Synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones via photoisomerization reaction

Abstract

A novel protocol has been developed for the synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones via the photoisomerization of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles. This procedure involves a photo-catalyzed ring opening of isoxazole and subsequent intramolecular nucleophilic addition between a 2H-azirine intermediate and a phenolic hydroxyl group. The synthetic method offers several notable advantages including operational simplicity, mild reaction conditions, moderate to high yields and environmental friendliness.

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Introduction

Organic photochemical reactions play an important role in the context of green chemistry and total synthesis.¹ The photons can be considered as the ideal clean reagent for organic synthesis in contrast to toxic chemical activators.² Another big advantage of reactions in the excited state is their applications in the synthesis of polycyclic or highly functionalized molecules which would be difficult to access with standard chemistry reactions in the ground state.³ Therefore, organic photochemical reactions have received considerable interest in the academic and industry.⁴

Aziridines, the smallest saturated aza-heterocycles, are versatile synthetic intermediates, which have been paid great attentions due to their important and wide applications in organic synthesis and medicinal chemistry.⁵ For an instance, natural fused aziridines such as mitomycins,⁶ azinomycins,⁷ ficellomycin⁸ displayed significant antibiotic and antitumor activities (Scheme 1).

Scheme 1 Fused aziridines in the natural products.

Current preparation of aziridines mainly included the cyclization reactions,⁹ nitrene addition to olefins,¹⁰ carbene and ylide addition to imines,^11,12 addition of azirines,¹³ aza-Darzen approaches¹⁴ and ring contraction.¹⁵ Another interesting methodology was affording 2H-azirines by the photo-cleavage reaction of isoxazoles (Scheme 2).¹⁶ It was known that 2H-azirines could be employed to synthesize aziridines by the addition with O-, S-, N-, C-nucleophiles and hydride.¹⁷ Although the preparation methods of aziridines and its derivatives have been well developed, the synthesis of fused aziridine derivatives was scarce due to the inconvenience of starting materials, nitrene source, catalysts and the harsh reaction conditions.

Scheme 2 The formation of 2H-azirines via photo-cleavage of isoxazoles.

In the previous work, we have synthesized 2H-phenanthro[9,10-c]pyrazoles by the intramolecular photocyclization and dehydration of 3,4-diaryl-1H-pyrazoles in EtOH–H₂O (1 : 1, v/v).¹⁸ Following our investigations on the development of new methodologies for the access to novel polyheterocyclic derivatives, we expect to synthesize phenanthro[9,10-c]isoxazoles 3 by the photocyclization of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles 2. Under the same conditions, the unexpected products, 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones 1 were isolated and identified instead of phenanthro[9,10-c]isoxazoles 3 (Scheme 3). As yet, only Buggle¹⁹ reported the synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one as a byproduct in 11% yield. In this paper, a series of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one analogues were successfully synthesized via the photoisomerization reaction of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles 2 in EtOH–H₂O (1 : 1, v/v).

Scheme 3 Irradiation of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles.

Results and discussion

Optimization of the photoisomerization conditions

On the basis of the literature,²⁰ 4-phenyl-5-(2-hydroxyphenyl)isoxazoles 2 were prepared by the condensation of isoflavones and hydroxylamine hydrochloride with Et₃N as a base in refluxing EtOH. The yields of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles 2 were in the range of 72–90%. Initially, 4-phenyl-5-(2-hydroxyphenyl)isoxazole (2a) was irradiated in EtOH–H₂O (1 : 1, v/v) with a 500 W middle-pressure mercury lamp at about 20 °C according to the methods of our previous work.¹⁸ Based on the careful isolation and characterization, it was surprised to find that 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1a) was obtained in 56% yield (entry 1) (Table 1).

Table 1 Optimization for the photoisomerization conditions of 2a^a

Entry	Solvent	Time (min)	Yield^b (%)
a The intermediate 2a (1 mmol) was dissolved in different solvents (100 mL). The solution was irradiated at λ ≥ 300 nm with a 500 W medium-pressure mercury lamp under an argon atmosphere at about 20 °C.b Yield of isolated product after column chromatography based on 2a.
1	EtOH–H2O (1 : 1)	60	56
2	CH2Cl2	120	21
3	Me2CO	120	28
4	MeCN	90	43
5	MeOH	90	34
6	EtOH	90	39
7	MeCN–H2O (1 : 1)	60	60
8	MeOH–H2O (1 : 1)	60	48

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Subsequently, other solvents were screened for the photoisomerization reaction. When replacing EtOH–H₂O (1 : 1, v/v) with organic solvent (CH₂Cl₂, Me₂CO, MeCN, MeOH, and EtOH), the yields of 1a were decreased (21–43%, entries 2–6). And using MeCN–H₂O (1 : 1, v/v) and MeOH–H₂O (1 : 1, v/v) as solvents, the yields of 1a were 60% and 48%, respectively (entries 7 and 8). Although the yield of 1a in MeCN–H₂O (1 : 1, v/v) was slightly higher than that in EtOH–H₂O (1 : 1, v/v), while from the economical and environmental point of view, EtOH–H₂O (1 : 1, v/v) was finally chosen as the reaction medium. Ultimately, the optimized conditions included the irradiation of 2a (C = 10⁻² mol L⁻¹) in EtOH–H₂O (1 : 1, v/v) with a 500 W medium-pressure mercury lamp (λ ≥ 300 nm) under an argon atmosphere at about 20 °C (entry 1).

The scope of the substrates

In order to explore the reaction scope, different substituted substrates 2a–w were irradiated under the optimized reaction conditions, which generated a structurally divergent 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones 1a–w in moderate to good yields (Table 2). Obviously, the reaction was practicable for substrates bearing either electron-donating or electron-withdrawing substituents. The functionalities such as Me, OMe, i-OPr, OBz, OH, F, Br and CF₃ were all tolerated, leading to the desired products 1a–w. While the electronic effect of different substituents had an influence on the yields of products 1. The substrates bearing electron-donating substituents including Me, OMe, i-OPr, OBz or OH gave the corresponding products in good yield (60–76%, entries 2–13), and when electron-withdrawing substituents such as F, Br or CF₃ were present, the corresponding products were obtained in relative lower yields (34–50%, entries 14–23).

Table 2 Synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones (1) via the photoisomerization^a

Entry	Substrate	R1	R2	R3	R4	Product	Time (min)	Yield^b (%)
a Conditions: 2 (1 mmol) was irradiated at λ ≥ 300 nm with a 500 W high-pressure mercury lamp in 100 mL EtOH–H2O (1 : 1, v/v) under an argon atmosphere at about 20 °C until it was consumed completely indicative by TLC.b Isolated yields after silica chromatography.
1	2a	H	H	H	H	1a	60	56
2	2b	i-OPr	H	H	H	1b	40	68
3	2c	i-OPr	H	H	OMe	1c	50	75
4	2d	i-OPr	H	H	Me	1d	70	71
5	2e	OH	H	H	H	1e	60	60
6	2f	OH	H	H	OMe	1f	70	72
7	2g	OMe	H	H	OMe	1g	70	74
8	2h	OBz	H	H	OMe	1h	70	70
9	2i	OMe	H	H	H	1i	50	73
10	2j	H	H	H	OMe	1j	60	61
11	2k	OMe	H	Me	H	1k	60	76
12	2l	OMe	H	H	Me	1l	70	72
13	2m	H	H	H	Me	1m	80	61
14	2n	H	Br	H	H	1n	80	44
15	2o	H	F	H	H	1o	90	35
16	2p	H	H	H	F	1p	100	39
17	2q	i-OPr	H	H	F	1q	90	43
18	2r	OMe	H	H	F	1r	90	46
19	2s	H	F	H	OMe	1s	80	40
20	2t	H	F	H	F	1t	120	34
21	2u	H	H	H	CF3	1u	120	41
22	2v	OMe	H	H	CF3	1v	100	50
23	2w	i-OPr	H	H	CF3	1w	100	48

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Characterization of the products

All the products, 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones 1 were characterized by IR, NMR and HRMS. In the ¹H NMR (DMSO-d₆) spectra of 1, two doublets showed up around 4.7 ppm and 5.4 ppm. When one drop of D₂O was added to the DMSO-d₆ solution, a doublet at about 4.7 ppm disappeared completely, and the doublet at 5.4 ppm was replaced by singlet. The results of deuterium (²H)-exchange showed that two signals around 4.7 ppm and 5.4 ppm belonged to –NH– and –CH– of aziridine ring. Simultaneously, in the ¹³C NMR spectra of 1, the peak at 188–191 ppm indicated the generation of carbonyl group (–C=O). The appearance of the peaks at about 47 ppm and 70 ppm validated the existence of two saturated carbon atoms (C_7a and C_1a) in the aziridine ring. In addition, the single crystal of 1k was obtained from methanol, and single crystal X-ray diffraction analysis also established the postulated structures unequivocally (Fig. 1).

Fig. 1 X-ray crystal structure of 1k showing 30% probability ellipsoids.

Mechanism of the photoisomerization reaction

A proposed mechanism for the formation of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one is depicted in Scheme 4. According to the photochemical characteristics of substituted isoxazoles,¹⁶ the first step of this photoisomerization reaction has been suggested to occur through homolytic cleavage of the labile N–O bond of isoxazole I with the generation of a diradical II, followed by giving a stable benzylic radical III, which then forms a 2H-azirine intermediate IV by coupling with the nitrogen centered radical. Second, an intramolecular nucleophilic addition reaction occurs between the phenolic hydroxyl group (–OH) of IV and the C=N bond of 2H-azirines ring. The N atom of 2H-azirine first accepts a proton from the phenolic hydroxyl group to produce an azirine carbonium ion V. Next, the phenoxyl anion attacks the carbonium ion to obtain the target product VI. Compared with the irradiation of 3,4-diaryl-1H-pyrazoles to synthesize 2H-phenanthro[9,10-c]pyrazoles,¹⁸ 4-phenyl-5-(2-hydroxyphenyl)isoxazoles have undergone the photoisomerization to produce the 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones owing to the labile N–O bonds.

Scheme 4 Proposed mechanism for the photoisomerization of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles.

Conclusions

In summary, a simple and efficient protocol has been developed for the synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones via the photoisomerization of 4-phenyl-5-(2-hydroxyphenyl)isoxazole in EtOH–H₂O (1 : 1, v/v) at λ ≥ 300 nm with a 500 W medium-pressure mercury lamp. This method offers several notable advantages including the operational simplicity, mild reaction conditions, moderate to high yields and friendly to environment. The formation of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one involves a photo-catalyzed ring opening of isoxazole and subsequent intramolecular nucleophilic addition between 2H-azirine intermediate and phenolic hydroxyl group.

Experimental sections

Melting points were measured by a X-5 micromelting point apparatus and are uncorrected. NMR spectra were recorded on a Bruker AM 300, 400 or 600 instrument using TMS as internal standard and DMSO-d₆ as solvent. High resolution mass spectrometry (HRMS) were recorded using electron-spray ionization (ESI) technique and IR spectra were recorded on a Nicollet 170SX FT-IR spectrophotometer with KBr pellets. The crystal diffraction data were collected on a Bruker Smart-1000 CCD diffractometer. All the irradiation experiments were performed in a BL-GHX-V photochemical reactor equipped with a 500 W medium-pressure mercury lamp. TLC was performed on silica gel 60-GF₂₅₄ plate. The silica gel (size 200–300 mesh) used for the column chromatography was purchased from Qingdao Haiyang Chemistry Plant (China).

General procedure for synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1a–w)

4-Phenyl-5-(2-hydroxyphenyl)isoxazoles 2a–w (1 mmol) was dissolved in 50 mL EtOH and 50 mL redistilled water. The solution was contained in 100 mL quartz tubes, deaerated by bubbling Ar for 30 min and irradiated at λ ≥ 300 nm with a 500 W medium-pressure mercury lamp, which were cooled to about 20 °C with tap water by means of an internal cold finger. The progress of reaction was monitored by TLC at regular intervals until the intermediate 2a–w has disappeared completely. Then, the solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel using petroleum ether–ethyl acetate to give the corresponding products (1a–w), and they are characterized by ¹H NMR, ¹³C NMR, IR and HRMS spectra.

7a-Phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1a). Yield: 56%; yellow solid; m.p. 80.6–81.5 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 4.61 (d, 1H, J = 6.6 Hz), 5.44 (d, 1H, J = 6.6 Hz), 7.09–7.17 (m, 2H), 7.34–7.42 (m, 5H), 7.61 (m, 1H), 7.80 (dd, 1H, J = 7.8, 1.6 Hz); ¹H NMR (600 MHz, DMSO-d₆ + D₂O), δ (ppm) 5.40 (s, 1H), 7.07 (d, 1H, J = 8.4 Hz), 7.12–7.15 (m, 1H), 7.32–7.38 (m, 5H), 7.58–7.61 (m, 1H), 7.76–7.77 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 48.3, 70.1, 117.8, 119.5, 122.2, 126.8, 128.0, 128.9, 134.1, 135.8, 155.4, 190.1; IR (KBr), ν (cm⁻¹) 3552, 3476, 3415, 1675, 1614, 1465, 1216, 751, 616; HRMS (m/z): calc. for C₁₅H₁₁NO₂ [M + Na]⁺ 260.0687, found 260.0675.

4-Isopropoxy-7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1b). Yield: 68%; white solid; m.p. 123.8–124.7 °C. ¹H NMR (300 MHz, DMSO-d₆), δ (ppm) 1.29 (d, 6H, J = 6.0 Hz), 4.59 (d, 1H, J = 6.6 Hz), 4.75 (m, 1H), 5.39 (d, 1H, J = 6.6 Hz), 6.58 (d, 1H, J = 2.1 Hz), 6.71 (dd, 1H, J = 8.8, 2.1 Hz), 7.36–7.40 (m, 5H), 7.73 (d, 1H, J = 8.8 Hz); ¹H NMR (400 MHz, DMSO-d₆ + D₂O), δ (ppm) 1.25 (d, 6H, J = 6.0 Hz), 4.67 (m, 1H), 5.35 (s, 1H), 6.52–6.54 (m, 1H), 6.67–6.69 (m, 1H), 7.33–7.36 (m, 5H), 7.69–7.71 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 21.6, 47.3, 70.1, 70.5, 102.2, 111.3, 112.5, 127.9, 128.0, 128.6, 128.8, 134.5, 157.5, 163.7, 188.6; IR (KBr), ν (cm⁻¹) 3473, 3250, 2978, 1651, 1617, 1444, 1381, 1297, 1246, 1113, 946, 853, 768, 691, 611; HRMS (m/z) calc. for C₁₈H₁₇NO₃ [M + H]⁺ 296.1287, found 296.1286.

4-Isopropoxy-7a-(4-methoxyphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1c). Yield: 75%; yellow solid; m.p. 128.3–128.8 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 1.29 (d, 6H, J = 6.0 Hz), 3.77 (s, 3H), 4.39 (d, J = 6.8 Hz, 1H), 4.74 (m, 1H), 5.38 (d, J = 6.8 Hz, 1H), 6.57 (d, J = 2.0 Hz, 1H), 6.70 (dd, J = 8.8, 2.0 Hz, 1H), 6.94 (d, J = 8.8 Hz, 2H), 6.94 (d, J = 8.8 Hz, 2H), 7.72 (d, J = 8.8 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 21.6, 46.9, 55.1, 70.1, 70.5, 102.1, 111.2, 112.6, 113.4, 126.4, 128.6, 130.1, 157.5, 158.9, 163.6, 188.8; IR (KBr), ν (cm⁻¹) 3553, 3477, 3415, 3265, 2977, 1649, 1613, 1515, 1448, 1292, 1246, 1188, 1115, 1028, 947, 825, 748, 624; HRMS (ESI) calc. for C₁₉H₁₉NO₄ [M + H]⁺ 326.1392, found 326.1379.

4-Isopropoxy-7a-(4-methylphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1d). Yield: 71%; white solid; m.p. 156.2–157.1 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 1.29 (d, 6H, J = 6.0 Hz), 2.32 (s, 3H), 4.43 (d, J = 6.0 Hz, 1H), 4.74 (m, 1H), 5.35 (d, J = 6.0 Hz, 1H), 6.58 (s, 1H), 6.70 (d, J = 8.8, 1H), 7.19 (d, J = 6.8 Hz, 2H), 7.28–7.30 (m, 2H), 7.72–7.74 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 20.7, 21.6, 47.1, 70.1, 70.5, 102.2, 111.3, 112.6, 128.5, 128.6, 128.7, 131.5, 137.1, 157.5, 163.7, 188.7; IR (KBr), ν (cm⁻¹) 3447, 3271, 2978, 2025, 1652, 1613, 1443, 1388, 1247, 1188, 1110, 854, 752, 642; HRMS (ESI) calc. for C₁₉H₁₉NO₃ [M + Na]⁺ 332.1263, found 332.1245.

4-Hydroxy-7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1e). Yield: 60%; white solid; m.p. 81.3–82.1 °C. ¹H NMR (600 MHz, DMSO-d₆), δ (ppm) 4.38 (d, J = 6.0 Hz, 1H), 5.38 (d, J = 6.0 Hz, 1H), 6.33 (s, 1H), 6.54 (d, J = 9.0 Hz, 1H), 7.29–7.34 (m, 5H), 7.62–7.64 (m, 1H); ¹³C NMR (150 MHz, DMSO-d₆), δ (ppm) 47.7, 70.9, 102.9, 111.8, 112.3, 128.3, 128.4, 129.3, 129.4, 135.2, 158.0, 164.9, 188.9; IR (KBr), ν (cm⁻¹) 3522, 3444, 3280, 2025, 1679, 1626, 1515, 1484, 1443, 1400, 1265, 1219, 1159, 995, 740, 565; HRMS (ESI) calc. for C₁₅H₁₁NO₃ [M + Na]⁺ 276.0637, found 276.0616.

4-Hydroxy-7a-(4-methoxyphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1f). Yield: 72%; yellow solid; m.p. 109.8–110.7 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.76 (s, 3H), 4.30 (d, J = 6.8 Hz, 1H), 5.32 (d, J = 6.8 Hz, 1H), 6.37 (d, J = 2.1 Hz, 1H), 6.58 (dd, J = 8.8, 2.1 Hz, 1H), 6.93 (d, J = 8.8 Hz, 2H), 7.31 (d, J = 8.8 Hz, 2H), 7.66–7.68 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 46.8, 55.1, 70.4, 102.4, 111.2, 111.8, 113.4, 126.6, 128.9, 130.1, 157.5, 158.9, 164.3, 188.7; IR (KBr), ν (cm⁻¹) 3553, 3477, 3415, 3239, 2929, 2361, 1616, 1254, 622, 483; HRMS (ESI) calc. for C₁₆H₁₃NO₄ [M + H]⁺ 284.0923, found 284.0909.

4-Methoxy-7a-(4-methoxyphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1g). Yield: 74%; yellow solid; m.p. 142.7–143.9 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.77 (s, 3H), 3.84 (s, 3H), 4.40 (d, J = 6.8 Hz, 1H), 5.40 (d, J = 6.8 Hz, 1H), 6.60 (d, J = 2.1 Hz, 1H), 6.94 (d, J = 8.8 Hz, 2H), 7.32 (dd, J = 8.8, 2.1 Hz, 2H) 7.74 (d, J = 8.8 Hz, 2H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 47.0, 55.1, 55.8, 70.5, 100.9, 110.4, 112.8, 113.4, 126.4, 128.5, 130.1, 157.5, 158.9, 165.3, 189.0; IR (KBr), ν (cm⁻¹) 3552, 3476, 3415, 3239, 2926, 2361, 1723, 1617, 1515, 1439, 1247, 1024, 838, 624, 482; HRMS (ESI) calc. for C₁₇H₁₅NO₄ [M + Na]⁺ 320.0899, found 320.0880.

4-Benzyloxy-7a-(4-methoxyphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1h). Yield: 70%; yellow solid; m.p. 101.8–102.7 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.77 (s, 3H), 4.41 (d, J = 6.8 Hz, 1H), 5.20 (s, 2H), 5.39 (d, J = 6.8 Hz, 1H), 6.69 (d, J = 2.1 Hz, 1H), 6.80–6.82 (m, 1H), 6.93–6.95 (m, 2H), 7.32–7.37 (m, 3H), 7.39–7.47 (m, 4H), 7.74 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 47.0, 55.1, 69.7, 70.5, 101.9, 110.0, 113.0, 113.4, 126.3, 127.8, 128.1, 128.5, 128.6, 130.1, 136.2, 157.4, 158.9, 164.3, 189.0; IR (KBr), ν (cm⁻¹) 3553, 3476, 3414, 3272, 2926, 1612, 1510, 1444, 1244, 1171, 1026, 835, 624; HRMS (ESI) calc. for C₂₃H₁₉NO₄ [M + H]⁺ 374.1392, found 374.1370.

4-Methoxy-7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1i). Yield: 73%; yellow solid; m.p. 103.0–104.1 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.84 (s, 3H), 4.51 (d, J = 6.4 Hz, 1H), 5.42 (d, J = 6.4 Hz, 1H), 6.62 (d, J = 2.1 Hz, 1H), 6.75 (dd, J = 8.8, 2.1 Hz, 1H), 7.36–7.41 (m, 5H), 7.76 (d, J = 8.8 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 47.4, 55.8, 70.5, 100.9, 110.5, 112.8, 127.9, 128.0, 128.5, 128.8, 134.4, 157.5, 165.4, 188.7; IR (KBr), ν (cm⁻¹) 3633, 3213, 1656, 1609, 1572, 1493, 1440, 1285, 1244, 1187, 1024, 839, 587; HRMS (ESI) calc. for C₁₆H₁₃NO₃ [M + H]⁺ 268.0974, found 268.0974.

7a-(4-Methoxyphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1j). Yield: 61%; yellow solid; m.p. 83.8–84.9 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.77 (s, 3H), 4.53 (d, J = 6.4 Hz, 1H), 5.44 (d, J = 6.4 Hz, 1H), 6.94–6.96 (m, 2H), 7.09–7.18 (m, 2H), 7.35 (d, J = 8.8 Hz, 2H), 7.61–7.64 (m, 1H), 7.80–7.82 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 47.9, 55.1, 70.2, 113.5, 117.8, 119.5, 122.2, 126.1, 126.8, 130.2, 135.7, 155.4, 159.0, 190.3; IR (KBr), ν (cm⁻¹) 3552, 3476, 3415, 2922, 1617, 1517, 1464, 1292, 1250, 1027, 825, 752, 623; HRMS (ESI) calc. for C₁₆H₁₃NO₃ [M + Na]⁺ 290.0793, found 290.0781.

4-Methoxy-6-methyl-7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1k). Yield: 76%; yellow solid; m.p. 132.1–132.9 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 2.51 (s, 3H), 3.81 (s, 3H), 4.37 (d, J = 6.4 Hz, 1H), 5.40 (d, J = 6.4 Hz, 1H), 6.47–6.56 (m, 2H), 7.35–7.40 (m, 5H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 22.0, 49.6, 55.6, 69.7, 99.5, 112.5, 112.9, 127.8, 127.9, 129.1, 134.7, 142.6, 158.4, 163.5, 190.6; IR (KBr), ν (cm⁻¹) 3479, 3247, 3056, 2924, 1665, 1610, 1573, 1446, 1356, 1281, 1245, 1205, 1142, 1045, 885, 840, 751, 698, 594; HRMS (ESI) calc. for C₁₇H₁₅NO₃ [M + H]⁺ 282.1130, found 282.1131.

4-Methoxy-7a-(4-methylphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1l). Yield: 72%; yellow solid; m.p. 135.7–136.5 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 2.32 (s, 3H), 3.84 (s, 3H), 4.46 (d, J = 6.0 Hz, 1H), 5.38 (d, J = 6.0 Hz, 1H), 6.61 (s, 1H), 6.73–6.75 (m, 1H), 7.18 (d, J = 8.8 Hz, 2H), 7.28–7.30 (m, 2H), 7.75 (dd, J = 8.8, 2.1 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 20.7, 47.2, 55.8, 70.5, 101.0.110.4, 112.9, 128.5, 128.7, 131.4, 137.2, 157.5, 165.3, 188.8; IR (KBr), ν (cm⁻¹) 3520, 3443, 3278, 2930, 2025, 1670, 1609, 1513, 1443, 1386, 1245, 1189, 1107, 836, 570; HRMS (ESI) calc. for C₁₇H₁₅NO₃ [M + Na]⁺ 304.0950, found 304.0934.

7a-(4-Methylphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1m). Yield: 61%; yellow solid; m.p. 83.8–84.9 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 2.32 (s, 3H), 4.58 (d, J = 6.4 Hz, 1H), 5.43 (d, J = 6.4 Hz, 1H), 7.10–7.21 (m, 4H), 7.30–7.32 (m, 2H), 7.61–7.65 (m, 1H), 7.82 (d, J = 8.0 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 20.7, 48.1, 70.2, 117.8, 119.5, 122.2, 126.8, 128.6, 128.8, 131.1, 135.7, 137.3, 155.4, 190.2; IR (KBr), ν (cm⁻¹) 3552, 3415, 3240, 2027, 1616, 1517, 1464, 1395, 1285, 1215, 1143, 986, 812, 752, 6; HRMS (ESI) calc. for C₁₆H₁₃NO₂ [M + H]⁺ 252.1025, found 252.1013.

5-Bromo-7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1n). Yield: 44%; yellow solid; m.p. 56.5–57.3 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 4.85 (d, J = 6.4 Hz, 1H), 5.52 (d, J = 6.4 Hz, 1H), 7.13 (d, J = 8.8 Hz, 1H), 7.37–7.45 (m, 5H), 7.79 (dd, J = 8.8, 2.1 Hz, 1H), 7.88 (d, J = 2.50 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 48.2, 70.4, 113.8, 120.6, 121.2, 128.0, 128.1, 128.8, 128.9, 133.6, 138.1, 154.5, 189.0; IR (KBr), ν (cm⁻¹) 3555, 3479, 3414, 3274, 3060, 2922, 2360, 1678, 1597, 1468, 1420, 1262, 1219, 1184, 1126, 982, 817, 746, 694, 587, 501; HRMS (ESI) calc. for C₁₅H₁₀BrNO₂ [M + H]⁺ 315.9973, found 315.9960.

10-Fluoro-7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1o). Yield: 35%; yellow solid; m.p. 78.9–80.2 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 4.73 (d, J = 6.8 Hz, 1H), 5.50 (d, J = 6.8 Hz, 1H), 7.21–7.22 (m, 1H), 7.39–7.44 (m, 5H), 7.51–7.53 (m, 2H); ¹H NMR (400 MHz, DMSO-d₆ + D₂O), δ (ppm) 5.46 (s, 1H), 7.15–7.19 (m, 1H), 7.36–7.40 (m, 5H), 7.47–7.52 (m, 2H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 48.1, 70.3, 111.7 (d, ²J = 23.6 Hz), 120.0, 120.1, 120.3 (d, ³J = 6.6 Hz), 123.2 (d, ²J = 24.2 Hz), 128.1 (d, ³J = 5.8 Hz), 128.9, 133.8, 151.7, 157.0 (d, ¹J = 238.4 Hz), 189.5; IR (KBr), ν (cm⁻¹) 3479, 3412, 3298, 3080, 1675, 1624, 1483, 1441, 1345, 1264, 1212, 1161, 1125, 1041, 990, 942, 822, 744, 699, 612; HRMS (ESI) calc. for C₁₅H₁₀FNO₂ [M + H]⁺ 256.0774, found 256.0765.

7a-(4-Fluorophenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1p). Yield: 39%; yellow solid; m.p. 104.0–104.9 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 4.65 (d, J = 6.8 Hz, 1H), 5.48 (d, J = 6.8 Hz, 1H), 7.11–7.25 (m, 4H), 7.47–7.51 (m, 2H), 7.62–7.66 (m, 1H), 7.81–7.83 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 47.7, 70.1, 114.9 (d, ²J = 21.3 Hz), 117.8, 119.4, 122.3, 126.8, 130.4, 131.1 (d, ³J = 8.4 Hz), 135.8, 155.4, 161.8 (d, ¹J = 242.9 Hz), 189.9; IR (KBr), ν (cm⁻¹) 3553, 3477, 3414, 3293, 3044, 1662, 1613, 1509, 1470, 1292, 1218, 1155, 1104, 1012, 899, 812, 747, 622; HRMS (ESI) calc. for C₁₅H₁₀FNO₂ [M + Na]⁺ 278.0593, found 278.0580.

4-Isopropoxy-7a-(4-fluorophenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1q). Yield: 43%; yellow solid. m.p. 98.8–99.5 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 1.29 (d, 6H, J = 6.0 Hz), 4.52 (d, J = 6.0 Hz, 1H), 4.75 (m, 1H), 5.41 (d, J = 6.0 Hz, 1H), 6.59 (s, 1H), 6.70–6.72 (m, 1H), 7.19–7.24 (m, 2H), 7.45–7.48 (m, 2H), 7.73 (dd, J = 8.8, 2.4 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 21.6, 46.7, 70.1, 70.4, 102.2, 111.3, 112.4, 114.8 (d, ²J = 21.3 Hz), 128.6, 130.7, 131.1 (d, ³J = 8.3 Hz), 157.5, 161.7 (d, ¹J = 242.8 Hz), 163.7, 188.4; IR (KBr), ν (cm⁻¹) 3520, 3443, 3277, 2930, 2025, 1670, 1609, 1514, 1443, 1385, 1245, 1189, 1159, 1110, 1017, 836, 769, 680, 570; HRMS (ESI) calc. for C₁₈H₁₆FNO₃ [M + Na]⁺ 336.1012, found 336.0989.

4-Methoxy-7a-(4-fluorophenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1r). Yield: 46%; yellow solid; m.p. 112.8–113.1 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.84 (s, 3H), 4.54 (d, J = 6.0 Hz, 1H), 5.44 (d, J = 6.0 Hz, 1H), 6.63 (s, 1H), 6.74–6.76 (m, 1H), 7.19–7.24 (m, 2H), 7.46–7.49 (m, 2H), 7.74–7.77 (m, 1H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 46.8, 55.8, 70.4, 101.0, 110.5, 112.7, 114.8 (d, ²J = 21.4 Hz), 128.6, 130.7, 131.1 (d, ³J = 8.4 Hz), 157.5, 161.8 (d, ¹J = 242.7 Hz), 165.4, 188.6; IR (KBr), ν (cm⁻¹) 3444, 3255, 2025, 1663, 1613, 1513, 1440, 1403, 1246, 1155, 1101, 1031, 831, 767, 679, 569; HRMS (ESI) calc. for C₁₆H₁₂FNO₃ [M + Na]⁺ 308.0699, found 308.0687.

4-Fluoro-7a-(4-methoxyphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1s). Yield: 40%; yellow solid; m.p. 57.9–58.2 °C. ¹H NMR (600 MHz, DMSO-d₆), δ (ppm) 3.76 (s, 3H), 4.60 (d, J = 6.6 Hz, 1H), 5.45 (d, J = 6.6 Hz, 1H), 6.93–6.94 (m, 2H), 7.15–7.18 (m, 1H), 7.33–7.34 (m, 2H), 7.48–7.51 (m, 2H); ¹³C NMR (150 MHz, DMSO-d₆), δ (ppm) 48.1, 55.6, 70.9, 112.2 (d, ²J = 23.7 Hz), 114.0, 120.5 (d, ³J = 7.6 Hz), 120.8 (d, ³J = 6.5 Hz), 123.6 (d, ²J = 24.3 Hz), 126.3, 130.7, 152.2, 157.5 (d, ¹J = 238.4 Hz), 159.6, 190.2; IR (KBr), ν (cm⁻¹) 3520, 3443, 3279, 2930, 2026, 1670, 1609, 1513, 1443, 1387, 1244, 1189, 1106, 1018, 836, 769, 681, 570; HRMS (ESI) calc. for C₁₆H₁₂FNO₃ [M + Na]⁺ 308.0699, found 308.0684.

4-Fluoro-7a-(4-fluorophenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1t). Yield: 34%; yellow solid; m.p. 139.5–140.1 °C. ¹H NMR (600 MHz, DMSO-d₆), δ (ppm) 4.72 (d, J = 6.60 Hz, 1H), 5.49 (d, J = 6.60 Hz, 1H), 7.17–7.22 (m, 3H), 7.48–7.53 (m, 4H); ¹³C NMR (150 MHz, DMSO-d₆), δ (ppm) 47.9, 70.8, 112.2 (d, ²J = 23.7 Hz), 115.3 (d, ²J = 21.3 Hz), 120.5 (d, ³J = 7.6 Hz), 120.7 (d, ³J = 6.9 Hz), 123.7 (d, ²J = 24.3 Hz), 130.6, 131.6 (d, ³J = 8.5 Hz), 152.1, 157.5 (d, ¹J = 238.5 Hz), 162.4 (d, ¹J = 243.5 Hz), 189.8; IR (KBr), ν (cm⁻¹) 3612, 3522, 3444, 3281, 2025, 1679, 1626, 1604, 1516, 1484, 1402, 1265, 1219, 1159, 1126, 995, 829, 763, 740, 700, 565; HRMS (ESI) calc. for C₁₅H₉F₂NO₂ [M + Na]⁺ 296.0499, found 296.0482.

7a-(4-Trifluoromethylphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1u). Yield: 41%; yellow solid. m.p. 106.1–107.0 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 4.82 (d, J = 6.6 Hz, 1H), 5.51 (d, J = 6.6 Hz, 1H) 7.13–7.21 (m, 2H), 7.64–7.70 (m, 3H), 7.77 (d, J = 8.4 Hz, 2H), 7.84 (dd, J = 8.0, 2.1 Hz, 1H); ¹³C NMR (150 MHz, DMSO-d₆), δ (ppm) 47.8, 70.1, 117.9, 119.3, 122.4, 124.2 (q, ¹J = 270.6 Hz), 124.8 (q, ³J = 3.8 Hz), 126.9, 128.5 (q, ²J = 31.5 Hz), 129.8, 136.0, 138.7, 155.3, 189.5; IR (KBr), ν (cm⁻¹) 3669, 3284, 3072, 1671, 1610, 1470, 1413, 1331, 1296, 1219, 1164, 1113, 1067, 1010, 903, 858, 815, 752; HRMS (ESI) calc. for C₁₆H₁₀F₃NO₂ [M + Na]⁺ 328.0561, found 328.0549.

4-Methoxy-(4-trifluoromethylphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1v). Yield: 50%; yellow solid; m.p. 132.3–133.2 °C. ¹H NMR (400 MHz, DMSO-d₆), δ (ppm) 3.85 (s, 3H), 4.72 (d, J = 6.6 Hz, 1H), 5.47 (d, J = 6.6 Hz, 1H), 6.65 (d, J = 2.4 Hz, 1H), 6.76 (dd, J = 8.4, 2.4 Hz, 1H), 7.68 (d, J = 8.4 Hz, 2H), 7.74–7.78 (m, 3H); ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm) 46.9, 55.9, 70.5, 101.0, 110.7, 112.6, 124.2 (q, ¹J = 270.5 Hz), 124.7 (q, ³J = 3.7 Hz), 128.5 (q, ²J = 31.6 Hz), 128.9, 129.8, 139.1, 157.5, 165.5, 188.1; IR (KBr), ν (cm⁻¹) 3553, 3476, 3414, 3238, 2361, 1616, 1440, 1325, 1247, 1171, 1134, 1065, 1022, 869, 832, 620, 482; HRMS (ESI) calc. for C₁₇H₁₂F₃NO₃ [M + Na]⁺ 358.0667, found 358.0650.

4-Isopropoxy-(4-trifluoromethylphenyl)-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-one (1w). Yield: 48%; yellow solid; m.p. 107.5–108.9 °C. ¹H NMR (600 MHz, DMSO-d₆), δ (ppm) 1.29 (d, 6H, J = 6.0 Hz), 4.69 (d, J = 6.8 Hz, 1H), 4.76 (m, 1H), 5.44 (d, J = 6.8 Hz, 1H), 6.61 (d, J = 2.4 Hz, 1H), 6.73 (dd, J = 8.4, 2.4 Hz, 1H), 7.66 (d, J = 8.4 Hz, 2H), 7.74–7.76 (m, 3H); ¹³C NMR (150 MHz, DMSO-d₆), δ (ppm) 21.5, 46.8, 70.1, 70.5, 102.2, 111.5, 112.3, 124.2 (q, ¹J = 270.6 Hz), 124.7 (q, ³J = 3.6 Hz), 128.4 (q, ²J = 31.6 Hz), 129.8, 139.1, 128.7, 157.5, 163.9, 188.0; IR (KBr), ν (cm⁻¹) 3553, 3477, 3415, 3290, 2986, 1654, 1617, 1572, 1500, 1445, 1331, 1279, 1248, 1165, 1113, 1066, 1015, 925, 856, 816, 610, 484; HRMS (ESI) calc. for C₁₉H₁₉F₃NO₃ [M + H]⁺ 364.1161, found 364.1147.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (no. 21372150), the Fundamental Funds Research for the Central Universities (no. GK261001095) and Innovation Funds of Graduate Programs of Shaanxi Normal University (no. 2012 CXB016) for financial support of this research.

Notes and references

1. (a) C. J. Mortko and M. A. Garcia-Garibay, J. Am. Chem. Soc., 2005, 127, 7994; (b) N. Hoffmann, Chem. Rev., 2008, 108, 1052; (c) M. Fleck and T. Bach, Angew. Chem., Int. Ed., 2008, 47, 6189; (d) V. Dichiarante, M. Fagnoni and A. Albini, Green Chem., 2009, 11, 942; (e) M. Fleck and T. Bach, Chem.–Eur. J., 2010, 16, 6015.
2. (a) W. M. Horspool, Synthetic Organic Photochemistry, Plenum Press, New York, 1984; (b) H. D. Roth, Angew. Chem., Int. Ed. Engl., 1989, 28, 1193; (c) Y. Inoue, Chem. Rev., 1992, 92, 741; (d) M. Fagnoni and A. Albini, Acc. Chem. Res., 2005, 38, 713; (e) A. G. Griesbeck and J. Mattay, Synthetic Organic Photochemistry, Marcel Dekker, New York, 2005, p. 141.
3. (a) P. Wessig and O. Muhling, Angew. Chem., Int. Ed., 2001, 40, 1064; (b) V. Dichiarante, M. Fagnoni and A. Albini, Angew. Chem., Int. Ed., 2007, 46, 6495; (c) S. Breitenlechner and T. Bach, Angew. Chem., Int. Ed., 2008, 47, 7957; (d) F. Family and M. A. Garcia-Garibay, J. Org. Chem., 2009, 74, 2476.
4. (a) H. E. Zimmerman and S. Shorunov, J. Org. Chem., 2009, 74, 5411; (b) S. Protti, M. Fagnoni and A. Albini, Angew. Chem., Int. Ed., 2005, 44, 5675; (c) D. Armesto, M. J. Ortiz, A. R. Agarrabeitia and M. Martin-Fontecha, J. Am. Chem. Soc., 2001, 123, 9920.
5. (a) A. Padwa, in Comprehensive Heterocyclic Chemistry III, ed. A. R. Katrizky, C. A. Ramsden, E. F. V. Scriven and R. J. K. Taylor, Elsevier, Amsterdam, 2008, vol. 1, p. 2; (b) J. B. Sweeney, in Science of Synthesis, ed. E. Schaumann and D. Enders, Georg Thieme Verlag, Stuttgart, Germany, 2008, vol. 40a, p. 643; (c) A. Padwa and S. S. Murphee, Prog. Heterocycl. Chem., 2003, 15, 75; (d) J. B. Sweeney, Chem. Soc. Rev., 2002, 31, 247; (e) B. Zwanenburg and P. ten Holte, Top. Curr. Chem., 2001, 216, 93; (f) U. M. Lindstrom and P. Somfai, Synthesis, 1998, 109; (g) P. Somfai and J. Ahman, Targets Heterocycl. Syst., 1999, 3, 341.
6. (a) M. Trost Barry, M. O'Boyle Brendan, T. Wildeliz and K. A. Michael, Chem.–Eur. J., 2011, 17, 7890; (b) N. Papaioannu, C. A. Evans, J. T. Blank and S. J. Miller, Org. Lett., 2001, 3, 2879; (c) K. J. Falci, R. W. Franck and S. P. Smith, J. Org. Chem., 1977, 42, 3317; (d) F. Nakatsubo, A. J. Cocuzza, D. E. Keeley and Y. Kishi, J. Am. Chem. Soc., 1977, 99, 4835; (e) T. R. Witty and W. A. Remers, J. Med. Chem., 1973, 16, 1280.
7. (a) K. Nagaoka, M. Matsumoto, J. Oono, K. Yokoi, S. Ishizeki and T. Nakashima, J. Antibiot., 1986, 39, 1527; (b) K. Yokoi, K. Nagaoka and T. Nakashima, Chem. Pharm. Bull., 1986, 34, 4554; (c) S. Ishizeki, M. Ohtsuka, K. Irinoda, K. Kukita, K. Nagaoka and T. Nakashima, J. Antibiot., 1987, 40, 60.
8. (a) A. D. Argoudelis, F. Reusser, H. A. Whaley, L. Baczynskyj, S. A. Mizsak and R. J. Wnuk, J. Antibiot., 1976, 29, 1001; (b) F. Reusser, Biochemistry, 1977, 16, 3406.
9. (a) S. J. Brois, J. Org. Chem., 1962, 27, 3532; (b) H. E. Baumgarten, R. L. Zey and U. Krolls, J. Am. Chem. Soc., 1961, 83, 4469.
10. (a) W. Lwowsky, Angew. Chem., Int. Ed. Engl., 1967, 6, 897; (b) J. U. Jeong, B. Tao, I. Sagasser, H. Henniges and K. B. Sharpless, J. Am. Chem. Soc., 1998, 120, 6844; (c) E. N. Jacobsen, in Comprehens iVe Asymmetric Catalysis, ed. E. N. Jacobsen, A. Pfaltz and H. amamoto, Springer, Berlin, 1999, vol. 2, p. 607; (d) Z. Li, K. R. Conser and E. N. Jacobsen, J. Am. Chem. Soc., 1993, 115, 5326.
11. L. Casarrubios, J. A. Perez, M. Brookhart and J. L. Templeton, J. Org. Chem., 1996, 61, 8358.
12. (a) D. K. Wang, L. X. Dai and X. L. Hou, Chem. Commun., 1997, 1231; (b) V. K. Aggarwal, A. Thompson, R. V. H. Jones and M. C. H. Standen, Phosphorus, Sulfur Silicon Relat. Elem., 1997, 120, 361.
13. F. Palacios, A. M. Ochoa de Retana, E. Martinez de Marigorta and J. Manuel de los Santos, Eur. J. Org. Chem., 2001, 2401.
14. (a) V. Reutrakul, V. Prapansiri and C. Panyachotipurio, Tetrahedron Lett., 1984, 25, 1949; (b) T. Satoh, T. Sato, T. Oahara and K. Yamakawa, J. Org. Chem., 1989, 54, 3973; (c) S. Florio, L. Troisi, V. Capriati and G. Ingrosso, Tetrahedron Lett., 1999, 40, 6101; (d) J. B. Sweeney, Eur. J. Org. Chem., 2009, 4911.
15. G. B. Mullen, G. A. Bennett and V. Georgiev, Eur. J. Org. Chem., 1990, 109.
16. (a) R. R. Sauers, L. M. Hadel, A. A. Scimone and T. A. Stevenson, J. Org. Chem., 1990, 55, 4011; (b) B. Singh and E. F. Ullman, J. Am. Chem. Soc., 1967, 89, 6911; (c) J. P. Ferris and R. W. Trimmer, J. Org. Chem., 1976, 41, 13; (d) A. Padwa, E. Chen and A. Ku, J. Am. Chem. Soc., 1975, 97, 6484; (e) S. Lopes, C. M. Nunes, A. Gómez-Zavaglia, T. M. Pinho e Melo and R. Fausto, J. Phys. Chem. A, 2011, 115, 1199.
17. M. J. Alves and F. T. Costa, Heterocyclic Targets in Advanced Organic Synthesis, ed. M. d. C. Carreiras and J. Marco-Contelles, Research Signpost, Trivandrum, India, 2011, p. 145.
18. Q. Y. Wang, Z. T. Zhang, Z. C. Du, H. L. Hua and S. S. Chen, Green Chem., 2013, 15, 1048.
19. K. Buggle and B. Fallon, J. Chem. Res., Synop., 1988, 349.
20. Y. M. Wu, K. Foleky and C. Borella, Patent, WO2008033449.

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Footnote

† Electronic supplementary information (ESI) available: Part experimental details, spectroscopic data. CCDC 1017876. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra12542h

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