z: calcd for C17H16BrO2 [M+H]+ 331.0334; found 331.0322.
6-Methoxy-4-methyl-2H-chromene (5p)20: Yellow oil; 1H NMR (300 MHz, CDCl3): δ = 6.79–6.67 (m, 3H), 5.61 (dd, J = 3.6, 1.8 Hz, 1H), 4.68 (dd, J = 3.6, 1.8 Hz, 2H), 3.76 (s, 3H), 2.00 (dd, J = 3.3, 1.8 Hz, 3H); IR (neat): υmax = 2830, 1581, 1493 cm−1; HRMS m/z: calcd for C11H12O2 [M ]+ 176.0837; found 176.0828.

ACKNOWLEDGEMENTS
This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT13095) and the Natural Science Foundation of China (NSFC 21162032).

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