HETEROCYCLES

Paper | Special issue | Vol. 82, No. 2, 2011, pp. 1239-1249
Received, 24th June, 2010, Accepted, 4th August, 2010, Published online, 6th August, 2010.
DOI: 10.3987/COM-10-S(E)68

■ Generation of 3-(1H-Isochromen-1-yl)-1H-indole via Silver Triflate-Catalyzed Tandem Reaction of 2-Alkynylbenzaldehyde with Indole

Banlai Ouyang, Jianjun Yuan, Qin Yang, Qiuping Ding, Yiyuan Peng,* and Jie Wu*

Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China

Abstract

A facile and efficient method is developed for synthesis of 3-(1H-isochromen-1-yl)-1H-indole via silver triflate-catalyzed tandem reaction of 2-alkynylbenzaldehyde with indole. The reaction proceeds smoothly in DMF at room temperature with good substrate generality. The availability of the starting materials combined with mild reaction conditions and the high efficiency of this synthetic route would be attractive and beneficial for the focused library construction.

INTRODUCTION
It is well known that natural product-like compounds can be utilized to dissect the circuitry of cells, which is analogous to the use of mutations in genetics.1 For access such small molecules capable of modulating any pathway or process of interest, diversity-oriented synthesis is an efficient device.2 Among the methods employed, tandem reactions from easily available starting materials provide an efficient and powerful tool to produce a collection of structurally diverse compounds.3,4 As we know, the 1H-isochromene core is regarded as a privileged scaffold which can be found in many natural products and drug candidates with remarkable biological activities.5 Thus, continuous efforts have been observed for the development of new methods for its construction.6 On the other hand, the indole skeleton is an important substructure as well in both natural products and therapeutic agents.7 As part of an ongoing program in our laboratory for accessing natural product-like compounds used in different biological assays,8,9 we conceived that the 1H-isochromene with an indole substituent might be a good candidate for library development. Therefore, we started to consider the possibility for the synthesis of this kind of compound.
Recently, 2-alkynylbenzaldehyde was discovered as a versatile building block for carbocycles and heterocycles formation.10,11 Its reliability and the ready availability of the starting materials made it attractive for reaction development. For instance, Yamamoto and Asao described the naphthyl ketones synthesis via AuCl3-catalyzed [4+2] benzannulation of 2-alkynylbenzaldehydes with alkynes.11a,b Very recently, Li and co-workers reported a PdCl2-catalyzed domino reaction of 2-alkynylbenzaldehyde with indole for generation of fluorescent 5H-benzo[b]carbazol-6-yl ketones.10i We also found that 1H-isochromen-1-ylphosphonates could be easily accessible via AgOTf-catalyzed reaction of 2-alkynylbenzaldehyde with diethyl phosphite. During the reaction process, it was recognized that the metal-catalyzed cyclization would occur firstly to generate an isobenzopyrylium complex, which then triggered intermolecular attack of the nucleophile thus affording 1H-isochromene derivatives. Encouraged by these results, we reasoned that 3-(1H-isochromen-1-yl)-1H-indoles could be synthesized via metal-catalyzed tandem reactions of 2-alkynylbenzaldehydes with indoles under suitable conditions. Herein, we would like to disclose our recent efforts for this transformation.

RESULTS AND DISCUSSION
2-Alkynylbenzaldehyde 1 was conveniently prepared from the commercially available 2-bromobenzaldehyde with alkyne via Sonogashira coupling reaction according to the literature method.12 The preliminary experiments were carried out for the reaction of 2-alkynylbenzaldehyde 1a with indole 2a catalyzed by different Lewis acids in various solvents at room temperature (Table 1). To our delight, the desired product 3a was afforded in 40% yield when the reaction was performed in MeCN in the presence of AgOTf (5 mol%) as a catalyst (Table 1, entry 1). Only a trace amount of product was detected when the catalyst was replaced by CuOTf, Cu(OTf)2, La(OTf)3, Sc(OTf)3, or Bi(OTf)3 (Table 1, entries 2-6). The product from double addition of indole to aldehyde was obtained when Dy(OTf)3, Yb(OTf)3, or Zn(OTf)2 was employed as the catalyst in the above reaction (Table 1, entries 7-9). The yield was increased to 56% when 10 mol% of AgOTf was used (Table 1, entry 10). Further screening of solvents demonstrated that the reaction worked the most efficiently in DMF, which furnished the corresponding product 3a in 66% yield (Table 1, entry 18). Inferior yields were generated when other solvents were utilized in the reaction.

Next, the scope of the tandem reaction of 2-alkynylbenzaldehyde 1 with indole 2 was examined and the results were presented in Table 2. From Table 2, we found that all reactions proceeded smoothly leading to the desired 3-(1H-isochromen-1-yl)-1H-indole 3 in good to excellent yields. For example, reaction of 2-alkynylbenzaldehyde 1a with 2-methylindole 2b gave rise to the 1H-isochromene 3b in 70% yield (Table 2, entry 2). A slightly lower yield was obtained when 5-methyl or 5-bromoindole was used as a replacement (Table 2, entries 3 and 4). Substrate 1b or 1c also worked well to generate the corresponding product in good yield (Table 2, entries 5-8). Under the standard conditions, the methyl-, fluoro-, or [1,3]dioxolyl-substituted 2-alkynylbenzaldehyde 1d-1f were tested as well in the reactions of various indoles (Table 2, entries 9-14). As expected, all reactions gave rise to the desired products in good yields. In addition, we did not observe the influence of the electron effect on the aromatic backbone of the substrates.

CONCLUSION
In summary, we have described an efficient AgOTf-catalyzed tandem reaction of 2-alkynylbenzaldehyde with indole, which generates the diverse 3-(1H-isochromen-1-yl)-1H-indoles in good yields. This transformation proceeds through cascade intramolecular cyclization and intermolecular nucleophilic addition. The easily availability of the starting materials combined with mild reaction conditions, good substrate generality, and the high efficiency of this synthetic route would be attractive and beneficial for the focused library construction.

EXPERIMENTAL
General experimental procedure for the synthesis of 3-(1H-isochromen-1-yl)-1H-indole 3 via silver triflate-catalyzed tandem reaction of 2-alkynylbenzaldehyde 1 with indole 2: Silver triflate (10 mol%) was added to a solution of 2-alkynylbenzaldehyde 1 (0.25 mmol) and indole 2 (0.3 mmol) in DMF. The mixture was stirred at room temperature. After completion of reaction as indicated by TLC, the mixture was diluted with water (10 mL), extracted with EtOAc (10 × 2 mL). The combined organic layers were then washed with brine, dried over anhydrous Na2SO4, and concentrated under reduced pressure. The residue was purified by flash column chromatography (n-hexane/ethyl acetate) to afford the desired product 3.
3-(3-Phenyl-1H-isochromen-1-yl)-1H-indole (3a): White solid, mp 153-154 °C; 1H NMR (400 MHz, CDCl3) δ 8.00 (s, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.64-7.66 (m, 2H), 7.35 (d, J = 8.0 Hz, 1H), 7.30-7.10 (m, 8H), 6.97 (d, J = 7.2 Hz, 1H), 6.85 (d, J = 2.4 Hz, 1H), 6.68 (s, 1H), 6.49 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 152.6, 136.6, 134.8, 132.0, 130.3, 128.6, 128.2, 128.1, 126.5, 126.4, 125.4, 125.1, 124.9, 123.6, 122.4, 120.1, 120.0, 115.3, 111.3, 100.8, 74.0; IR (KBr): ν (cm-1) 3415, 3022, 1622, 1491, 1455. HRMS calcd for C23H18NO (M+H): 324.1388, found: 324.1386.
2-Methyl-3-(3-phenyl-1H-isochromen-1-yl)-1H-indole (3b): Yellow solid, mp 63-65 °C; 1H NMR (400 MHz, CDCl3) δ 8.00 (s, 1H), 7.72 (d, J = 7.6 Hz, 2H), 7.44 (d, J = 4.0 Hz, 1H), 7.34-7.23 (m, 5H), 7.18-7.11 (m, 2H), 7.05-6.98 (m, 2H), 6.74 (d, J = 7.6 Hz, 1H), 6.57 (s, 1H), 6.50 (s, 1H), 2.41 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 154.3, 135.4, 134.5, 132.8, 130.8, 128.7, 128.2, 128.0, 127.8, 126.5, 125.2, 125.0, 123.4, 121.3, 120.1,119.7, 110.2, 109.5, 101.0, 74.5, 12.3; IR (KBr): ν (cm-1) 3400, 3057, 2918, 2849, 1621, 1492, 1460. HRMS calcd for C24H20NO (M+H): 338.1545, found: 338.1539.
5-Methyl-3-(3-phenyl-1H-isochromen-1-yl)-1H-indole (3c): White solid, mp 179-180 °C; 1H NMR (400 MHz, CDCl3) δ 7.93 (s, 1H), 7.67-7.63 (m, 3H), 7.29-7.09 (m, 7H), 7.03 (d, J = 8.4 Hz, 1H), 6.95 (d, J = 7.2 Hz, 1H), 6.82 (d, J = 2.4 Hz, 1H), 6.63 (s, 1H), 6.49 (s, 1H), 2.44 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.8, 134.9, 134.8, 132.1, 130.5, 129.3, 128.6, 128.2, 128.1, 126.7, 126.4, 125.5, 125.1, 125.0, 124.1, 123.6, 119.8, 114.7, 110.9, 100.9, 74.2, 21.6; IR (KBr): ν (cm-1) 3412, 3057, 2922, 2858, 1625, 1488, 1453. HRMS calcd for C24H20NO (M+H): 338.1545, found: 338.1541.
5-Bromo-3-(3-phenyl-1H-isochromen-1-yl)-1H-indole (3d): Green solid, mp 67-68 °C; 1H NMR (400 MHz, CDCl3) δ 8.08 (s, 1H), 7.96 (s, 1H), 7.63 (d, J = 7.2 Hz, 2H), 7.29-7.22 (m, 5H), 7.17 (d, J = 7.6 Hz, 1H), 7.13-7.10 (m, 2H), 6.91 (d, J = 7.2 Hz, 1H), 6.75 (d, J = 2.0 Hz, 1H), 6.57 (s, 1H), 6.48 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 152.5, 135.2, 134.6, 131.9, 130.0, 128.8, 128.3, 128.2, 128.1, 126.6, 126.1, 125.4, 125.3, 125.0, 123.8, 122.7, 115.0, 113.4, 112.9, 101.0, 73.8; IR (KBr): ν (cm-1) 3420, 3062, 1626, 1601, 1492, 1454. HRMS calcd for C23H17BrNO (M+H): 402.0494, found: 402.0499.
3-(3-p-Tolyl-1H-isochromen-1-yl)-1H-indole (3e): Yellow solid, mp 144-145 °C; 1H NMR (400 MHz, CDCl3) δ 8.01 (s, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.54 (d, J = 8.0 Hz, 2H), 7.33 (d, J = 8.0 Hz, 1H), 7.28-7.07 (m, 7H), 6.96 (d, J = 7.6 Hz, 1H), 6.84 (d, J = 2.0 Hz, 1H), 6.66 (s, 1H), 6.44 (s, 1H), 2.29 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.8, 138.6, 136.6, 132.2, 132.0, 130.3, 128.9, 128.1, 126.5, 126.2, 125.4, 125.1, 124.8, 123.5, 122.4, 120.2, 120.0, 115.4, 111.3, 100.1, 74.0, 21.3; IR (KBr): ν (cm-1) 3416, 3064, 3014, 2922, 1621, 1509, 1456. HRMS calcd for C24H20NO (M+H): 338.1545, found: 338.1557.
5-Methyl-3-(3-p-tolyl-1H-isochromen-1-yl)-1H-indole (3f): Brown solid, mp 148-149 °C; 1H NMR (400 MHz, CDCl3) δ 7.86 (s, 1H), 7.61 (s, 1H), 7.55 (d, J = 8.0 Hz, 2H), 7.25-6.91 (m, 8H), 6.78 (d, J = 2.4 Hz, 1H), 6.59 (s, 1H), 6.44 (s, 1H), 2.43 (s, 3H), 2.29 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 153.0, 138.7, 134.9,132.3, 132.0, 130.5, 129.3, 129.0, 128.1, 126.8, 126.2, 125.5, 125.1, 125.0, 124.0, 123.5, 119.8, 114.7, 111.0, 100.2, 74.2, 21.6, 21.3; IR (KBr): ν (cm-1) 3422, 3030, 2919, 2857, 1623, 1511, 1487. HRMS calcd for C25H22NO (M+H): 352.1701, found: 352.1709.
3-(3-(4-Chlorophenyl)-1H-isochromen-1-yl)-1H-indole (3g): White solid, mp 150-151 °C; 1H NMR (400 MHz, CDCl3) δ 8.02 (s, 1H), 7.83 (d, J = 7.2 Hz, 1H), 7.56 (d, J = 7.8 Hz, 2H), 7.34-7.11 (m, 8H), 6.96 (d, J = 7.6 Hz, 1H), 6.81 (d, J = 2.4 Hz, 1H), 6.67 (s, 1H), 6.45 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 151.5, 136.6, 134.3, 133.3, 131.7, 130.3, 128.4, 128.2, 126.6, 126.4, 125.2, 124.9, 123.8, 122.5, 120.1, 120.0, 115.3, 111.4, 101.2, 74.1; IR (KBr): ν (cm-1) 3391, 3066, 1623, 1490, 1455. HRMS calcd for C23H17ClNO (M+H): 358.0999, found: 358.0410.
3-(3-(4-Chlorophenyl)-1H-isochromen-1-yl)-5-methyl-1H-indole (3h): Yellow solid, mp 197-198 °C; 1H NMR (400 MHz, CDCl3) δ 7.97 (br, 1H), 7.62-7.57 (m, 2H), 7.28-7.11 (m, 6H), 7.05 (d, J = 8.4 Hz, 1H), 6.96 (d, J = 7.6 Hz, 1H), 6.83 (d, J = 2.4 Hz, 1H), 6.63 (s, 1H), 6.47 (s, 1H), 2.45 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 151.6, 134.9, 134.3, 133.3, 131.8, 130.4, 129.4, 128.4, 128.1, 126.7, 126.6, 125.2, 125.0, 124.1, 123.7, 119.7, 114.6, 111.0, 101.2, 74.2, 21.6; IR (KBr): ν (cm-1) 3415, 3072, 2919, 1626, 1490, 1405. HRMS calcd for C24H19ClNO (M+H): 372.1155, found: 372.1163.
3-(6-Methyl-3-phenyl-1H-isochromen-1-yl)-1H-indole (3i): Yellow solid, mp 152-153 °C; 1H NMR (400 MHz, CDCl3) δ 7.93 (s, 1H), 7.83 (d, J = 7.6 Hz, 1H), 7.62 (d, J = 6.8 Hz, 2H), 7.26-7.11 (m, 5H), 6.98 (s, 1H), 6.91 (d, J = 7.6 Hz, 1H), 6.82 (d, J = 7.6 Hz, 1H), 6.77 (d, J = 2.4 Hz, 1H), 6.62 (s, 1H), 6.43 (s, 1H), 2.34 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.6, 137.7, 136.6, 134.9, 132.0, 128.6, 127.7, 127.2, 126.5, 125.4, 125.1, 125.0, 124.4, 122.4, 120.2, 120.0, 115.4, 111.4, 100.9, 74.0, 21.4; IR (KBr): ν (cm-1) 3415, 3059, 2916, 1621, 1492, 1455. HRMS calcd for C24H20NO (M+H): 338.1545, found: 338.1551.
2-Methyl-3-(6-methyl-3-phenyl-1H-isochromen-1-yl)-1H-indole (3j) Yellow solid, mp 69-70 °C; 1H NMR (400 MHz, CDCl3) δ 7.92 (s, 1H), 7.71-7.69 (m, 2H), 7.44 (d, J = 7.6 Hz, 1H), 7.29-7.25 (m, 4H), 7.10 (t, J = 8.0 Hz, 1H), 7.00-7.01 (m, 2H), 6.82 (d, J = 7.6 Hz, 1H), 6.60 (d, J = 8.0 Hz, 1H), 6.52 (s, 1H), 6.46 (s, 1H), 2.33 (s, 3H), 2.32 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 154.3, 137.7, 135.4, 134.7, 134.6, 132.8, 128.7, 128.3, 128.2, 127.9, 127.2, 125.3, 125.0, 124.2, 121.3,120.2, 119.7, 110.4, 109.6, 101.1, 74.6, 21.3, 12.3; IR (KBr): ν (cm-1) 3403, 3058, 2920, 2851, 1622, 1460, 1046. HRMS calcd for C25H22NO (M+H): 352.1701, found: 352.1712.
5-Methyl-3-(6-methyl-3-phenyl-1H-isochromen-1-yl)-1H-indole (3k) Yellow solid, mp 172-173 °C; 1H NMR (400 MHz, CDCl3) δ 7.93 (s, 1H), 7. 65-7.62 (m, 3H), 7.26-7.18 (m, 4H), 7.02-6.99 (m, 2H), 6.91 (d, J = 8.0 Hz, 1H), 6.84-6.80 (m, 2H), 6.59 (s, 1H), 6.44 (s, 1H), 2.43 (s, 3H), 2.35 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.8, 137.6, 134.9, 132.0, 129.2, 128.6, 128.2, 127.8, 127.1, 126.8, 125.5, 125.0, 124.3, 124.0, 119.8, 114.8, 111.0, 100.9, 74.1, 21.6, 21.3; IR (KBr): ν (cm-1) 3419, 2922, 2851, 1624, 1493, 1450. HRMS calcd for C25H22NO (M+H): 352.1701, found: 352.1709.
5-Bromo-3-(6-methyl-3-phenyl-1H-isochromen-1-yl)-1H-indole (3l) Green solid, mp 64-65 °C; 1H NMR (400 MHz, CDCl3) δ 8.04 (br, 1H), 7.95 (s, 1H), 7.62 (d, J = 7.2 Hz, 2H), 7.28-7.21 (m, 4H), 7.08 (d, J = 8.4 Hz, 1H), 6.98 (s, 1H), 6.92 (d, J = 7.6 Hz, 1H), 6.78 (d, J = 7.6 Hz, 1H), 6.75 (s, 1H), 6.53 (s, 1H), 6.43 (s, 1H), 2.34 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.5, 137.9, 135.3, 134.7, 131.8, 128.7, 128.3, 128.1, 127.3, 126.1, 125.4, 125.3, 124.9, 124.8, 124.5, 122.7, 115.1, 113.3, 112.9, 101.1, 73.8, 21.4; IR (KBr): ν (cm-1) 3421, 2919, 1626, 1492, 1449. HRMS calcd for C24H19BrNO (M+H): 416.0650, found: 416.0656.
3-(7-Fluoro-3-phenyl-1H-isochromen-1-yl)-5-methyl-1H-indole (3m) Yellow solid, mp 198-199 °C; 1H NMR (400 MHz, CDCl3) δ 7.97 (s, 1H), 7.66-7.64 (m, 2H), 7.58 (s, 1H), 7.29-7.23 (m, 4H), 7.14-7.11 (m, 1H), 7.05 (d, J = 8.4 Hz, 1H), 6.99-6.94 (m, 1H), 6.89 (d, J = 1.6 Hz, 1H), 6.66 (d, J = 7.2 Hz, 1H), 6.56 (s, 1H), 6.48 (s, 1H), 2.44 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.3, 134.9, 134.6, 132.7, 129.5, 128.7, 128.4, 128.3, 128.2, 126.5, 125.3, 125.0, 124.9, 124.2, 119.7, 114.8 (d, 2JCF = 22.0 Hz), 113.9, 112.5 (d, 2JCF = 23.0 Hz), 111.0, 100.1, 74.0, 21.6; IR (KBr): ν (cm-1) 3412, 3065, 2929, 2855, 1609, 1494. HRMS calcd for C24H19FNO (M+H): 356.1451, found: 356.1455.
3-(7-Phenyl-5H-[1,3]dioxolo[4,5-g]isochromen-5-yl)-1H-indole (3n) Blue solid, mp 194-195 °C; 1H NMR (400 MHz, CDCl3) δ 7.99 (s, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.61 (d, J = 7.2 Hz, 2H), 7.32 (d, J = 8.0 Hz, 1H), 7.26-7.12 (m, 4H), 6.89 (s, 1H), 6.68 (s, 1H), 6.55 (s, 1H) , 6.46 (s, 1H) , 6.38 (s, 1H) , 5.89 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 151.3, 147.4, 146.1, 136.7, 134.7, 128.4, 128.2, 126.5, 126.4, 125.2, 124.8, 124.1, 122.4, 120.2, 120.1, 115.3, 111.3, 106.3, 104.5, 101.0, 100.9, 74.1; IR (KBr): ν (cm-1) 3384, 3069, 2900, 1640, 1499, 1482. HRMS calcd for C24H18NO3 (M+H): 368.1287, found: 368.1298.

ACKNOWLEDGEMENTS
Financial support from the National Natural Science Foundation of China (Nos. 20862009 and 20962010) and the Natural Science Foundation of Jiangxi Province (No. 2008GQH0026) is gratefully acknowledged.

References

1. (a) T. J. Mitchison, Chem. Biol., 1994, 1, 3; CrossRef (b) S. L. Schreiber, Bioorg. Med. Chem., 1998, 6, 1127; CrossRef (c) C. M. Crews and U. Splittgerber, Trends Biochem. Sci., 1999, 24, 317. CrossRef
2. (a) D. P. Walsh and Y.-T. Chang, Chem. Rev., 2006, 106, 2476; CrossRef (b) P. Arya, D. T. H. Chou, and M.-G. Baek, Angew. Chem. Int. Ed., 2001, 40, 339; CrossRef (c) S. L. Schreiber, Science, 2000, 287, 1964; CrossRef (d) M. R. Spaller, M. T. Burger, M. Fardis, and P. A. Bartlett, Curr. Opin. Chem. Biol., 1997, 1, 47. CrossRef
3. For selected examples, see: (a) J. Montgomery, Angew. Chem. Int. Ed., 2004, 43, 3890; CrossRef (b) E. Negishi, C. Coperet, S. Ma, S. Y. Liou, and F. Liu, Chem. Rev., 1996, 96, 365; CrossRef (c) L. F. Tietze, Chem. Rev., 1996, 96, 115; CrossRef (d) R. Grigg and V. Sridharan, J. Organomet. Chem., 1999, 576, 65; CrossRef (e) T. Miura and M. Murakami, Chem. Commun., 2007, 217; CrossRef (f) M. Malacria, Chem. Rev., 1996, 96, 289; CrossRef (g) K. C. Nicolaou, T. Montagnon, and S. A. Snyder, Chem. Commun., 2003, 551; CrossRef (h) K. C. Nicolaou, D. J. Edmonds, and P. G. Bulger, Angew. Chem. Int. Ed., 2006, 45, 7134; CrossRef (i) D. Enders, C. Grondal, and M. R. M. Hüttl, Angew. Chem. Int. Ed., 2007, 46, 1570; CrossRef (j) L. F. Tietze, G. Brasche, and K. Gericke, Domino Reactions in Organic Synthesis; Wiley-VCH: Weinheim, Germany, 2006. CrossRef
4. For recent selected examples, see: (a) F. Shi, X. Li, Y. Xia, L. Zhang, and Z.-X. Yu, J. Am. Chem. Soc., 2007, 129, 15503; CrossRef (b) S. W. Youn, J.-Y. Song, and D. I. Jung, J. Org. Chem., 2008, 73, 5658; CrossRef (c) K.-G. Ji, X.-Z. Shu, J. Chen, S.-C. Zhao, Z.-J. Zheng, L. Lu, X.-Y. Liu, and Y.-M. Liang, Org. Lett., 2008, 10, 3919; CrossRef (d) A. B. Beeler, S. Su, C. A. Singleton, and J. A. Porco, Jr., J. Am. Chem. Soc., 2007, 129, 1413; CrossRef (e) H.-S. Yeom, J.-E. Lee, and S. Shin, Angew. Chem. Int. Ed., 2008, 47, 7040; CrossRef (f) L.-Q. Lu, Y.-J. Cao, X.-P. Liu, J. An, C.-J. Yao, Z.-H. Ming, and W.-J. Xiao, J. Am. Chem. Soc., 2008, 130, 6946; CrossRef (g) X.-L. Sun and Y. Tang, Acc. Chem. Soc., 2008, 41, 937; CrossRef (h) S. H. Cho and S. Chang, Angew. Chem. Int. Ed., 2008, 47, 2836; CrossRef (i) S. H. Cho and S. Chang, Angew. Chem. Int. Ed., 2007, 46, 1897; CrossRef (j) E. J. Yoo, S. H. Park and S. Chang, Org. Lett., 2009, 11, 1155. CrossRef
5. (a) J. Jacobs, S. Claessens, and N. De Kimpe, Tetrahedron, 2008, 64, 412; CrossRef (b) S. Obika, H. Kono, Y. Yasui, R. Yanada, and Y. Takemoto, J. Org. Chem., 2007, 72, 4462; CrossRef (c) J. Barluenga, H. Vázquez-Villa, I. Merin, A. Ballesteros, and J. M. Gonzalez, Chem. Eur. J., 2006, 12, 5790; CrossRef (d) X. Yao and C.-J. Li, Org. Lett., 2006, 8, 1953; CrossRef (e) S. Mondal, T. Nogami, N. Asao, and Y. Yamamoto, J. Org. Chem., 2003, 68, 9496; CrossRef (f) K. Krohn and K. Vukics, Synthesis, 2007, 18, 2894. CrossRef
6. For selected examples, see: (a) J. Jacobs, S. Claessens, and N. De Kimpe, Tetrahedron, 2008, 64, 412; CrossRef (b) K. Krohn and K. Vukics, Synthesis, 2007, 2894; CrossRef (c) S. Obika, H. Kono, Y. Yasui, R. Yanada, and Y. Takemoto, J. Org. Chem., 2007, 72, 4462; CrossRef (d) J. Barluenga, H. Vazquez-Villa, I. Merino, A. Ballesteros, and J. M. Gonzalez, Chem. Eur. J., 2006, 12, 5790; CrossRef (e) A. V. Butin, V. T. Abaev, V. V. Melchin, and A. S. Dmitriev, Tetrahedron Lett., 2005, 46, 8439; CrossRef (f) L. R. Izquierdo, M. Kenneth, T. A. Grillo, and J. G. Luis, Synthesis, 2005, 1926; CrossRef (g) D. Yue, N. Della Ca, and R. C. Larock, Org. Lett., 2004, 6, 1581; CrossRef (h) S. Mo, S. Wang, G. Zhou, Y. Yang, Y. Li, X. Chen, and J. Shi, J. Nat. Prod., 2004, 67, 823; CrossRef (i) S. Mondal, T. Nogami, N. Asao, and Y. Yamamoto, J. Org. Chem., 2003, 68, 9496. CrossRef
7. (a) J. E. Saxton, Nat. Prod. Rep., 1997, 14, 559; CrossRef (b) M. Toyota and M. Ihara, Nat. Prod. Rep., 1998, 15, 327 and references therein. CrossRef
8. (a) S. Ye, X. Yang, and J. Wu, Chem. Commun., 2010, 46, 2950; CrossRef (b) X. Yu and J. Wu, J. Comb. Chem., 2010, 12, 238; CrossRef (c) X. Yu, Z. Chen, X. Yang, and J. Wu, J. Comb. Chem., 2010, 12, 374; CrossRef (d) S. Ye, K. Gao, H. Zhou, X. Yang, and J. Wu, Chem. Commun., 2009, 5406; CrossRef (e) Z. Chen, X. Yang, and J. Wu, Chem. Commun., 2009, 3469; CrossRef (f) Z. Chen, X. Yu, M. Su, X. Yang, and J. Wu, Adv. Synth. Catal., 2009, 351, 2702; CrossRef (g) Z. Chen, Q. Ding, X. Yu, and J. Wu, Adv. Synth. Catal., 2009, 351, 1692; CrossRef (h) Q. Ding, Z. Wang, and J. Wu, J. Org. Chem., 2009, 74, 921; CrossRef (i) Q. Ding, X. He, and J. Wu, J. Comb. Chem., 2009, 11, 587; CrossRef (j) X. Yu and J. Wu, J. Comb. Chem., 2009, 11, 895; CrossRef (k) Q. Ding, X. Huang, and J. Wu, J. Comb. Chem., 2009, 11, 1047. CrossRef
9. (a) K. Gao and J. Wu, Org. Lett., 2008, 10, 2251; CrossRef (b) Q. Ding and J. Wu, Adv. Synth. Catal., 2008, 350, 1850; CrossRef (c) Q. Ding and J. Wu, J. Comb. Chem., 2008, 10, 541; CrossRef (d) Z. Wang, R. Fan, and J. Wu, Adv. Synth. Catal., 2007, 349, 1943; CrossRef (e) L. Zhang and J. Wu, Adv. Synth. Catal., 2007, 349, 1047; CrossRef (f) L. Zhang, T. Meng, R. Fan, and J. Wu, J. Org. Chem., 2007, 72, 7279; CrossRef (g) Q. Ding, Y. Ye, R. Fan, and J. Wu, J. Org. Chem., 2007, 72, 5439; CrossRef (h) Z. Wang, B. Wang, and J. Wu, J. Comb. Chem., 2007, 9, 811. CrossRef
10. Selected examples for metal-catalyzed cyclization of 2-alkynylbenzaldehyde: (a) A. B. Beeler, S. Su, C. A. Singleton, and J. A. Porco, Jr., J. Am. Chem. Soc., 2007, 129, 1413 and references cited therein; CrossRef (b) N. Asao, Synlett, 2006, 1645; CrossRef (c) Q. Ding and J. Wu, Org. Lett., 2007, 9, 4959; CrossRef (d) K. Gao and J. Wu, J. Org. Chem., 2007, 72, 8611; CrossRef (e) W. Sun, Q. Ding, X. Sun, R. Fan, and J. Wu, J. Comb. Chem., 2007, 9, 690; CrossRef (f) X. Yu, Q. Ding, W. Wang, and J. Wu, Tetrahedron Lett., 2008, 49, 4390; CrossRef (g) F. Wang, Z. Miao, and R. Chen, Org. Biomol. Chem., 2009, 7, 2848; CrossRef (h) N. T. Patil and Y. Yamamoto, J. Org. Chem., 2004, 69, 5139; CrossRef (i) R.-Y. Tang and J.-H. Li, Chem. Eur. J., 2010, 16, 4733; CrossRef (j) G. Dyker, D. Hildebrandt, J. Liu, and K. Merz, Angew. Chem. Int. Ed., 2003, 42, 4399. CrossRef
11. (a) N. Asao, K. Takahashi, S. Lee, T. Kasahara, and Y. Yamamoto, J. Am. Chem. Soc., 2002, 124, 12650; CrossRef (b) N. Asao, T. Nogami, K. Takahashi, and Y. Yamamoto, Y. J. Am. Chem. Soc., 2002, 124, 764; CrossRef (c) N. Asao, T. Kasahara, and Y. Yamamoto, Angew. Chem. Int. Ed., 2003, 42, 3504; CrossRef (d) N. Asao, T. Nogami, S. Lee, and Y. Yamamoto, J. Am. Chem. Soc., 2003, 125, 10921; CrossRef (e) H. Kusama, H. Funami, J. Takaya, and N. Iwasawa, Org. Lett., 2004, 6, 605; CrossRef (f) N. Asao, H. Aikawa, and Y. Yamamoto, J. Am. Chem. Soc., 2004, 126, 7458; CrossRef (g) N. Asao, C. S. Chan, K. Takahashi, and Y. Yamamoto, Tetrahedron, 2005, 61, 11322; CrossRef (h) I. Nakamura, Y. Mizushima, I. D. Gridnev, and Y. Yamamoto, J. Am. Chem. Soc., 2005, 127, 9844; CrossRef (i) N. Kim, Y. Kim, W. Park, D. Sung, A.-K. Gupta, and C.-H. Oh, Org. Lett., 2005, 7, 5289; CrossRef (j) K. Sato, N. Asao, and Y. Yamamoto, J. Org. Chem., 2005, 70, 8977; CrossRef (k) N. Asao, K. Sato, Menggenbateer, and Y. Yamamoto, J. Org. Chem., 2005, 70, 3682. CrossRef
12. K. R. Roesch and R. C. Larock, J. Org. Chem., 2002, 67, 86. CrossRef