Molecular iodine induced/1,3-dipolar cycloaddition/oxidative aromatization sequence: an efficient strategy to construct 2-substituted benzo[f]isoindole-1,3-dicarboxylates

Abstract

A useful method for a molecular iodine induced 1,3-dipolar cycloaddition/oxidative aromatization sequence to construct 2-substituted-benzo[f]isoindole-1,3-dicarboxylates is reported. This is the first report of a molecular iodine induced 1,3-dipolar cycloaddition between quinone structures and diethyl N-substituted iminodiacetates.

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Introduction

Iminium ions are important reactive species in organic synthesis for the construction of carbon–carbon and carbon–heteroatom bonds.¹ Exploitation of these reactive intermediates by reacting with diverse nucleophiles to construct biologically relevant structural fragments has attracted great interest.² In the past few decades, the effective using of iminium ions as key intermediates has played an important role in organic reactions, such as the Aza-Henry reactions,³ Mannich reactions,⁴ Diels–Alder reactions,⁵ cross dehydrogenative coupling (CDC) reactions,⁶ Friedel–Crafts reactions,⁷ N-acyl iminium ion cyclizations,^1b,8 Ugi-type processes,^8e,9 Baylis–Hillman-type reactions,¹⁰ Pictet–Spengler reactions,¹¹ 1,3-dipolar cycloadditions,¹² intramolecular cyclizations.¹³ Since iodine is an inexpensive and environmentally benign reagent,¹⁴ molecular iodine-mediated reaction for synthesis of heterocycles has been widely reported.¹⁵ Moreover, molecular iodine catalyzed CDC reactions to produce iminium ions for the synthesizing a variety of functionalized tetrahydroisoquinolines have been illustrated by Itoh^6f and Prabhu.^6j

The benzo[f]isoindole framework is the core structure in a large number of natural products exhibiting important biological activities. For example, bhimamycin C and D,¹⁶ which display bioactivities against human ovarian cancer cell lines, are EP4 receptor agonists in the treatment of pain, and inhibitors of HIV-1 integrase (Fig. 1). Furthermore, benzo[f]isoindole framework frequently appears in a number of other anticancer compounds.^16b

Fig. 1 Representative examples of natural products.

Recently, Gong and co-workers¹⁷ had reported the chiral azomethine ylide dipoles reacted with quinone derivatives, and after a subsequent base promoted isomerization to generate chiral isoindolines. From our ongoing study of quinones,^2c,18 we herein reported a mechanism distinct method for the construction of 2-substituted benzo[f]isoindole-1,3-dicarboxylates 3 induced by molecular iodine.

Results and discussion

Our initial investigations were focused on examining the feasibility of the reaction of diethyl N-methyliminodiacetate 1a with 1,4-naphthoquinone 2a and optimizing the reaction conditions for application to construct a variety of 2-substituted benzo[f]isoindole-1,3-dicarboxylates.

To our delight, the proposed reaction between 1a and 2a did indeed occur in the presence of iodine (3 equiv.) in CH₃CN with NaHCO₃ as a base to afford 3aa in 60% (Table 1, entry 1). Some other common iodine-addition initiation systems were also investigated, including I₂/t-BuOCl and I₂/AgOAc, but these systems did not provide any improvement over molecular iodine only (Table 1, entries 2 and 3). Excitingly, the yield of 3aa was up to 75% when DBU was used instead of NaHCO₃ (Table 1, entry 4). Solvent screening studies revealed that xylene was the most suitable solvent as other solvents furnished lower yield of 3aa (Table 1, entries 4–8). When the temperature was dropped to 120 °C in xylene, the yield of 3aa was decreased to 80% (Table 1, entry 9). As the iodine dropped to 2 equiv., the yield of 3aa was decreased to 70% (Table 1, entry 10). The result showed that 3 equivalents of iodine was necessary in the reaction system. Finally, the best result was obtained in the presence of iodine (3 equiv.) in xylene with DBU as a base (Table 1, entry 8) and the isolated yield of 3aa was 86%. Besides, the yield of 3aa was decreased to 60% under nitrogen condition (Table 1, entry 11). Comparing to above result (Table 1, entry 8), which indicated oxygen played a role in the final aromatisation process. The structure of product 3ab was unambiguously established by X-ray crystallographic analysis.¹⁹

Table 1 Optimization of the reaction conditions^a

Entry	Base	Iodine (equiv.)	Solvent	T (°C)	Yield of 3aa^b (%)
a Reaction conditions: the mixture of 1a (3.0 mmol), 2a (1.0 mmol), base (3.0 mmol), iodine (3.0 mmol) in solvent (5.0 mL) was stirred for 5 h at reflux temperature under air condition.b Isolated yield.c 3 equiv. t-BuOCl was added.d 3 equiv. AgOAc was added.e Under nitrogen condition.
1	NaHCO3	I2 (3)	CH3CN	80	60
2^c	NaHCO3	I2 (3)	CH3CN	80	40
3^d	NaHCO3	I2 (3)	CH3CN	80	20
4	DBU	I2 (3)	CH3CN	80	75
5	DBU	I2 (3)	CH2Cl2	40	40
6	DBU	I2 (3)	CHCl3	60	49
7	DBU	I2 (3)	Toluene	110	70
8	DBU	I2 (3)	Xylene	140	86
9	DBU	I2 (3)	Xylene	120	80
10	DBU	I2 (2)	Xylene	140	70
11^e	DBU	I2 (3)	Xylene	140	60

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With the optimal reaction conditions established in hand, we then examined the substrate scope of this useful reaction. As highlighted in Table 2, a variety of ethyl N-substituted iminodiacetate 2 could react efficiently with 1a to give the corresponding products in moderate to good yields upon isolation (Table 2, entries 1–4). As the substituted group on N-atom increased, the yield of 3 was reduced. It was also found that the reaction proceeded smoothly with different quinone structures (Table 2, entries 5–12). However, the yield of corresponding products 3ad–dd were lower when 5-hydroxy-1,4-naphthoquinone 2d was employed because of some insoluble things and can not be identified by NMR or GC-MS (Table 2, entries 13–16 (Fig. 2)).

Table 2 Variation of the substrates

Entry	1	R^1	2	3	Yield of 3^a (%)
a Isolated yield.
1	1a	Me		3aa	86
2	1b	Et		3ba	70
3	1c	n-Pr		3ca	64
4	1d	n-Bu		3da	40
5	1a	Me		3ab	81
6	1b	Et		3bb	73
7	1c	n-Pr		3cb	61
8	1d	n-Bu		3db	57
9	1a	Me		3ac	70
10	1b	Et		3bc	63
11	1c	n-Pr		3cc	60
12	1d	n-Bu		3dc	52
13	1a	Me		3ad	40
14	1b	Et		3bd	34
15	1c	n-Pr		3cd	23
16	1d	n-Bu		3dd	16

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Fig. 2 X-ray structure of 3ab.

As various ethyl N-alkyl iminodiacetates 1 proceed with 2a–c in moderate to good yields, ethyl N-benzyl iminodiacetate 1e and ethyl N-isopropyl iminodiacetate 1f were chose to expand the applicability of this reaction under the optimal reaction conditions. However, only trace desired product 3e and 3f were observed by GC-MS (Scheme 1). The reason may lie in that the steric hindrance of phenyl group or isopropyl group made it difficult to form the corresponding product.

Scheme 1 Reaction of 2a with ethyl N-benzyl iminodiacetate 1e and ethyl N-isopropyl iminodiacetate 1f.

When 2,2′-azanediyldiacetate 1g was employed to react with 2a, a different product 3g was obtained in 50% (Scheme 2).

Scheme 2 Reaction of 2a with diethyl 2,2′-azanediyldiacetate 1g.

A mechanism for the resulting product 3 was depicted in Scheme 3. Referred to the literature, the iminium ion II could be generated by the reaction of tertiaryamine I in the presence of molecular iodine.^6f,6j Subsequently, 1,3-dipole azomethine III was afforded through a deprotonation process of the iminium intermediate II and reacted with 1,4-naphthoquinone 2a to furnish the [3 + 2] cycloaddition product. Finally, the corresponding product 3a was afforded by the co-oxidation of O₂ and I₂.^14b,20 The GC-MS analysis of the reaction mixture of 1a with a stoichiometric amount of I₂ had shown the formation of the 1,3-dipole azomethine III (molecular ion peak in 202.3 [M + 1]⁺, see ESI†).

Scheme 3 The possible mechanism for the cycloaddition/aromatization reaction induced by I₂.

Conclusions

In conclusion, we have developed a molecular iodine induced 1,3-dipolar cycloaddition/oxidative aromatization sequence to construct 2-substituted benzo[f]isoindole-1,3-dicarboxylates. This useful protocol provides a rapid and efficient strategy to construct biologically important compounds containing quinone structure. Moreover, we have developed a novel 1,3-dipolar cycloaddition reaction through using the intermedium of iminium ion induced by molecular iodine.

Experimental

General

All chemicals were purchased from commercial vendors and were used as received without further purification; any exceptions are noted within the text and the vendors are noted within the context of use. The ¹H and ¹³C NMR spectra were recorded at 500 and 125 MHz, respectively, in CDCl₃ using TMS as internal standard with a Bruker AM 500 spectrometer. Chemical shifts (δ) were reported as parts per million (ppm) and the following abbreviations were used to identify the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, b = broad and all combinations thereof can be explained by their integral parts. The GC-MS was taken on Agilent (GC431-MS210) and elementary analysis was on Thermo Electron Corporation Flash EA 1112, HRMS were recorded on a Bruker MicroTOF-QII mass instrument (ESI†).

General procedure for the preparation of 3

The mixture of diethyl N-substituented iminodiacetate (1a, 3.0 mmol, 3.0 equiv.), quinone (2, 1.0 mmol, 1.0 equiv.), DBU (3.0 mmol, 0.456 g, 3.0 equiv.), iodine (3.0 mmol, 0.762 g, 3.0 equiv.) and xylene (5.0 mL), was stirred for 5 h under refluxing temperature, determined by GC-MS and TLC. The reaction mixture was poured into 8 mL saturated aqueous sodium thiosulfate and was extracted (3 × 10 mL) with CH₂Cl₂. The combined extracts were dried over MgSO₄. The solvent was removed under vacuum, and the resulting crude product was purified by chromatography on silica gel eluted with CH₂Cl₂ to obtain 3 as yellow solid.

Diethyl 2-methyl-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3aa)

Yield 86%; mp 122–123 °C. IR (KBr): 2985, 1720, 1706, 1667, 1594, 1548, 1517, 1474, 1466, 1147, 1025, 1008, 800, 744, 703 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.22 (dd, J₁ = 3.0 Hz, J₂ = 7.5 Hz, 2H), 7.73 (dd, J₁= 3.5 Hz, J₂ = 7.5 Hz, 2H), 4.55 (q, J = 7.5 Hz, 4H), 3.93 (s, 3H), 1.50 (t, J = −7.5 Hz, 6H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 178.69 (2C), 160.84 (2C), 134.83 (2C), 133.44 (2C), 128.17 (2C), 127.05 (2C), 121.81 (2C), 62.70 (2C), 34.70, 14.01 (2C). GC-MS m/z 355.1 [M]⁺, 356.0, 310.3, 296.6, 237.5, 210.5, 206.6. HRMS (ESI-TOF) m/z calcd for C₁₉H₁₈NO₆ [M + H]⁺ 356.1129, found 356.1131.

Diethyl 2-ethyl-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3ba)

Yield 70%; mp 109–110 °C. IR (KBr): 2985, 1716, 1678, 1594, 1546, 1505, 1436, 1280, 1145, 1044, 1014, 743, 709 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.18 (dd, J₁ = 3.5 Hz, J₂ = 7.5 Hz, 2H), 7.69 (dd, J₁ = 3.5 Hz, J₂ = 7.0 Hz, 2H), 4.52 (q, J = 6.5 Hz, 4H), 4.33 (q, J = 7.0 Hz, 2H), 1.48–1.44 (m, 9H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 178.75 (2C), 160.98 (2C), 134.88 (2C), 133.33 (2C), 127.52 (2C), 126.93 (2C), 121.74 (2C), 62.49 (2C), 42.99, 16.54, 13.78 (2C). GC-MS m/z 370 [M + H]⁺, 369.0 [M]⁺, 342.2, 297.3, 296.2, 268.3, 197.3, 76.1. HRMS (ESI-TOF) m/z calcd for C₂₀H₁₉NO₆Na [M + Na]⁺ 392.1104, found 392.1112.

Diethyl 2-propyl-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3ca)

Yield 64%; mp 104–105 °C. IR (KBr): 2968, 1728, 1668, 1594, 1512, 1471, 1421, 1317, 1264, 1225, 1143, 1005, 798, 743, 713 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.20 (dd, J₁ = 4.0 Hz, J₂ = 7.0 Hz, 2H), 7.71 (dd, J₁ = 3.5 Hz, J₂ = 6.0 Hz, 2H), 4.54 (q, J = 7.5 Hz, 4H), 4.28 (t, J = 8.0 Hz, 2H), 1.87–1.81 (m, 2H), 1.48 (t, J = 7.5 Hz, 6H), 0.94 (t, J =7.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 178.79 (2C), 161.02 (2C), 134.89 (2C), 133.47 (2C), 127.72 (2C), 127.12 (2C), 121.83 (2C), 62.55 (2C), 48.96, 24.59, 13.81 (2C), 10.84. GC-MS m/z 384.1 [M + H]⁺, 383.0 [M]⁺, 325.3, 311.5, 211.3. HRMS (ESI-TOF) m/z calcd for C₂₁H₂₁NO₆Na [M + Na]⁺ 406.1261, found 406.1265.

Diethyl 2-butyl-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3da)

Yield 40.3%; mp 99–100 °C. IR (KBr): 2927, 1728, 1673, 1467, 1368, 1261, 1227, 1201, 1097, 1017, 800 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.21 (dd, J₁ = 3.0 Hz, J₂ = 6.0 Hz, 2H), 7.72 (dd, J₁ = 3.0 Hz, J₂ = 5.5 Hz, 2H), 4.54 (q, J = 6.5 Hz, 4H), 4.31 (t, J = 8.0 Hz, 2H), 1.81–1.75 (m, 2H), 1.49 (t, J = 6.5 Hz, 6H), 1.38–1.32 (m, 2H), 0.94 (t, J = 7.0 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 178.86 (2C), 161.11 (2C), 134.89 (2C), 133.41 (2C), 127.72 (2C), 127.09 (2C), 121.74 (2C), 62.68 (2C), 47.56, 33.47, 19.79, 13.99 (2C), 13.54. GC-MS m/z 398.5 [M + 1]⁺, 397.4 [M]⁺, 352.5, 325.5, 324.5 (100%), 296.8, 282.7, 254.7, 224.5. HRMS (ESI-TOF) m/z calcd for C₂₂H₂₃NO₆Na [M + Na]⁺ 420.1423, found 420.1427.

Diethyl 2-methyl-2H-naphtho[2,3-f]isoindole-4,11-dione-1,3-dicarboxylate (3ab)

Yield 81%; mp 176–177 °C. IR (KBr): 2927, 1720, 1705, 1674, 1621, 1513, 1472, 1293, 1242, 1208, 1186, 1107, 1038, 1022, 762, 747 cm⁻¹. ¹H NMR (CDCl₃, 500 MHz): 8.73 (s, 2H), 8.04 (dd, J₁ = 3.0 Hz, J₂ = 6.5 Hz 2H), 7.65 (dd, J₁ = 3.5 Hz, J₂ = 6.0 Hz, 2H), 4.57 (q, J = 7.0 Hz, 4H), 3.93 (s, 3H), 1.52 (t, J = 7.5 Hz, 6H). ¹³C NMR (CDCl₃, 125 MHz) 178.60 (2C), 160.97 (2C), 134.81 (2C), 131.26 (2C), 129.97 (2C), 129.25 (2C), 129.17 (2C), 128.29 (2C), 122.63 (2C), 62.73 (2C), 34.72 (1C), 14.05 (2C).

GC-MS m/z 405.9 [M + 1]⁺, 361.2, 333.2, 289.3, 288.4, 262.3, 261.3. Anal. calcd for C₁₈H₁₇NO₄S₂: C, 68.14; H, 4.72; N, 3.46. Found: C, 68.04; H, 4.88; N, 3.26%.

Diethyl 2-ethyl-2H-naphtho[2,3-f]isoindole-4,11-dione-1,3-dicarboxylate (3bb)

Yield 73%; mp 134–135 °C. IR (KBr): 2978, 1725, 1674, 1619, 1437, 1281, 1234, 1185, 1038, 1015, 862, 759 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.73 (s, 2H), 8.03 (dd, J₁ = 3.5 Hz, J₂ = 6.0 Hz, 2H), 7.64 (dd, J₁ = 3.5 Hz, J₂ = 6.5 Hz, 2H), 4.57 (q, J = 6.5 Hz, 4H), 4.36 (q, J = 7.5 Hz, 2H), 1.53–1.49 (m, 9H). ¹³C NMR (125 MHz, CDCl₃): δ(ppm) 178.72 (2C), 161.16 (2C), 134.81 (2C), 131.34 (2C), 129.96 (2C), 129.19 (2C), 129.13 (2C), 127.53 (2C), 122.64 (2C), 62.55 (2C), 43.12, 16.79, 14.01 (2C). GC-MS m/z 419.9 [M + H]⁺, 419.0 [M]⁺, 390.2, 375.1, 346.2, 318.3, 300.2, 274.4. HRMS (ESI-TOF) m/z calcd for C₂₄H₂₂NO₆ [M + H]⁺ 420.1442, found 420.1449.

Diethyl 2-propyl-2H-naphtho[2,3-f]isoindole-4,11-dione-1,3-dicarboxylate (3cb)

Yield 61%; mp 110–111 °C. IR (KBr): 2965, 1736, 1670, 1602, 1501, 1440, 1282, 1187, 1041, 1011, 917, 761 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.67 (s, 2H), 7.99 (dd, J₁ = 3.0 Hz, J₂ = 6.5 Hz, 2H), 7.60 (dd, J₁ = 3.0 Hz, J₂ = 6.5 Hz, 2H), 4.56 (q, J = 7.0 Hz, 4H), 4.28 (q, J = 7.5 Hz, 2H), 1.88–1.81 (m, 2H), 1.50 (q, J = 7.5 Hz, 6H), 0.95 (q, J = 7.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ(ppm) 178.58 (2C), 161.13 (2C), 134.71 (2C), 131.23 (2C), 129.91 (2C), 129.21 (2C), 129.14 (4C), 127.81 (2C), 122.49 (2C), 62.71 (2C), 49.20, 24.74, 14.02 (2C), 10.90. GC-MS: m/z 433.8 [M + H]⁺, 432.9[M]⁺, 404.2, 388.3, 347.2, 346.3, 300.2, 41.0. HRMS (ESI-TOF) m/z calcd for C₂₅H₂₄NO₆ [M + H]⁺ 434.1598, found 434.1606.

Diethyl 2-butyl-2H-naphtho[2,3-f]isoindole-4,11-dione-1,3-dicarboxylate (3db)

Yield 57%; mp 100–101 °C. IR (KBr): 2961, 1735, 1705, 1677, 1619, 1435, 1305, 1273, 1236, 1182, 1033, 1020, 753 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ(ppm) 8.69 (s, 2H), 8.00 (dd, J₁ = 3.0 Hz, J₂ = 6.0 Hz, 2H), 7.61 (dd, J₁ = 3.0 Hz, J₂ = 6.5 Hz, 2H), 4.56 (q, J = 7.5 Hz, 4H), 4.30 (q, J = 8.0 Hz, 2H), 1.82–1.76 (m, 2H), 1.51 (q, J = 7.0 Hz, 6H), 1.38–1.31 (m, 2H), 0.94 (q, J = 7.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ(ppm) 178.55 (2C), 161.08 (2C), 134.69 (2C), 131.20 (2C), 129.87 (2C), 129.11 (2C), 129.08 (2C), 127.74 (2C), 122.46 (2C), 62.66 (2C), 47.54, 33.42, 19.74, 13.98 (2C), 13.51. GC-MS m/z 448.0 [M + H]⁺, 446.9 [M]⁺, 418.0, 402.1, 375.1, 374.3 (100%), 304.3, 41.0. HRMS (ESI-TOF) m/z calcd for C₂₆H₂₆NO₆ [M + H]⁺ 448.1755, found 448.1750.

Diethyl 2-methyl-5-nitro-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3ac)

Yield 70%; mp 85–86 °C. IR (KBr): 2983, 1723, 1676, 1593, 1544, 1509, 1375, 1306, 1252, 1226, 1144, 1026, 912, 798, 712 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.40 (dd, J₁ = 1.0 Hz, J₂ = 7.5 Hz, 1H), 7.83 (t, J = 7.5 Hz, 1H), 7.70 (dd, J₁ = 1.0 Hz, J₂ = 8.0 Hz, 1H), 4.54 (q, J = 7.0 Hz, 2H), 4.47 (q, J = 7.0 Hz, 2H), 3.96 (s, 3H), 1.49 (t, J = 7.0 Hz, 3H), 1.43 (t, J = 6.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.18, 175.21, 160.25, 160.08, 149.43, 135.97, 133.77, 129.58, 128.65, 128.51, 127.35, 126.75, 121.63, 120.67, 62.91, 62.87, 34.95, 13.96, 13.73. GC-MS m/z 400.8 [M]⁺, 399.8, 384.2, 356.2, 339.3, 309.1, 284.1, 256.1. HRMS (ESI-TOF) m/z calcd for C₁₉H₁₆N₂O₈Na [M + Na]⁺ 423.0799, found 423.0807.

Diethyl 2-ethyl-5-nitro-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3bc)

Yield 63%; mp 75–76 °C. IR (KBr): 2982, 1730, 1681, 1531, 1440, 1309, 1233, 1141, 1018, 798, 711 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.39 (dd, J₁ = 0.5 Hz, J₂ = 7.0 Hz, 1H), 7.82 (t, J = 8.0 Hz, 1H), 7.70 (dd, J₁ = 1.5 Hz, J₂ = 8.5 Hz, 1H), 4.54 (q, J = 7.5 Hz, 2H), 4.47 (q, J = 7.5 Hz, 2H), 4.40 (q, J = 7.0 Hz, 2H), 3.96 (s, 3H), 1.48 (t, J = 7.0 Hz, 6H), 1.43 (t, J = 7.0 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.24, 175.26, 160.38, 160.16, 149.38, 136.00, 133.77, 129.54, 128.04, 127.79, 127.32, 126.79, 121.75, 120.71, 62.90, 62.89, 43.44, 16.75, 13.93, 13.71.

GC-MS m/z 414.8 [M]⁺, 413.8, 396.0, 386.0, 369.2, 342.3, 325.3, 295.1. HRMS (ESI-TOF) m/z calcd for C₂₀H₁₈N₂O₈Na [M + Na]⁺ 437.0955, found 437.0958.

Diethyl 2-propyl-5-nitro-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3cc)

Yield 60%; mp 70–71 °C. IR (KBr): 2977, 1734, 1682, 1532, 1438, 1367, 1315, 1227, 1142, 1018, 797, 712 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.40 (dd, J₁ = 1.0 Hz, J₂ = 8.0 Hz, 1H), 7.83 (t, J = 8.0 Hz, 1H), 7.70 (dd, J₁ = 0.5 Hz, J₂ = 7.5 Hz, 1H), 4.54 (q, J = 7.5 Hz, 2H), 4.48 (q, J = 7.0 Hz, 2H), 4.37 (t, J = 7.5 Hz, 2H), 4.31–4.26 (m, 2H), 1.80–1.74 (m, 2H), 1.48 (t, J = 7.5 Hz, 3H), 1.43 (t, J = 7.0 Hz, 3H), 0.94 (t, J = 7.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.30, 175.34, 160.48, 160.26, 149.47, 136.06, 133.77, 129.58, 128.33, 128.11, 127.34, 126.85, 121.73, 120.70, 62.94, 62.89, 49.34, 24.82, 13.95, 13.74, 10.90. GC-MS m/z 429.1 [M + H]⁺, 428.1, 400.2, 383.3, 342.3, 341.3, 295.4, 206.4. HRMS (ESI-TOF) m/z calcd for C₂₁H₂₁N₂O₈ [M + H]⁺ 429.1293, found 429.1321.

Diethyl 2-butyl-5-nitro-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3dc)

Yield 52%; mp 65–66 °C. IR (KBr): 2966, 1740, 1620, 1544, 1445, 1373, 1220, 1018, 800, 711 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ(ppm) 8.40 (dd, J₁ = 1.0 Hz, J₂ = 7.5 Hz, 1H), 7.83 (t, J = 7.5 Hz, 1H), 7.70 (dd, J₁ = 1.5 Hz, J₂ = 8.0 Hz, 1H), 4.54 (q, J = 7.0 Hz, 2H), 4.48 (q, J = 7.0 Hz, 2H), 4.34 (t, J = 7.5 Hz, 2H), 1.85–1.79 (m, 2H), 1.49 (t, J = 7.5 Hz, 3H), 1.43 (t, J = 7.0 Hz, 3H), 0.94 (t, J = 7.0 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.27, 175.28, 160.45, 160.24, 149.46, 136.05, 133.71, 129.53, 128.26, 128.05, 127.30, 126.84, 121.68, 120.65, 62.88, 62.83, 47.69, 33.45, 19.72, 13.93, 13.71, 13.49. GC-MS m/z 443.1 [M + H]⁺, 412.3, 397.2, 370.2, 369.2 (100%), 297.5, 269.3. HRMS (ESI-TOF) m/z calcd for C₂₂H₂₃N₂O₈ [M + H]⁺ 443.1449, found 443.1461.

Diethyl 2-methyl-5-hydroxy-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3ad)

Yield 40%; mp 133–134 °C. IR (KBr): 2964, 1705, 1669, 1634, 1455, 1262, 1082, 1021, 802, 695 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 12.65 (s, 1H), 7.73 (dd, J₁ = 1.0 Hz, J₂ = 7.5 Hz, 1H), 7.59 (t, J = 8.0 Hz, 1H), 7.22 (dd, J₁ = 1.0 Hz, J₂ = 8.5 Hz, 1H), 4.56–4.51 (m, 4H), 3.91 (s, 3H), 1.50–1.47 (m, 4H). ¹³C NMR (125 MHz, CDCl₃): δ(ppm) 184.77, 177.87, 162.73, 160.62, 160.60, 135.99, 135.16, 128.72, 128.22, 123.95, 121.77, 120.90, 119.35, 116.95, 62.84, 62.73, 34.75, 13.99, 13.98. GC-MS m/z 372.2 [M + H]⁺, 371.3, 326.4, 325.5, 299.5, 253.5, 225.5, 63.1. HRMS (ESI-TOF) m/z calcd for C₁₉H₁₇NO₇Na [M + Na]⁺ 394.0907, found 394.0897.

Diethyl 2-ethyl-5-hydroxy-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3bd)

Yield 34%; mp 100–101 °C. IR (KBr): 2977, 1735, 1705, 1674, 1633, 1555, 1365, 1350, 1263, 1226, 1075, 801, 715 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 12.67 (s, 1H), 7.73 (dd, J₁ =1.5 Hz, J₂ = 7.5 Hz, 1H), 7.59 (t, J = 8.5 Hz, 1H), 7.22 (dd, J₁ = 1.0 Hz, J₂ = 8.5 Hz, 1H), 4.56–4.52 (m, 4H), 4.34 (q, J = 6.5 Hz, 2H), 1.50–1.46 (m, 9H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 184.86, 177.98, 162.74, 160.81, 160.75, 135.98, 135.22, 128.05, 127.55, 123.96, 62.87, 62.76, 43.32, 16.73, 13.97 (2C). GC-MS m/z 386.3 [M + H]⁺, 385.5 (100%), 339.6, 312.5, 266.5, 239.3, 183.2, 155.5. HRMS (ESI-TOF) m/z calcd for C₂₀H₂₀NO₇ [M + H]⁺ 386.1235, found 386.1250.

Diethyl 2-propyl-5-hydroxy-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3cd)

Yield 23%; mp 100–101 °C. IR (KBr): 2974, 1732, 1637, 1618, 1557, 1509, 1444, 1356, 1342, 1268, 1218, 1081, 1010, 834, 748 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 12.67 (s, 1H), 7.73 (dd, J₁ = 1.5 Hz, J₂ = 7.5 Hz, 1H), 7.59 (t, J = 8.5 Hz, 1H), 7.22 (dd, J₁ = 1.0 Hz, J₂ = 8.5 Hz, 1H), 4.56–4.51 (m, 4H), 4.26 (q, J = 8.5 Hz, 2H), 1.92–1.80 (m, 2H), 1.48 (t, J = 7.0 Hz, 3H), 0.95 (t, J = 7.5 Hz, 3H), 0.94 (t, J = 7.5Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 184.89, 178.05, 162.75, 160.86, 160.82, 136.00, 135.23, 128.32, 127.82, 123.97, 121.72, 120.81, 119.36, 116.99, 62.87, 62.78, 49.34, 24.76, 13.98, 13.96, 10.93. GC-MS m/z 400.1 [M + H]⁺, 399.3, 370.3, 353.5, 312.3, 266.2, 254.3, 41.1. HRMS (ESI-TOF) m/z calcd for C₂₁H₂₁NO₇Na [M + Na]⁺ 422.1210, found 422.1218.

Diethyl 2-butyl-5-hydroxy-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3dd)

Yield 16%; mp 85–86 °C. IR (KBr): 2964, 1712, 1671, 1632, 1560, 1511, 1439, 1367, 1349, 1262, 1204, 1079, 1019, 801 cm⁻¹. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 12.67 (s, 1H), 7.73 (dd, J₁ = 1.0 Hz, J₂ = 7.5 Hz, 1H), 7.59 (t, J = 7.5 Hz, 1H), 7.22 (dd, J₁ = 1.5 Hz, J₂ = 8.0 Hz, 1H), 4.56–4.51 (m, 4H), 4.29 (t, J = 7.5 Hz, 2H), 1.81–1.75 (m, 2H), 1.48 (t, J = 7.0 Hz, 6H), 1.40–1.32 (m, 2H), 0.95 (t, J = 7.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 183.85, 177.00, 161.72, 159.83, 159.78, 134.95, 134.21, 127.25, 126.77, 122.94, 120.67, 119.78, 118.32, 115.97, 61.82, 61.73, 46.67, 32.40, 18.75, 12.95 (2C), 12.51. GC-MS m/z 414.1 [M + H]⁺, 413.2, 384.2, 368.2, 340.2, 312.2, 298.2, 270.2. HRMS (ESI-TOF) m/z calcd for C₂₂H₂₃NO₇Na [M + Na]⁺ 436.1367, found 436.1376.

Diethyl 2-(2-ethoxy-2-oxoethyl)-2H-benzo[f]isoindole-4,9-dione-1,3-dicarboxylate (3g)

Yield 50%; mp 137–138 °C. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.23 (dd, J₁ = 3.0 Hz, J₂ = 5.5 Hz, 2H), 7.74 (dd, J₁ = 3.5 Hz, J₂ = 6.5 Hz, 2H), 5.23 (s, 2H), 4.51 (q, J = 7.0 Hz, 4H), 4.27 (q, J = 7.5 Hz, 2H), 1.48 (t, J = 7.0 Hz, 6H), 1.30 (t, J = 7.5 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm); 178.47 (2C), 166.69, 160.59 (2C), 134.80 (2C), 133.47 (2C), 127.83 (2C), 127.03 (2C), 122.60 (2C), 62.70 (2C), 62.26, 48.52, 14.03, 13.88 (2C). HRMS (ESI-TOF) m/z calcd for C₂₂H₂₁NO₈Na [M + Na]⁺ 450.1165, found 450.1167.

Acknowledgements

We gratefully acknowledge the financial supported by the Natural Science Foundation of China (21176223) and the National Natural Science Foundation of Zhejiang (LY13B020016) and the Key Innovation Team of Science and Technology in Zhejiang Province(2010R50018).

Notes and references

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19. ESI.†.
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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic data (CIF files) of 3ab, spectral data of all compounds and copies of ¹H and ¹³C NMR spectra of products 3. CCDC 915882. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra01593b

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