8798 J. Org. Chem. 2009, 74, 8798–8801 Published on Web 10/23/2009 DOI: 10.1021/jo9015634 r 2009 American Chemical Society pubs.acs.org/joc
Synthesis of 3-Alkoxymethylcoumarin from
3-Cyanochromene via a Novel Intermediate
2-Phenylimino-3-alkoxymethylchromene
Jui-Chi Tsai,†,‡,)Sie-Rong Li,†,‡,)Michael Y. Chiang,§Lian-Yeu Chen,†,‡Po-Yuan Chen,†
Yi-Fang Lo,†Chen-Hao Wang,†Chun-Nan Lin,‡and Eng-Chi Wang*,†
†Faculty of Medicinal and Applied Chemistry and‡Institute of
Pharmaceutical Sciences, Kaohsiung Medical University, Kaohsiung City 807, Taiwan, and§Department of Chemistry,
National Sun Yat-Sen University, Kaohsiung City 804, Taiwan. )These authors contributed equally to the paper.
[email protected] Received July 22, 2009
In this paper a concise, efficient, and environmentally benign method for the synthesis of 3-alkoxymethylcou-marin is described. From the reaction of 3-cyanochro-mene with an alkoxide and arylamine in THF, (Z)-2-phenylimino-3-alkoxymethylchromene was obtained as a novel intermediate via an isomerization of the double bond, a 1,2-addition of alkoxide, a Michael-type addition of aniline, an another isomerization of double bond and an elimination of ammonia. Subsequently, the intermedi-ate was converted into the desired coumarin by treatment with 15% HCl in THF in good yield.
Coumarins, which are chemical derivatives of benzo-2-pyrones or chromen-2-ones, are widely distributed in various species of plants.1,2 In addition, diverse approaches have been used to synthesize numerous artificial coumarins. Both naturally occurring and synthetic coumarins have attracted intensive attention from chemists due to their broad spectral properties and potential for biological activities. For exam-ple, the bioactivity of coumarins includes steroid sulfatase inhibitory activity,3 lipid-lowering activity,4
anticarcino-genic activity,5fungicidal activity,6acaricidal activity,7 po-tential anticonvulsant and analgesic activities,8 as well as other activities.9 The major synthetic methods for the pre-paration of coumarins include the Pechmann reaction and its modifications,10 the Knoevenagel condensation,11
the Wittig reaction,12 the Claisen rearrangement,13 the Vilsmeier-Haack and Suzuki cross-coupling reactions,14 Pd-catalyzed site-selective cross-coupling reactions,15as well as other reactions. Despite the large number of syntheses reported, the synthesis of 3-alkoxymethylcoumarins has gained little attention. Up to the present, only two methods which were reported included the chloromethylation of coumarin, followed by the reaction with alcohol16aand the reaction of coumarin with an chloromethyl ether.16b
How-ever, only a few target compounds were synthesized in these reported methods. Other drawbacks include the tedious reaction condition, long reaction time, and low overall yield. The recent increased interest is due to the potential antic-ancer activity exhibited by some coumarin-3-N-arylsulfona-mides17 and coumarin-3-carboxamides.18 Thus, develop-ment of a concise, unique, and efficient synthesis for 3-alkoxymethylcoumarins is requisite and significant from both chemical and biological viewpoints. Herein, we disclose
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a complete new method for the synthesis of 3-alkoxymethyl-coumarins via a novel intermediate.
Our synthetic strategy involves the reaction of 3-cyano-chromene with an alkoxide and aniline in either THF or alcohol to yield various 2-phenylimino-3-alkoxymethyl-2H-chromenes as intermediates. The resultant intermediates were treated, respectively, with 15% HCl in THF to yield the title compounds (Scheme 1).
3-Cyanochromenes 2a-f, which were prepared according to previous reports,9b,19,20were reacted with sodium alkox-ide 3a-d and arylamine 4a,b to generate 2(Z)-phenylimino-3-alkoxymethylchromenes 5a-j stereospecifically. In order to optimize the yield of 5, compounds 2a-d were studied in a model reaction using sodium alkoxide 3 and aniline (4a) in either refluxing THF or anhydrous alcohol. The results are summarized in Table 1. From the results, THF was found to be a better solvent than anhydrous alcohol. The alkoxide was also varied, and it was found that the reactivities followed the trend n-butoxide > isopropoxide > ethoxide > methoxide. The proposed reaction mechanism for the generation of compound 5 is shown below (Scheme 2). 3-Cyanochromene (2) initially undergoes alkoxide-mediated isomerization to form I, which is subsequently converted to II by attack of the alkoxide on the cyano group. TheR,γ-unsaturated function-ality of II then undergoes a Michael addition by the soft base to afford the enamine in chromane III. With the existing alkoxide in solution, theπ-bond of intermediate III migrates to form IV, which has both enamine and hemiaminal func-tionality. Due to the formation of a hydrogen bond between enamine hydrogen and the hemiaminal nitrogen, the lone-pair electrons of nitrogen on enamine functional group moves and pushes π electrons into the chromene ring to eliminate ammonia to generate V. Through the sequential
steps described above, the oxygen of the chroman ring and the N-aryl group in the structure V located on the same side is established. In order to clarify if the NH2 or OR group is
participating in the hydrogen bonding to the HNC6H5in IV,
the heats of formation of both rotomers were calculated by PM3. The conformation of IV which a hydrogen bond between the NH2and HNC6H5showed a length of 1.870 A˚
with heat of formation of Hf = -7.04157 kcal/mol. The
other conformation in which the hydrogen bond is between the OR and HNC6H5showed a length of 1.911 A˚ with heat of
formation of Hf= -5.98868 kcal/mol. The
energy-mini-mized structures of compounds IV and IV0 are shown in SCHEME1. Synthesis of 3-Alkoxymethylcoumarin via a Novel
Intermediate
SCHEME2. Proposed Reaction Mechanism of the Formation of Compound 5 from 2
TABLE1. Results of the Reaction of Compounds 2a-d with a Sodium Alkoxide 3 and Aniline (4a) in Refluxing Alcohol or THFa
entry compd R0ONa/solventb time (h)reactionc (% yield)productd
1 2a MeONa/MeOH 3 5a (48) 2 2a MeONa/THF 3 5a (65) 3 2b EtONa/EtOH 2 5b (62) 4 2b EtONa/THF 2 5b (79) 5 2c i-PrONa/i-PrOH 1 5c (68) 6 2c i-PrONa/THF 1 5c (82) 7 2d n-BuONa/n-BuOH 1 5d (73) 8 2d n-BuONa/THF 1 5d (86) a
Reaction conditions: 2 (5.0 mmol), 3 (10 mmol), and 4a (7.5 mmol) dissolved in ROH (30 mL) or THF (30 mL) was heated to reflux.bTHF is
dried from sodium and benzophenone; alcohols are dried from anhy-drous calcium oxide.cReaction time is determined by checking the
formation of product and almost consumption of starting material by TLC.dPercent yield is determined by isolating the product from column
chromatography.
(19) Kaye, P. T.; Nocanda, X. W. J. J. Chem. Soc., Perkin Trans. 1 2002, 10, 1318–1323.
(20) (a) Mouysset, G.; Payard, M.; Grassy, G.; Tronche, P.; Dabire, H.; Mouille, P.; Schmitt, H. Eur. J. Med. Chem. 1987, 22, 539–544. (b) Larsen, M.; Kromann, H.; Kharazmi, A.; Nielsen, S. F. Bioorg. Med. Chem. Lett. 2005, 15, 4858–4861.
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Tsai et al.Figure 1. The results of the PM3 calculation are consistent with the proposed mechanism requiring the conformation shown in compound IV. In the final step of the mechanism, the double bond of intermediate V undergoes an alkoxide-mediated isomerization to provide conjugation with the benzene ring, thereby forming stable product 5.
Since the N-phenyl group is on the same side of chro-mene as the oxygen, 5 is obtained stereospecifically in the Z-configuration. For the structural determination of com-pounds 5a-j, the data of the1H NMR,13C NMR, EI-MS,
and HRMS or elemental analyses were acquired and corro-borate the proposed structures. The structure of 3-ethoxy-methyl-2(Z)-(4-methylphenyl)imino-2H-chromene (5g) which was crystallized from dichloromethane and n-hexane was further confirmed by X-ray crystallography. The final struc-tural model was refined by full-matrix, least-squares proce-dures to give an R1value of 0.041 and an Rwof 0.122. The
molecules in the yellow prismatic crystal (0.8 0.7 0.5 mm) crystallized in space group P21/m with lattice parameters a =
9.856(2) A˚, b = 7.395(2) A˚, c = 11.096(2) A˚,β = 101.07(2)°, V= 793.7(3) A˚3, and Z = 2. The data were collected on a Rigaku AFC7S diffractometer with graphite-monochronated Mo KR (0.71073 A˚) radiation. The selected data of bond lengths (A˚) and bond angles (deg) of 5g are depicted in Table 2, respectively.
The crystal structure of 5g contains a chroman ring with an N-aryl and an ethoxymethyl group. The crystallographic plane where the chroman ring and the ethoxymethyl group lie cuts the perpendicular N-aryl group in half. The double bond of C(1)dN(1) is in the Z-configuration. The ORTEP diagram of 5g is shown in Figure 2.21
In conclusion, we have established a novel reaction for the formation of 3-alkoxymethyl-2(Z)-(4-aryl)imino-2H-chro-mene 5a-j from 3-cyanochro3-alkoxymethyl-2(Z)-(4-aryl)imino-2H-chro-mene (2), an alkoxide 3, and arylamine 4 through multiple mechanistic steps including isomerization of the double bond, a 1,2-addition of alkoxide, a Michael-type addition of arylamine, another isomerization of the double bond, and an elimination of ammonia. We initially converted compound 5 into a series of 3-alkoxy-methylcoumarins 6a-j by treating 5a-j with 15% HCl in THF resulting in yields of 68-87%. All prepared coumarins have satisfactory spectral data such as1H NMR,13C NMR, EI-MS, and HRMS or elemental analysis. In conclusion, we have established a novel route for the synthesis of 3-alkoxy-methyl-2(Z)-(4-aryl)imino-2H-chromene 5a-j independent of previous studies. Furthermore 5 can be smoothly used in the synthesis of 3-alkoxymethylcoumarins 6a-j. In addition, arylamine, which is a known environmental pollutant, can be recovered, thereby providing an environmentally friendly synthetic route.
Experimental Section
General Procedure for the Preparation of Substituted 3-Alkoxy-methyl-2-phenyliminochromenes 5a-j. Under the protection of dried argon, to a solution of 3-cyano-2H-chromene 2a-f (5.0 mmol) in anhydrous THF (30 mL) were added sodium alkoxide (10.0 mmol) 3a-d and arylamine 4a,b (7.5 mmol). The reaction mixture was stirred and heated at reflux for 1-3 h until TLC analysis showed the consumption of the starting material. The resultant reaction mixture was concentrated in vacuo to remove the solvent. The residue was purified by silica gel column chromatography (ethyl acetate/n-hexane = 1:20) to give 5a-j. The physical and spectral data of selected 5a and 5b are illustrated as follows.
3-Methoxymethyl-2(Z)-phenylimino-2H-chromene (5a). Com-pound 5a (0.86 g, 65%) was obtained as yellow crystals: mp 64-65 °C; Rf= 0.51 (ethyl acetate/n-hexane = 1: 6); IR (KBr) ν 2936, 2869, 1644, 1585, 1487, 1451, 1403, 1225, 1180, 1115, 1058, 761, 705 cm-1;1H NMR (400 MHz, CDCl3)δ 3.53 (s, 3H), 4.45 (d, 2H, J = 1.6 Hz), 7.00 (d, 1H, J = 8.0 Hz), 7.06-7.12 (m, 2H), 7.18-7.21 (m, 2H), 7.25 (td, 1H, J = 7.2, 1.6 Hz), 7.29 (dd, 1H, J= 7.6, 1.6 Hz), 7.32-7.36 (m, 3H); 13C NMR (100 MHz, CDCl3)δ 59.0, 69.6, 115.3, 119.8, 122.9, 123.6, 123.7, 127.1, 128.5, 128.9, 129.6, 129.7, 146.0, 147.7, 152.2; MS (EI) 265 (Mþ, 10), 250 (100), 235 (27). Anal. Calcd for C17H15NO2: C, 76.96; H, 5.70; N,
5.28. Found: C, 76.93; H, 5.68; N, 5.22.
3-Ethoxymethyl-2(Z)-phenylimino-2H-chromene (5b). Com-pound 5b (1.10 g, 79%) was obtained as yellow crystals: mp FIGURE1. Conformation of energy-minimized compounds IV
and IV0.
TABLE2. Selected Bond Length (A˚) and Bond Angles (deg) of Com-pound 5g
bond length bond angle
atom-atom length atom angle
O(1)-C(1) 1.374 (2) C(1)-O(1)-C(9) 121.46(14) O(1)-C(9) 1.382 (2) C(10)-O(2)-C(11) 112.26(16) O(2)-C(10) 1.408 (2) C(1)-N(1)-C(13) 121.06(16) O(2)-C(11) 1.417 (2) N(1)-C(1)-O(1) 120.01(17) N(1)-C(1) 1.266 (3) N(1)-C(1)-C(2) 121.57(17) N(1)-C(13) 1.424 (2) O(1)-C(1)-C(2) 118.42(16) C(1)-C(2) 1.458 (3) C(3)-C(2)-C(1) 119.94(18) C(2)-C(3) 1.332 (3) C(3)-C(2)-C(10) 124.27(18) C(3)-C(4) 1.444 (3) C(1)-C(2)-C(10) 115.79(16)
FIGURE2. Molecular plot of 3-ethoxymethyl-2-(4-methylphenyl)-imino-2H-chromene (5g).
(21) The CIF file has been deposited at the Cambridge Crystallographic Centre (CCDC), UK, and received the CCDC no. 680478.
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71-72 °C; Rf= 0.56 (ethyl acetate/n-hexane = 1:6); IR (KBr) ν 2978, 2861, 1640, 1583, 1486, 1449, 1386, 1227, 1182, 1116, 1064, 758, 703 cm-1;1H NMR (400 MHz, CDCl3)δ 1.37 (t, 3H, J= 6.8 Hz), 3.74 (q, 2H, J = 6.8 Hz), 4.53 (d, 2H, J = 2.0 Hz), 7.04 (d, 1H, J = 8.0 Hz), 7.11-7.16 (m, 2H), 7.21-7.24 (2H, m), 7.29 (1H, td, J = 8.0, 1.2 Hz), 7.34-7.41 (m, 3H), 7.42 (t, 1H, J= 1.6 Hz);13C NMR (100 MHz, CDCl 3)δ 15.6, 67.1, 67.8, 115.7, 120.2, 123.2, 123.9, 124.0, 127.5, 128.9, 129.6, 129.9, 130.0, 146.4, 148.2, 152.6; MS (EI) 279 (Mþ, 0.3), 250 (100), 236 (49), 235 (58), 233 (47). Anal. Calcd for C18H17NO2: C,
77.40; H, 6.13; N, 5.01. Found: C, 77.46; H, 6.21; N, 4.82. General Procedure for the Preparation of 3-Alkoxymethylcou-marins 6a-j. 3-Alkoxymethyl-2-phenyliminochromene 5a-j (1.0 mmol) dissolved in THF (10 mL) was stirred and cooled in an ice bath. To the cooled solution was added 15% HCl (20 mL) in drops. The reaction mixture was continually stirred at room temperature for 1 h. To the resultant mixture was added saturated NaCl solution (50 mL) and the mixture extracted with dichloromethane (50 mL 3). The organic extracts were combined and washed with NaHCO3(50 mL 3), dried with
anhydrous MgSO4, and filtered. The filtrate was concentrated in
vacuo to remove the solvent to give a crude crystal. The resultant crystal was recrystallized from ethyl acetate and n-hexane to yield pure coumarin (6a-j). The physical and spectral data of selected 6a and 6b are illustrated as follows.
3-Methoxymethylcoumarin (6a). Compound 6a (0.13 g, 68%) was obtained as colorless crystals: mp 72-73 °C (lit.16a mp
126°C, lit.16bmp 70°C); R f= 0.33 (ethyl acetate/n-hexane = 1:5); IR (KBr)ν 2957, 2919, 1713, 1603, 1454, 1394, 1168, 1115, 1043, 1010, 925, 780, 630 cm-1;1H NMR (400 MHz, CDCl3) δ 3.52 (s, 3H), 4.41 (d, 2H, J = 1.6 Hz), 7.28 (td, 1H, J = 7.6, 0.8 Hz), 7.34 (d, 1H, J = 8.4 Hz), 7.48-7.53 (m, 2H), 7.78 (t, 1H, J= 1.6 Hz);13C NMR (100 MHz, CDCl3)δ 59.1, 69.0, 116.5, 119.2, 124.5, 126.0, 127.7, 131.1, 138.2, 153.1, 160.4; MS (EI) 190 (Mþ, 2), 175 (68), 160 (100), 103 (41). Anal. Calcd for C11H10O3: C, 69.46; H, 5.30. Found: C, 69.21; H, 5.42.
3-Ethoxymethylcoumarin (6b). Compound 6b (0.15 g, 73%) was obtained as colorless crystals: mp 95-96 °C; Rf = 0.38
(ethyl acetate/n-hexane = 1: 5); IR (KBr) v 2970, 2924, 2863, 2341, 1717, 1605, 1574, 1447, 1384, 1283, 1172, 1116, 1061, 919, 756, 630 cm-1;1H NMR (400 MHz, CDCl3)δ 1.31 (t, 3H, J = 7.0 Hz), 3.68 (q, 2H, J = 7.0 Hz), 4.46 (d, 2H, J = 1.6 Hz), 7.28 (td, 1H, J = 7.6, 1.2 Hz), 7.34 (d, 1H, J = 8.0 Hz), 7.48-7.52 (m, 2H), 7.81 (t, 1H, J = 1.6 Hz);13C NMR (100 MHz, CDCl3) δ 15.2, 66.9, 66.9, 116.5, 119.2, 124.4, 126.4, 127.7, 131.0, 138.1, 153.1, 160.5; MS (EI) 175 ([M- (C2H5)]þ, 53), 160 (100), 132
(51), 131 (42). Anal. Calcd for C12H12O3: C, 70.57; H, 5.92.
Found: C, 70.60; H, 5.95.
Acknowledgment. The financial support from the National Science Council (grant no. NSC 97-2113-M-037-001-MY2) of Taiwan is grateful. We also thank the National Center of High-Performance Computing for computer time and facil-ities. Furthermore, we thank the anonymous reviewers for their valuable comments to improve the quality of this work. Supporting Information Available: General experimental methods; characterization data for 4a-f, 5c-j, and 6c-j; copies of1H and13C NMR spectra; and crystallographic information files (CIFs) of 5g are available. This material is available free of charge via the Internet at http://pubs.acs.org.
