Metal-Free Oxidative Decarbonylative [3+2] Annulation of Terminal

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Cite This: J. Org. Chem. 2018, 83, 8581−8588

Metal-Free Oxidative Decarbonylative [3+2] Annulation of Terminal Alkynes with Tertiary Alkyl Aldehydes toward Cyclopentenes Hua-Xu Zou,†,‡ Yang Li,*,†,‡ Xu-Heng Yang,†,‡ Jiannan Xiang,† and Jin-Heng Li*,†,‡,§ †

State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China § State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China Downloaded via UNIV OF CAMBRIDGE on August 3, 2018 at 08:48:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

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ABSTRACT: A new metal-free oxidative decarbonylative [3+2] annulation of terminal alkynes with tertiary alkyl aldehydes is presented, which features broad substrate scope and excellent selectivity. The selectivity of this reaction toward cyclopentenes and indenes relies on the nature of the aldehyde substrates. While treatment of tertiary γ,δ-unsaturated aldehydes with common terminal alkynes assembles cyclopentenes, 2-methyl-2-arylpropanals succeed in accessing indenes.

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INTRODUCTION Five-membered carbocycles, including cyclopentenes and indenes, are important structural motifs that widely exist in natural products and pharmaceuticals (Figure 1).1,2 In particular, cyclopentenes and their polycycles, such as carbovir,2a 4-ACPCA,2b gukulenin A/B,2c taiwaniaquinol D,2f and (±)-japonicols C,2g exhibit significant biological activities and are versatile intermediates for the synthesis of natural products, bioactive compounds, functional materials, and significant chiral ligands.1 Accordingly, construction of such cyclopentene backbones continues to be a major focus of chemical synthesis, and conventional synthetic methodologies are concentrated on reactions that involve intra- or intermolecular annulation processes.1,3,4 Despite impressive advances in the field, efficient intermolecular approaches to selectively access cyclopentene structures, especially structures that own an all-carbon quaternary center, are challenging,3 and examples of radical-mediated annulation for such purposes are quite rare.4 Recently, remarkable progress in oxidative radical catalysis has been made for the intramolecular annulation of unsaturated molecules (e.g., alkynes and alkenes) initiated by the alkyl carbon-centered radical systems;5 however, there has been no corresponding success with intermolecular annulation reactions, wherein the intramolecular version is usually preferential. We envisioned that the alkyl carbon-centered radical systems having a radical-accepted functional group could be used to competitively trap the radical intermediate from their initial reaction with unsaturated molecules, and consequently, the intramolecular process might be suppressed. Very recently, the Tang group 4g and our group 4h independently developed new strategies, including the classical Cu-catalyzed halogen atom transfer strategy and the Ag© 2018 American Chemical Society

catalyzed oxidative decarboxylation strategy, for the synthesis of cyclopentenes via metal-catalyzed [3+2] annulations of terminal alkynes with unsaturated α-halogenocarbonyls or tertiary unsaturated acids (Scheme 1a, eq 1). In 2016, we developed a new metal-free radical-mediated oxidative decarbonylative hydroalkylation of terminal and internal alkynes with alkyl aldehydes, including benzyl, secondary, and tertiary alkyl aldehydes using a di-tert-butyl peroxide (DTBP) oxidant, which enables the simultaneous formation of a C−C bond and a C−H bond through a sequence of decarbonylation, radical addition, and protonation (Scheme 1b, eq 2). Subsequently, we have reported a manganesecatalyzed intermolecular oxidative acylative annulation of alkynes with vinyl aldehydes for the divergent synthesis of bridged tricarbocycles and bridged bicyclic and tricyclic carbocycles. This reaction can be appliedto secondary γ,δvinyl aldehydes; thus, three chemical bonds, including two C− C bonds and one C−H bond, are formed via aldehyde C(sp2)−H oxidative functionalization, [4+2] annulation, and protonation cascades (Scheme 1b, eq 3). However, using a tertiary γ,δ-vinyl aldehyde, the reaction proceeded via a decarbonylative [3+2] annulation process. On this basis, aldehydes can undergo decabonylation and acylation selectively, which rely on the electronic and steric hindrance nature of steraldehyes as well as the oxidative reaction conditions. Because of continuous interest in the oxidative transformations of aldehydes with unsaturated hydrocarbons, we here report a new metal-free oxidative decarbonylative [3+2] annulation of terminal alkynes with tertiary alkyl aldehydes for Received: May 3, 2018 Published: June 5, 2018 8581

DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

Article

The Journal of Organic Chemistry

Figure 1. Important molecules containing five-membered carbocycle moieties.

Scheme 1. Radical-Mediated [3+2] Annulation and Transformation of Aldehydes

they were inferior to TBHP (5 M in decane). We found that a smaller amount of TBHP (2 equiv) had a negative effect (entry 8), and a larger amount of TBHP (4 equiv) delivered results identical to those seen with 3 equiv of TBHP (entry 9). A screen of the solvent effect revealed that the use of CH2ClCH2Cl resulted in no formation of 3aa (entry 10), and two other solvents, including MeCN and DMF, were less efficient than PhCl (entries 11 and 12, respectively). Among the reaction temperature effects examined, the reaction at 100 °C (entry 1) took precedence over that at either 110 or 90 °C (entry 13 or 14, respectively). It is noteworthy that the reaction can be applied to a 1 mmol scale of alkyne 2a, accessing 3aa in good yield (entry 15). Having established the optimal reaction conditions, we turned our attention to the scope of this [3+2] annulation with regard to alkynes 1 and aldehydes 2 (Table 2, Scheme 2, and Scheme 3). In the presence of 2,2-dimethylpent-4-enal (2a), a wide range of electron-rich and electron-deficient terminal alkynes 1b−p were found to be compatible with the optimal conditions [3ba−pa, respectively (Table 2)]. For aryl alkynes 1b−h, various substituents, including Me, MeO, Br, and Cl, on the aryl moiety were well tolerated (3ba−ha). It was noted that the substitution electronic nature had a fundamental

the synthesis of diverse polysubstituted cyclopentenes. This reaction allows the formation of two C−C bonds through a sequence of decarbonylation,6,7 annulation, and protonation and represents the first example of intermolecular [3+2] annulation of terminal alkynes using an oxidative radicalmediated decarbonylation strategy. Furthermore, this reaction could be extended to access indenes involving aryl C(sp2)−H functionalization via the design of the substrates (Scheme 1c, eq 4).

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RESULTS AND DISCUSSION We initiated our investigations of the [3+2] annulation by combining phenylacetylene (1a) with 2,2-dimethylpent-4-enal (2a) in the presence of oxidants (Table 1). After various reaction parameters had been evaluated, alkyne 1a reacted with 2,2-dimethylpent-4-enal (2a), and 3 equiv of TBHP (tert-butyl hydroperoxide, 5 M in decane) in PhCl at 100 °C for 12 h was preferred and gave the desired cyclopentene 3aa in 74% yield (entry 1). However, the reaction could not occur in the absence of TBHP (entry 2). Other alternative oxidants, such as aqueous TBHP, DTBP (di-tert-butyl peroxide), TBPB (tertbutyl peroxybenzoate), CHP (cumene hydroperoxide), and K2S2O8, displayed reactivity (entries 3−7, respectively), but 8582

DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

Article

The Journal of Organic Chemistry Table 1. Screening of the Reaction Conditionsa

entry

variation from the standard conditions

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15b

none without TBHP TBHP (70% in H2O) DTBP instead of TBHP TBPB instead of TBHP CHP instead of TBHP K2S2O8 instead of TBHP TBHP (2 equiv) TBHP (4 equiv) CH2ClCH2Cl instead of PhCl MeCN instead of PhCl DMF instead of PhCl 110 °C 90 °C none

74 0 26 48 36 51 14 52 72 trace 62 49 73 58 70

As shown in Scheme 3, treatment of alkyne 1a with pivalaldehyde 2j and TBHP afforded hydroalkylation product 5aj in 70% yield (eq 5).6j For 2-phenylacetaldehyde 2k, a primary aldehyde, decarbonylative hydroalkylation of 4ethynyltoluene (1b) also occurred, offering alkene 5bk in 40% yield (eq 6). However, the reaction of alkyne 1a with aldehyde 2a was inhibited when a stoichiometric amount of radical inhibitors, including TEMPO, hydroquinone, and 2,6di-tert-butyl-4-methylphenol (BHT), was added (eq 7). These results suggest that this [3+2] annulation reaction involves a free radical process. To further understand the mechanism, deuterium labeling experiments were performed (eqs 8 and 9). In the presence of D2O, a number of deuterium atoms were incorporated into resulting product 3aa (eq 8). However, no deuterium-labeled product was observed when using CD3CN as the medium (eq 9). A mechanism for this [3+2] annulation reaction was proposed (Scheme 4).4−6 In the presence of peroxide, aldehyde 2a is easily transformed into carbonyl radical A under heating conditions,5,6 which sequentially undergoes decarbonylation to afford alkyl radical intermediate B. Intermolecular addition of intermediate B across the CC bond of alkyne 1a selectively generates vinyl radical intermediate C, followed by rapid annulation with the CC bond in aldehyde 2a to give new alkyl radical intermediate D. Finally, protonation of intermediate D produces desired product 3aa.

a Reaction conditions: 1a (0.2 mmol), 2a (2 equiv), TBHP (5 M in decane, 3 equiv), PhCl (2 mL), argon, 100 °C, and 12 h. b1a (1 mmol), PhCl (5 mL), and 36 h.

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influence on the reactivity, but the substituted position showed a slight effect. While aryl alkyne 1b having an electrondonating p-Me group delivered 3ba in 81% yield, aryl alkyne 1d with a weak electron-withdrawing group was converted into 3da in 60% yield. It was noted that 1-ethynylnaphthalene 1i was transformed into 7,7a,8,9-tetrahydrocyclopenta[a]phenalene (3ia), a cyclopentene-fused polycycle, via oxidative decarbonylation, annulation, protonation, and C−H functionalization cascades. Heteroaryl alkynes 1j and 1k were both viable for giving 3ja and 3ka, respectively, in good yields. Two functionalized alkynes, including vinyl alkyne 1l and 1,3-diyne 1m, were viable to furnish functionalized cyclopentenes 3la and 3ma, respectively, which made this [3+2] annulation more attractive. Unfortunately, aliphatic alkyne 1n was not reactive (3na). For electron-deficient alkynes, namely, 1-phenylprop-2yn-1-one 1o and N-methyl-N-phenylpropiolamide 1p, were suitable substrates (3oa and 3pa). Notably, propiolamide 1p gave 3pa in moderate yield along with an inherent C−H functionalization product 4pa. It was noted that internal alkyne 1q was not a suitable substrate (3qa). The feasibility of the [3+2] annulation with other alkyl aldehydes 2b−g was next investigated (3ab−ad, 3je, 3af, and 3ag). Tertiary alkyl aldehydes, such as 2,2-diethylpent-4-enal (2b), 2-ethyl-2-methylpent-4-enal (2c), 2,2-diethylhex-4-enal 2d, and 2-methyl-2-phenylpent-4-enal 2e, react with alkyne 1a or 1j efficiently, affording the corresponding products 3ab−ad and 3je in moderate yields. However, secondary alkyl aldehyde 1f was inert and could be recovered (3af). Furthermore, an attempt to build a six-membered ring from 2,2-dimethylhex-5enal (2g) failed (3ag). Importantly, this reaction could be applied to 2-methyl-2phenylpropanal (2h). In the presence of TBHP, 2-methyl-2phenylpropanal (2h) could smoothly undergo the [3+2] annulation through C−H functionalization to assemble indenes 3jh and 3ph. However, secondary 2-phenylpropanal 2i failed to execute the reaction under the optimal conditions (3ji).

CONCLUSIONS In summary, we have developed a novel oxidative decarbonylative [3+2] annulation of terminal alkynes with tertiary alkyl aldehyde for the synthesis of polysubstituted cyclopentenes and indenes. This reaction enables the formation of two new C−C bonds using a metal-free oxidative decarbonylation strategy and represents a new oxidative radical-mediated intermolecular [3+2] annulation of alkynes with alkyl radicals that are formed from decarbonylation of tertiary alkyl aldehydes having an aldehyde group connected to the tertiary sp3-hybridized carbon atom. Importantly, selectivity toward cyclopentenes and indenes can be controlled and relies on the nature of substrates, which makes this type of transformation useful in synthetic and medicinal chemistry.

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EXPERIMENTAL SECTION

General Considerations. The 1H and 13C NMR spectra were recorded in a CDCl3 solvent on a NMR spectrometer using TMS as the internal standard. HRMS was measured on an electrospray ionization (ESI) apparatus using time-of-flight (TOF) mass spectrometry. Melting points are uncorrected. Typical Experimental Procedure for the Synthesis of Tertiary Alkyl Aldehydes (2).8 To a solution of aldehyde (10 mmol) in CH2Cl2 (50 mL) was added in one portion KOtBu (13 mmol), followed immediately by allyl bromide (20 mmol). The resulting mixture was stirred for 2 h at room temperature, and then water (20 mL) was added. The organic layer was further washed with water (3 × 10 mL), dried over MgSO4, filtered, and concentrated in vacuum. The resulting residue was purified by flash column chromatography using a petroleum ether/Et2O (9:1) eluent to give title compound 2. Typical Experimental Procedure for Decarbonylative Annulation of Terminal Alkynes with Tertiary Alkyl Aldehydes toward Cyclopentenes. To a Schlenk tube were added alkyne 1 (0.2 mmol), aldehyde 2 (2 equiv), TBHP (5 M in decane, 3 equiv), and PhCl (2 mL). Then the tube was charged with argon, and the contents were stirred at 100 °C (oil bath temperature) under an argon 8583

DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

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The Journal of Organic Chemistry Table 2. Variations of the Terminal Alkynes (1) and Vinyl Alkyl Aldehydes (2)a

Reaction conditions: 1 (0.2 mmol), 2 (2 equiv), TBHP (5 M in decane, 3 equiv), PhCl (2 mL), argon, 100 °C, and 12 h.

a

(3,3,5-Trimethylcyclopent-1-en-1-yl)benzene (3aa). 27.5 mg, 74%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.37 (d, J = 7.6 Hz, 2H), 7.30 (t, J = 7.2 Hz, 2H), 7.20 (t, J = 7.2 Hz, 1H), 5.79 (s, 1H), 3.31 (dd, J = 13.2, 6.8 Hz, 1H), 2.12 (dd, J = 11.2, 10.0 Hz, 1H), 1.47 (dd, J = 12.8, 4.8 Hz, 1H), 1.18 (s, 3H), 1.12 (d, J = 6.8 Hz, 3H), 1.09 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 144.8, 137.4, 136.9, 128.4 (2C), 126.8, 126.5 (2C), 48.2, 44.3, 39.4, 30.3, 29.4, 21.4; LRMS (EI, 70 eV) m/z (%) 186 (M+, 19), 171 (100), 91 (17); HRMS m/z (ESI) calcd for C14H19 ([M + H]+) 187.1481, found 187.1488. 1-Methyl-4-(3,3,5-trimethylcyclopent-1-en-1-yl)benzene (3ba). 32.4 mg, 81%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.27 (d, J = 8.0 Hz, 2H), 7.12 (d, J = 7.2 Hz, 2H), 5.74 (s, 1H), 3.29 (dd, J = 12.8, 6.4 Hz, 1H), 2.33 (s, 3H), 2.14−2.08 (m, 1H), 1.47 (dd, J = 12.4, 4.0 Hz, 1H), 1.17 (s, 3H), 1.12 (d, J = 7.2 Hz, 3H), 1.09 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 144.6, 136.5, 136.5, 134.0, 129.1 (2C), 126.4 (2C), 48.2, 44.2, 39.4, 30.4, 29.5, 21.4, 21.3; LRMS (EI, 70 eV) m/z (%) 200 (M+, 21), 185 (100), 143 (19); HRMS m/z (ESI) calcd for C15H21 ([M + H]+) 201.1638, found 201.1642. 5-(4-Methoxyphenyl)-1,2-diphenylpentane-1,5-dione (3ca). 29.4 mg, 68%; colorless oil; 1H NMR (500 MHz, CDCl3) δ 7.30 (d, J = 8.4 Hz, 2H), 6.85 (d, J = 8.4 Hz, 2H), 5.68 (s, 1H), 3.80 (s, 3H), 3.26 (d, J = 5.6 Hz, 1H), 2.10 (dd, J = 12.4, 8.8 Hz, 1H), 1.47 (dd, J = 12.8, 4.4 Hz, 1H), 1.17 (s, 3H), 1.11 (d, J = 6.7 Hz, 3H), 1.09 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 158.5, 144.1, 135.4, 129.4, 127.5 (2C),

Scheme 2. Decarbonylation Annulation with 2Phenylpropanals (2)

atmosphere for the indicated time until the starting material was completely consumed as monitored by TLC and/or GC-MS analysis. After the reaction had reached completion, the reaction mixture was cooled to room temperature, diluted in diethyl ether, and washed with brine. The aqueous phase was re-extracted with EtOAc (3 × 10 mL). The combined organic extracts were dried over MgSO4 and concentrated in vacuum. The resulting residue was purified by silica gel flash column chromatography (50:1 hexane/ethyl acetate) to afford desired product 3. 8584

DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

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The Journal of Organic Chemistry Scheme 3. Other Aldehydes and Control Experiments

Scheme 4. Possible Mechanism

124.5, 48.0, 44.2, 39.3, 30.0, 29.2, 21.1; LRMS (EI, 70 eV) m/z (%) 222 (M+ + 2, 7), 220 (M+, 23), 205 (100); HRMS m/z (ESI) calcd for C14H18Cl ([M + H]+) 221.1092, found 221.1095. 1-Chloro-2-(3,3,5-trimethylcyclopent-1-en-1-yl)benzene (3ga). 34.3 mg, 78%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.34 (d, J = 7.2 Hz, 1H), 7.18−7.13 (m, 3H), 5.60 (s, 1H), 3.48−3.40 (m, 1H), 2.08 (dd, J = 12.4, 8.0 Hz, 1H), 1.39 (dd, J = 12.4, 7.6 Hz, 1H), 1.19 (s, 3H), 1.11 (s, 3H), 0.92 (d, J = 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 144.0, 141.1, 137.1, 132.5, 130.7, 129.6, 127.8, 126.2, 48.6, 44.3, 41.0, 29.7, 28.1, 20.0; LRMS (EI, 70 eV) m/z (%) 222 (M+ + 2, 7), 220 (M+, 21), 205 (100); HRMS m/z (ESI) calcd for C14H18Cl ([M + H]+) 221.1092, found 221.1095. 2,4-Dimethoxy-1-(3,3,5-trimethylcyclopent-1-en-1-yl)benzene (3ha). 32.5 mg, 66%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.13 (d, J = 8.8 Hz, 1H), 6.44 (d, J = 6.8 Hz, 2H), 5.72 (s, 1H), 3.80 (s, 6H), 3.43−3.34 (m, 1H), 2.03 (dd, J = 12.4, 8.0 Hz, 1H), 1.36 (dd, J = 12.4, 6.8 Hz, 1H), 1.17 (s, 3H), 1.08 (s, 3H), 0.97 (d, J = 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 159.6, 158.2, 142.7, 138.4, 130.1, 119.4, 104.0, 98.8, 55.3, 48.2, 43.9, 40.5, 30.1, 28.9, 20.8; LRMS (EI, 70 eV) m/z (%) 246 (M+, 24), 231 (100), 93 (11); HRMS m/z (ESI) calcd for C16H23O2 ([M + H]+) 247.1693, found 247.1699. 9,9-Dimethyl-7,7a,8,9-tetrahydrocyclopenta[a]phenalene (3ia). 21.5 mg, 46%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.73− 7.65 (m, 3H), 7.43 (t, J = 7.6 Hz, 1H), 7.37 (t, J = 7.6 Hz, 1H), 7.23 (s, 1H), 6.10 (s, 1H), 3.34 (dd, J = 14.8, 5.6 Hz, 1H), 3.28−3.22 (m, 1H), 2.81 (t, J = 13.6 Hz, 1H), 2.22 (dd, J = 12.4, 7.2 Hz, 1H), 1.57− 1.52 (m, 2H), 1.26 (s, 3H), 1.16 (s, 3H); 13C NMR (100 MHz,

113.7 (2C), 55.3, 48.1, 44.1, 39.4, 30.3, 29.5, 21.4; LRMS (EI, 70 eV) m/z (%) 216 (M+, 18), 201 (100), 172 (10); HRMS m/z (ESI) calcd for C15H21O ([M + H]+) 217.1587, found 217.1592. 1-Bromo-4-(3,3,5-trimethylcyclopent-1-en-1-yl)benzene (3da). 31.7 mg, 60%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.42 (d, J = 8.0 Hz, 2H), 7.23 (d, J = 8.0 Hz, 2H), 5.79 (s, 1H), 3.26 (dd, J = 12.4, 6.0 Hz, 1H), 2.15−2.09 (m, 1H), 1.47 (dd, J = 12.8, 4.8 Hz, 1H), 1.18 (s, 3H), 1.09 (d, J = 2.4 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 143.8, 138.2, 135.8, 131.4 (2C), 128.1 (2C), 120.5, 48.1, 44.4, 39.3, 30.2, 29.3, 21.2; LRMS (EI, 70 eV) m/z (%) 266 (M+ + 2, 16), 264 (M+, 16), 251 (82), 170 (100); HRMS m/z (ESI) calcd for C14H18Br ([M + H]+) 265.0586, found 265.0588. 1-Chloro-4-(3,3,5-trimethylcyclopent-1-en-1-yl)benzene (3ea). 31.7 mg, 72%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.27 (d, J = 5.6 Hz, 4H), 5.77 (s, 1H), 3.26 (dd, J = 12.8, 6.4 Hz, 1H), 2.14− 2.09 (m, 1H), 1.47 (dd, J = 12.8, 3.2 Hz, 1H), 1.17 (s, 3H), 1.09 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 143.8, 138.0, 135.3, 132.4, 128.5 (2C), 127.8 (2C), 48.2, 44.4, 39.4, 30.2, 29.4, 21.2; LRMS (EI, 70 eV) m/z (%) 222 (M+ + 2, 17), 220 (M+, 52), 176 (21), 105 (100); HRMS m/z (ESI) calcd for C14H18Cl ([M + H]+) 221.1092, found 221.1095. 1-Chloro-3-(3,3,5-trimethylcyclopent-1-en-1-yl)benzene (3fa). 30.8 mg, 70%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.34 (s, 1H), 7.23 (d, J = 4.4 Hz, 1H), 7.18 (dd, J = 6.0, 3.2 Hz, 1H), 5.81 (s, 1H), 3.31−3.22 (m, 1H), 2.12 (dd, J = 12.8, 8.4 Hz, 1H), 1.47 (dd, J = 12.8, 5.2 Hz, 1H), 1.18 (s, 3H), 1.10 (d, J = 7.6 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 143.6, 138.7, 134.1, 129.4, 126.6, 126.5, 8585

DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

Article

The Journal of Organic Chemistry CDCl3) δ 139.4, 135.9, 133.7, 133.3, 130.6, 129.2, 127.3, 126.0, 125.6, 125.5, 124.3, 120.9, 47.5, 44.9, 41.8, 39.1, 29.9, 28.0; LRMS (EI, 70 eV) m/z (%) 234 (M+, 28), 219 (100), 101 (10); HRMS m/z (ESI) calcd for C18H19 ([M + H]+) 235.1481, found 235.1484. 3-(3,3,5-Trimethylcyclopent-1-en-1-yl)pyridine (3ja). 26.9 mg, 72%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 8.63 (s, 1H), 8.44 (s, 1H), 7.63 (d, J = 8.0 Hz, 1H), 7.27−7.21 (m, 1H), 5.87 (s, 1H), 3.31 (dd, J = 13.6, 6.8 Hz, 1H), 2.14 (dd, J = 12.8, 8.4 Hz, 1H), 1.51− 1.48 (m, 1H), 1.19 (s, 3H), 1.11 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 147.7, 147.6, 141.6, 139.1, 133.4, 132.3, 123.1,47.9, 44.3, 39.1, 29.9, 29.0, 20.8; LRMS (EI, 70 eV) m/z (%) 187 (M+, 18), 172 (100), 144 (11); HRMS m/z (ESI) calcd for C13H18N ([M + H]+) 188.1434, found 188.1439. 3-(3,3,5-Trimethylcyclopent-1-en-1-yl)thiophene (3ka). 25.0 mg, 65%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.24 (t, J = 4.0 Hz, 1H), 7.18 (d, J = 4.8 Hz, 1H), 7.09 (s, 1H), 5.73 (s, 1H), 3.21−3.13 (m, 1H), 2.09 (dd, J = 12.8, 8.8 Hz, 1H), 1.47 (dd, J = 12.8, 4.4 Hz, 1H), 1.20 (d, J = 7.2 Hz, 3H), 1.18 (s, 3H), 1.09 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 139.8, 138.2, 136.3, 126.5, 125.1, 119.8, 48.0, 44.1, 40.2, 30.2, 29.6, 21.7; LRMS (EI, 70 eV) m/z (%) 192 (M+, 26), 177 (100), 143 (36); HRMS m/z (ESI) calcd for C12H17S ([M + H]+) 193.1045, found 193.1047. 1-(3,3,5-Trimethylcyclopent-1-en-1-yl)cyclohex-1-ene (3la). 21.7 mg, 57%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 5.71 (s, 1H), 5.36 (s, 1H), 3.01−2.96 (m, 1H), 2.27−2.21 (m, 1H), 2.14−2.06 (m, 3H), 1.98 (dd, J = 12.8, 9.2 Hz, 1H), 1.70−1.64 (m, 2H), 1.62−1.55 (m, 2H), 1.40 (dd, J = 12.8, 2.8 Hz, 1H), 1.14 (d, J = 7.2 Hz, 3H), 1.12 (s, 3H), 1.04 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 146.2, 133.9, 132.5, 124.5, 47.7, 43.9, 38.7, 30.4, 30.3, 26.5, 25.7, 22.9, 22.5, 22.4; LRMS (EI, 70 eV) m/z (%) 190 (M+, 30), 175 (100), 133 (20); HRMS m/z (ESI) calcd for C14H23 ([M + H]+) 191.1794, found 191.1798. [(3,3,5-Trimethylcyclopent-1-en-1-yl)ethynyl]benzene (3ma). 17.2 mg, 41%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.45− 7.43 (m, 2H), 7.30 (d, J = 5.6 Hz, 3H), 5.92 (s, 1H), 2.04−1.98 (m, 1H), 2.01 (dd, J = 12.4, 4.0 Hz, 1H), 1.35−1.30 (dd, J = 12.8, 7.6 Hz, 1H), 1.21 (d, J = 6.8 Hz, 3H), 1.14 (s, 3H), 1.07 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 147.4, 131.5 (2C), 128.2 (2C), 127.9, 127.3, 123.6, 91.2, 86.1, 47.9, 44.9, 42.1, 29.5, 27.8, 20.2; LRMS (EI, 70 eV) m/z (%) 210 (M+, 45), 195 (100), 151 (13); HRMS m/z (ESI) calcd for C16H19 ([M + H]+) 211.1481, found 211.1487. Phenyl(3,3,5-trimethylcyclopent-1-en-1-yl)methanone (3oa). 19.7 mg, 46%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.76 (d, J = 7.2 Hz, 2H), 7.53 (t, J = 7.2 Hz, 1H), 7.44 (t, J = 7.6 Hz, 2H), 6.17 (s, 1H), 3.38−3.30 (m, 1H), 2.10 (dd, J = 12.8, 8.0 Hz, 1H), 1.44 (dd, J = 12.8, 7.2 Hz, 1H), 1.25 (s, 3H), 1.20 (d, J = 6.8 Hz, 3H), 1.06 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 195.0, 154.8, 145.2, 139.1, 132.0, 129.0 (2C), 128.2 (2C), 47.4, 45.4, 39.2, 29.2, 27.6, 20.1; LRMS (EI, 70 eV) m/z (%) 214 (M+, 26), 105 (100), 77 (42); HRMS m/z (ESI) calcd for C15H19O ([M + H]+) 215.1430, found 215.1427. N,3,3,5-Tetramethyl-N-phenylcyclopent-1-ene-1-carboxamide (3pa). 22.8 mg, 47%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.33 (t, J = 7.6 Hz, 2H), 7.27 (s, 1H), 7.15 (d, J = 7.6 Hz, 2H), 5.43 (s, 1H), 3.35 (s, 3H), 2.56−2.51 (m, 1H), 1.74 (dd, J = 12.4, 7.6 Hz, 1H), 1.12 (dd, J = 12.0, 8.0 Hz, 1H), 1.04 (d, J = 6.8 Hz, 3H), 0.90 (s, 3H), 0.80 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 168.8, 145.9, 144.4, 141.0, 129.0 (2C), 127.1 (2C), 126.9, 47.6, 44.5, 39.9, 37.4, 28.7, 26.7, 19.7; LRMS (EI, 70 eV) m/z (%) 243 (M+, 19), 137 (100), 77 (13); HRMS m/z (ESI) calcd for C16H22NO ([M + H]+) 244.1696, found 244.1701. 2,2,5-Trimethyl-2,5,10,10a-tetrahydrobenzo[b]cyclopenta[e]azepin-4(1H)-one (4pa). 10.1 mg, 21%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.27−7.24 (m, 1H), 7.11 (t, J = 4.2 Hz, 3H), 5.65 (d, J = 1.6 Hz, 1H), 3.71 (dd, J = 15.2, 7.2 Hz, 1H), 3.37 (s, 3H), 3.21 (dd, J = 13.6, 6.4 Hz, 1H), 2.39 (d, J = 13.6 Hz, 1H), 1.79 (dd, J = 12.0, 7.2 Hz, 1H), 1.36 (t, J = 13.2 Hz, 1H), 1.03 (s, 3H), 0.81 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 169.2, 143.7, 142.7, 138.8, 133.4, 130.7, 127.3, 125.3, 122.4, 49.9, 44.6, 44.1, 35.3, 34.3, 28.8, 26.0; LRMS (EI, 70 eV) m/z (%) 241 (M+, 100), 199 (23), 132 (12);

HRMS m/z (ESI) calcd for C16H20NO ([M + H]+) 242.1539, found 242.1542. (3,3-Diethyl-5-methylcyclopent-1-en-1-yl)benzene (3ab). 24.8 mg, 58%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.36 (d, J = 8.0 Hz, 2H), 7.30 (t, J = 7.6 Hz, 2H), 7.20 (t, J = 7.2 Hz, 1H), 5.73 (s, 1H), 3.31−3.22 (m, 1H), 2.08 (dd, J = 13.2, 8.8 Hz, 1H), 1.51−1.37 (m, 5H), 1.09 (d, J = 6.8 Hz, 3H), 0.88 (t, J = 7.6 Hz, 3H), 0.82 (t, J = 7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 146.1, 137.0, 134.6, 128.2 (2C), 126.6, 126.4 (2C), 51.6, 42.7, 39.2, 32.3, 31.8, 21.2, 9.3, 9.0; LRMS (EI, 70 eV) m/z (%) 214 (M+, 4), 185 (100), 143 (14); HRMS m/z (ESI) calcd for C16H23 ([M + H]+) 215.1794, found 215.1799. (3-Ethyl-3,5-dimethylcyclopent-1-en-1-yl)benzene (3ac). 26.0 mg, 65%; dr 1:1; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.37 (t, J = 7.2 Hz, 2.1H), 7.30 (t, J = 7.6 Hz, 2.1H), 7.20 (t, J = 7.2 Hz, 1.0H), 5.77 (s, 1H), 3.35−3.24 (m, 1.0H), 2.19 (dd, J = 12.8, 8.8 Hz, 0.5H), 2.01 (dd, J = 12.8, 8.4 Hz, 0.5H), 1.54−1.33 (m, 3.5H), 1.13 (d, J = 6.0 Hz, 2.1H), 1.10 (d, J = 7.2 Hz, 2.1H), 1.05 (s, 1.5H), 0.91 (t, J = 7.6 Hz, 1.6H), 0.85 (t, J = 7.6 Hz, 1.6H); 13C NMR (100 MHz, CDCl3) δ 145.4, 145.2, 137.0, 136.8, 136.3, 135.7, 128.2 (3C), 126.6, 126.4 (2C), 47.9, 47.8, 45.5, 39.4, 38.7, 34.9, 34.8, 27.8, 26.3, 21.5, 20.9, 9.6, 9.4; LRMS (EI, 70 eV) m/z (%) 200 (M+, 6), 171 (100), 143 (22); HRMS m/z (ESI) calcd for C15H21 ([M + H]+) 201.1638, found 201.1642. (3,3,5-Triethylcyclopent-1-en-1-yl)benzene (3ad). 23.3 mg, 51%; colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.35−7.30 (m, 4H), 7.20 (t, J = 6.8 Hz, 1H), 5.71 (s, 1H), 3.11 (d, J = 8.0 Hz, 1H), 1.98 (dd, J = 13.2, 8.8 Hz, 1H), 1.74−1.72 (m, 1H), 1.52−1.35 (m, 5H), 1.19− 1.09 (m, 1H), 0.90−0.80 (m, 9H); 13C NMR (100 MHz, CDCl3) δ 145.3, 137.4, 135.2, 128.1 (2C), 126.6, 126.4 (2C), 51.5, 46.4, 39.4, 32.3, 31.9, 27.5, 11.8, 9.3, 9.0; LRMS (EI, 70 eV) m/z (%) 228 (M+, 4), 199 (100), 143 (14); HRMS m/z (ESI) calcd for C17H25 ([M + H]+) 229.1951, found 229.1954. 3-(3,5-Dimethyl-3-phenylcyclopent-1-en-1-yl)pyridine (3je). 18.4 mg, 37%; dr 1:1; yellow oil; 1H NMR (400 MHz, CDCl3) δ 8.70 (s, 1.0H), 8.49 (d, J = 4.0 Hz, 1.0H), 7.71 (d, J = 7.6 Hz, 1.0H), 7.39 (d, J = 8.0 Hz, 1.0H), 7.37−7.26 (m, 4.2H), 7.23−7.16 (m, 1.0H), 6.20 (s, 0.5H), 6.17 (s, 0.5H), 3.44−3.33 (m, 1.0H), 2.61 (dd, J = 13.2, 8.8 Hz, 0.5H), 2.49 (dd, J = 12.8, 8.0 Hz, 0.5H), 1.94 (dd, J = 12.8, 6.0 Hz, 0.5H), 1.85 (dd, J = 13.2, 5.2 Hz, 0.5H), 1.60 (s, 1.5H), 1.49 (s, 1.5H), 1.18 (d, J = 6.8 Hz, 1.5H), 1.05 (d, J = 6.8 Hz, 1.5H); 13C NMR (100 MHz, CDCl3) δ 150.3, 149.8, 148.1, 148.0, 147.9, 146.5, 143.6, 136.8, 136.6, 133.7, 128.3, 128.3, 125.9, 125.8, 125.7, 123.2, 51.8, 51.5, 49.8, 49.8, 39.4, 39.0, 29.8, 28.5, 20.8, 19.9; LRMS (EI, 70 eV) m/z (%) 249 (M+, 16), 211 (M+, 27), 173 (100); HRMS m/z (ESI) calcd for C18H20N ([M + H]+) 250.1590, found 250.1597. 3-(1,1-Dimethyl-1H-inden-3-yl)pyridine (3jh). 23.0 mg, 52%; yellow oil; 1H NMR (400 MHz, CDCl3) δ 8.85 (s, 1H), 8.60 (d, J = 4.4 Hz, 1H), 7.89 (d, J = 7.6 Hz, 1H), 7.45−7.37 (m, 3H), 7.30− 7.28 (m, 2H), 6.49 (s, 1H), 1.41 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 154.2, 148.8, 148.7, 145.4, 141.0, 137.7, 134.8, 131.6, 126.5, 125.8, 123.4, 121.7, 120.2, 48.8, 24.6; LRMS (EI, 70 eV) m/z (%) 221 (M+, 13), 177 (15), 143 (100); HRMS m/z (ESI) calcd for C16H16N ([M + H]+) 222.1277, found 222.1281. N,1,1-Trimethyl-N-phenyl-1H-indene-3-carboxamide (3ph). 28.3 mg, 51%; dr 2:1; yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.42 (t, J = 7.6 Hz, 3.0H), 7.32−7.17 (m, 4.5H), 7.09 (t, J = 7.6 Hz, 0.3H), 7.00 (t, J = 7.6 Hz, 0.7H), 6.75−6.69 (m, 1.0H), 6.58 (t, J = 7.6 Hz, 0.3H), 6.21 (d, J = 7.6 Hz, 0.3H), 3.23 (s, 1.0H), 3.13 (s, 2.0H), 1.79 (s, 4.0H), 1.65 (s, 2.1H); 13C NMR (100 MHz, CDCl3) δ 13C NMR (101 MHz, CDCl3) δ 164.9, 164.8, 150.9, 150.8, 149.9, 149.8, 147.7, 142.5, 128.8, 128.7, 128.7, 128.0, 127.4, 127.1, 126.3, 126.0 (2C), 125.9, 125.8, 123.8, 121.6, 120.5, 119.8, 118.7, 107.7, 107.6, 41.3, 41.2, 30.5, 29.2, 26.1, 25.8; LRMS (EI, 70 eV) m/z (%) 277 (M+, 27), 219 (100), 175 (18); HRMS m/z (ESI) calcd for C19H20NO ([M + H]+) 278.1539, found 278.1543. (3,3-Dimethylbut-1-en-1-yl)benzene (5aj). 22.4 mg, 70%; dr 1.25:1; colorless liquid; 1H NMR (400 MHz, CDCl3) δ 7.37−7.35 (m, 1.0H), 7.31−7.23 (m, 2.1H), 7.22−7.17 (m, 2.1H), 6.41 (d, J = 12.8 Hz, 0.5H), 6.29 (s, 0.5H), 6.27 (s, 0.4H), 5.60 (d, J = 12.4 Hz, 8586

DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

Article

The Journal of Organic Chemistry 0.5H), 1.12 (s, 4.2H), 0.98 (s, 5H); 13C NMR (100 MHz, CDCl3) δ 142.6, 141.8, 139.4, 138.0, 128.9, 128.5, 127.5, 127.1, 126.7, 126.1, 126.0, 124.5 (2C), 34.2, 33.3, 31.2, 29.6; LRMS (EI, 70 eV) m/z (%) 160 (M+, 30), 145 (100), 117 (31); HRMS m/z (ESI) calcd for C12H17 ([M + H]+) 161.1325, found 161.1331. 1-Methyl-4-(3-phenylprop-1-en-1-yl)benzene (5bk). 16.6 mg, 40%; dr 2:1; colorless liquid; 1H NMR (400 MHz, CDCl3) δ 7.32−7.09 (m, 9.7H), 6.56 (d, J = 11.2 Hz, 0.6H), 6.43 (d, J = 16.0 Hz, 0.3H), 6.34−6.28 (m, 0.3H), 5.85−5.78 (m, 0.6H), 3.69 (d, J = 7.6 Hz, 1.3H), 3.54 (d, J = 6.4 Hz, 0.6H), 2.35 (s, 2.0H), 2.32 (s, 0.9H); 13C NMR (100 MHz, CDCl3) δ 141.8, 140.9, 136.5, 134.4, 130.9, 130.0, 129.8, 129.2, 128.9, 128.6, 128.6, 128.5, 128.4 (2C), 128.3, 128.2, 126.1, 126.0, 125.9, 39.3, 37.9, 34.7, 21.2; LRMS (EI, 70 eV) m/z (%) 208 (M+, 91), 193 (92), 178 (38), 115 (100); HRMS m/z (ESI) calcd for C16H17 ([M + H]+) 209.1325, found 209.1332.

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Chebib, M.; Hibbs, D. E.; Kim, H.-L.; Johnston, G. A. R.; Salam, N. K.; Hanrahan, J. R. Novel γ-Aminobutyric Acid ρ1 Receptor Antagonists; Synthesis, Pharmacological Activity and StructureActivity Relationships. J. Med. Chem. 2008, 51, 3825−3840 and references cited therein. Gukulenin: (c) Park, S. Y.; Choi, H.; Hwang, H.; Kang, H.; Rho, J.-R. Gukulenins A and B, Cytotoxic Tetraterpenoids from the Marine Sponge Phorbas gukulensis. J. Nat. Prod. 2010, 73, 734−737. A chemokine receptor modulator: (d) Williams, R. M.; Cox, R. J. Paraherquamides, Brevianamides, and Asperparalines: Laboratory Synthesis and Biosynthesis. An Interim Report. Acc. Chem. Res. 2003, 36, 127−139. Aldosterone synthase inhibitors: (e) Voets, M.; Antes, I.; Scherer, C.; Müller-Vieira, U.; Biemel, K.; Marchais-Oberwinkler, S.; Hartmann, R. W. Synthesis and Evaluation of Heteroaryl-Substituted Dihydronaphthalenes and Indenes: Potent and Selective Inhibitors of Aldosterone Synthase (CYP11B2) for the Treatment of Congestive Heart Failure and Myocardial Fibrosis. J. Med. Chem. 2006, 49, 2222−2231. Taiwaniaquinol D: (f) Deng, J.; Zhou, S.; Zhang, W.; Li, J.; Li, R.; Li, A. Total Synthesis of Taiwaniadducts B, C, and D. J. Am. Chem. Soc. 2014, 136, 8185−8188. (±)-Japonicols C: (g) Hu, L.; Xue, Y.; Zhang, J.; Zhu, H.; Chen, C.; Li, X.-N.; Liu, J.; Wang, Z.; Zhang, Y.; Zhang, Y. (±)-Japonicols A-D, Acylphloroglucinol-Based Meroterpenoid Enantiomers with Anti-KSHV Activities from Hypericum japonicum. J. Nat. Prod. 2016, 79, 1322−1328. For a review, see: (h) Faulkner, D. J. Marine Natural Products. Nat. Prod. Rep. 1988, 5, 613−663. (i) Straus, S.; Glass, C. K. Cyclopentenone Prostaglandins: New Insights on Biological Activities and Cellular Targets. Med. Res. Rev. 2001, 21, 185−210. (3) For selected reviews and papers on the synthesis of cyclopentene derivatives via Lu [3+2] annulation reactions, see: (a) Zhang, C.; Lu, X. Phosphine-Catalyzed Cycloaddition of 2,3-Butadienoates or 2Butynoates with Electron-Deficient Olefins. A Novel [3 + 2] Annulation Approach to Cyclopentenes. J. Org. Chem. 1995, 60, 2906−2908. (b) Lu, X.; Du, Y.; Lu, C. Synthetic Methodology Using Tertiary Phosphines as Nucleophilic Catalysts. Pure Appl. Chem. 2005, 77, 1985−1990. (c) Ye, L.-W.; Zhou, J.; Tang, Y. PhosphineTriggered Synthesis of Functionalized Cyclic Compounds. Chem. Soc. Rev. 2008, 37, 1140−1152. (d) Cowen, B. J.; Miller, S. J. Enantioselective Catalysis and Complexity Generation from Allenoates. Chem. Soc. Rev. 2009, 38, 3102−3116. Via another organocatalyzed [3+2] annulation, see: (e) Jin, Z.; Chen, S.; Wang, Y.; Zheng, P.; Yang, S.; Chi, Y. R. β-Functionalization of Carboxylic Anhydrides with β-Alkyl Substituents through Carbene Organocatalysis. Angew. Chem., Int. Ed. 2014, 53, 13506−13509 and references cited therein. Via transition metal catalysis, see: (f) Welker, M. E. 3 + 2 Cycloaddition Reactions of Transition-Metal 2-Alkynyl and η1-Allyl Complexes and Their Utilization in Five-Membered-Ring Compound Syntheses. Chem. Rev. 1992, 92, 97−112. (g) Lautens, M.; Klute, W.; Tam, W. Transition Metal-Mediated Cycloaddition Reactions. Chem. Rev. 1996, 96, 49−92. (h) Gothelf, K. V.; Jørgensen, K. A. Asymmetric 1, 3-Dipolar Cycloaddition Reactions. Chem. Rev. 1998, 98, 863−910. (i) Pandey, G.; Banerjee, P.; Gadre, S. R. Construction of Enantiopure Pyrrolidine Ring System via Asymmetric [3 + 2]-Cycloaddition of Azomethine Ylides. Chem. Rev. 2006, 106, 4484−4517. (j) Wang, Y.; Muratore, M. E.; Rong, Z.; Echavarren, A. M. Formal (4 + 1) Cycloaddition of Methylenecyclopropanes with 7-Aryl-1,3,5-Cycloheptatrienes by Triple Gold(I) Catalysis. Angew. Chem., Int. Ed. 2014, 53, 14022−14026. (4) For papers on the synthesis of cyclopentenes via radicalmediated [3+2] annulation of 2-vinylcyclopropane-1,1-dicarboxylate with alkynes irradiated with 300 nm ultraviolet light, see: (a) Chuang, C.-P.; Ngoi, T. H. J. Free-Radical Reaction of 2-Vinylcyclopropane1,1-dicarboxylate with Alkynes. J. Chin. Chem. Soc. 1991, 38, 379− 381. Via [3+2] annulation of cyclopropylanilines with alkynes via visible light photocatalysis, see: (b) Nguyen, T. H.; Morris, S. A.; Zheng, N. Intermolecular [3 + 2]-Annulation of Cyclopropylanilines with Alkynes, Enynes, and Diynes via Visible Light Photocatalysis. Adv. Synth. Catal. 2014, 356, 2831−2837. (c) Morris, S.; Wang, J.; Zheng, N. The Prowess of Photogenerated Amine Radical Cations in

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01130. Copies of 1H and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jin-Heng Li: 0000-0001-7215-7152 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grants 21472039 and 21625203) and the Jiangxi Province Science and Technology Project (Grants 20165BCB18007 and 20171ACB20015) for financial support.

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REFERENCES

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DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588

Article

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DOI: 10.1021/acs.joc.8b01130 J. Org. Chem. 2018, 83, 8581−8588