Anionic Annulation of 3-Cyanophthalides with Allene Carboxylates: A

Mar 22, 2018 - For the formation of quinol 8, the addition of H2O to the double bond of ... 1,4-dihydroxynaphthol 8b were obtained in 56% and 25% yiel...
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Article Cite This: J. Org. Chem. 2018, 83, 4537−4544

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Anionic Annulation of 3‑Cyanophthalides with Allene Carboxylates: A Carbon-Conserved Synthesis of Naphtho[b]furanones Supriti Jana and Dipakranjan Mal* Department of Chemistry, Indian Institute of Technology Kharagpur, 721302, India S Supporting Information *

ABSTRACT: The reaction of 3-cyanophthalides with allene carboxylates in the presence of tBuOLi results in a tandem annulation furnishing naphtho[b]furanones in good yields with no loss of carbon. The carbon economy is explained by a tandem process, in which the initially expelled cyanide induces the second annulation.

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INTRODUCTION In a recent report, we reported the anionic annulation of unactivated phthalides 1 with allene carboxylates 2 providing 9hydroxynaphtho[c]furanones 3 in good yields. This tandem reaction was interpreted in terms of formal [4+2] annulation to form intermediates A, followed by transposition of the incipient hydroxy group (Scheme 1, eq 1).1 A short synthesis of

cyanophthalides provided the desired naphtho[c]furanones (cf. 5). Scheme 2. Annulation of Cyanophthalide 6 with Allene Carboxylate 2b and Optimization of Reaction Conditions

Scheme 1. Synthetic Background and Working Hypothesis

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RESULTS AND DISCUSSION We initiated this annulation study with racemic allene-1carboxylates 2.1 Accordingly, 3-cyanophthalide (6)3a was reacted with allene carboxylate 2b in the presence of LDA in THF at −78 °C, as previously applied to unactivated phthalides 1. Routine workup of the reaction mixture followed by chromatographic purification afforded two solid products. The preliminary examination of the 1H NMR spectra of the purified products revealed no correspondence to the desired product (cf. 5). The more polar product displayed a downfield −CH group at δ 5.3 ppm and an exchangeable hydrogen in the spectrum leading to the proposal of structure 7 for the major product. The IR band of this product at 1803 cm−1 supported the presence of a 5-membered lactone ring of benzofuranones.4 Finally, the structure 7 was confirmed by a single crystal X-ray data analysis (vide Supporting Information for details, Figure S1, Table S1) and its HRMS data. The minor and less polar product corresponded to the structure 8. It was fully

justicidin B was achieved by application of the annulation. In a more recent report,2 Ham et al. demonstrated the synthetic utility of 4,9-dihydroxynaphtho[c]furanones 5 (Scheme 1, eq 2), which differ from the naphthofuranones 3 in an additional hydroxy group. In order to access the former by extension of our annulation chemistry of allene carboxylates 2, we envisaged their reaction with activated phthalides 4 in the place of 1. The probable intermediate B could undergo Michael addition with water followed by lactonization to give dihydroxynaphthofuranones 5 (Scheme 1). However, the hypothesis proved to be wrong. The attempted annulation reactions of 2 with phthalides 4 in the presence of tBuOLi afforded naphtho[b]furanones (cf. 7) in good yields in single operations (Scheme 2). On the contrary, the reaction of 4-methoxy and 6-methoxy substituted © 2018 American Chemical Society

Received: January 30, 2018 Published: March 22, 2018 4537

DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

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The Journal of Organic Chemistry

benzofuranones are frequently found as core structural motifs in many natural products and drug scaffolds, including hopeahainol, abesinol E, ferrubietolide, etc.5 Strikingly, the related naphthofuranone6 moieties are very rare in natural products. To generalize the reaction (Scheme 2), 3-cyanophthalide (6) was submitted to the reaction with 4-methoxyphenyl substituted allenoate 2c under the influence of tBuOLi. Naphthofuranone 7a was obtained as the sole product in 87% yield (Table 1, entry 1). When the reaction was carried out with 5-bromocyanophthalide 183b with allene carboxylates 2b, compounds 7b and 8a were obtained in 63% and 19% yields, respectively (Table 1, entry 2). Likewise, naphthofuranone 7c was obtained in 62% yield when the annulation of 5,7dimethoxycyanophthalide 193c with allene carboxylate 2b (Table 1, entry 3) was conducted. When 5-methylcyanophthalide 203b was subjected to react with allene carboxylates 2b, naphtho[b]furanone 7d and 1,4-dihydroxynaphthol 8b were obtained in 56% and 25% yields, respectively (Table 1, entry 4). Very interestingly, when benzo fused cyanophthalide 21 was reacted with the allene carboxylate 2b, we obtained two products, i.e., anthrafuranones 7e and 1,4-dihydroxy anthraquinone 8c in 59% and 27% yields, respectively, according to our expectation (Table 1, entry 5). In conjugation with the study of activated naphthalide, i.e., 21, we cursorily looked into the reactivity of unactivated naphthalide, i.e., 23. The attempted annulations between 23 and allene carboxylates 2a and 2b turned out to be unsuccessful. Naphthalide 23 was prepared by LDA mediated annulation of phthalide 22 with allene carboxylate 2a, followed by Omethylation reaction by the treatment of K2CO3 and MeI in overall 70% yield (Scheme 5). For a large-scale synthesis of the phthalide 23, Yu lactone annulation7 was applied to 1-methoxy2-naphthoic acid (25), which was synthesized in 92% yield from commercially available 1-hydroxy-2-naphthoic acid (24) by the treatment with dimethyl sulfate and K2CO3 in acetone under reflux followed by the ester hydrolysis with LiOH in a THF/H2O mixture. Treatment of 1-methoxy-2-naphthoic acid (25) with 10 mol % Pd(OAc)2 and KHCO3 in dibromomethane at 140 °C for 36 h furnished naphthofuranone 23 in 84% yield. Compound 23 was then treated8 with N,Ndiethylamine and anhydrous AlCl3 in DCM as a solvent to give amide derivative 26 in 94% yield. Compound 26 was oxidized9 by PCC in DCM, to provide aldehyde 27 in 89% yield. Finally, the aldehyde 27 was reacted8 with TMSCN and a catalytic amount of KCN and 18-crown-6 in DCM as a solvent, followed by evaporation of DCM, and the resultant reaction mixture was stirred in AcOH for 15 h to furnish cyanophthalide 21 in 89% yield. The tandem annulation described in the foregoing section is found to be very sensitive to substituents of phthalide components (Scheme 6). Cyanophthalides like 4-methoxy and 6-methoxy substituted cyanophthalides provided the desired naphtho[c]furanones 29. Naphthofuranone 29a was obtained in 82% yield, when the annulation of 4-methoxycyanophthalide 28a3b with allene carboxylate 2b (Table 2, entry 1) was conducted. The annulation of 6,7-dimethoxy cyanophthalide 28b3b with the allene carboxylates 2b under the same set of reaction conditions (Table 2, entry 2) furnished 1,4-dihydroxynaphthalide 29b in 65% yield, contrary to the synthesis of naphtho[b]furanones. Similarly, naphthofuranone 29c was

characterized by NMR and HRMS data. We optimized the reaction with bases like LiHMDS, NaH, and tBuOLi. From the results (Scheme 2), it is apparent that entry 3 is the best set of conditions for the annulation. The one-pot formation of naphtho[b]furanone 7 can be explained by the mechanism shown in Scheme 3. The Scheme 3. Probable Mechanism for the Formation of Naphtho[b]furanone 7

annulation cascade is initiated by tBuOLi promoted generation of 3-lithiophthalide 6a, which undergoes cycloaddition with 2b to give tricyclic intermediate 9, collapsing to tetralone 10. As cyano group is a good leaving group, it is expelled from the intermediate 10 to give quinone methide 11, and then the expelled cyanide ion adds to the double bond of the intermediate 11, forming the naphthoquinone dianion 12. Under acidic workup conditions, the dioxide and the cyano groups in 12 are protonated to form 13. Intramolecular imidation to 14 followed by hydrolysis during acidic workup conditions gives naphthofuranone 7. Alternatively, the addition of the cyanide group might take place in the acidic conditions. For the formation of quinol 8, the addition of H2O to the double bond of 11 forming naphthoquinol 15 followed by aerial oxidation of naphthoquinol 15 to naphthoquinone 16 is proposed (Scheme 4). Under acidic workup conditions, the 1,2-hydride shift of intermediate 17a leads 17b. Aromatization of the intermediate 17b produces compound 8. Intrigued by this unprecedented tandem annulation, we decided to test the generality of the reaction. Furthermore, Scheme 4. Probable Mechanism for the Formation of Quinol 8

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DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

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The Journal of Organic Chemistry Table 1. Substrate Scope of the Annulationa Leading to Naphtho[b]furanones

a

All reactions were carried out in the presence of tBuOLi (3 equiv), THF, −78 °C to room temperature, 6−7 h, quenched with 3 M HCl.

Scheme 5. Synthesis of Cyanonaphthofuranone 21

Scheme 6. General Scheme for the Annulation towards 29

obtained in 79% yield, when the annulation of 4,7dimethoxycyanophthalide 28c3d with allene carboxylate 2b (Table 2, entry 3) was conducted. When allene acceptor 2c was used in the place of 2b, the reaction provided 1,4-dihydroxy naphtho[2,3-c]furan-1(3H)-one 29d in 78% yield (Table 2,

entry 4). No naphtho[b]furanones were formed in the above annulations. Next, we explored sulfonyl phthalide 28d as the donor. Unlike the cyanophthalide analogue 6, it delivered naphthofuranone 29e in 89% yield (Table 2, entry 5). No product with incorporation of the sulfonyl group was obtained. This can be explained by the poorer nucleophilicity of the expelled arylsulfinate ion. We also performed the annulation reaction between 3-phenylthiophthalide and ethyl 4-phenyl-2,3butadienoate (2b) with the optimized reaction conditions, but we did not get the Michael addition product of SPh. This donor is not at all suitable for this type of annulation. We obtained 4539

DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

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The Journal of Organic Chemistry Table 2. Scope of the Annulation Leading to 29a

Scheme 7. Explanations of Naphtho[c]furanones Formation for 4-Methoxy and 6-Methoxy Substituted Cyanophthalides

furanones 29a−d (Scheme 7). When the electron donating methoxy group is not at the 4 or 6 position of cyanophthalides as in 19, 20, and 21, the expelled cyanide can add to the quinone methide unites (cf. 32, 33, and 34), and naphtho[b]furanones were obtained. In order to divert the above reactions to the formation of naphtho[b]furanones, we considered effecting cyanide addition from an additive in the reaction. Accordingly, we treated cyanophthalide 28b with allene carboxylate 2b followed by the treatment with a combination3b of PIDA-TMSCN-BF3·Et2O and tetrabutylammonium cyanide separately, but the reactions afforded naphtho[c]furanone 29b. There was no indication of any other product formation in TLC experiment. In another experiment, we considered ZnCl2 as an additive,9 anticipating it would coordinate with nuclear methoxy group, thus making the intermediates (cf. 11) more electrophilic in nature, so that addition of cyanide is possible. However, the experiment conducted with 6,7-dimethoxy cyanophthalide 28b, showed no departure from the previous result. In this case also, only naphtho[c]furanone 29b was obtained in 60% yield. This is due to the fact that the intrinsic reactivity of the quinone methide 31 is not good enough for the external cyanide to attack. As a result, during the acidic workup condition, H2O was added to 31 and naphtho[c]furanones were obtained. The driving force for the formation of naphtho[c]furanones might be the lactonization of intermediate hydroxy acid. To increase the yield of the major product, i.e., naphtho[b]furanone 7, we used tetrabutylammonium cyanide as an additional cyanide source, but this was of no avail.

a

All reactions were carried out in the presence of tBuOLi (3 equiv), THF, −78 °C to room temperature, 6−7 h, quenched with 3 M HCl.

only less than 15% of the H2O addition product, i.e., naphtho[c]furanone 29e, and self-decomposition product of the donor. The formation of naphtho[c]furanone 29e may be explained by preferential addition of H2O under acidic workup conditions, when PhSH is less nucleophilic than H2O. To trap the Michael type acceptor intermediate, we performed the reaction between the cyanophthalide 20 and ethyl 4-phenyl-2,3-butadienoate (2b) with the optimized reaction conditions followed by the addition of nBuNH2 as a nucleophile. Here, the added amine did not affect the product formation and also the yield of the products; both of the products 7d and 8b were obtained in 50% and 19% yields, respectively. Under acidic conditions, amines remain protected, and no addition takes place. It is apparent that the formation of naphtho[b]furanones (Table 1) and naphtho[c]furanones (Table 2) is dependent on the effect of the substituents of the cyanophthalides. When the electron donating methoxy group is at the 4 or 6 position of cyanophthalides as in 28a−c, the resonance effect exerted by the methoxy groups reduces the electron density of the carbonyl group of the quinone methide unit (cf. 30 and 31) (Scheme 7). Consequently, the possibility of the conjugate addition of the cyanide ion is reduced, and the formation of naphtho[b]furanones is precluded. In these cases, during acidic workup, H2O was added to the double bond of the quinone methide unit (i.e., 30 and 31), forming the naphtho[c]-

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CONCLUSION In conclusion, we have identified two trails in the reactions between 3-cyanophthalides and allene carboxylates. One leads to the formation of naphtho[b]furanones and the other to naphtho[c]furanones. This divergence of the tandem annulations is due to the in situ Michael addition of CN−, which is released in the first annulation. The cyanide addition is seemingly controlled by the reactivity of the quinone methide intermediate formed after the first annulation. The results would serve as a starting point for further investigations for the development of naphtho[b]furanones, which are scarcely studied. The easy accessibility of the starting materials and 4540

DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

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The Journal of Organic Chemistry

159.2, 142.9, 136.5, 130.6, 129.0, 127.9, 127.4, 127.1, 125.3, 124.7, 123.5, 121.2, 117.3, 102.2, 61.6, 52.8, 13.6; HRMS (ESI) calcd for C21H17O5 [M + H]+ 349.1076, found 349.1083. Ethyl 3-Benzoyl-1,4-dihydroxy-2-naphthoate (8). According to the general procedure for annulation, the condensation of cyanophthalide 6 (189 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 3-benzoyl-1,4-dihydroxy-2-naphthoate (8) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a yellow solid (84 mg, 25%): mp 186−188 °C; νmax (KBr, cm−1) 3120, 2922, 1725, 1662, 1412, 1234, 1102, 771; 1H NMR (600 MHz, CDCl3) δ 11.30 (s, 1H), 10.90 (s, 1H), 8.56−8.33 (m, 2H), 7.81−7.65 (m, 4H), 7.57−7.47 (m, 1H), 7.43 (td, J = 7.3, 1.2 Hz, 2H), 3.68 (brs, 2H), 0.92 (t, J = 7.2 Hz, 3H); 13C NMR (150 MHz, CDCl3) δ 197.7, 170.1, 153.4, 152.1, 140.3, 132.4, 129.9, 129.7, 128.6, 128.5, 128.3, 127.9, 124.3, 124.2, 110.5, 103.9, 61.6, 13.4; HRMS (ESI) calcd for C18H11O4 [M + H − EtOH]+ 291.0657, found 291.0653. Ethyl 5-Hydroxy-3-(4-methoxyphenyl)-2-oxo-2,3dihydronaphtho[1,2-b]furan-4-carboxylate (7a). According to the general procedure for annulation, the condensation of cyanophthalide 6 (189 mg, 1 mmol) with ethyl 4-(4-methoxyphenyl)buta-2,3-dienoate 2c (298.0 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 5-hydroxy-3-(4-methoxyphenyl)-2-oxo-2,3dihydronaphtho[1,2-b]furan-4-carboxylate (7a) (purified by flash column chromatography (hexanes/EtOAc 18:1)) as a yellow solid (329.2 mg, 87%): mp 165−167 °C; νmax (KBr, cm−1) 3430, 2983, 2836, 1797, 1654, 1600, 1511, 1450, 1397, 1378, 1339, 1251, 1133, 1073, 1021, 908, 907, 769; 1H NMR (400 MHz, CDCl3) δ 12.17 (s, 1H), 8.47 (d, J = 8.4 Hz, 1H), 7.99 (d, J = 8.2 Hz, 1H), 7.76−7.72 (m, 1H), 7.66−7.62 (m, 1H), 7.10−6.97 (m, 2H), 6.87−6.81 (m, 2H), 5.14 (s, 1H), 4.15−4.10 (m, 2H), 3.76 (s, 3H), 0.99 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 175.8, 170.0, 159.3, 159.142.8, 130.5, 128.5, 128.4, 127.0, 125.2, 124.7, 123.4, 121.2, 117.7, 114.4, 102.2, 61.7, 55.3, 52.0, 13.8; HRMS (ESI) calcd for C22H19O6 [M + H]+ 379.1182, found 379.1186. Ethyl 8-Bromo-5-hydroxy-2-oxo-3-phenyl-2,3-dihydronaphtho[1,2-b]furan-4-carboxylate (7b). According to the general procedure for annulation, the condensation of 5-bromo cyanophthalide 18 (238 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 8-bromo-5-hydroxy-2-oxo-3-phenyl-2,3-dihydronaphtho[1,2-b]furan4-carboxylate (7b) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a white solid (269.0 mg, 63%): mp 182− 184 °C; νmax (KBr, cm−1) 2925, 2370, 1804, 1648, 1440, 1425, 1331, 770; 1H NMR (400 MHz, CDCl3) δ 12.19 (s, 1H), 8.33 (d, J = 8.9 Hz, 1H), 8.16 (d, J = 1.9 Hz, 1H), 7.71 (dd, J = 9.0, 1.9 Hz, 1H), 7.30 (d, J = 6.9 Hz, 3H), 7.11 (dd, J = 7.1, 2.0 Hz, 2H), 5.23 (s, 1H), 4.10 (qd, J = 7.1, 1.7 Hz, 2H), 0.91 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 174.9, 169.7, 159.0, 141.8, 136.1, 130.6, 129.1, 128.0, 127.3, 126.5, 125.7, 124.2, 123.7, 123.7, 118.9, 102.7, 61.8, 52.8, 13.6; HRMS (ESI) calcd for C21H16BrO5 [M + H]+ 427.0181, found 427.0185. Ethyl 3-Benzoyl-6-bromo-1,4-dihydroxy-2-naphthoate (8a). According to the general procedure for annulation, the condensation of 5-bromo cyanophthalide 18 (238 mg, 1 mmol) with ethyl 4phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 3-benzoyl-6-bromo1,4-dihydroxy-2-naphthoate (8a) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a yellow solid (79.0 mg, 19%): mp 183−185 °C; νmax (KBr, cm−1) 2927, 1728, 1652, 1582, 1475, 1403, 1263, 1155, 1107, 1022, 774; 1H NMR (400 MHz, CDCl3) δ 12.52 (s, 1H), 8.33 (d, J = 2.0 Hz, 1H), 7.98 (d, J = 8.4 Hz, 1H), 7.84 (dd, J = 8.4, 2.1 Hz, 1H), 7.70 (s, 1H), 7.45−7.12 (m, 5H), 3.63 (q, J = 7.1 Hz, 2H), 0.65 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 185.5, 170.4, 160.6, 140.5, 137.7, 136.7, 134.1, 131.4, 130.3, 127.4, 128.9, 128.7, 128.5, 126.9, 126.3, 101.0, 61.4, 13.1; HRMS (ESI) calcd for C20H16BrO5 [M + H]+ 415.0181, found 415.0185. Ethyl 5 -Hydrox y-6,8-dimet h o x y - 2 - o x o - 3 -p h e n y l - 2 , 3 dihydronaphtho[1,2-b]furan-4-carboxylate (7c). According to the general procedure for annulation, the condensation of cyanophthalide 19 (219 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8

the mild reaction conditions combined with the importance of naphthofuranones make this new annulation a useful advancement.

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EXPERIMENTAL SECTION

General Experimental Methods. Melting points were determined in open end-capillary tubes and are uncorrected. Solvents were dried and distilled following the standard procedures. TLC was carried out on precoated plates (silica gel 60, GF254), and the spots were visualized with UV and fluorescent lights. Column chromatography was performed on silica gel (60−120 or 230−400 mesh). 1H and 13C NMR spectra for all of the compounds were recorded at 400/500/600 and 100/125/150 MHz. IR spectra were recorded on an FT-IR instrument using a KBr pellet. Mass analyzer type was “TOF MS ESI” in every case. The phrase “usual workup” or “worked up in usual manner” refers to washing of the organic phase with water (2 × 1/3 the volume of the organic phase) and brine (1 × 1/4 the volume of the organic phase), drying (anhydrous Na2SO4), filtration, and concentration under reduced pressure. Method A: General Annulation Procedure with LDA. In a flame-dried flask, LDA (3.2 mmol) was prepared by adding nBuLi (3.2 mmol, 1.6 M in hexanes) in a solution of diisopropylamine (3.2 mmol) in THF (10 mL) at −78 °C under a nitrogen atmosphere. After 30 min at −78 °C, an appropriate phthalide (1 mmol) in THF (5 mL) was added dropwise over 15 min. The reaction mixture was stirred at −78 °C for 30 min, and then a solution of the appropriate Michael acceptor (1.2 mmol) in THF (5 mL) was added dropwise over 15 min at −78 °C. The reaction mixture was further stirred for 1 h at the same temperature and allowed to warm under ambient conditions to room temperature over 5−6 h. The solution was then quenched with 3 M HCl (15 mL). The resulting mixture was concentrated under reduced pressure, and the residue was extracted with ethyl acetate (3 × 50 mL). The combined extracts were washed with brine, dried (Na2SO4), and concentrated to obtain the crude product, which was purified by column chromatography on silica gel using ethyl acetate and hexanes (5−20% ethyl acetate in hexanes). Method B: General Annulation Procedure with LiHMDS and t BuOLi. To a stirred solution of LiHMDS/tBuOLi (9.84 mmol) in THF (40 mL) at −78 °C (ethyl acetate/liquid nitrogen bath) under an inert atmosphere was added a solution of a phthalide (3.28 mmol) in THF (10 mL). The resulting yellowish solution was stirred at −78 °C for 30 min, after which a solution of a Michael acceptor (1.0−1.5 equiv unless otherwise stated) in THF (10 mL) was added to it. The cooling bath was removed after about 1 h at −78 °C, and the reaction mixture was brought to room temperature over a period of 1 h and further stirred for 6−8 h. The reaction was then quenched with 3 M HCl (15 mL), and the resulting solution was concentrated. The residue was diluted with ethyl acetate (50 mL), and the layers were separated. The aqueous layer was extracted with ethyl acetate (3 × 25 mL). The combined extracts were washed with H2O (15 mL), brine (15 mL), dried (anhydrous. Na2SO4), and concentrated under reduced pressure. The crude product was purified by column chromatography using ethyl acetate and hexanes on silica gel (5−25% ethyl acetate in hexanes) or by recrystallization to obtain a pure product. Ethyl 5-Hydroxy-2-oxo-3-phenyl-2,3-dihydronaphtho[1,2-b]furan-4-carboxylate (7). According to the general procedure for annulation, the condensation of cyanophthalide 6 (189 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 5-hydroxy-2oxo-3-phenyl-2,3-dihydronaphtho[1,2-b]furan-4-carboxylate (7) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a white solid (219 mg, 63%), mp 163−165 °C. The structure of compound 7 was confirmed by the single crystal data analysis (vide Supporting Information for details): νmax (KBr, cm−1) 2983, 2931, 1803, 1659, 1598, 1453, 1397, 1378, 1335, 1240, 1132, 1020, 777, 713; 1 H NMR (600 MHz, CDCl3) δ 12.20 (s, 1H), 8.51 (dt, J = 8.4, 0.9 Hz, 1H), 8.03 (dt, J = 8.2, 0.9 Hz, 1H), 7.77 (m, 1H), 7.66 (m, 1H), 7.39− 7.20 (m, 3H), 7.18−7.02 (m, 2H), 5.24 (s, 1H), 4.13−4.07 (m, 2H), 0.91 (t, J = 7.1 Hz, 3H); 13C NMR (150 MHz, CDCl3) δ 175.5, 170.0, 4541

DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

Article

The Journal of Organic Chemistry mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 5-hydroxy-6,8-dimethoxy-2-oxo-3-phenyl-2,3dihydronaphtho[1,2-b]furan-4-carboxylate (7c) (purified by flash column chromatography (hexanes/EtOAc 15:1)) as a dark brown solid (253 mg, 62%): mp 177−179 °C; νmax (KBr, cm−1) 3425, 2913, 2851, 1799, 1725, 1652, 1584, 1443, 1284, 1036, 772; 1H NMR (400 MHz, CDCl3) δ 12.71 (s, 1H), 7.35−7.27 (m, 3H), 7.13 (dd, J = 7.6, 1.8 Hz, 2H), 6.90 (d, J = 2.3 Hz, 1H), 6.61 (d, J = 2.3 Hz, 1H), 5.24 (s, 1H), 4.11−3.99 (m, 5H), 3.95 (s, 3H), 0.89 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 175.5, 169.9, 162.1, 160.8, 160.7, 142.1, 136.2, 129.0, 127.9, 127.4, 127.1, 119.3, 111.8, 100.9, 100.0, 92.8, 61.3, 56.4, 55.8, 53.2, 13.6; HRMS (ESI) calcd for C23H21O7 [M + H]+ 409.1287, found 409.1283. Ethyl 5-Hydroxy-8-methyl-2-oxo-3-phenyl-2,3-dihydronaphtho[1,2-b]furan-4-carboxylate (7d). According to the general procedure for annulation, the condensation of 5-methylcyanophthalide 20 (173 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 5-hydroxy-8-methyl-2-oxo-3-phenyl-2,3-dihydronaphtho[1,2-b]furan4-carboxylate (7d) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a white solid (203 mg, 56%): mp 165− 167 °C; νmax (KBr, cm−1) 2984, 2926, 1806, 1646, 1470, 1338, 1238, 1142, 1072, 944, 802, 712; 1H NMR (400 MHz, CDCl3) δ 12.18 (s, 1H), 8.38 (d, J = 8.6 Hz, 1H), 7.80 (s, 1H), 7.48 (dd, J = 8.5, 1.7 Hz, 1H), 7.30−7.26 (m, 3H), 7.14−7.11 (m, 2H), 5.22 (s, 1H), 4.09 (qd, J = 7.2, 2.9 Hz, 2H), 2.59 (s, 3H), 0.90 (t, J = 7.2 Hz, 3H); 13C NMR (126 MHz, CDCl3) δ 175.7, 170.1, 159.4, 142.6, 141.5, 136.7, 129.3, 129.1, 127.9, 127.5, 124.7, 123.8, 123.5, 120.5, 117.5, 101.6, 61.6, 53.0, 22.2, 13.7; HRMS (ESI) calcd for C22H19O5 [M + H]+ 363.1232, found 363.1246. Ethyl 3-Benzoyl-1,4-dihydroxy-6-methyl-2-naphthoate (8b). According to the general procedure for annulation, the condensation of 5-methylcyanophthalide 20 (173 mg, 1 mmol) with ethyl 4phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 3-benzoyl-1,4dihydroxy-6-methyl-2-naphthoate (8b) (purified by flash column chromatography (hexanes/EtOAc 25:1)) as a dark yellow solid (88.0 mg, 25%): mp 186−188 °C; νmax (KBr, cm−1) 2982, 1738, 1646, 1552, 1494, 1286, 1238, 1024, 772, 696; 1H NMR (400 MHz, CDCl3) δ 12.59 (s, 1H), 8.02−7.99 (m, 2H), 7.66 (s, 1H), 7.54 (dd, J = 8.0, 1.8 Hz, 1H), 7.43−7.32 (m, 4H), 7.31−7.26 (m, 1H), 3.63 (q, J = 7.2 Hz, 2H), 2.49 (s, 3H), 0.65 (t, J = 7.2 Hz, 3H); 13C NMR (126 MHz, CDCl3) δ 187.4, 170.7, 162.0, 142.3, 139.3, 138.2, 134.8, 132.9, 130.2, 129.5, 128.7, 128.6, 127.8, 125.4, 100.0, 61.3, 21.8, 13.3; HRMS (ESI) calcd for C21H18O5Na [M + Na]+ 373.1052, found 373.1047. 4-Methoxy-3-oxo-1,3-dihydronaphtho[2,3-c]furan-1-carbonitrile (21). To a stirred solution of formyl benzamide 27 (0.15 g, 0.56 mmol, 1.0 equiv) in CH2Cl2 (11 mL) at 0 °C were added KCN (7.28 mg, 0.112 mmol, 0.2 equiv) and 18-crown-6 (29.0 mg, 0.112 mmol, 0.2 equiv), followed by TMSCN (0.08 mL, 0.672 mmol, 1.2 equiv). The resulting mixture was warmed to room temperature and stirred at the same temperature for 2.0 h. The solvent was removed under reduced pressure, and the residue was coevaporated with toluene (2 × 2 mL) to remove all traces of TMSCN. The resulting brown oil was dissolved in AcOH (3 mL). The resulting mixture was stirred for 24 h at 30 °C. The solvent was removed under reduced pressure, and the obtained residue was purified by flash column chromatography (silica gel, 20− 30% EtOAc in hexanes). Pure cyano phthalide 21 was obtained as a white solid (0.12 g, 89% yield): mp 167−169 °C; νmax (KBr, cm−1) 3435, 2942, 1767, 1642, 1584, 1338, 1291, 1272, 1084, 1030, 888, 763; 1 H NMR (400 MHz, CDCl3) δ 8.47 (d, J = 8.5 Hz, 1H), 7.93 (d, J = 8.1 Hz, 1H), 7.83−7.71 (m, 2H), 7.66 (dd, J = 8.5, 6.7 Hz, 1H), 6.16 (s, 1H), 4.46 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 165.5, 159.1, 137.8, 136.0, 130.7, 128.6, 128.1, 127.7, 124.6, 116.2, 114.7, 107.3, 65.2, 64.2; HRMS (ESI) calcd for C14H10NO3 [M + H]+ 240.0661, found 240.0659. Ethyl 5-Hydroxy-6-methoxy-2-oxo-3-phenyl-2,3-dihydroanthra[1,2-b]furan-4-carboxylate (7e). According to the general procedure for annulation, the condensation of cyanophthalide 21 (239 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol)

in the presence of tBuOLi (256 mg, 3.2 mmol) produced ethyl 5hydroxy-6-methoxy-2-oxo-3-phenyl-2,3-dihydroanthra[1,2-b]furan-4carboxylate (7e) (purified by flash column chromatography (hexanes/ EtOAc 25:1)) as a light brown solid (252 mg, 59%): mp 179−181 °C; νmax (KBr, cm−1) 3429, 2924, 1802, 1729, 1599, 1461, 1377, 1245, 1218, 1064, 1031, 774; 1H NMR (400 MHz, CDCl3) δ 13.30 (s, 1H), 8.48 (d, J = 8.5 Hz, 1H), 8.35 (s, 1H), 8.04 (d, J = 8.2 Hz, 1H), 7.63 (dddd, J = 15.0, 8.2, 6.8, 1.6 Hz, 2H), 7.31 (td, J = 7.0, 6.6, 3.5 Hz, 3H), 7.21−7.13 (m, 2H), 5.29 (s, 1H), 4.19 (s, 3H), 4.11 (m, 2H), 0.93 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 175.6, 170.5, 162.1, 157.4, 142.9, 136.4, 135.2, 129.0, 128.5, 128.3, 127.9, 127.4, 126.6, 123.7, 122.4, 116.5, 116.1, 115.0, 101.8, 100.1, 64.3, 61.6, 53.3, 13.6; HRMS (ESI) calcd for C26H21O6 [M + H]+ 429.1338, found 429.1345. Ethyl 3-Benzoyl-1,4-dihydroxy-9-methoxyanthracene-2-carboxylate (8c). According to the general procedure for annulation, the condensation of cyanophthalide 21 (239 mg, 1 mmol) with ethyl 4phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of t BuOLi (256 mg, 3.2 mmol) produced ethyl 3-benzoyl-1,4-dihydroxy9-methoxyanthracene-2-carboxylate (8c) (purified by flash column chromatography (hexanes/EtOAc 25:1)) as a dark yellow solid (112 mg, 27%): mp 177−179 °C; νmax (KBr, cm−1) 3420, 2928, 1738, 1653, 1403, 1265, 1216, 1102, 1022, 771; 1H NMR (400 MHz, CDCl3) δ 13.37 (s, 1H), 8.56 (s, 1H), 8.37 (d, J = 8.1 Hz, 1H), 8.09−8.00 (m, 1H), 7.74−7.63 (m, 2H), 7.57 (s, 1H), 7.45 (d, J = 7.5 Hz, 2H), 7.37 (dd, J = 8.4, 6.7 Hz, 2H), 7.33−7.25 (m, 1H), 4.19 (s, 3H), 3.71 (q, J = 7.2 Hz, 2H), 0.68 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 187.4, 171.1, 163.3, 157.2, 137.8, 136.4, 134.9, 131.8, 130.2, 129.9, 129.3, 129.2, 129.0, 129.0, 128.5, 128.4, 125.4, 123.9, 118.5, 99.7, 64.0, 61.3, 13.2; HRMS (ESI) calcd for C25H21O6 [M + H]+ 417.1338, found 417.1349. 9-Methoxynaphtho[2,3-c]furan-1(3H)-one (23).10 A 60 mL reaction tube equipped with a magnetic stir bar was charged with Pd(OAc)2 (100 mg, 0.445 mmol, 10 mol %) followed by KHCO3 (1.10 g, 11.14 mmol), 1-methoxy-2-naphthoic acid (25) (0.9 g, 4.45 mmol), and dibromomethane (18.0 mL). The reaction tube was sealed with a Teflon tube, and the reaction mixture was stirred at 140 °C for 36 h, after which it was filtered through a small pad of Celite. The filtrate was concentrated in vacuo. The residue was purified by silica gel flash column chromatography (20% ethyl acetate in hexanes) to give 9-methoxynaphtho[2,3-c]furan-1(3H)-one (23) as a white solid (0.80 g, 84%): νmax (KBr, cm−1) 3473, 2946, 2853, 1749, 1636, 1462, 1329, 1290, 1208, 1087, 958, 763; 1H NMR (400 MHz, CDCl3) δ 8.42 (d, J = 8.5 Hz, 1H), 7.85 (d, J = 8.2 Hz, 1H), 7.65 (ddd, J = 8.3, 6.7, 1.3 Hz, 1H), 7.56 (ddd, J = 8.2, 6.7, 1.2 Hz, 1H), 7.52 (s, 1H), 5.40 (s, 2H), 4.39 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 168.8, 157.9, 141.3, 137.8, 129.5, 127.8, 127.7, 126.3, 124.3, 114.9, 110.7, 69.0, 63.9. We note that our NMR and IR data do not coincide with the values11 given by Kraus et al. The 1H NMR data perfectly matched with the values10 given by Rickborn et al., but the IR data do not match. HRMS (ESI) calcd for C13H11O3 [M + H]+ 215.0708, found 215.0699. N,N-Diethyl-3-(hydroxymethyl)-1-methoxy-2-naphthamide (26). To a stirred suspension of aluminum trichloride (0.346 g, 2.6 mmol, 2.6 equiv) in dichloromethane (10 mL) at 0 °C was added diethylamine (0.53 mL, 5.0 mmol, 5.0 equiv). The resulting mixture was warmed to room temperature and stirred at the same temperature for 30 min. A solution of phthalide 23 (0.214 g, 1.0 mmol, 1.0 equiv) in dichloromethane (5 mL) was added to the preformed mixture at 0 °C. After 45 min, the reaction was quenched by adding ice water (15 mL). The mixture was stirred for 30 min from 0 °C to room temperature and filtered through Celite. The filtrate was extracted with EtOAc (3 × 25 mL). The combined organic layers were washed with brine (10 mL), dried over Na2SO4, and concentrated. The obtained residue was purified by flash column chromatography (silica gel, 50% EtOAc in hexanes) to give pure hydroxy benzamide 26 (2.70 g, 94%) as a colorless oil: νmax (KBr, cm−1) 3386, 2976, 2936, 1759, 1608, 1459, 1368, 1290, 1220, 1129, 1086, 1029, 772; 1H NMR (400 MHz, CDCl3) δ 8.17−7.94 (m, 1H), 7.92−7.77 (m, 1H), 7.67 (s, 1H), 7.52 (hept, J = 5.1 Hz, 2H), 4.71 (d, J = 12.7 Hz, 1H), 4.55 (d, J = 12.7 Hz, 1H), 4.13 (brs, 1H), 3.97 (s, 3H), 3.80 (dt, J = 14.2, 7.1 Hz, 1H), 3.51 4542

DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

Article

The Journal of Organic Chemistry (dq, J = 14.0, 7.1 Hz, 1H), 3.27 (dt, J = 14.4, 7.2 Hz, 1H), 3.16 (dt, J = 14.5, 7.2 Hz, 1H), 1.33 (t, J = 7.1 Hz, 3H), 1.01 (t, J = 7.1 Hz, 3H); 13 C NMR (100 MHz, CDCl3) δ 168.7, 151.9, 136.4, 134.8, 128.2, 127.2, 127.0, 126.5, 125.6, 124.1, 122.3, 63.9, 62.9, 43.4, 39.4, 13.9, 12.7; HRMS (ESI) calcd for C17H20NO2 [M + H − H2O]+ 270.1494, found 270.1486. N,N-Diethyl-3-formyl-1-methoxy-2-naphthamide (27). To a stirred solution of hydroxy benzamide 26 (0.176 g, 0.613 mmol, 1.0 equiv) in CH2Cl2 (10 mL) were added PCC (0.40 g, 1.84 mmol, 3.0 equiv) and 3 Å molecular sieves (0.3 g) at room temperature. The suspension was stirred overnight at room temperature, then filtered through a short pad of silica gel, rinsed with CH2Cl2, and concentrated. The residue was purified by flash column chromatography (silica gel, 40% EtOAc in hexanes) to give formyl benzamide 27 (0.155 mg, 88.7% yield) as a colorless oil: νmax (KBr, cm−1) 3446, 2933, 1697, 1629, 1457, 1363, 1273, 1088, 772; 1H NMR (400 MHz, CDCl3) δ 10.09 (s, 1H), 8.22 (s, 1H), 8.17 (dd, J = 8.5, 1.1 Hz, 1H), 8.02 (d, J = 8.0 Hz, 1H), 7.71 (ddd, J = 8.3, 6.9, 1.3 Hz, 1H), 7.63 (ddd, J = 8.1, 6.8, 1.3 Hz, 1H), 4.03 (s, 3H), 3.77−3.62 (m, 2H), 3.18−3.10 (m, 2H), 1.37 (t, J = 7.1 Hz, 3H), 1.01 (t, J = 7.2 Hz, 3H); 13 C NMR (100 MHz, CDCl3) δ 190.6, 166.6, 152.8, 133.6, 131.4, 130.9, 130.4, 130.0, 129.6, 127.9, 125.6, 122.8, 63.3, 43.2, 39.2, 13.7, 12.6; HRMS (ESI) calcd for C17H20NO3 [M + H]+ 286.1443, found 286.1439. 4,9-Dihydroxy-5-methoxy-3-phenylnaphtho[2,3-c]furan-1(3H)one (29a). According to the general procedure for annulation, the condensation of 4-dimethoxy cyanophthalide 28a (95 mg, 0.5 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (113 mg, 0.6 mmol) in the presence of tBuOLi (128 mg, 1.6 mmol) produced 4,9-dihydroxy-5methoxy-3-phenylnaphtho[2,3-c]furan-1(3H)-one (29a) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a pink solid (132 mg, 82%): mp 182−184 °C; νmax (KBr, cm−1) 3445, 3372, 2938, 1738, 1657, 1611, 1458, 1417, 1391, 1290, 1214, 1104, 1053, 968, 780, 578; 1H NMR (400 MHz, DMSO-d6) δ 10.13 (s, 1H), 9.09 (s, 1H), 7.93 (d, J = 8.5 Hz, 1H), 7.48 (t, J = 8.1 Hz, 1H), 7.38−7.36 (m, 3H), 7.32−7.30 (m, 2H), 7.15 (d, J = 7.7 Hz, 1H), 6.60 (s, 1H), 3.96 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ 169.0, 156.2, 145.2, 140.5, 137.1, 128.8, 128.5, 128.2, 127.7, 126.2, 123.9, 118.8, 116.5, 108.0, 107.0, 80.5, 56.2; HRMS (ESI) calcd for C19H15O5 [M + H]+ 323.0919, found 323.0904. 4,9-Dihydroxy-7,8-dimethoxy-3-phenylnaphtho[2,3-c]furan1(3H)-one (29b). According to the general procedure for annulation, the condensation of 6,7-dimethoxy cyanophthalide 28bb (219 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced 4,9dihydroxy-7,8-dimethoxy-3-phenylnaphtho[2,3-c]furan-1(3H)-one (29b) (purified by flash column chromatography (hexanes/EtOAc 20:1)) as a yellow solid (229 mg, 65%): mp 181−183 °C; νmax (KBr, cm−1) 3341, 3259, 2942, 1730, 1655, 1617, 1399, 1378, 1279, 1221, 1065, 984, 916; 1H NMR (400 MHz, DMSO-d6) δ 10.18 (s, 1H), 9.33 (s, 1H), 7.94 (d, J = 9.3 Hz, 1H), 7.63 (d, J = 9.3 Hz, 1H), 7.51−7.28 (m, 3H), 7.33−7.21 (m, 2H), 6.59 (s, 1H), 4.03 (s, 3H), 3.97 (s, 3H); 13 C NMR (100 MHz, DMSO-d6) δ 168.2, 148.9, 148.1, 144.9, 138.9, 137.8, 129.2, 129.0, 128.1, 127.0, 124.2, 119.7, 119.5, 118.1, 106.2, 80.0, 62.5, 57.3; HRMS (ESI) calcd for C20H17O6 [M + H]+ 353.1025, found 353.1031. 4,9-Dihydroxy-5,8-dimethoxy-3-phenylnaphtho[2,3-c]furan1(3H)-one (29c). According to the general procedure for annulation, the condensation of 4,7-dimethoxy cyano phthalide 28c (219 mg, 1 mmol) with ethyl 4-phenylbuta-2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced 4,9dihydroxy-5,8-dimethoxy-3-phenylnaphtho[2,3-c]furan-1(3H)-one (29c) (purified by flash column chromatography (hexanes/EtOAc 15:1)) as a red solid (278 mg, 79%): mp 179−181 °C; νmax (KBr, cm−1) 3458, 3318, 1749, 1612, 1438, 1385, 1374, 1154, 1040, 1024, 735, 624; 1H NMR (400 MHz, CDCl3) δ 10.20 (s, 1H), 9.17 (s, 1H), 7.35 (m, 5H), 6.78 (d, J = 8.6 Hz, 1H), 6.72 (d, J = 8.7 Hz, 1H), 6.42 (s, 1H), 4.05 (s, 3H), 3.94 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 168.8, 152.8, 151.1, 148.1, 140.7, 136.6, 128.9, 128.5, 128.4, 127.6,

120.4, 117.3, 108.0, 106.8, 104.7, 80.7, 56.9, 56.5; HRMS (ESI) calcd for C20H17O6 [M + H]+ 353.1025, found 353.1035. 4,9-Dihydroxy-5,8-dimethoxy-3-(4-methoxyphenyl)naphtho[2,3c]furan-1(3H)-one (29d). According to the general procedure for annulation, the condensation of 4,7-dimethoxy cyanophthalide 28c (219 mg, 1 mmol) with ethyl 4-(4-methoxyphenyl)buta-2,3-dienoate 2c (298.0 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced 4,9-dihydroxy-5,8-dimethoxy-3-(4-methoxyphenyl)naphtho[2,3-c]furan-1(3H)-one (29d) (purified by flash column chromatography (hexanes/EtOAc 10:1)) as a yellow solid (298 mg, 78%): mp 186−188 °C; νmax (KBr, cm−1) 3398, 2934, 1747, 1634, 1430, 1223, 1134, 1033, 914, 834, 774, 726; 1H NMR (400 MHz, DMSO-d6) δ 10.29 (s, 1H), 9.24 (s, 1H), 7.20 (d, J = 8.3 Hz, 2H), 7.05 (d, J = 8.8 Hz, 1H), 6.98 (d, J = 8.5 Hz, 1H), 6.91 (d, J = 8.4 Hz, 2H), 6.48 (s, 1H), 4.03 (s, 3H), 3.92 (s, 3H), 3.74 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ 167.9, 160.0, 152.4, 150.8, 147.8, 140.2, 129.5, 127.6, 120.3, 117.0, 114.4, 108.4, 107.7, 106.1, 79.8, 57.3, 57.0, 55.6; HRMS (ESI) calcd for C21H19O7 [M + H]+ 383.1131, found 383.1125. 4,9-Dihydroxy-3-phenylnaphtho[2,3-c]furan-1(3H)-one (29e). According to the general procedure for annulation, the condensation of sulfones phthalide 28d (288 mg, 1 mmol) with ethyl 4-phenylbuta2,3-dienoate 2b (225.8 mg, 1.2 mmol) in the presence of tBuOLi (256 mg, 3.2 mmol) produced 4,9-dihydroxy-3-phenylnaphtho[2,3-c]furan1(3H)-one (29e) (purified by flash column chromatography (hexanes/EtOAc 15:1)) as a white solid (260 mg, 89%): mp 181− 183 °C; νmax (KBr, cm−1) 3468, 3247, 1707, 1392, 1330, 1226, 1145, 1082, 964, 855, 765, 708; 1H NMR (600 MHz, DMSO-d6) δ 10.17 (s, 1H), 9.35 (s, 1H), 8.34 (d, J = 8.6 Hz, 1H), 8.16 (d, J = 8.4 Hz, 1H), 7.69−7.67 (m, 1H), 7.62−7.56 (m, 1H), 7.37 (d, J = 6.8 Hz, 3H), 7.31 (dd, J = 7.5, 2.1 Hz, 2H), 6.68 (s, 1H); 13C NMR (150 MHz, DMSOd6) δ 169.8, 147.5, 139.5, 137.7, 130,9, 129.3, 129.0, 128.8, 128.2, 126.7, 126.3, 125.2, 124.1, 122.7, 106.0, 80.8; HRMS (ESI) calcd for C18H13O4 [M + H]+ 293.0814, found 293.0807.

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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/acs.joc.8b00272. ORTEP view of 7, NMR spectra, and crystal data (PDF) Crystal data of 7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dipakranjan Mal: 0000-0001-6634-9932 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Council of Science & Industrial Research, New Delhi, India, for financial support. S.J. is thankful to IIT KHARAGPUR for her research fellowship and contingency grant. We are also thankful to the DST-FIST for financial support for establishing an NMR facility.

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REFERENCES

(1) Mal, D.; Jana, S. J. Org. Chem. 2016, 81, 11857. (2) Kim, T.; Jeong, K.-H.; Kang, K.-S.; Nakata, M.; Ham, J. Eur. J. Org. Chem. 2017, 2017, 1704. (3) (a) Morrow, G. W.; Swenton, J. S. J. Org. Chem. 1987, 52, 713. (b) Ghosh, B.; Chakraborty, S.; Mal, D. ChemistrySelect 2016, 1, 3097. (c) Brimble, M. A.; Houghton, S. I.; Woodgate, P. D. Tetrahedron 2007, 63, 880. (d) Freskos, J. N.; Morrow, G. W.; Swenton, J. S. J. Org. 4543

DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544

Article

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DOI: 10.1021/acs.joc.8b00272 J. Org. Chem. 2018, 83, 4537−4544