HETEROCYCLES

Note | Regular issue | Vol. 81, No. 11, 2010, pp. 2609-2616
Received, 6th August, 2010, Accepted, 6th September, 2010, Published online, 7th September, 2010.
DOI: 10.3987/COM-10-12040

■ Regioselectivity in Direct Arylation of Benzanilide Possessing Oxygen Substituents in the Benzoyl Part Using Palladium without Phosphine Ligand

Takashi Harayama,* Chie Nagura, Taeko Miyagoe, Yuko Kawata, Hitoshi Abe, and Yasuo Takeuchi

Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki City, Kagawa 769-2193, Japan

Abstract

The direct arylation of benzanilide (1) possessing ether oxygen(s) in benzoyl part was examined under phosphine ligand-free conditions, which afforded ortho-product (2) and para-product (3) in very similar ratio to presence of P(o-Tol)3 conditions. This results suggest that bulkiness of palladium under phosphine-free conditions would be comparable to that of P(o-Tol)3 conditions.

Direct arylation (aryl-aryl coupling reaction)1c of a nonactivated aryl C-H bond with an activated arene by palladium-phosphine reagent has been used to synthesize many condensed aromatic compounds.1 We reported that an intramolecular direct arylation of 2-halo-N-arylbenzamides was a convenient method for synthesizing polycyclic aromatic lactams.2 Moreover, we successfully synthesized benzonaphthazepine,3 pyrrolophenanthridine alkaloids,3 quinazoline alkaloids4 and benzpyranones4,5 utilyzing palladium reagent.
Recently, we reported that the oxygen substituent(s) of N-(2-iodophenyl)benzamide (1) and 2-iodo-N-methylbenzanilide (A) affected the regioselectivy of coupling position in the direct arylation using Pd-phosphine reagent and the ratio of ortho-product (2) to para-product (3) was influenced by the coordinating ability of oxygen substituent(s) to the PdII complex and by the steric relationship between the substituent(s) and the phosphine ligand in the complex (B).6,7
It is well known that aryl iodide produces a coupling product without phosphine ligand.8 Then, we investigated whether the ratio of ortho-product (2) to para-product (3) is influenced by presence or absence of phosphine ligand in the aryl-aryl coupling reaction of benzanilide (1) possessing ether oxygen(s) in the benzoyl part.

The results of the coupling reactions using phosphine ligand-free Method A [Pd(OAc)2 (10 mol%), and K2CO3 (200 mol%)] in DMF under reflux are summarized in Table 1.
The phosphine-free conditions afforded ortho-product (2) and para-product (3) in very similar ratio to the P(o-Tol)3 conditions (Method C in Table 1). We proposed tentatively the δ-bond metathesis mechanism for the direct arylation.6b,6c,9 According to this hypothesis, this results indicate that bulkiness of palladium complex under phosphine-free conditions would be comparable to that of P(o-Tol)3 conditions by solvation as shown in B (L=solvent) in Scheme 1.
Direct arylation of benzanilide possessing other substituents under phosphine ligand-free is now in progress..

EXPERIMENTAL
Melting points were measured on a micro-melting point hot-stage apparatus (Yanagimoto) and are uncorrected. IR spectra were recorded on a JASCO FT/IR 350 spectrophotometer and 1H-NMR spectra in deuteriochloroform on Varian Mercury 300 or VXR-500 spectrometers. NMR spectral data are reported in parts per million downfield from tetramethylsilane as the internal standard (δ 0.0), and the coupling constants are given in Hertz. MS spectra were obtained on a VG-70SE. Analytical HPLC was performed with a Shimadzu SPD-6A on a silica gel column (Chemcosorb 5Si-U). Column chromatography was carried out on a Merck silica gel (230–400 mesh). All the extracts were washed with brine, dried over anhydrous MgSO4, and filtered; the filtrate was concentrated to dryness under reduced pressure.

N-(2-Iodophenyl)-N-methyl-3,4-methylenedioxylbenzamide (1a)
Methyl iodide (0.55 mL, 8.16 mmol) was added to a suspension of N-(2-iodophenyl)-3,4-methylenedioxylbenzamide8b (2.0 g, 5.44 mmol) and NaH (60%, 0.49 g, 16. 2 mmol) in dry DMF (100 mL). After stirring for 15 min at rt, the excess NaH was decomposed with ice water, and extracted with Et2O. The residue in Et2O was subjected to column chromatography on a silica gel. Elution with hexane:AcOEt (4:1) gave 1a (1.82 g, 89%) as colorless needles (from hexane), mp 95-96 ˚C (lit.,10 mp 97-97.5˚C).
3,4-Ethylenedioxy-N-(2-iodophenyl)-N-methylbenzamide (1b)
A mixture of 3,4-ethylenedioxybenzoic acid11 (0.50 g, 2.8 mmol) in five drops of dry DMF and thionyl chloride (0.3 mL, 4.2 mmol) was first refluxed for 15 min and then concentrated to dryness under reduced pressure. A solution of 2-iodo-N-methylaniline12 (0.65 g, 3.36 mmol) in dry CH2Cl2 (3.5 mL) and dry NEt3 (0.3 mL, 3.36 mmol) was added to this residue and the whole was stirred for 1.5 h at rt. The reaction mixture was concentrated to dryness and diluted with AcOEt, and then washed with 10% HCl and brine. The residue in CHCl3 was subjected to column chromatography on silica gel. The elution with hexane:AcOEt (1:1) gave 1b (0.67 g, 61%) as colorless needles (from CHCl3), mp 162–163 ˚C. IR (KBr) cm-1: 1630. 1H-NMR (300 MHz) δ : 3.34 (3H, s), 4.18 (4H, s), 6.59 (1H, br d, J = 7.8 Hz), 6.80 (1H, br d, J = 7.2 Hz), 6.93 (1H, br t, J = 7.5 Hz), 6.97 (1H, br s), 7.10 (1H, br d, J = 7.2 Hz), 7.25 (1H, br t, J = 7.5 Hz), 7.83 (1H, br d, J = 7.5 Hz). Anal. Calcd for C16H14NO3I: C, 48.63; H, 3.57; N, 3.54. Found: C, 48.35; H, 3.60; N, 3.65.
3,4-Dimethoxy-N-(2-iodophenyl)-N-methylbenzamide (1c)
A few drops of dry DMF and oxalyl chloride (2.0 mL, 21.6 mmol) were added to a solution of 3,4-dimethoxybenzoic acid (2.0g, 10.8 mmol) in dry CH2Cl2 (100 mL) and the mixture was stirred for 3 h under ice cooling. Then, the reaction mixture was concentrated to dryness under reduced pressure. A solution of 2-iodoaniline (2.40 g, 10.8 mmol) in dry CH2Cl2 (50 mL) and dry NEt3 (1.8 mL, 12.9 mmol) was added to this residue and the whole was stirred for 3 h at rt. The reaction mixture was concentrated to dryness, diluted with CHCl3, and then washed with 10% HCl, 5% aqueous NaOH solution, and brine. The residue in CHCl3 was subjected to column chromatography on silica gel. Elution with hexane:AcOEt (2:1) gave 3,4-dimethoxy-N-(2-iodophenyl)benzamide (3.04 g, 72%) as colorless needles (from Et2O), mp 154–156 ˚C. IR (KBr) cm-1: 3250, 1640, 1515. 1H-NMR (500 MHz) δ: 3.96 (3H, s), 3.98 (3H, s), 6.87 (1H, ddd, J = 8.0, 8.0, 1.3 Hz), 6.97 (1H, d, J = 8.3 Hz), 7.40 (1H, ddd, J = 8.0, 8.0, 1.1 Hz), 7.54 (1H, dd, J = 8.0, 1.3 Hz), 7.55 (1H, d, J = 1.3 Hz), 7.82 (1H, dd, J = 8.0, 1.1 Hz), 8.26 (1H, brs), 8.45 (1H, dd, J = 8.3, 1.3 Hz). Anal. Calcd for C15H14NO3I: C, 47.02; H, 3.68; N, 3.66. Found: C, 47.00; H, 3.96; N, 3.53.
Methyl iodide (0.55 mL, 8.0 mmol) was added to a suspension of 3,4-dimethoxy-N-(2-iodophenyl)benzamide (2.00g, 5.48 mmol) and NaH (60%, 0.49 g, 16.2 mmol) in dry DMF (100 mL). After stirring for 15 min under ice cooling, the excess NaH was decomposed with ice water, and extracted with Et2O. The residue in CHCl3 was subjected to column chromatography on a silica gel. Elution with hexane:AcOEt (1:1) gave 1c (1.82 g, 89%) as colorless needles (from CHCl3-hexane), mp 125–126.5 ˚C. IR (KBr) cm-1: 1630. 1H-NMR (500 MHz) δ: 3.36 (3H, s), 3.70 (3H, s), 3.81 (3H, s), 6.65 (1H, d, J = 7.5 Hz), 6.90-6.92 (2H, m), 7.01 (1H, d, J = 7.5 Hz), 7.06 (1H, d, J = 7.5 Hz), 7.22 (1H, dd, J = 7.5, 7.5 Hz), 7.85 (1H, d, J = 7.5 Hz). Anal. Calcd for C16H16NO3I: C, 48.38; H, 4.06; N, 3.53. Found : C, 48.34; H, 4.11; N, 3.45.
3-Methoxy-N-(2-iodophenyl)-N-methylbenzamide (1d)
The compound (1d) was prepared according to reference 9.
3-Isopropoxy-4-methoxy-N-(2-iodophenyl)-N-methylbenzamide (1e)
A mixture of 3-isopropoxy-4-methoxybenzoic acid (4.2 g, 20 mmol) in five drops of dry DMF and thionyl chloride (2.17 mL, 30 mmol) was first refluxed for 20 min and then concentrated to dryness under reduced pressure. A solution of 2-iodo-N-methylaniline (5.25 g, 26 mmol) in dry CH2Cl2 (30 mL) and dry NEt3 (3.62 mL, 26 mmol) was added to this residue and the whole was stirred for 40 min at rt. The reaction mixture was concentrated to dryness and diluted with AcOEt, and then washed with 10% HCl, 5% aqueous NaHCO3 solution, and brine. The residue was recrystallized from Et2O to afford 3-isopropoxy-4-methoxy-N-(2-iodophenyl)lbenzamide (6.32 g, 77 %) as colorless needles, mp 131-132 ˚C. IR (KBr) cm-1: 1645, 1506. 1H-NMR (300 MHz) δ: 1.42 (6H, d, J = 6.0 Hz ), 3.93 (3H, s), 4.67 (1H, septet, J = 6.0 Hz ), 6.87 (1H, ddd, J = 8.1, 7.8, 2.1 Hz), 6.97 (1H, d, J = 8.4 Hz), 7.40 (1H, ddd, J = 8.1, 8.1, 1.5 Hz), 7.53 (1H, dd, J = 8.1, 2.1 Hz), 7.56 (1H, d, J = 1.5 Hz), 7.81(1H, dd, J = 7.8, 1.5 Hz), 8.24 (1H, br s), 8.45 (1H, dd, J = 8.4, 1.5 Hz). Anal. Calcd for C17H18NO3I: C, 49.65; H, 4.41; N, 3.41. Found: C, 49.50; H, 4.33; N, 3.37.
Methyl iodide (1.0 mL, 15.0 mmol) was added to a suspension of 3-isopropoxy-4-methoxy-N-(2-iodophenyl)lbenzamide (4.11g, 10.0 mmol) and NaH (60%, 1.16 g, 30 mmol) in dry DMF (50 mL). After stirring for 45 min at rt, the excess NaH was decomposed with ice water, and extracted with AcOEt. The residue in CHCl3 was subjected to column chromatography on a silica gel. Elution with hexane:AcOEt (4:1) gave 1e (4.09 g, 96%) as a pale yellow oil. IR (KBr) cm-1: 1645. 1H-NMR (300 MHz) δ: 1.18 (3H, d, J = 6.0 Hz), 1.28 (3H, d, J = 6.0 Hz), 3.35 (3H, s), 3.78 (3H, s), 4.30 (1H, septet, J = 6.0 Hz), 6.66 (1H, d, J = 8.4 Hz), 6.87-7.25 (5H, m), 7.83 (1H, d, J = 6.9 Hz). High resolution MS (FAB) m/z: Calcd for C18H21NO3 I [M+1]+: 426.0566. Found: 426.0654.
3-tert-Butoxy-N-(2-iodophenyl)-N-methylbenzamide (1f)
A solution of methyl 3-tert-butoxybenzoate (2.86 g, 13.7 mmol) in dry DMF (100 mL) was added to a suspension of NaH (60%, 1.22 g, 31.7 mmol) and 2-iodoaniline in dry DMF (30 mL) and the reaction mixture was stirred at rt for 72 h under an argone atmosphere. The excess NaH was decomposed with ice water, and extracted with AcOEt. The residue in CHCl3 was subjected to column chromatography on a silica gel. Elution with hexane:AcOEt (5:1) gave 3-tert-butoxy-N-(2-iodophenyl)benzamide (4.50 g, 83%) as colorless needles (from Et2O-hexane), mp 61–63 ˚C. IR (KBr) cm-1: 3390, 1680, 1515. Anal. Calcd for C17H18NO2I: C, 51.66; H, 4.59; N, 3.54. Found: C, 51.49; H, 4.61; N, 3.38.
Methyl iodide (1.74 mL, 26.0 mmol) was added to a suspension of 3-tert-butoxy-N-(2-iodophenyl)benzamide (4.11g, 10.4 mmol) and NaH (60%, 1.2 g, 31.2 mmol) in dry DMF (100 mL). After stirring for 45 min under ice cooling, the excess NaH was decomposed with ice water, and extracted with AcOEt. The residue in CHCl3 was subjected to column chromatography on a silica gel. Elution with hexane:AcOEt (3:1) gave 1f (3.29 g, 77%) as colorless prisms (from Et2O-hexane), mp 183–185 ˚C. IR (KBr) cm-1: 1630. 1H-NMR (300 MHz) δ: 1.18 (9H, s), 3.37 (3H, s), 6.84-7.22 (6H, m), 7.80 (1H, d, J = 7.8 Hz). Anal. Calcd for C18H20NO3I: C, 52.83; H, 4.93; N, 3.42. Found: C, 52.81; H, 5.08; N, 3.52.
General Procedure for the Direct Arylation of Benzamides (1)
Coupling reaction was carried out under the reaction conditions indicated in Table 1. Then, the reaction mixture was diluted with AcOEt, and the precipitates were removed by filtration. The filtrate was washed with brine.
Direct arylation of N-(2-iodophenyl)-N-methyl-3,4-methylenedioxylbenzamide (1a)
The residue was dissolved in CHCl3 and was subjected to column chromatography on silica gel. Elution with hexane:AcOEt (8:1) gave 5-methyl-9,10-methylendioxyphenanthridin-6(5H)-one (2a) and successive elution with the same solvent gave 5-methyl-8,9-methylendioxyphenanthridin-6(5H)-one (3a).
5-Methyl-9,10-methylendioxyphenanthridin-6(5H)-one (2a): colorless needles (from CHCl3), mp 179-181 ˚C. IR (KBr) cm-1: 1600. 1H-NMR (500 MHz) δ: 3.78 (3H, s), 6.28 (2H, s), 7.10 (1H, d, J = 8.5 Hz), 7.29 (1H, dd, J = 7.8, 7.8 Hz), 7.37 (1H, d, J=8.0, Hz), 7.53 (1H, dd, J = 7.8, 7.8 Hz), 8.21 (1H, d, J = 8.5 Hz), 8.65 (1H, d, J = 7.8 Hz). Anal. Calcd for C15H11NO3: C, 71.14; H, 4.38; N, 5.53. Found: C, 71.27; H, 4.29; N, 5.51.
5-Methyl-8,9-methylendioxyphenanthridin-6(5H)-one (3a): colorless needles (from CHCl3-hexane), mp 245-247˚C (lit.,13 238 ˚C). Anal. Calcd for C15H11NO3: C, 71.14; H, 4.38; N, 5.53. Found: C, 71.07; H, 4.60; N, 5.47.
Direct arylation of 3,4-ethylenedioxy-N-(2-iodophenyl)-N-methylbenzamide (1b)
The residue was dissolved in CHCl3 and subjected to column chromatography on silica gel. Elution with hexane:AcOEt (2:1) gave 9,10-ethylendioxy-5-methylphenanthridin-6(5H)-one (2b) and successive elution with the same solvent gave 8,9-methylendioxy-5-methylphenanthridin-6(5H)-one (3b).
9,10-Ethylendioxy-5-methylphenanthridin-6(5H)-one (2b): colorless needles (from CHCl3-hexane), mp 219-221 ˚C. IR (KBr) cm-1: 1650. 1H-NMR (300 MHz) δ: 3.81 (3H, s), 4.43-4.53 (4H, m), 7.13 (1H, d, J = 8.7 Hz), 7.28 (1H, ddd, J = 8.4, 8.4, 1.2 Hz), 7.41 (1H, dd, J = 8.4, 1.2 Hz), 7.53 (1H, ddd, J = 8.4, 8.4 1.5 Hz), 8.19 (1H, d, J = 8.7 Hz), 9.20 (1H, dd, J = 8.4, 1.5 Hz). Anal. Calcd for C16H13NO3: C, 71.90; H, 4.90; N, 5.24. Found: C, 71.66; H, 4.99; N, 5.36.
8,9-Ethylendioxy-5-methylphenanthridin-6(5H)-one (3b): colorless needles (from CHCl3-hexane), mp 183-184 ˚C. IR (KBr) cm-1: 1650. 1H-NMR (300 MHz) δ: 3.80 (3H, s), 4.36-4.41 (4H, m), 7.28 (1H, br t, J = 7.8, 6.9 Hz), 7.38 (1H, br d, J = 7.8Hz), 7.50 (1H, br t, J = 7.8, 6.9 Hz), 7.69 (1H, s), 8.02 (1H, s), 8.09 (1H, br d, J = 7.8 Hz). High resolution MS (FAB) m/z: Calcd for C16H14NO3 [M+1]+: 268.0974. Found: 268.1007.
Direct arylation of 3,4-dimethoxy-N-(2-iodophenyl)-N-methylbenzamide (1c)
The residue was dissolved in CHCl3 and subjected to column chromatography on silica gel. Elution with hexane:AcOEt (2:1) gave 9,10-dimethoxy-5-methylphenanthridin-6(5H)-one (2c) and successive elution with the same solvent gave 8,9-dimethoxy-5-methylphenanthridin-6(5H)-one (3c).
9,10-Dimethoxy-5-methylphenanthridin-6(5H)-one (2c): colorless needles (from CHCl3-hexane), mp 129-130 ˚C. IR (KBr) cm-1: 1595. 1H-NMR (500 MHz) δ: 3.79 (3H, s), 3.90 (3H, s), 4.03 (3H, s), 7.22 (1H, d, J = 8.6 Hz), 7.32 (1H, ddd, J = 7.9, 7.9, 1.3 Hz), 7.40 (1H, dd, J = 7.9, 1.3 Hz), 7.55 (1H, ddd, J = 7.9, 7.9, 1.4 Hz), 8.42 (1H, d, J = 8.6 Hz), 9.28 (1H, dd, J = 7.9, 1.4 Hz). High resolution MS (FAB) m/z: Calcd for C16H16NO3 [M+1]+: 270.1130. Found: 270.1117.
8,9-Dimethoxy-5-methylphenanthridin-6(5H)-one (3c): colorless needles (from CHCl3-hexan), mp 221.5-222.5˚C (lit.,14 219-220˚C).
Direct arylation of3-Isopropoxy-4-methoxy-N-(2-iodophenyl)-N-methylbenzamide (1d)
The residue was dissolved in CHCl3 and was subjected to column chromatography on silica gel. Elution with hexane:AcOEt:CHCl3 (3:1:5) gave 8-isopropoxy-9-methoxy-5-methylphenanthridin-6(5H)-one (3d) and successive elution with the same solvent gave 10-isopropoxy-9-methoxy-5-methylphenanthridin-6(5H)-one (2d).
10-Isopropoxy-9-methoxy-5-methylphenanthridin-6(5H)-one (2d): yellow oil. IR (KBr) cm-1: 1645. 1H-NMR (300 MHz) δ: 1.29 (6H, d, J = 6.3 Hz), 3.79(3H, s), 4.00 (3H, s), 4.55 (1H, sep, J = 6.3 Hz), 7.20 (1H, br d, J = 7.5 Hz), 7.27 (1H, br t, J = 7.5 Hz), 7.38 (1H, br d, J = 8.1 Hz), 7.52 (1H, br t, J = 7.2 Hz), 8.39 (1H, d, J = 8.1 Hz), 9.48 (1H, br d, J = 7.8 Hz). High resolution MS (FAB) m/z: Calcd for C18H2NO3 I[M+1]+: 298.1443. Found: 298.1446.
8-Isopropoxy-9-methoxy-5-methylphenanthridin-6(5H)-one (3d): colorless prisms (from CHCl3-hexane), mp 160 ˚C. IR (KBr) cm-1: 1644. 1H-NMR (300 MHz) δ: 1.54 (6H, d, J = 6.0 Hz), 3.83 (3H, s), 4.07 (3H, s), 4.83 (1H, sep, J = 6.0 Hz), 7.32 (1H, br d, J = 7.2 Hz), 7.42 (1H, br d, J = 7.8 Hz), 7.52 (1H, br t, J = 7.4 Hz), 7.62 (1H, s), 7.98 (1H, s), 8.17 (1H, br d, J = 7.2 Hz). Anal. Calcd for C18H19NO3: C, 72.71; H, 6.44; N, 4.71. Found: C, 72.71; H, 6.49; N, 4.75.
Direct arylation of 3-methoxy-N-(2-iodophenyl)-N-methylbenzamide (1e)
The residue was dissolved in CHCl3 and was subjected to column chromatography on silica gel. Elution with hexane:isopropyl ether (2:1) gave 10-methoxy-5-methylphenanthridin-6(5H)-one (2e)9 and successive elution with the same solvent gave 8-methoxy-5-methylphenanthridin-6(5H)-one (3e).9
Direct arylation of 3-tert-butoxy-N-(2-iodophenyl)-N-methylbenzamide (1f)
The residue was dissolved in CHCl3 and subjected to column chromatography on silica gel. Elution with hexane:AcOEt (5:1) gave a mixture of coupling products, which was separated by preparative thin layer chromatography using CHCl3:CH2Cl2:hexane (3:3:2). The upper zone gave 10-tert-butoxy-5-methylphenanthridin-6(5H)-one (2f) and the lower zone gave 8-tert-butoxy-5-methylphenanthridin-6(5H)-one (3f).
10-tert-Butoxy-5-methylphenanthridin-6(5H)-one (2f): colorless needles (from CHCl3-hexane), mp 89-91 ˚C. IR (KBr) cm-1: 1650. 1H-NMR (300 MHz) δ: 1.43 (9H, s), 3.81 (3H, s), 7.25-7.54 (4H, m), 8.36 (1H, br d, J = 8.5 Hz), 9.52 (1H, dd, J = 8.3, 1.8 Hz). Anal. Calcd for C18H19NO2.1/2H2O: C, 74.46; H, 6.94; N, 4.82. Found: C, 74.58; H, 6.70; N, 4.88. FAB-MS (positive ion mode) m/z: 282 [M+1] +.
10-tert-Butoxy-5-methylphenanthridin-6(5H)-one (3f): colorless oil. IR (KBr) cm-1: 1650. 1H-NMR (300 MHz) δ: 1.45 (9H, s), 3.82 (3H, s), 7.31 (1H, dd, J = 7.5, 7.5 Hz), 7.37-7.54 (3H, m), 8.14-8.23 (2H, m). FAB-MS (positive ion mode) m/z: 282 [M+1] +.

References

. Present address: Graduate School of Science and Engineering, University of Toyama, Gofuku, Toyama 930-8555, Japan.
1. a) J. Hassan, M. Sévignon, C. Gozzi, E. Schulz, and M. Lemaire, Chem. Rev., 2002, 102, 1359; CrossRef b) G. A. M. Echavarren, B. Gómez-Lor, J. González, and Ó. de Frutos, Synlett, 2003, 585; CrossRef c) D. Arberico, M. E. Scott, and M. Lautens, Chem. Rev, 2007, 107, 174. CrossRef
2. T. Harayama, Heterocycles, 2005, 65, 697. CrossRef
3. T. Harayama, Recent Res. Devel. Organic Chem., 2005, 9, 15.
4. H. Abe and T. Harayama, Heterocycles, 2008, 75, 1305. CrossRef
5. H. Abe, M. Arai, Y. Takeuchi, and T. Harayama, Heterocycles, 2009, 77, 1409. CrossRef
6. a) T. Harayama, Y. Kawata, C. Nagura, T. Sato, T. Miyagoe, H. Abe, and Y. Takeuchi, Tetrahedron Lett., 2005, 46, 6091; CrossRef b) T. Harayama, M. Asai, T. Miyagoe, H. Abe, Y. Takeuchi, A. Yamaguchi, and S. Fujii, Heterocycles, 2010, 81, 1881; CrossRef c) T. Harayama, Yakugaku Zasshi, 2006, 126, 543. CrossRef
7. C. A. Tolmann, Chem. Rev., 1977, 77, 313. CrossRef
8. a) W. Cabri and I. Candiani, Acc. Chem. Res, 1995, 28, 2 and references cited therein; CrossRef b) T. Harayama, H. Akamatsu, K. Okamura, T. Miyagoe, T. Akiyama, H. Abe, and Y. Takeuchi, J. Chem. Soc., Perkin Trans. 1, 2001, 523. CrossRef
9. H. Nishioka, C. Nagura, H. Abe, Y. Takeuchi, and T. Harayama, Heterocycles, 2006, 70, 549. CrossRef
10. W. R. Bowman, H. Heaney, and B. M. Jordan, Tetrahedron, 1991, 48, 10119. CrossRef
11. D. E. Thurston, D. S. Bose, P. W. Howard, T. C. Jenkins, A. Leoni, P. G. Baraldi, A. Guiotto, B. Cacciari, L. R. Kelland, M. Foloppe, and S. Raul, J. Med. Chem., 1999, 42, 1951. CrossRef
12. R. C. Larock, E. K. Yum, and M. D. Refvik, J. Org. Chem., 1998, 63, 7652. CrossRef
13. A. Mondon and K. Krohn, Chem. Ber., 1972, 105, 3726 CrossRef