Efficient catalytic-free method to produce α-aryl cycloalkanones through highly chemoselective coupling of aryl compounds with oxyallyl cations

Abstract

Catalytic-free coupling of aryl compounds and α-halo cycloketones via in situ generated oxyallyl cation intermediates is reported here. The reactions efficiently afford α-naphthol cycloalkanones with moderate to excellent yields. Electron-rich aromatic compounds are also used to produce the corresponding α-aryl cycloalkanones, and in some cases, analytically pure products are obtained after simple filtration followed by evaporation.

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Introduction

α-Aryl cycloalkanones are important building blocks for concise preparation of medicines or organic functional materials, such as naphthonone,¹ brazan,² and electroluminescence devices.³ The synthesis of α-aryl cycloalkanones, particularly α-naphthol cycloalkanones, can be realized by nucleophilic substitution of α-hydroxylketones with oxygen protected naphthols via Grignard reagents and final deprotection,⁴ or by direct electrophilic substitution of α-hydroxylketones with phenols under acidic conditions.⁵ α-Arylation of β-dicarbonyl compounds, as a powerful technology, needs to be catalyzed by transition metals,⁶ organocatalysts⁷ or enzymes.⁸ Pappo and colleagues have recently reported an efficient cross-dehydrogenative coupling reaction between phenols and α-substituted β-ketoesters catalyzed by iron trichloride.⁹

It has been reported that reaction of β-naphthol with α-cyclohexanone in xylene under refluxing temperature produces compound V directly.¹⁰ However, a high temperature is necessary, the yield is moderate (22–57%), and a large amount of by-product III is evolved. It is worth noting that direct substitution of α-haloketones with naphthols under basic conditions produces phenol ethers III (Scheme 1) as the main product.¹¹

Scheme 1 Different reaction pathways of α-haloketones with naphthols.

Here we report an efficient catalyst-free coupling of unprotected naphthols and oxyallyl cation intermediates generated from α-halocycloketones at room temperature. Oxyallyl cations have been extensively explored for more than half a century.¹² The literature is replete with examples of cycloaddition reactions of such species, especially (4 + 3) cycloaddition reactions.¹³ Moreover, aromatic groups have been extensively used to trap oxyallyl cations in the context of Nazarov cyclization.¹⁴ However, there are only a few reports on the interrupted cycloaddition reactions of aromatic compounds with oxyallyl cations derived from α-halo ketones.¹⁵ Recently, MacMillan and colleagues and ourselves both reported an efficient synthesis of α-indole carbonyl compounds via electrophilic aromatic substitution of unprotected indoles to in situ generated oxyallyl cations.¹⁶ To further expand the reaction scope, we started to use naphthols as nucleophiles to react with oxyallyl cations generated from α-halo cycloalkanones. To our delight, the reaction proceeded smoothly and produced α-naphthol cycloalkanones IV (Scheme 1) which are eventually transformed to product V (Scheme 1) with high yield.

Results and discussion

We first explored the reaction between 2-chlorocyclohexanone (Ia) and 2-naphthol (IIb). At room temperature, most of the starting materials remain unreacted in common organic solvents such as DMF, DMSO, THF, toluene, Et₂O, CH₂Cl₂, EtOAc, CH₃CN, toluene and xylene (Table 1, entries 1 and 2). Although cough medicine, product 1, is evolved when xylene is used as a solvent under high temperature, 33% yield of ether by-product 2, which is not transformed to product 1 within the prolonged reaction time, is also obtained (Table 1, entry 3). In the case where water was used as a solvent, hydrolysis of 2-chlorocyclohexanone becomes the preferred reaction pathway to produce compound 3 serving as the main product,¹⁷ while very little of compound 1 could be isolated (Table 1, entry 4). Owing to their high ionizing power and low nucleophilicity, fluorinated alcohols, such as trifluoroethanol (TFE) and hexafluoro isopropanol (HFIP), are the best solvents for generation of oxyallyl cations.¹⁸ So the fluorinated alcohols were then evaluated (Table 1, entries 5–11). Under the reaction conditions of Na₂CO₃ in TFE at room temperature, a clean reaction with high yield is realized (Table 1, entry 5). Higher temperature (Table 1, entry 6) or higher ionizing power (Table 1, entry 7) is beneficial for the reaction rate rather than the reaction efficiency. With TFE as the solvent, the choice of base has a significant effect on the reaction purity and yield. Organic bases, e.g. pyrrolidine and Et₃N, effectively initiate the reaction (Table 1, entries 10 and 11). However, when a relatively weak base, e.g. sodium bicarbonate, was chosen, virtually no reaction takes place (Table 1, entry 8). In contrast, when a strong base, e.g. NaOH, was used, the reaction becomes complex with formation of Favorskii rearrangement adduct 5 among the side products (Table 1, entry 9).¹⁹

Table 1 Model reaction optimization^a

Entry	Base	Solvent	Temperature (°C)	Time (h)	Product^b (yield%)
a Reaction condition: I (0.5 mmol), II (0.5 mmol), base (0.6 mmol) in solvent (1 mL).b Isolated yields.c DMF, DMSO, THF, Et2O, toluene, CH2Cl2, EtOAc, CH3CN.d Yield of minor product was determined by crude NMR integration.e No other product was isolated. TFE = 2,2,2-trifluoroethanol; HFIP = hexafluoro-2-propanol.
1	Na2CO3	Solvents^c	25	48	1 (<5)
2	Na2CO3	Xylene	25	48	1 (<5)
3	Na2CO3	Xylene	140	24	1/2 (42/33)^d
4	Na2CO3	H2O	25	12	1/3 (8/85)^d
5	Na2CO3	TFE	25	12	1 (91)^e
6	Na2CO3	TFE	80	10	1/4 (88/5)^d
7	Na2CO3	HFIP	25	10	1 (86)^e
8	NaHCO3	TFE	25	48	1 (<5)
9	NaOH	TFE	25	4	1/5 (48/5)^d
10	Pyrrolidine	TFE	25	48	1/4 (71/6)^d
11	Et3N	TFE	25	48	1/4 (80/6)^d

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Then we examined various naphthols in reaction with 2-chlorocyclopentanone. Both protected and unprotected β-naphthols are alkylated at C-1 to produce compounds IV or V in moderate to excellent yields (Table 2, entries 1–6). Bromo-substituted naphthols show excellent outcomes no matter what the substitution position of bromo is (Table 2, entries 3–5). For α-naphthols, the reaction shows excellent chemoselectivity, naphthols are regioselectively alkylated at C-4, and no C-2 alkylated product is obtained (Table 2, entries 7 and 8). To further examine the regioselectivity, 4-chloro-1-naphthol was selected as substrate (Table 2, entry 8). No C-2 alkylated product was detected under our standard reaction conditions. Most of the starting material is recovered after reacting for 12 h.

Table 2 Addition of naphthols to α-halo alkanones^a,b

Entry	Haloketone	Naphthol	Product	Yield
a I (0.5 mmol), II (0.5 mol), Na2CO3 (0.6 mmol) in TFE (1 mL).b Isolated yields.c Ratio of diastereomers was determined by integration of benzylic proton signals in ^1H NMR spectrum.d No another isomer was detected.e The structure is confirmed by 2D NMR spectra (COSY, HMQC and HMBC).f No desired product is obtained.g The ^1H and ^13C NMR spectra are the same as that reported in the literature.^4ah Ic (1.0 mmol), IIi (0.5 mol), Na2CO3 (1.1 mmol) in HFIP (1 mL).i The ether product (15), 2-(naphthalen-2-yloxy)pentan-3-one, is also isolated.
1				91% (dr > 20 : 1)^c
2				92% (dr = 10 : 1)^c
3				93% (dr = 10 : 1)^c
4				93% (dr = 8 : 1)^c
5				94% (dr = 8 : 1)^c
6				88%
7				89%^d^,^e
8				—^f
9				87%^d^,^g
10				11%^h^,^i

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Noncyclic haloketones were also examined. Unfortunately, when 1,1-dichloropropan-2-one or 1,3-dichloropropan-2-one was used, we found that most of the 2-naphthol remained unreacted. Similarly, for substrates 1,3-dibromo-3-methylbutan-2-one and 1-chloro-1,3-diphenylpropan-2-one, a small amount of 2-naphthol is consumed without isolating any desired products. Only 2-bromo-3-pentanone produces a small amount of condensed product 14 in the presence of excessive haloketone (Table 2, entry 10).

To explore the reaction scope further, we screened different hydroxyquinolines, wherein 5-hydroxy and 7-hydroxy isoquinoline gave acceptable yields of α-aryl cycloalkanones along with their corresponding ether products III (Table 3, entries 1 and 2). It should be noted that product 16 is the ketone isomer, not the hemiketal form. When excessive haloketone Ib was used, disubstituted product 22 was obtained as a mixture of stereoisomers (Table 3, entry 3).

Table 3 Addition of hydroxyquinolines to α-chloropentanone^a

Entry	Quinine	Product IV	Product III	Product VI
a Isolated yield.b I (0.5 mmol), II (0.5 mol), Na2CO3 (0.6 mmol) in TFE (1 mL).c I (1.5 mmol), II (0.5 mol), Na2CO3 (1.6 mmol) in TFE (1 mL).d Yield of product 15 was determined by integration of proton signals in ^1H NMR spectrum.e Not detected.f Ratio of diastereomers was determined by integration of proton signals in ^1H NMR spectrum.g The structure is confirmed by 2D NMR spectra (COSY, HMQC and HMBC).
1^b				—^e
2^b				—^e
3^c

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We think that the reaction mechanism is just like that of Friedel–Crafts alkylation, so we used different types of aromatic compound to react with 2-chlorocyclopentanone. We found that an electron-rich system of aromatic compounds plays an important role in the reaction efficiency. When phenol (IIm) was used as a substrate, only ether product 23 is obtained (Table 4, entry 1). For the reaction of substrates IIn and IIo, the main products are, respectively, ether 24 and amine 25, both of which are accompanied by several other complex compounds. However, when the more electron-rich compound IIp was used, an almost quantitative reaction takes place. After simple filtration followed by evaporation, analytically pure product 26 is obtained (Table 4, entry 4). The reaction efficiency is dramatically lowered if there is an electron-withdrawing group on the arene ring. Moderate yield is obtained in the presence of substrate IIq, while no reaction occurs for substrate IIr (Table 4, entries 5 and 6).

Table 4 Addition of aryl compounds to α-chloropentanone^a,b

Entry	Aryl compounds	Product	Yield
a I (0.5 mmol), II (0.5 mol), Na2CO3 (0.6 mmol) in TFE (1 mL).b Isolated yields.c Not detected.
1			46%
2			38%
3			32%
4			99%
5			48%
6			—^c

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Conclusions

We developed a highly efficient and practical method for chemoselective coupling of α-halo cycloalkanones with electron-rich aromatic compounds, especially naphthols, at room temperature. No protection of naphthol substrates and no catalysts are required in our protocol. α-Aryl cycloalkanone 26 is obtained in quantitative yield, and analytically pure products are obtained after simple filtration followed by evaporation. An electron-rich system of aryl compounds plays an important role in the reaction efficiency. Further research on the interrupted cycloaddition reaction of oxyallyl cations is being pursued in our lab.

Experimental section

General information

Nuclear magnetic resonance spectra (¹H and ¹³C) were recorded on JEOL ECA400 (400 MHz), Bruker AV300 (300 MHz), AV400 (400 MHz), AV500 (500 MHz) or BBFO400 (400 MHz) spectrometers. ¹H NMR chemical shifts were recorded in parts per million (ppm, δ) relative to tetramethylsilane (δ 0.00) or DMSO (δ = 2.50, singlet), and the splitting patterns were designated as singlet (s), doublet (d), triplet (t), quartet (q), dd (doublet of doublets); m (multiplets), etc. All first-order splitting patterns were assigned on the basis of the appearance of the multiplet. Splitting patterns that could not be easily interpreted were designated as multiplet (m) or broad (br). High resolution mass spectral analysis (HRMS) was performed on a Finnigan MAT 95 XP mass spectrometer (Thermo Electron Corporation). Analytical thin-layer chromatography (TLC) was carried out on Merck 60 F254 pre-coated silica gel plates (0.2 mm thickness). Visualization was performed using a UV lamp or chemical stains such as KMnO₄ and 2,4-dinitrophenyl hydrazine solutions.

Commercially available materials purchased from Alfa Aesar or Aldrich were used as received, apart from α-haloketones that were further purified via distillation or column chromatography over silica gel prior to use. Some of the α-chloroketones (2-bromo-3-pentanone and 1,3-dibromo-3-methylbutan-2-one) were prepared using a literature method.²⁰

Typical procedure for metal-free coupling of aromatic compounds with α-haloketones

A 4 mL vial equipped with a magnetic stir bar was charged with fresh distilled α-halo ketone I (0.5–1.5 mmol), aromatic compound II (0.5 mmol) and TFE or HFIP (1.0 mL). Anhydrous Na₂CO₃ (0.6–1.6 mmol) was added to the reaction mixture and stirred at room temperature. After completion of the reaction (about 12–24 h, monitored by TLC or crude ¹H NMR analysis), the reaction mixture was filtered through a celite pad using Et₂O or CH₂Cl₂ and the filtrate was concentrated under reduced pressure. The crude residue was further purified by silica gel flash chromatography using EtOAc/hexanes as eluent to give pure products. In some cases, analytically pure products could be obtained merely by simple filtration and evaporation under reduced pressure.

7a,8,9,10,11,11a-Hexahydronaphtho[2,1-b]benzofuran-7a-ol (1). White solid, Mp: 136–137 °C; ¹H NMR (400 MHz, CDCl₃) δ = 7.81 (d, J = 8.3 Hz, 1H), 7.69 (t, J = 9.2 Hz, 2H), 7.45 (ddd, J = 8.2, 6.9, 1.2 Hz, 1H), 7.31 (ddd, J = 8.1, 6.8, 1.1 Hz, 1H), 7.17 (d, J = 8.7 Hz, 1H), 3.41 (dd, J = 10.2, 6.8 Hz, 1H), 3.26 (d, J = 3.1 Hz, 1H), 2.46–2.27 (m, 2H), 1.90 (ddd, J = 14.1, 12.3, 5.2 Hz, 1H), 1.85–1.74 (m, 1H), 1.64–1.28 (m, 4H), 1.15 (dddd, J = 14.0, 12.0, 10.3, 3.9 Hz, 1H) ppm. ¹³C NMR (100 MHz, CDCl₃) δ = 154.23, 130.61, 129.91, 129.04, 126.72, 124.64, 123.18, 122.79, 113.05, 109.78, 46.62, 33.37, 30.86, 21.79, 21.76 ppm. HRMS (ESI) calcd for C₁₆H₁₇O₂ (M + 1)⁺: 241.1229, found: 241.1233.

2-(Naphthalen-2-yloxy)cyclohexanone (2)^11a. White solid, Mp: 104–106 °C; ¹H NMR (400 MHz, CDCl₃) δ = 7.66 (dd, J = 2.8, 9.2 Hz, 2H), 7.58 (d, J = 8.0 Hz, 1H), 7.33 (t, J = 7.2 Hz, 1H), 7.23 (t, J = 7.2 Hz, 1H), 7.10 (dd, J = 2.4, 8.8 Hz, 1H), 6.93 (d, J = 2.0 Hz, 1H), 4.72–4.65 (m, 1H), 2.59–2.52 (m, 1H), 2.34–2.21 (m, 2H), 2.01–1.89 (m, 3H), 1.74–1.62 (m, 2H) ppm.

2-Hydroxycyclohexanone (3)²¹. White solid; Mp: 110–112 °C; ¹H NMR (400 MHz, CDCl₃) δ = 4.13 (dd, J = 11.9, 7.0 Hz, 1H), 3.64 (s, 1H), 2.57 (ddt, J = 13.8, 4.3, 2.3 Hz, 1H), 2.52–2.43 (m, 1H), 2.36 (tdd, J = 13.7, 6.4, 1.5 Hz, 1H), 2.16–2.06 (m, 1H), 1.96–1.84 (m, 1H), 1.81–1.68 (m, 1H), 1.68–1.58 (m, 2H), 1.56–1.40 (m, 2H) ppm.

2-(2,2,2-Trifluoroethoxy)cyclohexanone (4)²². Colorless oil; ¹H NMR (400 MHz, CDCl₃) δ = 4.20 (dq, J = 12.7, 9.0 Hz, 1H), 4.01 (dd, J = 10.4, 5.8 Hz, 1H), 3.76 (dq, J = 12.7, 8.5 Hz, 1H), 2.58–2.45 (m, 1H), 2.30 (dtd, J = 12.3, 5.4, 2.4 Hz, 2H), 2.08–1.88 (m, 2H), 1.85–1.62 (m, 3H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 208.47, 128.00, 125.23, 122.45, 119.68, 83.86, 67.88, 67.54, 67.20, 66.86, 40.53, 34.23, 27.25, 23.22 ppm. HRMS (ESI) calcd for C ₈H₁₂F₃O₂ (M + 1)⁺: 197.0789, found: 197.0784.

8,9,10,10a-Tetrahydro-7aH-cyclopenta [b] naphtha [1,2-d]furan-7a-ol (6). White solid; Mp: 128–129 °C; further purification could be realized by recrystallization using ethanol. ¹H NMR (400 MHz, CDCl₃) δ = 7.80 (d, J = 8.2 Hz, 1H), 7.69 (d, J = 8.8 Hz, 1H), 7.63 (d, J = 8.4 Hz, 1H), 7.46 (ddd, J = 8.2, 6.9, 1.2 Hz, 1H), 7.31 (ddd, J = 8.1, 6.9, 1.1 Hz, 1H), 7.09 (d, J = 8.8 Hz, 1H), 3.84 (dd, J = 9.3, 3.0 Hz, 1H), 3.33 (s, 1H), 2.49–2.19 (m, 2H), 2.11 (ddd, J = 13.0, 10.9, 6.4 Hz, 1H), 1.93 (dddd, J = 8.6, 6.5, 5.4, 3.3 Hz, 1H), 1.81 (dtd, J = 9.6, 6.3, 3.0 Hz, 1H), 1.72–1.58 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 155.38, 130.48, 129.62, 129.35, 128.96, 126.78, 123.01, 122.44, 121.74, 121.24, 111.83, 51.48, 40.03, 32.71, 24.84 ppm. HRMS (ESI) calcd for C₁₅H₁₅O₂ (M + 1)⁺: 227.1072, found: 227.1069.

3-Bromo-8,9,10,10a-tetrahydro-7aH-cyclopenta[b]naphtha [1,2-d]furan-7a-ol (7). White solid; Mp: 86–87 °C; ¹H NMR (400 MHz, CDCl₃) δ = 7.94 (s, 1H), 7.58 (d, J = 8.8 Hz, 1H), 7.49 (q, J = 8.7 Hz, 2H), 7.08 (d, J = 8.8 Hz, 1H), 3.79 (d, J = 8.9 Hz, 1H), 3.60–3.30 (brs, 1H), 2.48–2.17 (m, 2H), 2.10 (dd, J = 18.1, 11.9 Hz, 1H), 1.94–1.73 (m, 2H), 1.74–1.52 (m, 2H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 155.76, 130.87, 130.72, 130.03, 128.93, 128.53, 124.11, 121.86, 121.58, 116.51, 112.87, 51.38, 40.07, 32.78, 24.87 ppm. HRMS (ESI) calcd for C₁₅H₁₄BrO₂ (M + 1)⁺: 306.1745, found: 306.1744.

2-Bromo-8,9,10,10a-tetrahydro-7aH-cyclopenta[b]naphtha [1,2-d]furan-7a-ol (8). Colorless oil; ¹H NMR (500 MHz, CDCl₃) δ = 7.75 (s, 1H), 7.65 (dd, J = 8.6, 3.6 Hz, 2H), 7.37 (dd, J = 8.7, 1.8 Hz, 1H), 7.07 (dd, J = 8.7, 3.4 Hz, 1H), 3.81–3.70 (m, 1H), 3.50–3.40 (brs, 1H), 2.49–2.34 (m, 1H), 2.34–2.20 (m, 1H), 2.19–2.04 (m, 1H), 1.94–1.72 (m, 2H), 1.64 (dq, J = 10.6, 6.3 Hz, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 156.09, 131.62, 130.61, 129.39, 127.93, 126.38, 124.57, 122.02, 121.22, 120.68, 112.18, 51.13, 40.02, 32.70, 24.80 ppm. HRMS (ESI) calcd for C₁₅H₁₄BrO₂ (M + 1)⁺: 306.1745, found: 306.1742.

6-Bromo-8,9,10,10a-tetrahydro-7aH-cyclopenta[b]naphtha [1,2-d]furan-7a-ol (9). Colorless oil, 94% yield. ¹H NMR (500 MHz, CDCl₃) δ = 7.90 (s, 1H), 7.71 (t, J = 10.8 Hz, 1H), 7.60 (d, J = 8.3 Hz, 1H), 7.47 (dd, J = 8.1, 7.1 Hz, 1H), 7.33 (dd, J = 8.1, 7.0 Hz, 1H), 3.99–3.88 (m, 1H), 3.51 (d, J = 10.4 Hz, 1H), 2.52–2.35 (m, 2H), 2.19–2.08 (m, 1H), 1.99–1.88 (m, 1H), 1.83 (ddd, J = 12.7, 6.4, 3.5 Hz, 1H), 1.76–1.64 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 152.39, 131.06, 130.53, 129.43, 128.02, 127.00, 123.93, 122.92, 122.61, 122.23, 104.75, 52.34, 40.20, 32.80, 24.94 ppm. HRMS (ESI) calcd for C₁₅H₁₄BrO₂ (M + 1)⁺: 306.1745, found: 306.1740.

2-(2-Methoxynaphthalen-1-yl)cyclopentanone (10). White solid; Mp: 92–93 °C; ¹H NMR (400 MHz, CDCl₃) δ = 7.79 (dd, J = 8.5, 4.4 Hz, 2H), 7.47 (t, J = 7.3 Hz, 1H), 7.39–7.30 (m, 1H), 7.30–7.22 (m, 2H), 3.85 (s, 3H), 2.60 (ddd, J = 18.4, 11.9, 8.6 Hz, 1H), 2.45 (ddd, J = 19.3, 12.4, 8.0 Hz, 2H), 2.33–2.10 (m, 2H), 2.09–1.84 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 219.87, 129.62, 128.98, 128.76, 126.69, 123.51, 122.48, 114.12, 56.07, 47.79, 38.22, 30.83, 22.08 ppm. HRMS (ESI) calcd for C₁₅H₁₇O₂ (M + 1)⁺: 241.1229, found: 241.1230.

2-(1-Hydroxynaphthalen-4-yl)cyclopentanone (11). Yellow solid; Mp: 102–103 °C; ¹H NMR (400 MHz, CDCl₃) δ = 8.22–8.08 (m, 1H), 7.78 (dd, J = 8.4, 3.4 Hz, 1H), 7.57–7.34 (m, 2H), 6.96 (dd, J = 9.8, 7.9 Hz, 1H), 6.59 (dd, J = 15.7, 7.8 Hz, 1H), 5.99 (brs, 1H), 4.06–3.88 (m, 1H), 2.71–2.53 (m, 2H), 2.53–2.36 (m, 1H), 2.30–2.10 (m, 2H), 2.10–1.89 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 221.02, 151.09, 132.87, 127.01, 126.41, 125.37, 125.12, 124.74, 123.33, 122.64, 108.31, 52.34, 39.10, 32.48, 21.06 ppm. HRMS (ESI) calcd for C₁₆H₁₇O₂ (M + 1)⁺: 241.1229, found: 241.1231.

2-(4-Methoxy naphthalen-2-yl)cyclopentanone (13)^4a. Colorless oil, ¹H NMR (400 MHz, CDCl₃) δ = 8.31 (dd, J = 8.3, 1.2 Hz, 1H), 7.81 (d, J = 8.0 Hz, 1H), 7.60–7.38 (m, 2H), 7.14 (d, J = 8.0 Hz, 1H), 6.77 (d, J = 8.0 Hz, 1H), 4.01–3.93 (m, 1H), 3.98 (s, 3H), 2.68–2.51 (m, 2H), 2.51–2.35 (m, 1H), 2.30–2.12 (m, 2H), 2.04 (dddd, J = 14.1, 12.2, 7.1, 3.3 Hz, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 219.12, 154.76, 132.88, 127.33, 126.58, 126.16, 125.12, 125.02, 123.46, 122.78, 103.47, 55.54, 52.03, 38.94, 32.45, 21.08 ppm. HRMS (ESI) calcd for C₁₅H₁₇O₂ (M + 1)⁺: 241.1229, found: 241.1234.

2-Ethyl-1-methyl-1,2-dihydronaphtho[2,1-b] furan-2-ol (14). Freshly distilled 2-chloro-3-pentanone (1.0 mmol) was added into the solution of 2-naphthol (0.5 mmol), anhydrous Na₂CO₃ (0.6 mmol) and HFIP (1.0 mL). The mixture was stirred at r.t. over 2 days, and then filtered through a celite pad using CH₂Cl₂. The filtrate was concentrated under reduced pressure and separated by silica gel flash chromatography to give two pure products. Product 13 was white solid, Mp: 101–103 °C, 11% yield. ¹H NMR (400 MHz, CDCl₃) δ = 8.38 (d, J = 8.4 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.66–7.50 (m, 3H), 7.44 (t, J = 7.6 Hz, 1H), 2.84 (dd, J = 15.2, 7.6 Hz, 2H), 2.57 (s, 3H), 1.33 (t, J = 7.6 Hz, 3H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 162.04, 157.26, 130.32, 128.43, 127.51, 125.75, 124.22, 124.10, 123.09, 122.21, 121.72, 113.01, 27.90, 25.00, 23.93 ppm. HRMS (ESI) calcd for C₁₇H₁₉O₂ (M + 1)⁺: 211.1123, found: 211.1125.

2-(Naphthalen-2-yloxy)pentan-3-one (15). Brown oil, 7% yield. ¹H NMR (500 MHz, CDCl₃) δ = 7.81–7.72 (m, 2H), 7.69 (d, J = 8.2 Hz, 1H), 7.48–7.41 (m, 1H), 7.39–7.30 (m, 1H), 7.17 (dd, J = 8.9, 2.5 Hz, 1H), 6.98 (d, J = 2.4 Hz, 1H), 4.80 (q, J = 6.9 Hz, 1H), 2.73 (dq, J = 18.9, 7.2 Hz, 1H), 2.47 (dq, J = 18.9, 7.3 Hz, 1H), 2.20–2.04 (m, 1H), 1.56 (d, J = 7.0 Hz, 3H), 1.01 (t, J = 7.3 Hz, 3H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 213.20, 155.48, 134.33, 129.94, 129.26, 127.63, 126.89, 126.63, 124.10, 118.75, 107.42, 79.13, 29.89, 18.07, 7.11 ppm. HRMS (ESI) calcd for C₁₅H₁₇O₂ (M + 1)⁺: 229.1229, found: 229.1224.

2-(7-Hydroxyisoquinolin-8-yl)cyclopentanone (16). Yellow solid; Mp: 92–93 °C, ¹H NMR (400 MHz, CDCl₃) δ = 9.28 (s, 1H), 8.50 (d, J = 5.8 Hz, 1H), 8.14 (d, J = 5.8 Hz, 1H), 7.16 (dd, J = 17.2, 8.2 Hz, 1H), 7.07–6.96 (m, 1H), 4.14–4.02 (m, 1H), 2.71–2.52 (m, 2H), 2.52–2.34 (m, 1H), 2.22 (tt, J = 17.4, 6.0 Hz, 2H), 2.12–2.03 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 218.72, 152.19, 147.69, 140.26, 129.17, 128.13, 127.12, 126.39, 116.83, 112.65, 51.10, 38.65, 32.18, 20.98 ppm. HRMS (ESI) calcd for C₁₄H₁₄NO₂ (M + 1)⁺: 228.1025, found: 228.1022.

2-(5-Hydroxyisoquinolin-8-yl)cyclopentanone (18). White solid, Mp: 103–104 °C, 36% yield. ¹H NMR (400 MHz, CDCl₃) δ = 8.92 (s, 1H), 8.15 (d, J = 5.6 Hz, 1H), 7.54 (d, J = 8.8 Hz, 1H), 7.45 (d, J = 5.6 Hz, 1H), 7.21 (d, J = 5.6 Hz, 1H), 3.87 (t, J = 6.8 Hz, 1H), 2.43–2.34 (m, 2H), 2.23–2.18 (m, 1H), 1.84–1.81 (m, 2H), 1.71–1.52 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 205.73, 157.01, 146.54, 138.91, 131.63, 130.83, 130.24, 127.65, 121.76, 108.57, 50.42, 39.91, 33.32, 24.56 ppm. HRMS (ESI) calcd for C₁₄H₁₄NO₂ (M + 1)⁺: 228.1025, found: 228.1026.

2-(Isoquinolin-5-yloxy)cyclopentanone (19). Colorless oil, 15% yield. ¹H NMR (400 MHz, CDCl₃) δ = 9.14 (s, 1H), 8.42 (d, J = 5.7 Hz, 1H), 7.75 (d, J = 9.0 Hz, 1H), 7.59 (d, J = 5.7 Hz, 1H), 7.42 (dd, J = 8.9, 2.5 Hz, 1H), 7.34 (d, J = 2.4 Hz, 1H), 4.79 (t, J = 8.4 Hz, 1H), 2.70–2.57 (m, 1H), 2.51–2.31 (m, 2H), 2.31–2.17 (m, 1H), 2.13–1.88 (m, 2H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 213.42, 156.68, 151.17, 141.44, 131.80, 129.63, 128.24, 123.89, 120.26, 107.63, 79.54, 35.28, 29.42, 17.33 ppm. HRMS (ESI) calcd for C₁₄H₁₄NO₂ (M + 1)⁺: 228.1025, found: 228.1027.

2-(8-Hydroxy-2-methylquinolin-5-yl)cyclopentanone (20). With an excess amount of α-haloketone, three products were obtained. Product 19 was a white solid, Mp: 114–115 °C, 15% yield. ¹H NMR (400 MHz, CDCl₃) δ = 8.08 (d, J = 8.7 Hz, 1H), 7.32 (d, J = 8.7 Hz, 1H), 7.15 (d, J = 7.9 Hz, 1H), 7.08 (d, J = 7.9 Hz, 1H), 3.88 (dd, J = 10.4, 8.7 Hz, 1H), 2.72 (s, 3H), 2.63–2.48 (m, 2H), 2.41 (ddd, J = 19.1, 10.4, 8.6 Hz, 1H), 2.32–2.13 (m, 2H), 2.13–1.95 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 218.07, 156.53, 150.89, 138.08, 133.04, 125.50, 124.98, 124.87, 122.50, 109.19, 51.27, 38.51, 31.50, 24.73, 21.04 ppm. HRMS (ESI) calcd for C₁₅H₁₆NO₂ (M + 1)⁺: 242.1181, found: 242.1177.

2-((2-Methylquinolin-8-yl)oxy)cyclopentanone (21). White solid, Mp: 87–89 °C, 43% yield. ¹H NMR (400 MHz, CDCl₃) δ = 8.01 (d, J = 8.4 Hz, 1H), 7.47–7.33 (m, 2H), 7.33–7.28 (m, 1H), 7.22 (dd, J = 7.5, 1.2 Hz, 1H), 5.09–4.89 (m, 1H), 2.77 (s, 3H), 2.72–2.58 (m, 1H), 2.46–2.36 (m, 1H), 2.28 (dddd, J = 14.1, 10.3, 6.6, 4.7 Hz, 2H), 2.13–1.79 (m, 2H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 214.01, 158.31, 153.25, 140.07, 136.16, 127.82, 125.49, 122.51, 121.08, 112.90, 81.11, 35.38, 29.37, 25.60, 17.20 ppm. HRMS (ESI) calcd for C₁₅H₁₆NO₂ (M + 1)⁺: 242.1181, found: 242.1180.

2-((2-Methyl-5-(2-oxocyclopentyl)quinolin-8-yl)oxy)cyclopentanone (22). White solid, mixture of two diastereomers (dr = 1 : 1), Mp: 76–79 °C, 11% yield. ¹H NMR (400 MHz, CDCl₃) δ = 8.08 (dd, J = 11.5, 8.8 Hz, 1H), 7.31 (dd, J = 8.7, 3.2 Hz, 1H), 7.20–7.08 (m, 2H), 5.07–4.82 (m, 1H), 3.92 (dd, J = 18.8, 8.8 Hz, 1H), 2.76 (d, J = 2.1 Hz, 3H), 2.67–2.49 (m, 3H), 2.49–2.33 (m, 3H), 2.33–2.13 (m, 4H), 2.13–1.98 (m, 1H), 1.98–1.80 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 218.07, 217.80, 214.13, 213.94, 157.91, 157.86, 152.57, 152.51, 140.74, 140.52, 132.69, 132.52, 128.47, 128.26, 126.70, 126.58, 124.15, 123.85, 122.30, 122.21, 113.27, 112.37, 81.38, 81.16, 51.47, 51.07, 38.71, 38.61, 35.44, 35.39, 31.80, 31.69, 29.40, 25.40, 21.04, 21.02, 17.22 ppm. HRMS (ESI) calcd for C₂₀H₂₂NO₃ (M + 1)⁺: 324.1600, found: 324.1603.

2-Phenoxycyclopentanone (23)^11a. White solid; Mp: 69–71 °C; ¹H NMR (300 MHz, CDCl₃) δ = 7.28 (t, J = 8.1 Hz, 2H), 6.96–7.00 (m, 3H), 4.58–4.63 (m, 1H), 2.46–2.55 (m, 1H), 2.33–2.41 (m, 2H), 2.11–2.19 (m, 1H), 1.89–2.04 (m, 2H).

2-(2-Methoxyphenoxy)cyclopentanone (24). Colorless oil; ¹H NMR (400 MHz, CDCl₃) δ = 7.08–6.94 (m, 2H), 6.94–6.82 (m, 2H), 4.60 (dd, J = 9.2, 8.0 Hz, 1H), 3.85 (s, 3H), 2.51–2.40 (m, 1H), 2.35 (dt, J = 9.5, 6.8 Hz, 2H), 2.21–2.08 (m, 1H), 2.10–1.98 (m, 1H), 1.95–1.76 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 213.77, 150.31, 147.32, 122.81, 120.82, 117.09, 112.34, 81.00, 55.95, 35.24, 29.53, 17.20 ppm. HRMS (ESI) calcd for C₁₂H₁₅O₃ (M + 1)⁺: 207.1021, found: 207.1024.

2-((3-Methoxyphenyl) amino)cyclopentanone (25). White solid; Mp: 121–122 °C; ¹H NMR (400 MHz, CDCl₃) δ = 7.10 (t, J = 8.1 Hz, 1H), 6.37–6.27 (m, 2H), 6.22 (t, J = 2.3 Hz, 1H), 4.42 (s, 1H), 3.77 (s, 3H), 3.69 (dd, J = 11.4, 7.9 Hz, 1H), 2.76 (ddd, J = 12.5, 7.6, 6.3 Hz, 1H), 2.56–2.41 (m, 1H), 2.21 (dd, J = 19.5, 9.4 Hz, 1H), 2.17–2.07 (m, 1H), 1.99–1.84 (m, 1H), 1.58 (qd, J = 12.3, 6.8 Hz, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 215.63, 160.86, 148.85, 130.09, 106.57, 103.21, 99.66, 61.96, 55.15, 34.68, 31.83, 17.79 ppm. HRMS (ESI) calcd for C₁₂H₁₆NO₂ (M + 1)⁺: 206.1181, found: 206.1180.

2-(2,4,6-Trimethoxyphenyl)cyclopentanone (26). Freshly distilled α-chlorocyclopentanone (0.5 mmol) was added into the solution of 1,3,5-trimethoxybenzene (0.5 mmol), anhydrous Na₂CO₃ (0.6 mmol) and TFE (1.0 mL). After stirring at r.t. for 8 h, a white solid was obtained merely by filtration through a celite pad using dichloromethane and evaporation under reduced pressure. Mp: 117–118 °C, ¹H NMR (400 MHz, CDCl₃) δ = 6.12 (s, 2H), 3.79 (s, 3H), 3.74 (s, 6H), 3.68 (dd, J = 11.0, 8.7 Hz, 1H), 2.37 (dd, J = 9.6, 5.5 Hz, 2H), 2.28–2.17 (m, 1H), 2.09 (ddt, J = 16.8, 11.7, 4.2 Hz, 2H), 1.93–1.75 (m, 1H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ = 220.63, 160.28, 158.55, 109.34, 91.18, 55.61, 55.35, 45.06, 38.05, 29.92, 21.82 ppm. HRMS (ESI) calcd for C₁₄H₁₉O₄ (M + 1)⁺: 251.1283, found: 251.1280.

2-(3-Acetyl-2,4,6-trimethoxyphenyl)cyclopentanone (27). White solid, Mp: 86–87 °C, 48% yield. ¹H NMR (500 MHz, CDCl₃) δ = 6.25 (s, 1H), 3.82 (s, 3H), 3.78 (s, 3H), 3.67 (s, 3H), 3.57 (dd, J = 10.6, 9.0 Hz, 1H), 2.50 (s, 3H), 2.38 (dd, J = 9.8, 4.5 Hz, 2H), 2.31–2.20 (m, 1H), 2.20–2.08 (m, 2H), 1.95–1.81 (m, 1H) ppm. ¹³C NMR (126 MHz, CDCl₃) δ = 219.98, 201.97, 159.24, 157.22, 157.11, 114.77, 91.82, 55.90, 55.63, 45.66, 37.96, 32.51, 30.25, 21.72 ppm. HRMS (ESI) calcd for C₁₆H₂₁O₅ (M + 1)⁺: 293.1389, found: 293.1390.

Acknowledgements

The project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the Scientific and Technological Research Program of Chongqing Municipal Education Commission.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Chemical shifts assignments, and spectral data for compounds. See DOI: 10.1039/c4ra01043d

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