Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans

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Article Cite This: ACS Omega 2017, 2, 7525-7531

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Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening Nagendra Nath Yadav,† Hyeonsu Jeong,‡ and Hyun-Joon Ha*,‡ †

Department of Chemistry, North Eastern Regional Institute of Science and Technology, Nirjuli, Arunachal Pradesh 791109, India Department of Chemistry, Hankuk University of Foreign Studies, Yongin 17035, Korea

‡

S Supporting Information *

ABSTRACT: Multisubstituted furans were prepared from dialkyl 2(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3· OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.

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metallic catalysts including Ag, In, and Pt.9 However, a metalfree method is highly desired to avoid metal impurities in the synthetic products, especially in biologically relevant compounds.10 A few methods other than the Paal−Knorr furan synthesis were developed for the synthesis of substituted furans without metal catalysts.6c−e,11 Most of them proceeded in an intramolecular fashion with strong acid or iodine as reagents. Some of the reactions required a prerequisite of highly reactive intermediates including silyl enol ether, acid chloride, and so forth.12 Thereby, a mild and metal-free synthetic method for the preparation of multisubstituted furan is highly desirable for further development of this useful heterocycle. A few previous synthetic methods with the breakage of highly strained cyclopropane or epoxide have been reported in the literature based on the cycloisomerization.8,9 All of these methods proceed by the intermediacy of A, which is more or less similar to the reactive intermediate derived from the acyclic starting material. Oxygen from the carbonyl group of the starting substrate adjacent to cyclopropane or ring-opened epoxide attacks the activated alkynic or allenic appendage with the assistance of metal catalysts such as Cu, Ag, Au, and In.5−9 However, in this study, we developed a completely different method. The suitably disposed carbonyl oxygen ended in the furan ring attacks nonactivated aziridine which is activated by BF3·OEt2 followed by aromatization to yield furan. This strategy is depicted as B in Figure 2. The cyclization with the opening of aziridine for a new heterocyclic compound was observed in our early investigations.13 However, it has not been shown to make a new ring through the cyclization with the breakage of the aziridine ring in an intramolecular fashion. This extraordinary reaction pathway involving cyclization by heteroatoms at the tether of the starting material with

INTRODUCTION Furans as oxygen-containing five-membered heterocycles are an important class of biologically active compounds.1,2 They are found in nature and in various pharmaceuticals as well as in flavor and fragrance.3 Synthesis of multisubstituted furans is important not only because of their presence in various biologically active compounds but also because they may serve as key synthetic precursors.4 Especially, 2-aminomethylfurans are found in many important drugs and natural products (Figure 1).2 2-Amino-methylated furans are well accessible

Figure 1. Drugs and natural products containing the 2-aminomethylfuran motif.

from readily available furan building blocks such as furfural via reductive amination reaction, but the synthesis of substituted 2aminomethylfurans requires multiple steps.1a,2g−i Owing to their importance, ample examples are reported in the literature detailing the synthesis of substituted furans. Most of the synthetic methods are based on the addition of oxygen to the activated alkene,5 alkyne,6 or allene.7 Opening of the cyclopropane ring and cycloisomerization in inter- or intramolecular fashion are also well-documented in the literature.8 The majority of synthetic methods require metals as reagents or catalysts such as Ag, Pd, Rh, Cu, Pt, and Fe. A few methods starting from epoxides have also been reported, requiring © 2017 American Chemical Society

Received: October 13, 2017 Accepted: October 24, 2017 Published: November 1, 2017 7525

DOI: 10.1021/acsomega.7b01542 ACS Omega 2017, 2, 7525−7531

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OEt2 and FeCl3 yielded the expected product, and BF3·OEt2 was the most effective. Table 1. Formation of Multisubstituted Furans (3a) from 2(Aziridin-2-ylmethylene)malonate (2a) with Various Lewis Acids Figure 2. Highly substituted furan (1) and its general synthetic method involving metal-mediated cyclization (A) compared to the method (B) derived from the breakage of an aziridine ring.

concomitant breakage of the highly strained ring shows a possibility to develop new methods for the preparation of various heterocycles. The key for the success is the breaking of the aziridine ring by the nearby oxygen. Normally, most of the “activated” aziridines readily react with the incoming nucleophile at the less hindered site without any controllable regiochemical diversity.14 Another chemical method to make ring opening easy is to design the starting substrate with olefin for allylic activation.15 Even though we have studied aziridine for quite a long time,13 we were not able to open the “unactivated” aziridine rings with the internal oxygen so far because of the intrinsic inertness of the ring opening. Aziridine in this synthesis is “unactivated” with the electron-donating substituent at the ring nitrogen and is quite stable and inert to almost all nucleophiles.15,16 These “unactivated” aziridines should be activated before they react with nucleophiles.17 However, they have a big advantage on the regiochemical diversity, that is, their ring can be opened either at C2 or at C3 and is completely controllable by the selection of activators and nucleophiles.15−17

entry

Lewis acid (1.05 equiv)

time (h)

yield (%)

1 2 3 4 5

BF3·OEt2 AlCl3 CeCl3 I2 FeCl3

3.0 12.0 4.0 12 4.5

85 0 0 0 36

Several different aziridines were utilized for the intramolecular ring-opening reaction, leading to the synthesis of various aminomethylfurans in good to moderate yields (Table 2). The optimized reaction protocol was applied for the starting materials with various substituents, that is, R1 at the nitrogen of 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione including Table 2. Synthesis of Multisubstituted Furans (3) from 2(Aziridin-2-ylmethylene)malonate or 1,3-Dione (2) with BF3·OEt2a

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RESULTS AND DISCUSSION At first, we prepared diethyl 2-(aziridin-2-ylmethylene)malonate (2a) having suitably disposed carbonyl oxygen located near aziridine being ready to attack for the ring opening. Initially, aziridinyl malonate (2a) was treated with 1.05 equiv of BF3·OEt2 and sodium acetate in CH2Cl2 at room temperature (rt) for 4 h to expect the usual intermolecular aziridine ring-opening product 3a′. Much to our surprise, only intramolecular aziridine ring-opening product 3a was formed in 85% yields without furnishing any traces of the product arising from the intermolecular ring opening (Scheme 1). Once the structure of furan 3a was confirmed, the same reaction was performed without using external nucleophile sodium acetate and the same results were observed as above. To improve the reaction yield for the conversion of aziridines to furan derivatives, various Lewis acids such as AlCl3, CeCl3, I2, and/or FeCl3 were screened (Table 1). Among them, only BF3· Scheme 1. Synthesis of Multisubstituted Furan 3a from Diethyl 2-[((R)-(1-(R)-(1-Phenylethyl)aziridin-2yl)methylene)]malonate (2a)

a Reaction conditions: 1,3-dione 2 (1.0 equiv) and BF3·OEt2 (1.05 equiv) in CH2Cl2 (0.3 M) at rt. bStarting ketone 2p was prepared in situ and directly converted to furan derivative 3p.

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Scheme 2. Mechanism of the Formation of Furan 3a and γ-Lactone 5 from Unsaturated Ester 2a and Saturated Ester 4, Respectively

Scheme 3. Conversion of Furan 3b to Michael Adduct 6 and Polycyclic Compound 7

nucleophile, that is, the suitably disposed carbonyl oxygen with the breakage of the carbon−nitrogen bond in aziridine as in I to yield II. This dominant regioselectivity is directed by the allylic activation and by the preference of 5-exo-tet cyclization according to Baldwin’s rule.15,16,19 Then, the aromatization via deprotonation of the acidic proton next to the aminomethyl substituent as in II afforded furan 3. A corroborating evidence of this mechanism was found from the same reaction with the saturated substrate, diethyl 2(aziridin-2-ylmethyl)malonate 4, affording γ-lactone 5 instead of furan (Scheme 2). When the proton at C2 of aziridine is not acidic enough, dealkylation to form γ-lactone 5 is more dominant rather than aromatization. Therefore, furan 3 and lactone 5 from dialkyl 2-(aziridin-2-ylmethylene)malonate 2a and dialkyl 2-(aziridin-2-ylmethyl)malonate 4 were obtained from the similar intermediate I in the presence of Lewis acid BF3OEt2, respectively. This reaction proceeded without any metallic additives as catalysts or reagents with complete conservation of all of the starting atoms to the products, that is, as a high degree of atom-economical synthesis. Taking advantage of this synthetic method to get highly substituted furan especially with aminomethyl appendage at C5, we treated aminomethylfuran 3b with 1.0 equiv of dimethyl acetylenedicarboxylate (DMAD) and obtained Michael addition product 6 in quantitative yields. The reaction of aminomethylfuran 3b with 2.0 equiv of DMAD afforded highly functionalized polycyclic compound 7 as a single isomer, confirmed by the NMR spectra (Scheme 3). The crystalline structure of the single stereoisomeric product 7 confirmed four new stereocenters generated from the

2-phenylethyl (2a, 2b, 2c, 2l, 2m, 2n, 2o, and 2p), benzyl (2d and 2e), t-butyl (2f, 2g, and 2h), c-hexyl (2i and 2j), and nhexyl (2k) groups without altering the reaction yields. No difference was observed in the reaction yields by changing the alkyl esters (R3 and/or R4) of 2-(aziridin-2-ylmethylene)malonate, whether they are methyl (2b and 2f), ethyl (2a, 2d, 2g, 2i, 2l, and 2m), or i-Pr (2c, 2e, 2h, 2j, and 2k). The same starting malonates decorated with the additional methyl (2l, R2 = Me) and i-Pr (2m, R2 = i-Pr) groups at the β-position aiming for the substituent at C4 of the furan ring yielded the expected products (3l and 3m) with the same efficiency. Under the same reaction conditions, the starting substrate (2n) bearing ester (R4 = OEt) and ketone (R3 = Me) at each side (E-/Z-mixture) also afforded the product (3n) in 80% yield. The product 3n retaining the methylcarbonyl group at the C3 position of furan suggested us that the aziridine ring opening proceeded by more electron-rich ester oxygen instead of the ketone oxygen. A striking feature of this reaction was represented by the reactions with ketones, as shown in 2o (R3 and R4 = Me) and 2p (R3 and R4 = −CH2CH2CH2−) to produce furans 3o and 3p in 85 and 77% yields, respectively. The formation of these products is ascribed to the breakage of the aziridine ring by ketone oxygen. All of the reactions afforded the expected products in >75% yield with complete conservation of the starting atoms to the products, that is, with high degree of atom economy (Table 2). A plausible mechanism of this reaction is detailed in Scheme 2. Lewis acid activates the aziridine ring (I) of the starting material 2 by coordinating with the lone pair electrons in the ring nitrogen which was observed in our earlier study.18 At this stage, the ring is highly activated toward the incoming 7527

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Scheme 4. Plausible Reaction Mechanism for the Formation of Compound 7

stereoselective conjugate addition20 and cycloaddition reactions.21 Furthermore, it should be noted that all of the atoms in one furan and two acetylenedicarboxylates are completely retained in the adduct without the loss of any atom. On the basis of the above observation, the plausible reaction mechanism for the formation of compound 7 from aminomethylfuran 3b via Michael adduct 6 is proposed (Scheme 4).

2-ylmethylene)malonate or 2-(aziridin-2-ylmethylene)-1,3dione 2 (500 mg, 1.57 mmol) in CH2Cl2 (5.0 mL) was added BF3·OEt2 (0.20 mL, 1.65 mmol) at ambient temperature. The reaction mixture was allowed to stir for 4 h (Table 2). After completion of the reaction as confirmed by TLC (50% EtOAc/hexane), the mixture was quenched with aq NaHCO3 (5.0 mL) and extracted with CH2Cl2 (10 × 3 mL). The combined organic layer was dried over anhydrous Na2SO4, and the solvent was removed in vacuo. This crude product was purified by column chromatography to provide analytically pure furan 3. (R)-Ethyl 2-Ethoxy-5-[(1-phenylethylamino)methyl]furan3-carboxylate (3a). [α]20 D = +71.8 (c = 0.98, CHCl3); Rf = 0.48 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.39−7.21 (m, 5H), 6.32 (s, 1H), 4.43 (q, J = 7.1 Hz, 2H), 4.25 (q, J = 7.1 Hz, 2H), 3.80 (q, J = 6.6 Hz, 1H), 3.50 (ABq, J = 14.8 Hz, 2H), 1.75 (br s, 1H), 1.45 (t, J = 7.1 Hz, 3H), 1.36 (d, J = 6.6 Hz, 3H), 1.32 (t, J = 7.1 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 163.0, 161.2, 144.7, 143.4, 128.3, 126.9, 126.5, 108.2, 92.6, 67.9, 59.6, 56.6, 43.4, 24.1, 14.8, 14.2; HRMSMALDI (m/z): [M + Na]+ calcd for C18H23NO4Na, 340.1520; found, 340.1537. (R)-Methyl 2-Methoxy-5-[(1-phenylethylamino)methyl]furan-3-carboxylate (3b). [α]20 D = +57.0 (c = 1.08, CHCl3); Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.36−7.27 (m, 4H), 7.26−7.19 (m, 1H), 6.32 (s, 1H), 4.05 (s, 3H), 3.83−3.70 (m, 4H), 3.47 (ABq, J = 14.9 Hz, 2H), 1.70 (br s, 1H), 1.34 (d, J = 6.7 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 162.9, 161.1, 144.5, 143.3, 128.0, 126.6, 126.2, 108.1, 91.0, 57.6, 56.4, 50.6, 43.1, 23.9; HRMS-MALDI (m/z): [M + Na]+ calcd for C16H19NO4Na, 312.1207; found, 312.1203. (R)-Isopropyl 2-Isopropoxy-5-[(1-phenylethylamino)methyl]-furan-3-carboxylate (3c). [α]20 D = +57.7 (c = 1.65, CHCl3); Rf = 0.55 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.36−7.29 (m, 4H), 7.27−7.22 (m, 1H), 6.30 (s, 1H), 5.18−5.07 (m, 1H), 4.93−4.83 (m, 1H), 3.79 (q, J = 6.6 Hz, 1H), 3.50 (ABq, J = 14.8 Hz, 2H), 1.73 (br s, 1H), 1.40 (d, J = 6.2 Hz, 6H), 1.35 (d, J = 6.6 Hz, 3H), 1.29 (d, J = 6.3 Hz, 6H); 13C NMR (CDCl3, 101 MHz): δ 162.7, 160.8, 144.8, 143.8, 128.4, 127.0, 126.6, 108.1, 95.1, 77.0, 66.9, 56.6, 43.6, 24.1, 22.3, 22.0; HRMS-MALDI (m/z): [M + Na]+ calcd for C20H27NO4Na, 368.1833; found, 368.1820. Ethyl 5-[(Benzylamino)methyl]-2-ethoxyfuran-3-carboxylate (3d). Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.34−7.30 (m, 4H), 7.27−7.22 (m, 1H), 6.39 (s, 1H), 4.43 (q, J = 7.1 Hz, 2H), 4.25 (q, J = 7.1 Hz, 2H), 3.78 (s, 2H), 3.66 (d, J = 0.7 Hz, 2H), 1.73 (br s, 1H), 1.44 (t, J = 7.1 Hz, 3H), 1.32 (t, J = 7.1 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 163.0, 161.3, 143.2, 139.6, 128.3, 128.1, 127.0, 108.6, 92.7, 68.0, 59.7, 52.4, 45.0, 14.9, 14.3; HRMS-MALDI (m/z): [M + Na]+ calcd for C17H21NO4Na, 326.1363; found, 326.1352.

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CONCLUSIONS In conclusion, we developed a new synthetic method for highly substituted furans from 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione via aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2 followed by aromatization. This synthetic method is free from any metal and is highly atom-economical with all of the atoms in the starting material retained in the final product.

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EXPERIMENTAL SECTION General Information. Chiral aziridines are available from Sigma-Aldrich as reagents and from Imagene Co., Ltd. (http:// www.imagene.co.kr/) in bulk quantities. All commercially available compounds were used as received unless stated otherwise. All reactions were carried out under a nitrogen atmosphere in oven-dried glassware with a magnetic stirrer. Dichloromethane was distilled from calcium hydride. Reactions were monitored by thin-layer chromatography (TLC) with 0.25 mm E Merck precoated silica gel plates (60 F254). Visualization was accomplished either with UV light or by immersion in solutions of ninhydrin, p-anisaldehyde, or phosphomolybdic acid, followed by heating on a hot plate for about 10 s. Purification of reaction products was carried out by flash chromatography using Kieselgel 60 Art 9385 (230−400 mesh). 1 H NMR and 13C NMR spectra were obtained using a Varian unity lNOVA 400 WB (400 MHz) or Bruker AVANCE III HD (400 MHz) spectrometer. Chemical shifts are reported relative to chloroform (δ = 7.26) for 1H NMR and chloroform (δ = 77.2) for 13C NMR and acetonitrile (δ = 1.94) for 1H NMR and acetonitrile (δ = 1.32) for 13C NMR. Data are reported as (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, p = quintet, and m = multiplet). Coupling constants are given in hertz. Ambiguous assignments were resolved based on standard one-dimensional proton decoupling experiments. Optical rotations were obtained using a RUDOLPH AUTOPOL III digital polarimeter and a JASCO P-2000 polarimeter. Optical rotation data were reported as follows: [α]20 (concentration c = g/100 mL, solvent). High-resolution mass spectra were recorded on a 4.7 Tesla IonSpec ESI-TOFMS instrument, and JEOL (JMS-700) and AB SCIEX 4800 Plus MALDI TOF (2,5-dihydroxybenzoic acid) matrix were used to prepare samples for mass spectrometry. Data were obtained in the reflector positive mode with internal standards for calibration. General Procedure for the Synthesis of Multisubstituted Furan (3). To a stirred solution of dialkyl 2-(aziridin7528

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[M + Na]+ calcd for C18H31NO4Na, 348.2146; found, 348.2153. (R)-Ethyl 2-Ethoxy-4-methyl-5-[(1-phenylethylamino)methyl]furan-3-carboxylate (3l). [α]20 D = +44.0 (c = 1.14, CHCl3); Rf = 0.50 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.38−7.29 (m, 4H), 7.27−7.22 (m, 1H), 4.39 (q, J = 7.1 Hz, 2H), 4.25 (q, J = 7.1 Hz, 2H), 3.75 (q, J = 6.6 Hz, 1H), 3.50 (ABq, J = 14.7 Hz, 2H), 2.36 (br s, 1H), 1.95 (s, 3H), 1.43 (t, J = 7.1 Hz, 3H), 1.37−1.29 (m, 6H); 13C NMR (CDCl3, 101 MHz): δ 163.8, 161.6, 144.7, 139.1, 128.4, 127.0, 126.6, 118.1, 92.7, 67.4, 59.4, 56.6, 40.8, 24.2, 14.9, 14.3, 10.0; HRMS-MALDI (m/z): [M + Na]+ calcd for C19H25NO4Na, 354.1676; found, 354.1669. (R)-Ethyl 2-Ethoxy-4-isopropyl-5-[(1-phenylethylamino)methyl]furan-3-carboxylate (3m). [α]20 D = +18.5 (c = 0.97, CHCl3); Rf = 0.55 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.37−7.30 (m, 4H), 7.28−7.22 (m, 1H), 4.36 (q, J = 7.1 Hz, 2H), 4.25 (q, J = 7.1 Hz, 2H), 3.77 (q, J = 6.6 Hz, 1H), 3.55 (ABq, J = 14.6 Hz, 2H), 3.15−3.04 (m, 1H), 1.87 (br s, 1H), 1.42 (t, J = 7.1 Hz, 3H), 1.36−1.30 (m, 6H), 1.22− 1.17 (m, 6H); 13C NMR (CDCl3, 101 MHz): δ 163.7, 161.7, 145.0, 138.9, 128.4, 128.0, 127.0, 126.6, 92.0, 67.3, 59.5, 56.9, 42.3, 24.8, 24.3, 22.1, 14.9, 14.3; HRMS-MALDI (m/z): [M + Na]+ calcd for C21H29NO4Na, 382.1990; found, 382.2005. (R)-1-(2-Ethoxy-5-[(1-phenylethylamino)methyl]furan-3yl)ethanone (3n). [α]20 D = +69.0 (c = 1.07, CHCl3); Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.38− 7.29 (m, 4H), 7.28−7.21 (m, 1H), 6.38 (s, J = 3.9 Hz, 1H), 4.48−4.41 (m, 2H), 3.79 (q, J = 6.6 Hz, 1H), 3.50 (ABq, J = 14.9 Hz, 2H), 2.36 (s, 3H), 1.79 (br s, 1H), 1.46 (t, J = 7.1 Hz, 3H), 1.35 (d, J = 6.6 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 191.4, 161.3, 144.7, 144.1, 128.3, 126.9, 126.5, 107.2, 102.4, 67.3, 56.6, 43.4, 28.3, 24.1, 14.8; HRMS-MALDI (m/z): [M + Na]+ calcd for C17H21NO3Na, 310.1414; found, 310.1413. (R)-1-(2-Methyl-5-[(1-phenylethylamino)methyl]furan-3yl]ethanone (3o). [α]20 D = +53.3 (c = 4.12, CHCl3); Rf = 0.45 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.39− 7.16 (m, 5H), 6.31 (s, 1H), 3.79 (q, J = 6.6 Hz, 1H), 3.56 (ABq, J = 14.6 Hz, 2H), 2.54 (s, 3H), 2.34 (s, 3H), 1.74 (br s, 1H), 1.36 (d, J = 6.6 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 193.7, 157.3, 151.6, 144.5, 128.2, 126.7, 126.3, 121.5, 107.1, 56.9, 43.4, 28.7, 24.0, 14.0; HRMS-MALDI (m/z): [M + Na]+ calcd for C16H19NO2Na, 280.1308; found, 280.1300. (R)-2-[(1-Phenylethylamino)methyl]-6,7-dihydrobenzofuran-4(5H)-one (3p). Piperidine (10 μL, 0.1 mmol) was added to a stirred solution of aziridinyl aldehyde (175 mg, 1.0 mmol), cyclohexane-1,3-dione (118 mg, 1.05 mmol), and 4 Å molecular sieves (dry powder, 0.2 g) in CH2Cl2 (4.0 mL) at rt, and the reaction mixture was allowed to stir for 12 h. BF3· OEt2 (0.16 mL, 1.3 mmol) was added to the reaction mixture and stirred for 6.0 h. The reaction mixture was filtered through Celite and washed with CH2Cl2 (3 × 15 mL). The combined organic layer was washed with aq NaHCO3 (5.0 mL), and the solvents were removed under vacuum to get a crude product, which was purified by column chromatography on silica gel to get pure product 3p (207 mg, 77%). [α]20 D = +73.6 (c = 1.02, CHCl3); Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.36−7.18 (m, 5H), 6.35 (s, 1H), 3.78 (q, J = 6.6 Hz, 1H), 3.58 (ABq, J = 14.7 Hz, 2H), 2.80 (t, J = 6.3 Hz, 2H), 2.46−2.38 (m, 2H), 2.15−2.04 (m, 2H), 1.82 (br s, 1H), 1.34 (d, J = 6.7 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 193.7, 166.1, 153.9, 144.3, 128.0, 126.5, 126.2, 121.2, 102.9,

Isopropyl 5-[(Benzylamino)methyl]-2-isopropoxyfuran-3carboxylate (3e). Rf = 0.60 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.35−7.27 (m, 4H), 7.26−7.20 (m, 1H), 6.39 (s, 1H), 5.18−5.08 (m, 1H), 4.94−4.84 (m, 1H), 3.76 (s, 2H), 3.64 (s, 2H), 1.77 (br s, 1H), 1.40 (d, J = 6.2 Hz, 6H), 1.29 (d, J = 6.3 Hz, 6H); 13C NMR (CDCl3, 101 MHz): δ 162.4, 160.6, 143.4, 139.5, 128.1, 127.9, 126.7, 108.1, 94.9, 76.8, 66.6, 52.2, 44.8, 22.1, 21.7; HRMS-MALDI (m/z): [M + Na]+ calcd for C19H25NO4Na, 354.1676; found, 354.1660. Methyl 5-[(tert-Butylamino)methyl]-2-methoxyfuran-3carboxylate (3f). Rf = 0.50 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 6.38 (s, 1H), 4.10 (s, 3H), 3.80 (s, 3H), 3.64 (d, J = 0.7 Hz, 2H), 1.60 (br s, 1H), 1.15 (s, 9H); 13C NMR (CDCl3, 101 MHz): δ 163.4, 161.4, 144.3, 107.9, 91.6, 58.1, 51.1, 50.6, 39.6, 28.8; HRMS-MALDI (m/z): [M + Na]+ calcd for C12H19NO4Na, 264.1207; found, 264.1214. Ethyl 5-[(tert-Butylamino)methyl]-2-ethoxyfuran-3-carboxylate (3g). Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 6.39 (s, 1H), 4.44 (q, J = 7.1 Hz, 2H), 4.25 (q, J = 7.1 Hz, 2H), 3.64 (d, J = 0.7 Hz, 2H), 1.76 (br s, 1H), 1.45 (t, J = 7.1 Hz, 3H), 1.32 (t, J = 7.1 Hz, 3H), 1.16 (s, 9H); 13C NMR (CDCl3, 101 MHz): δ 163.1, 161.3, 144.1, 107.7, 93.0, 68.2, 59.7, 50.7, 39.7, 28.8, 14.9, 14.3; HRMSMALDI (m/z): [M + Na]+ calcd for C14H23NO4Na, 292.1520; found, 292.1534. Isopropyl 5-[(tert-Butylamino)methyl]-2-isopropoxyfuran3-carboxylate (3h). Rf = 0.50 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 6.37 (s, 1H), 5.18−5.07 (m, 1H), 4.96− 4.85 (m, 1H), 3.64 (d, J = 0.6 Hz, 2H), 1.77 (br s, 1H), 1.40 (d, J = 6.2 Hz, 6H), 1.29 (d, J = 6.3 Hz, 6H), 1.15 (s, 9H); 13C NMR (CDCl3, 101 MHz): δ 162.7, 160.7, 144.5, 107.4, 95.3, 77.1, 66.9, 50.6, 39.7, 28.8, 22.3, 21.9; HRMS-MALDI (m/z): [M + Na]+ calcd for C16H27NO4Na, 320.1833; found, 320.1848. Ethyl 5-[(Cyclohexylamino)methyl]-2-ethoxyfuran-3-carboxylate (3i). Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 6.38 (s, 1H), 4.43 (q, J = 7.1 Hz, 2H), 4.25 (q, J = 7.1 Hz, 2H), 3.69 (s, 2H), 2.51−2.40 (m, 1H), 1.91−1.83 (m, 2H), 1.79 (br s, 1H), 1.77−1.70 (m, 2H), 1.65− 1.57 (m, 1H), 1.44 (t, J = 7.1 Hz, 3H), 1.32 (t, J = 7.1 Hz, 3H), 1.27−1.16 (m, 3H), 1.15−1.04 (m, 2H); 13C NMR (CDCl3, 101 MHz): δ 163.1, 161.3, 143.5, 108.3, 92.8, 68.0, 59.8, 55.3, 42.8, 33.1, 26.0, 24.8, 14.9, 14.3; HRMS-MALDI (m/z): [M + Na]+ calcd for C16H25NO4Na, 318.1676; found, 318.1677. Isopropyl 5-[(Cyclohexylamino)methyl]-2-isopropoxyfuran-3-carboxylate (3j). Rf = 0.60 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 6.35 (s, 1H), 5.17−5.07 (m, 1H), 4.89 (sep, J = 6.2 Hz, 1H), 3.68 (d, J = 0.6 Hz, 2H), 2.44 (tt, J = 10.3, 3.8 Hz, 1H), 1.89−1.81 (m, 2H), 1.77 (br s, 1H), 1.75− 1.68 (m, 2H), 1.65−1.57 (m, 1H), 1.40 (d, J = 6.2 Hz, 6H), 1.29 (d, J = 6.3 Hz, 6H), 1.27−1.16 (m, 3H), 1.15−1.03 (m, 2H); 13C NMR (CDCl3, 101 MHz): δ 162.7, 160.7, 144.1, 107.9, 95.1, 77.0, 66.9, 55.1, 42.8, 33.1, 25.9, 24.8, 22.3, 21.9; HRMS-MALDI (m/z): [M + Na]+ calcd for C18H29NO4Na, 346.1990; found, 346.2002. Isopropyl 5-[(Hexylamino)methyl]-2-isopropoxyfuran-3carboxylate (3k). Rf = 0.60 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 6.40 (s, 1H), 5.17−5.07 (m, 1H), 4.96− 4.86 (m, 1H), 3.68 (s, 2H), 2.60 (t, J = 7.3 Hz, 2H), 2.12 (br s, 1H), 1.55−1.46 (m, 2H), 1.40 (d, J = 6.2 Hz, 6H), 1.33−1.24 (m, 12H), 0.88 (t, J = 6.8 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 162.7, 160.9, 108.7, 95.1, 77.1, 67.0, 48.5, 45.6, 31.6, 29.6, 29.5, 26.9, 22.5, 22.3, 22.0, 13.9; HRMS-MALDI (m/z): 7529

DOI: 10.1021/acsomega.7b01542 ACS Omega 2017, 2, 7525−7531

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56.5, 43.2, 37.0, 23.8, 22.8, 22.1; HRMS-MALDI (m/z): [M + Na]+ calcd for C17H19NO2Na, 270.1489; found, 270.1477. (R)-Ethyl 2-Oxo-5-[(1-phenylethylamino)methyl]tetrahydro-furan-3-carboxylate (5). Procedure A. BF3·OEt2 (0.16 mL, 1.3 mmol) was added to a solution of ester 4 (160 mg, 0.5 mmol) in CH2Cl2 (4.0 mL), and the mixture was stirred for 24 h at rt. The reaction mixture was quenched with aq NaHCO3 and extracted with CH2Cl2, and the combined organic layer was dried over Na2SO4. The solvents were removed under vacuum to get a crude product, which was purified by column chromatography to yield pure lactone 5 (81 mg, 56%). Procedure B. BF3·OEt2 (0.16 mL, 1.3 mmol) was added to a solution of ester 4 (160 mg, 0.5 mmol) in acetonitrile (4.0 mL), and the mixture was refluxed for 2.0 h at 90 °C. The reaction mixture was cooled to rt and quenched with aq NaHCO3. The reaction mixture was extracted with CH2Cl2 (3 × 10 mL), the combined organic layer was dried over Na2SO4, and the solvents were removed under vacuum to get a crude product, which was purified by column chromatography to yield pure lactone 5 (125 mg, 86%). Rf = 0.30 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CD3CN): δ 7.37−7.30 (m, 4H), 7.27−7.20 (m, 1H), 4.68−4.59 (m, 0.5H), 4.55−4.46 (m, 0.5H), 4.24− 4.13 (m, 2H), 3.76 (dq, J = 10.2, 6.6 Hz, 1H), 3.67 (dt, J = 10.6, 8.1 Hz, 1H), 2.81−2.71 (m, 1H), 2.61−2.21 (m, 3H), 2.12 (br s, 1H), 1.33−1.21 (m, 6H); HRMS-MALDI (m/z): [M + Na]+ calcd for C16H21NO4Na, 314.1363; found, 314.1373. (R)-Dimethyl 2-[((5-Methoxy-4-(methoxycarbonyl)furan2-yl)methyl)(1-phenylethyl)amino]-maleate (6). A mixture of aminomethylfuran 3b (200 mg, 0.69 mmol) and DMAD (84 μL, 0.69 mmol) in a sealed tube was stirred for 1 h at 90 °C. The reaction mixture was purified by column chromatography to get pure compound 6 (288 mg, 97% yield). [α]20 D = +66.7 (c = 1.0, CHCl3); Rf = 0.40 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.45−7.25 (m, 5H), 6.25 (s, 1H), 4.86 (s, 1H), 4.78 (q, J = 6.8 Hz, 1H), 4.02 (s, 3H), 3.96 (s, 3H), 3.92 (s, 2H), 3.76 (s, 3H), 3.64 (s, 3H), 1.64 (d, J = 6.9 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 167.8, 166.0, 163.0, 161.0, 153.7, 138.7, 138.6, 128.6, 128.0, 127.3, 110.2, 91.5, 86.4, 58.7, 57.9, 53.0, 51.1, 50.8, 41.5, 17.0; HRMS-MALDI (m/z): [M + Na]+ calcd for C22H25NO8Na, 454.1473; found, 454.1460. (4aS,5R,6S,8aR)-Pentamethyl 6-Methoxy-2-((R)-1-phenylethyl)-2,4a,5,6-tetrahydro-1H-6,8a-epoxyisoquinoline3,4,5,7,8-pentacarboxylate (7). A mixture of aminomethylfuran 3b (200 mg, 0.69 mmol) and DMAD (0.17 mL, 1.4 mmol) in a sealed tube was stirred for 12 h at 90 °C. The reaction mixture was purified by column chromatography to get pure compound 7 (285 mg, 72% yield). [α]20 D = +1.6 (c = 0.62, CHCl3); Rf = 0.35 (EtOAc/hexane, 50%); 1H NMR (400 MHz, CDCl3): δ 7.43−7.28 (m, 5H), 4.68 (q, J = 6.8 Hz, 1H), 3.94 (s, 3H), 3.79 (s, 3H), 3.73 (s, 3H), 3.72 (s, 3H), 3.59 (s, 3H), 3.54 (s, 3H), 3.25 (ABq, J = 14.4 Hz, 2H), 3.11 (d, J = 4.2 Hz, 1H), 3.03 (d, J = 4.2 Hz, 1H), 1.56 (d, J = 6.8 Hz, 3H); 13C NMR (CDCl3, 101 MHz): δ 170.6, 167.1, 166.2, 162.3, 162.0, 148.1, 145.5, 141.0, 138.4, 128.7, 128.0, 127.5, 112.6, 96.7, 80.1, 58.2, 57.3, 54.8, 53.0, 52.6, 52.4, 52.2, 51.0, 44.4, 41.5, 17.3; HRMS-MALDI (m/z): [M + Na]+ calcd for C28H31NO12Na, 574.1920; found, 574.1913.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01542. General experimental procedures for starting materials (2a−2o), 1H and 13C NMR spectra of all prepared compounds, and X-ray crystallographic data for compound 7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF-2012M3A7B4049645 and 2014R1A5A1011165 with Centre for New Directions in Organic Synthesis).

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REFERENCES

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DOI: 10.1021/acsomega.7b01542 ACS Omega 2017, 2, 7525−7531

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DOI: 10.1021/acsomega.7b01542 ACS Omega 2017, 2, 7525−7531