Copper-catalyzed synthesis of 1,3,5-triarylpentane-1,5-diones from α,β-unsaturated ketones

Abstract

An efficient method for copper catalyzed synthesis of 1,3,5-triarylpentane-1,5-diones using α,β-unsaturated ketones as unique starting materials is described. The protocol offers several advantages such as simple, inexpensive reagents, mild reaction conditions and simple work-up procedure.

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Introduction

Michael addition of α,β-unsaturated carbonyl compounds as substrates is a beneficial method to obtain 1,5-dicarbonyl compounds,¹ which are important synthetic building blocks and have attracted the attention of many organic chemists.

The reported synthetic methods for 1,3,5-triarylpentane-1,5-diones (Scheme 1) include: (1) using benzaldehyde and acetophenone as the raw materials by Michael addition reaction;² (2) using α,β-unsaturated ketones (or benzaldehyde) and silyl enol ethers as the starting materials in the presence of magnesium chloride (or sodium hydroxide);³ (3) using α,β-unsaturated ketones and diphenacyl sulfides as the starting materials under microwave irradiation.⁴ However, some of these methods use air- and moisture-sensitive materials and require manipulation using complex techniques; some of these methods use materials with unpleasant odors that often cause environment pollution. In this work, a novel method for the synthesis of 1,3,5-triarylpentane-1,5-diones is reported using α,β-unsaturated ketones as the unique starting materials and inexpensive cuprous iodide as the catalyst (Scheme 2).

Scheme 1 The common synthetic methods for 1,3,5-triphenylpentane-1,5-dione.

Scheme 2 Synthesis of 1,3,5-triphenylpentane-1,5-dione from α,β-unsaturated ketone.

Results and discussion

Initially, the synthesis of 1,3,5-triarylpentane-1,5-diones were investigated using chalcone (1a) as a selected substrate. The reaction was performed under the conditions of a wide range of catalysts, bases and solvents.

It was found that the reaction desired to proceed under an appropriate catalyst (Table 1), and the copper(I) salts, such as CuCl, CuBr, CuI, CuOTf and CuCN, were revealed certain activities for the synthesis of 1,3,5-triphenylpentane-1,5-dione (2a) (Table 1, entries 2–6). Among them, the best yield was obtained from CuI as a catalyst (Table 1, entry 4). However, copper(II) salts and other metal compounds tested had no activations for the reaction (Table 1, entries 7–9).

Table 1 The effect of reaction conditions on the yield of 2a^a

Entry	Catalyst	Base	Yield ^(%)
a Reaction condition: 1a (0.2 mmol), base (0.6 mmol), and catalyst (0.02 mmol) were stirred at 120 °C in DMF–H2O (50 : 1) for 8 h under air.b Isolated yields.
1	None	NaOH	0
2	CuCl	NaOH	41
3	CuBr	NaOH	55
4	CuI	NaOH	83
5	CuOTf	NaOH	62
6	CuCN	NaOH	56
7	Cu(OAc)2	NaOH	0
8	FeCl2	NaOH	0
9	AgCl	NaOH	0
10	CuI	Ca(OH)2	45
11	CuI	Na2CO3	42
12	CuI	KOH	70
13	CuI	t-BuONa	78
14	CuI	t-BuOK	76
15	CuI	None	0

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The bases were also indispensable for the reaction, and NaOH, KOH, Ca(OH)₂, Na₂CO₃, t-BuONa and t-BuOK were tested for the reaction using CuI as a catalyst. It was found that these bases had certain effects for the reaction (Table 1, entries 4, 10–14). Among them, NaOH was the most suitable base for the reaction (Table 1, entry 4).

Meanwhile the solvents also played an important role in the reaction. It was found that the yield of 2a could be significantly elevated if addition of some water to organic solvents, such as DMF and DMSO. However, the highest yield for 2a was obtained in DMF–H₂O (50 : 1). Other solvents, such as 1,4-dioxane, PhMe, MeCN and EtOH in the presence of a certain amount of water were found no effects on the reaction.

In addition, it was worthy of mentioning that keeping the reaction under air condition at 120 °C were also an essential condition for the reaction.

Based on the above findings, a series of α,β-unsaturated ketones were investigated to synthesize 1,3,5-triarylpentane-1,5-diones using CuI as a catalyst and NaOH as a base in DMF–H₂O (50 : 1) (Table 2). It was found that α,β-unsaturated ketones with electron-donating (CH₃, CH₃O) or electron-withdrawing (F, Cl, Br) groups on the aromatic rings could be successfully converted to the corresponding products in good to excellent yields. And the substituent groups at o-, m-, p-position on the aromatic rings did not show strong steric effects. In addition, α,β-unsaturated ketones bearing heterocyclyl such as thiophen-2-yl could also converted into the corresponding product in good yield (Table 2, entry 16).

Table 2 Scope of α,β-unsaturated ketones^a

Entry	R^1	R^2	Yield ^(%)	Mp (°C)
a Reaction condition: α,β-unsaturated ketones (0.2 mmol), NaOH (0.6 mmol), and CuI (0.02 mmol) in DMF (3 mL) and H2O (0.06 mL) were stirred at 120 °C for 8 h under air.b Isolated yields.
1	C6H5	C6H5	83	63–65
2	4-CH3C6H4	C6H5	72	102–104
3	4-CH3OC6H4	C6H5	70	100–102
4	2-CH3OC6H4	C6H5	68	106–108
5	4-ClC6H4	C6H5	65	96–98
6	2-ClC6H4	C6H5	67	100–102
7	4-BrC6H4	C6H5	58	90–92
8	C6H5	4-CH3C6H4	76	104–106
9	C6H5	4-CH3OC6H4	74	105–107
10	4-CH3C6H4	4-CH3C6H4	78	88–90
11	4-CH3OC6H4	4-CH3OC6H4	80	Oil
12	4-CH3C6H4	4-CH3OC6H4	78	Oil
13	4-FC6H4	4-CH3C6H4	77	108–110
14	2,4-Cl2C6H3	3-CH3OC6H4	71	Oil
15		4-ClC6H4	73	86–88
16		C6H5	63	89–91

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A plausible mechanism was proposed for the synthesis of 1,3,5-triarylpentane-1,5-diones (Scheme 3). First, α,β-unsaturated ketones 1 were coordinated with Cu(I) to form Cu(I) complexes A. Complexes A were attacked by the hydroxide ion in a conjugate fashion to give the intermediates B. B were protonated to form β-hydroxy ketones C.⁵ The base-induced retro-aldol reactions of C would provide the corresponding benzaldehydes and acetophenones. The resulting acetophenones could undergo base-catalyzed conjugate additions to 1 to yield the observed compounds 2. And the benzaldehydes subsequently were oxidized by air in the system to benzoic acids, which could reduce the concentration of benzaldehydes and render the reaction equilibrium favorable to the formation of 2.

Scheme 3 Plausible mechanism.

Conclusions

A novel method for the synthesis of 1,3,5-triarylpentane-1,5-diones using α,β-unsaturated ketones as unique starting materials and copper(I) as a catalyst under air condition has been developed. The advantages of this protocol are the use of inexpensive and non-toxic raw materials, mild conditions and simple work-up procedure.

Experimental

IR spectra were recorded using KBr pellets on an Alpha Centauri FTIR spectrophotometer. ¹H NMR and ¹³C NMR spectra were obtained with Mercury-400 BB or Mercury-600BB instrument using CDCl₃ as solvent and Me₄Si as the internal standard. Melting points were observed in an electrothermal melting point apparatus. α,β-Unsaturated ketones were prepared according to literature procedure.⁶

The general procedure for the preparation of 1,3,5-triarylpentane-1,5-diones

The mixture of α,β-unsaturated ketones (0.2 mmol), NaOH (0.6 mmol) and CuI (0.02 mmol) in DMF (3 mL) and H₂O (0.06 mL) was stirred under air at 120 °C for 8 h. After the completion of the reaction, the mixture was cooled to room temperature, diluted with ethyl acetate and washed with saturated sodium carbonate solution. The resulting organic phase was dried over anhydrous magnesium sulfate, and concentrated under reduced pressure. The residue was isolated by column chromatography using petroleum ether and ethyl acetate (30 : 1) as eluent to give pure product. The analytical data for products are given below.

1,3,5-Triphenylpentane-1,5-dione (2a)

Gray solid; Mp: 63–65 °C; IR (KBr, cm⁻¹): ν 1684; ¹H NMR (400 MHz, CDCl₃): δ 7.96 (d, J = 7.2 Hz, 4H, ArH), 7.53–7.57 (m, 2H, ArH), 7.42–7.47 (m, 4H, ArH), 7.28 (d, J = 13.6 Hz, 4H, ArH), 7.19–7.20 (m, 1H, ArH), 4.06–4.11 (m, 1H, CH), 3.55 (dd, J = 7.2, 16.8 Hz, 2H, CH₂), 3.37 (d, J = 6.8, 16.4 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 198.5, 143.8, 137.0, 133.0, 128.6, 128.5, 128.1, 127.5, 126.7, 44.9, 37.2.

1,5-Diphenyl-3-p-tolylpentane-1,5-dione (2b)

White solid; Mp: 102–104 °C; IR (KBr, cm⁻¹): ν 1674; ¹H NMR (400 MHz, CDCl₃): δ 7.95 (d, J = 7.6 Hz, 4H, ArH), 7.52–7.57 (m, 2H, ArH), 7.42–7.46 (m, 4H, ArH), 7.17 (d, J = 6.8 Hz, 2H, ArH), 7.08 (d, J = 7.2 Hz, ArH), 4.02–4.05 (m, 1H, CH), 3.48 (dd, J = 6.8, 16.4 Hz, 2H, CH₂), 3.32 (dd, J = 6.8, 16.4 Hz, 2H, CH₂), 2.28 (s, 1H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 198.6, 140.7, 136.9, 136.1, 133.0, 129.2, 128.5, 128.1, 127.2, 45.0, 36.8, 21.0.

3-(4-Methoxyphenyl)-1,5-diphenylpentane-1,5-dione (2c)

White solid; Mp: 100–102 °C; IR (KBr, cm⁻¹): ν 1677; ¹H NMR (400 MHz, CDCl₃): δ 7.95 (d, J = 7.6 Hz, 4H, ArH), 7.53–7.56 (m, 2H, ArH), 7.42–7.46 (m, 4H, ArH), 7.19 (d, J = 8.4 Hz, 2H, ArH), 6.81 (d, J = 8.0 Hz, 2H, ArH), 4.00–4.04 (m, 1H, CH), 3.76 (s, 3H, OCH₃), 3.47 (dd, J = 6.8, 16.4 Hz, 2H, CH₂), 3.31 (dd, J = 7.2, 16.8 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 198.7, 158.2, 137.0, 135.8, 133.0, 128.5, 128.4, 128.1, 114.0, 55.2, 45.1, 36.5.

3-(2-Methoxyphenyl)-1,5-diphenylpentane-1,5-dione (2d)

White solid; Mp: 106–108 °C; IR (KBr cm⁻¹): ν 1680; ¹H NMR (400 MHz, CDCl₃): δ 7.99 (d, J = 7.2 Hz, 4H, ArH); 7.52–7.54 (m, 2H, ArH); 7.43–7.46 (m, 4H, ArH); 7.15–7.30 (m, 2H, ArH); 6.82–6.89 (m, 2H, ArH); 4.33–4.36 (m, 1H, CH); 3.77 (s, 3H, OCH₃); 3.41–3.53 (m, 4H, 2CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 199.1, 157.0, 137.0, 132.8, 131.3, 128.4, 128.1, 127.6, 120.6, 110.7, 55.1, 43.0, 32.7.

3-(4-Chlorophenyl)-1,5-diphenylpentane-1,5-dione (2e)

Yellow solid; Mp: 96–98 °C; IR (KBr, cm⁻¹): ν 1674; ¹H NMR (400 MHz, CDCl₃): δ 7.94 (d, J = 7.2 Hz, 4H, ArH), 7.53–7.57 (m, 2H, ArH), 7.42–7.46 (m, 4H, ArH), 7.22–7.23 (m, 4H, ArH), 4.04–4.07 (m, 1H, CH), 3.48 (dd, J = 5.6, 16.8 Hz, 2H, CH₂), 3.33 (dd, J = 6.0, 16.0 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 198.1, 142.3, 136.8, 133.1, 132.3, 128.9, 128.7, 128.6, 128.0, 44.7, 36.5.

3-(2-Chlorophenyl)-1,5-diphenylpentane-1,5-dione (2f)

Yellow solid; Mp: 100–102 °C; IR (KBr, cm⁻¹): ν 1677; ¹H NMR (400 MHz, CDCl₃): δ 7.94 (d, J = 7.6 Hz, 4H, ArH), 7.50–7.54 (m, 2H, ArH), 7.39–7.43 (m, 4H, ArH), 7.30–7.33 (m, 2H, ArH), 7.14–7.18 (m, 1H, ArH), 7.08–7.11 (m, 1H, ArH), 4.48–4.55 (m, 1H, CH), 3.51 (dd, J = 7.2, 17.2 Hz, CH₂), 3.43 (dd, J = 6.8, 17.2 Hz, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 198.3, 140.8, 136.8, 133.7, 133.1, 130.0, 128.5, 128.4, 128.1, 127.7, 127.0, 42.9, 34.0.

3-(4-Bromophenyl)-1,5-diphenylpentane-1,5-dione (2g)

Red solid; Mp: 90–92 °C; IR (KBr, cm⁻¹): ν 1674; ¹H NMR (400 MHz, CDCl₃): δ 7.95 (d, J = 6.8 Hz, 4H, ArH), 7.55–7.58 (m, 2H, ArH), 7.44–7.47 (m, 4H, ArH), 7.39 (d, J = 6.8 Hz, 2H, ArH), 7.18 (d, J = 6.8 Hz, ArH), 4.04–4.07 (m, 1H, CH), 3.49 (dd, J = 6.8, 16.8 Hz, 2H, CH₂), 3.33 (dd, J = 6.0, 16.8 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 198.1, 142.8, 136.8, 133.2, 131.6, 129.3, 128.6, 128.1, 120.4, 44.6, 36.5.

1,5-Bis-(4-methoxyphenyl)-3-phenylpentane-1,5-dione (2h)

Yellow solid; Mp: 104–106 °C; IR (KBr, cm⁻¹): ν 1680; ¹H NMR (400 MHz, CDCl₃): δ 7.85 (d, J = 6.0 Hz, 4H, ArH), 7.17–7.27 (m, 9H, ArH), 4.03–4.06 (m, 1H, CH), 3.45 (dd, J = 6.8, 16.4 Hz, 2H, CH₂), 3.30 (dd, J = 4.8, 14.4 Hz, 2H, CH₂), 2.39 (m, 6H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 198.2, 143.9, 143.8, 134.3, 129.2, 128.5, 128.2, 127.4, 126.5, 44.8, 37.2, 21.6.

1,5-Bis(4-methoxyphenyl)-3-phenylpentane-1,5-dione (2i)

White solid; Mp: 105–107 °C; IR (KBr, cm⁻¹): ν 1670; ¹H NMR (600 MHz, CDCl₃): δ 7.93 (d, J = 8.4 Hz, 4H, ArH), 7.24–7.26 (m, 4H, ArH), 7.15–7.17 (m, 1H, ArH), 6.91 (d, J = 9.0 Hz, ArH), 4.0–4.05 (m, 1H, CH), 3.84 (s, 6H, OCH₃), 3.42 (dd, J = 7.2, 16.2 Hz, 2H, CH₂), 3.27 (dd, J = 6.6, 16.2 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 197.1, 163.4, 144.0, 130.4, 130.0, 128.5, 127.4, 126.5, 113.6, 55.4, 44.6, 37.6.

1,3,5-Tri-p-tolylpentane-1,5-dione (2j)

Yellow solid; Mp: 88–90 °C; IR (KBr, cm⁻¹): ν 1674; ¹H NMR (400 MHz, CDCl₃): δ 7.85 (d, J = 7.6 Hz, 4H, ArH), 7.23 (d, J = 7.6 Hz, 4H, ArH), 7.16 (d, J = 7.2 Hz, 2H, ArH), 7.07 (d, J = 7.2 Hz, 2H, ArH), 3.99–4.02 (m, 1H, CH), 3.43 (dd, J = 7.2, 16.4 Hz, 2H, CH₂), 3.28 (dd, J = 6.8, 16.4 Hz, 2H, CH₂), 2.36 (s, 6H, CH₃), 2.27 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 198.2, 143.7, 140.9, 136.0, 134.5, 129.2, 129.2, 128.2, 127.2, 44.9, 37.0, 21.5, 20.9.

1,3,5-Tris(4-methoxyphenyl)pentane-1,5-dione (2k)

Yellow liquid; IR (KBr, cm⁻¹): ν 1670; ¹H NMR (400 MHz, CDCl₃): δ 7.94 (d, J = 6.8 Hz, 4H, ArH), 7.18 (d, J = 6.8 Hz, 2H, ArH), 6.89 (d, J = 6.8 Hz, 4H, ArH), 6.79 (d, J = 6.8 Hz, 2H, ArH), 3.97–4.00 (m, 1H, CH), 3.83 (s, 6H, OCH₃); 3.73 (s, 3H, OCH₃), 3.40 (dd, J = 6.4, 16.0 Hz, 2H, CH₂), 3.23 (dd, J = 5.6, 15.6 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 197.3, 163.4, 158.1, 136.0, 130.4, 130.1, 128.4, 113.9, 113.7, 55.4, 55.1, 44.9, 37.0.

1,5-Bis(4-methoxyphenyl)-3-p-tolylpentane-1,5-dione (2l)

Yellow liquid; IR (KBr, cm⁻¹): ν 1670; ¹H NMR (400 MHz, CDCl₃): δ 7.94 (d, J = 7.6 Hz, 4H, ArH), 7.16 (d, J = 6.8 Hz, 2H, ArH), 7.07 (d, J = 7.2 Hz, 2H, ArH), 6.90 (d, J = 7.2 Hz, 4H, ArH), 3.98–4.01 (m, 1H, CH), 3.83 (s, 6H, OCH₃), 3.42 (dd, J = 6.8, 16.0 Hz, 2H, CH₂), 3.24 (dd, J = 6.4, 16.0 Hz, 2H, ArH), 2.29 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 197.3, 163.4, 140.9, 136.0, 130.4, 130.1, 129.2, 127.3, 113.7, 55.4, 44.8, 37.3, 21.0.

3-(4-Fluorophenyl)-1,5-di-p-tolylpentane-1,5-dione (2m)

Yellow solid; Mp: 108–110 °C; IR (KBr, cm⁻¹): ν 1680; ¹H NMR (600 MHz, CDCl₃): δ 7.83 (d, J = 8.4 Hz, 4H, ArH), 7.22–7.25 (m, 6H, ArH), 6.91–6.94 (m, 2H, ArH), 4.01–4.06 (m, 1H, CH), 3.43 (dd, J = 7.2, 16.8 Hz, 2H, CH₂), 3.27 (dd, J = 7.8, 16.8 Hz, 2H, CH₂), 2.38 (s, 6H, CH₃); ¹³C NMR (150 MHz, CDCl₃): δ 193.3, 139.2, 134.9, 134.8, 129.6, 124.5, 124.2, 124.1, 123.5, 110.6, 110.5, 40.1, 31.9, 16.8.

3-(2,4-Dichlorophenyl)-1,5-bis-(3-methoxyphenyl)pentane-1,5-dione (2n)

Yellow liquid; IR (KBr, cm⁻¹): ν 1680; ¹H NMR (600 MHz, CDCl₃): δ 7.53–7.55 (m, 2H, ArH), 7.46–7.47 (m, 2H, ArH), 7.33–7.36 (m, 3H, ArH), 7.24–7.25 (m, 1H, ArH), 7.15–7.17 (m, 1H, ArH), 7.08–7.10 (m, 2H, ArH), 4.45–4.50 (m, 1H, CH), 3.82 (s, 6H, 2OCH₃), 3.49 (dd, J = 7.2, 17.4 Hz, 2H, CH₂), 3.40 (dd, J = 7.2, 17.4 Hz, 2H, CH₂); ¹³C NMR (150 MHz, CDCl₃): δ 198.1, 160.0, 139.7, 138.1, 134.6, 133.0, 130.0, 129.9, 129.5, 127.5, 121.0, 120.2, 112.4, 55.7, 43.1, 33.7.

3-Benzo[1,3]dioxol-5-yl-1,5-bis(4-chlorophenyl)pentane-1,5-dione (2o)

Yellow solid; Mp: 86–88 °C; IR (KBr, cm⁻¹): ν 1681; ¹H NMR (600 MHz, CDCl₃): δ 7.87 (d, J = 8.4 Hz, 4H, ArH), 7.41 (d, J = 8.4 Hz, 4H, ArH), 6.74 (s, 1H, ArH), 6.67 (s, 2H, ArH), 5.88 (s, 2H, CH₂), 3.90–3.95 (m, 1H, CH), 3.39 (dd, J = 7.2, 16.8 Hz, 2H, CH₂), 3.23 (dd, J = 7.2, 16.8 Hz, 2H, CH₂); ¹³C NMR (150 MHz, CDCl₃): δ 197.2, 147.8, 146.3, 139.6, 137.2, 135.1, 129.5, 128.9, 120.4, 108.3, 107.7, 100.9, 45.0, 37.0.

1,5-Diphenyl-3-thiophen-2-yl-pentane-1,5-dione (2p)

Gray solid; Mp: 89–91 °C; IR (KBr, cm⁻¹): ν 1686; ¹H NMR (400 MHz, CDCl₃): δ 7.96 (d, J = 7.6 Hz, 4H, ArH), 7.53–7.55 (m, 2H, ArH), 7.43–7.46 (m, 4H, ArH), 7.10–7.11 (m, 1H, ThH), 6.88–6.89 (m, 2H, ThH), 4.42–4.45 (m, 1H, CH), 3.53 (dd, J = 6.8, 16.8 Hz, 2H, CH₂), 3.42 (dd, J = 6.8, 16.8 Hz, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃): δ 198.0, 147.4, 136.8, 133.1, 128.5, 128.1, 126.6, 124.2, 123.2, 45.5, 23.4.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21162024, 21362034, 21462038) and Key Laboratory of Eco-Environment-Related Polymer Materials for Ministry of Education for the financial support of this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Copies of IR, ¹H and ¹³C NMR spectra of all compounds. See DOI: 10.1039/c5ra09155a

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