HETEROCYCLES

Paper | Regular issue | Vol. 89, No. 9, 2014, pp. 2105-2121
Received, 30th July, 2014, Accepted, 26th August, 2014, Published online, 28th August, 2014.

■ Hypervalent Iodine Mediated One-Pot C-H Functionalization at 2α- or 3α-Position of Indole Derivatives

Kazuhiro Higuchi,* Masato Inaba, Asuka Naganuma, Takako Ishizaki, Masanori Tayu, and Tomomi Kawasaki*

Department of Pharmaceutical Sciences, Meiji Pharmaceutical University, 2-522-1 Noshio, Kiyose, Tokyo 204-8588, Japan

Abstract

The one-pot 2α- and 3α-functionalization of 2,3-disubstituted indoles using a hypervalent iodine reagent has been developed. The substitution at the 2α-position of indoles took place using phenyliodinebis(trifluoroacetate) with oxygen and carbon nucleophiles in moderate yields. The combination of iodosobenzene and trimethylsilyl azide afforded 3α-azide derivatives preferentially. The latter reaction was applied to other 2,3-disubstituted indoles.

INTRODUCTION
Hypervalent iodine reagents1 having a strong leaving-group ability and electrophilicity allow a wide range of reactions, such as oxidations and C-C coupling reactions under mild conditions with a tolerance for a wide range of other functional groups. The reaction between arenes and hypervalent iodine is attractive for one of the interesting research area.2 Especially the application of the electron-rich arenes such as indoles give rise to the substitution via cation radical intermediate,3 indolyliodonium fragmentation,4 and so on.5 Recently, Ishibashi’s group revealed the combination of phenyliodine diacetate (PIDA) and tetrabutylammonium iodide (TBAI) reacted with tetrahydrocarbazoles to give 2α-acetoxy derivatives.6 Alternatively, Du Bois and co-workers developed that the reaction of tetrahydrocarbazoles using imino λ3-iodanes with a rhodium catalyst affords 2α- and 3α-amino derivatives.7
Recently, we developed the concise 2α-functionalization of indole derivatives using a thionium species generated from DMSO-TFAA (Scheme 1).8

This method is useful for the introduction of various substituents to the 2α-position9 in one-pot procedure under mild reaction conditions. However, we require a more practical method to access to 2α-functionalized indoles for the synthesis of biologically active compounds.10 We report herein the 2α- and 3α-functionalization of indole derivatives using a hypervalent iodine reagent.

RESULTS AND DISCUSSION
Optimization of Reaction Conditions
To obtain the desired 2α-methoxy indole derivatives more conveniently, we investigated the reactivity of hypervalent iodine reagents toward N-p-methoxybenzyl (PMB)-tetrahydrocarbazole 1a (Table 1). To a solution of 1a in CH2Cl2 added one equivalent of hypervalent iodine reagent at 0 °C. After consumption of 1a, ten equivalents of MeOH as a nucleophile was added to the reaction mixture. When Dess-Martin periodinane (DMP, entry 1) and phenyliodine diacetate (PIDA, entry 2) were used, carbazole 4 and dimer 3 was obtained respectively instead of the desired 2α-methoxy product 2a. PhI(OH)OTs (Koser’s reagent)11 afforded a trace amount of 2a along with dimer 3 (22%) and carbazole 4 (5%), respectively (entry 3). The use of iodopentafluorobenzene bis(trifluoroacetate) (FPIFA) as a more electrophilic λ3−iodan produced 2a and 3 in 8% and 29% yields, respectively (entry 4). In the case of phenyliodine bis(trifluoroacetate) (PIFA) gave 2a in 35% yield (entry 5). The substituent effect at the indole nitrogen was examined for PIFA. N-Unsubstituted tetrahydrocarbazole gave a complex-mixture (entry 6) and Boc and acetyl derivatives produced a trace amount of 2α-methoxy compounds 2c and 2d (entries 7, 8).
Subsequently, we examined the effect of reaction temperature using PIFA (Table 2). At room temperature, the reaction afforded no desired product 2a and gave dimer 3 in 43% yield (entry 1). When the reaction was performed at –20 °C, 2a was given in 30% yield (entry 3). At –30 °C, the 2α-methoxy compound 2a was obtained in 24% yield without the formation of dimer 3 (entry 4). At –40 °C, carbazole 4 was the sole product (entry 5). We concluded that the condition proceeded with PIFA at 0 °C is an optimized condition (entry 2).

The plausible reaction mechanism is as follows (Scheme 2). First, the 3-position of 1a attacks an iodine atom of PIFA to generate iminium intermediate 5. Transformation of iminium 5 to enamine 6, followed by the nucleophilic attack of MeOH afforded 2α-methoxy compound 2a. At room temperature (Table 2, entry 1), the immediate attack of unreacted 1a to intermediate 6 produced dimer 3 before addition of MeOH. Since 1a was consumed at 0 °C and –20 °C (checked by TLC), the regeneration of 1a would be caused by the attack of MeOH on the iodine atom in iminium 5. The addition of MeOH to 6 is faster than 1a, therefore, both yield of 2a and ratio of 2a/3 were increased (entries 2 and 3). At –30 °C and –40 °C, starting material 1a was not consumed completely and nucleophiles (1a and/or MeOH) react more slowly with 6. Consequently, the elimination of iodobenzene from intermediates and the subsequent oxidation occurs to give carbazole 4 (entries 4 and 5). The structures of 2a-d and 3 were determined by differential nOe and H-H cosy spectra (Figure 1).

Study of Nucleophiles
With the optimized conditions in hand, we investigated the scope and limitation of nucleophiles for this reaction (Table 3). As is the case in MeOH, an isopropoxy group was introduced to give 2e in 41% yield (entry 1). The carbon nucleophiles MeMgBr and Me2Zn gave 2α-methyl derivative 2f in 26% and 21% yields, respectively (entries 2, 3). Vinyl and allyl groups were also introduced at the 2α-position to give products 2g (39%) and 2h (31%) (entries 4, 5). Additionally, the use of N-methylindole as an aryl nucleophile provided compound 2i in 9% yield (entry 6). Also, the nitrogen nucleophiles, aniline and benzylamine afforded the products 2j and 2k in 5% and 8% yields, respectively (entries 7, 8).

3α-Azide Substituent
When TMSN3 was used as a nucleophile at 0 °C for substrate 1a, 2α-azide product 2l was obtained in 20% yield (Table 4, entry 1, Method A). At –20 °C, 3α-azide product 7a was produced in 45% yield along with 2α-product 2l in 26% yield (entry 2). For N-Boc derivative 1c under this reaction condition, both 2α- and 3α-substituted products 2m and 7c were obtained in 22% and 25% yields, respectively (entry 3).

Since the treatment of 1c with MeOH produced trace of 2α-derivative 2c (3% yield, Table 1, entry 7), we hypothesized that the mechanism of azidation is different from that of other nucleophiles as shown in Table 3. It is known that combinations of iodosobenzene (PhIO) and TMSN3 generate phenyliodine bisazide (PhI(N3)2),12 which is an extremely labile intermediate for producing radical species.13 Thus, we investigated the use of preformed PhI(N3)2 for the azidation14 of 1 (Table 4, entries 4-8, Method B). To a solution of PIFA in MeCN was added TMSN3 at –40 °C and the mixture was stirred for 5 min. After the addition of 1a, 2α-azide 2l and 3α-azide 7a were formed in 9% and 33% yields, respectively (entry 4). The combination of PhIO and TMSN3 increased the yield of 7a to 43% (entry 5). The other substrates 1c-e gave 3α-azide compounds 7c-e in 16-30 % yields (entries 6-8). The position of the substituted azide group in 2 and 7 was determined by differential nOe and H-H cosy spectra (Figure 2).

Based on these results, we suggest the following plausible mechanism included both ionic and radical pathways (Scheme 3). In the ionic pathway, 1a and PIFA generate iminium 5 followed by formation of enamine 6 and then SN2’-type reaction between the azide ion and 6 gives 2α-compound 2l. In the radical

pathway, the iodine atom of iminium 5 or PIFA reacts with TMSN3 to produce PhI(N3)2 8. Then, the azide radical from 8 reacts with tetrahydrocarbazole 1a at 2α and/or 3α position and then the generated benzylic radical coupled with the iodo radical to form intermediates 9 and 10. Finally, reductive elimination of iodobenzene gave the corresponding products 2l and 7a.

Using the combination of iodosobenzene and TMSN3 (Table 4, entry 5), we studied the scope and limitations of the substrates for a variety of 2,3-disubstituted indoles (Table 5). N-Boc cyclopenta[b]indole 1g and cyclohepta[b]indoles 1i produced the corresponding 2α- and 3α-azide products 2 and 7 in higher yield than N-PMB derivatives 1f and 1h (entries 1 vs 2, 3 vs 4). The reaction of cyclohepta[b]indole 1h was accompanied by formation of olefin 11 in 23% yield (entry 3). In the case of 2,3-dialkyl substituted indoles, N-PMB derivatives gave better yields and selectivity than the N-Boc indoles (entries 5 vs 6, 7 vs 8). In particular, N-PMB-2-propyl-3-ethylindole 1n gave only 3α-product 7n in 75% yield (entry 9).

CONCLUSION
We studied the one-pot functionalization of 2,3-substituted indoles with hypervalent iodine reagents. We developed the substitution at 2α-position of indoles using PIFA with oxygen and carbon nucleophiles, which afforded 2α-derivatives in moderate yields. In the case of the combination of PhIO and TMSN3, the radical mechanism was also included to afford 3α-azide derivatives preferentially. These results provide an interesting complementary approach to our previous method using thionium species, which afforded 2α-azide derivatives.
EXPERIMENTAL
All melting points were measured on a Yanagimoto micro melting point apparatus, and are uncorrected. IR spectra were recorded on a Shimadzu IR Prestige-21 spectrophotometer. 1H and 13C NMR spectra were measured on a JEOL JNM-AL300 (300 MHz), a JEOL JNM-AL400 (400 MHz), a JEOL JNM-ECS400 (400 MHz), or a JEOL JNM-LA500 (500 MHz) spectrometer with tetramethylsilane as an internal standard. J-Values are given in Hertz. Mass spectra were recorded on a JEOL JMS 700 instrument with a direct inlet system. Thin layer chromatography (TLC) was carried out on a Merck silica gel plate 60F254. Column chromatography was carried out on a silica gel [Fuji Silysia Co. Inc. (silica gel PSQ 60B)]. All solvents were purified by standard procedures prior to use. The following compounds were characterized by the previous reports: 1a-d, 1f-m, 2a, 2d-m, 3, 4.6,8,15

Repesentative procedure for C-H functionalization of indole derivatives with hypervalent iodine reagent (Table 1-3, Table 4 entries 1-3)
Under argon atmosphere, to a solution of 1a (92 mg, 0.31 mmol) in CH2Cl2 (0.1 M) was added PIFA (0.13 g, 0.31 mmol) at 0 °C. After 10 min stirring, MeOH (0.13 mL, 3.1 mmol) was added to the reaction mixture. The mixture was stirred for 10 min and quenched by aqueous sodium sulfite (3 mL) and then extracted with CH2Cl2 (10 mL, 3 times). The organic layer was washed by brine and dried over MgSO4. The concentrated residue was purified by silica gel chromatography (AcOEt/n-hexane = 1/20) to give 2a8 (34 mg, 35%); 38 (24 mg, 27%); 415 (3.5 mg, 4%).

9-(tert-Butoxycarbonyl)-1-methoxy-1,2,3,4-tetrahydro-9H-carbazole (2c)
Colorless oil. IR (CHCl3): 2982, 2934, 1722, 1454, 1371, 1314 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.69 (9H, s, tBu), 1.78-2.04 (3H, m, CH2CH2CHOMe, CH2CH2CHOMe), 2.28 (1H, ddt, J = 3.0, 6.6, 13.8 Hz, CH2CHOMe), 2.53 (1H, ddd, J = 6.3, 11.4, 16.8 Hz, CCH2CH2), 2.80 (1H, ddd, J = 2.1, 5.4, 16.2 Hz, CCH2CH2), 3.50 (3H, s, OCH3), 5.04 (1H, t, J = 3.0 Hz, CHOMe), 7.19 (1H, dt, J = 0.9, 7.2 Hz, Ar-H), 7.27 (1H, dt, J = 1.5, 7.2 Hz, Ar-H), 7.43 (1H, dd, J = 0.6, 8.1 Hz, Ar-H), 8.02 (1H, d, J = 8.1 Hz, Ar-H). 13C NMR (75 MHz, CDCl3): δ 17.2, 21.1, 27.1, 28.3, 56.3, 70.6, 83.2, 115.7, 118.5, 119.8, 122.2, 128.9, 124.5, 134.2, 136.2, 150.3. MS (EI): m/z (%) 301 (M＋, 34), 214 (11), 201 (54), 170 (63), 169 (100), 168 (44), 167 (13), 57 (29). HRMS (EI): m/z Calcd for C18H23NO3: 301.1678; Found: 301.1680.

9-(4-Methoxybenzyl)-1-phenylamino-1,2,3,4-tetrahydro-9H-carbazole (2j)
Yellow oil. IR (CHCl3): 2928, 2839, 1601, 1512, 1501, 1464, 1246 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.77-1.91 (3H, m, CH2CH2CHN, CH2CH2CHNHAr), 2.17-2.27 (1H, m, CH2CHNHAr), 2.67 (1H, dt, J = 5.4, 8.7 Hz, CCH2CH2), 2.90 (1H, dt, J = 4.2, 16.2 Hz, CCH2CH2), 3.75 (3H, s, OCH3), 3.83 (1H, brs, NHPh), 4.65 (1H, brs, CHNHAr), 5.20 (1H, d, J = 16.8 Hz, NCH2Ar), 5.31 (1H, d, J = 16.8 Hz, NCH2Ar), 6.53 (2H, d, J = 7.8 Hz, Ar-H), 6.67-6.71 (1H, m, Ar-H), 6.77-6.73 (2H, m, Ar-H), 6.84-6.86 (2H, m, Ar-H), 7.11 (1H, ddd, J = 0.9, 6.6, 7.5 Hz, Ar-H), 7.17 (2H, dd, J = 7.5, 8.4 Hz, Ar-H), 7.18 (1H, dt, J = 1.2, 6.6 Hz, Ar-H), 7.26 (1H, dt, J = 0.9, 7.2 Hz, Ar-H), 7.57 (1H, dt, J = 0.8, 7.5 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 18.4, 21.0, 28.5, 44.9, 45.8, 55.2, 109.5, 112.6, 113.0, 114.0, 117.4, 118.6, 119.0, 122.1, 126.6, 127.3, 129.3, 130.4, 134.2, 137.1, 146.3, 158.7. MS (EI) m/z (%): 382 (M+, 1), 290 (23), 289 (56), 121 (100), 93 (13). HRMS (EI): m/z Calcd for C26H26N2O: 382.2045; Found: 382.2040.

Representative procedure for C-H functionalization of indole derivatives with hypervalent iodine reagent (Table 4 entries 4-8, Table 5)
Under argon atmosphere, a solution of iodosobenzene (PhIO, 57 mg, 0.26 mmol) in CH2Cl2 (2.2 mL) was added trimethylsilyl azide (TMSN3, 72 μL, 0.52 mmol) and stirred for 5 min. Subsequently, 1a (63 mg, 0.22 mmol) was added to the reaction mixture and stirred for 60 min at –40 °C. The mixture was quenched by aqueous sodium sulfite (2 mL) and then extracted with CH2Cl2 (10 mL, 3 times). The organic layer was washed by brine and dried by MgSO4. The concentrated residue was purified by silica gel chromatography (AcOEt/n-hexane = 1/20) or preparative silica gel chromatography to give 2l (6.4 mg, 9%); 7a (32 mg, 43%).

4-Azido-9-(4-methoxybenzyl)-1,2,3,4-tetrahydro-9H-carbazole (7a)
Brown oil. IR (CHCl3): 2947, 2837, 2097, 1730, 1612, 1512, 1464 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.88-2.15 (4H, m, CHN3CH2CH2, CHN3CH2CH2), 2.53-2.63 (1H, m, CCH2), 2.73 (1H, dt, J = 4.8, 16.2 Hz, CCH2), 3.73 (3H, s, OCH3), 4.88 (1H, t, J = 4.2 Hz, CHN3), 5.19 (2H, d, J = 1.8 Hz, NCH2Ar), 6.78-6.83 (2H, m, Ar-H), 6.90-6.95 (2H, m, Ar-H), 7.12-7.20 (2H, m, Ar-H), 7.23-7.29 (1H, m, Ar-H), 7.68-7.73 (1H, m, Ar-H). 13C NMR (75 MHz, CDCl3): δ 19.0, 21.9, 29.9, 45.8, 54.9, 55.2, 107.5, 109.4, 114.1, 118.2, 119.9, 121.6, 126.6, 127.3, 129.4, 136.6, 138.2, 158.8. HRMS (FAB): m/z Calcd for C20H20N4O: 332.1637; Found: 332.1635.

4-Azido-9-(tert-butoxycarbonyl)-1,2,3,4-tetrahydro-9H-carbazole (7c)
Yellow oil. IR (CHCl3): 3009, 2980, 2945, 2934, 2100, 1724, 1454, 1369, 1314 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.71 (9H, s, tBu), 1.93-2.12 (4H, m, CHN3CH2CH2, CHN3CH2CH2), 2.84-3.04 (1H, m, CCH2), 3.18 (1H, dt, J = 4.8, 18.0 Hz, CCH2), 4.72 (1H, t, J = 3.9 Hz, CHN3), 7.22-7.32 (2H, m, Ar-H), 7.57-7.67 (1H, m, Ar-H), 8.08-8.18 (1H, m, Ar-H).
13C NMR (75 MHz, CDCl3): δ 19.5, 25.6, 28.2, 28.9, 54.2, 84.0, 114.1, 115.6, 118.0, 123.0, 124.0, 128.2, 135.8, 138.7, 150.4. MS (EI): m/z (%) 312 (19), 270 (16), 215 (14), 214 (100), 213 (19), 170 (66), 169 (20), 168 (30), 167 (12), 57 (41). HRMS (EI): m/z Calcd for C17H20N4O2: 312.1586; Found: 312.1585.

1-Azido-9-(methoxycarbonyl)-1,2,3,4-tetrahydro-9H-carbazole (2n)
White solid. mp 93-95 ºC. IR (CHCl3): 2953, 2100, 1734, 1456, 1443, 1369 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.89-2.05 (3H, m, CH2CH2CHN3), 2.15-2.29 (1H, m, CH2CH2CHN3), 2.50-2.65 (1H, m, CCH2), 2.85 (1H, dt, J = 3.6, 16.8 Hz, CCH2), 4.10 (3H, s, OCH3), 5.29 (1H, t, J = 3.0 Hz, CHN3), 7.27 (1H, dt, J = 0.9, 7.5 Hz, Ar-H), 7.36 (1H, dt, J = 1.2, 7.2 Hz, Ar-H), 7.47 (1H, dt, J = 0.6, 7.8 Hz, Ar-H), 8.15 (1H, d, J = 8.1 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 17.4, 20.8, 30.7, 53.8, 55.1, 116.0, 118.8, 121.3, 123.1, 125.5, 128.8, 131.4, 136.1, 152.0. MS (EI): m/z (%) 270 (M+, 15), 229 (15), 228 (100), 168 (16), 167 (11). HRMS (EI): m/z Calcd for C14H14N4O2: 270.1117; Found: 270.1115.

4-Azido-9-(methoxycarbonyl)-1,2,3,4-tetrahydro-9H-carbazole (7e)
Yellowish white solid. mp 77-79 ºC. IR (CHCl3): 2955, 2359, 2340, 2099, 1734, 1458, 1443, 1368 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.90-2.17 (4H, m, CHN3CH2CH2), 2.80-3.04 (1H, m), 3.20 (1H, dt, J = 4.5, 18.3 Hz, CCH2), 4.05 (3H, s, OCH3), 4.72 (1H, t, J = 4.2 Hz, CHN3), 7.30 (1H, t, J = 3.3 Hz, Ar-H), 7.30 (1H, ddd, J = 1.8, 7.2, 17.1 Hz, Ar-H), 7.56-7.67 (1H, m, Ar-H), 8.07-8.20 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): 19.4, 25.2, 28.9, 53.5, 54.1, 114.8, 115.6, 118.2, 123.3, 124.3, 128.3, 135.7, 138.6, 152.4. MS (EI): m/z (%) 270 (M+, 12), 229 (15), 228 (100), 168 (16), 167 (12). HRMS (EI): m/z Calcd for C14H14N4O2: 270.1117; Found: 270.1118.

9-Acetyl-1-azido-1,2,3,4-tetrahydro-9H-carbazole (2o)
Brown oil. IR (CHCl3): 2947, 2930, 2100, 1697, 1460, 1373, 1308 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.91-2.05 (3H, m, CH2CH2CHN3), 2,12- 2.25 (1H, m, CH2CH2CHN3), 2.504-2.70 (1H, m, CCH2), 2.84 (3H, s, COCH3), 2.81-2.91 (1H, m, CCH2), 5.39 (1H, t, J = 3.0 Hz, CHN3), 7.29 (1H, dt, J = 0.9, 7.5 Hz, Ar-H), 7.37 (1H, dt, J = 1.2, 7.2 Hz, Ar-H), 7.51 (1H, dt, J = 0.6, 7.8 Hz, Ar-H), 7.76 (1H, dt, J = 0.6, 8.1 Hz, Ar-H). 13C NMR (75 MHz, CDCl3): δ 17.4, 20.8, 27.4, 30.7, 55.3, 114.7, 119.5, 121.8, 123.1, 125.3, 129.5, 132.7, 135.7, 169.6. MS (EI): m/z (%) 254 (M+, 25), 213 (11), 212 (74), 211 (17), 184 (18), 183 (17), 171 (13), 170 (100), 169 (38), 168 (56), 167 (23), 156 (12). HRMS (EI): m/z Calcd for C14H14N4O: 254.1168; Found: 254.1167.

9-Acetyl-4-azido-1,2,3,4-tetrahydro-9H-carbazole (7d)
Colorless solid. mp 76-79 ºC. IR (CHCl3): 3009, 2949, 2099, 1701, 1371, 1304 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.90-2.30 (4H, m, CHN3CH2CH2), 2.74 (3H, s, COCH3), 2.91-3.04 (1H, m, CCH2), 3.17 (1H, dt, J = 4.5, 17.7 Hz, CCH2), 4.73 (1H, t, J = 3.9 Hz, CHN3), 7.31 (1H, t, J = 3.6 Hz, Ar-H), 7.31 (1H, ddd, J = 2.1, 7.2, 15.6 Hz, Ar-H), 7.60-7.70 (1H, m, Ar-H), 7.92-8.02 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 19.8, 26.3, 27.4, 28.7, 54.2, 115.2, 115.5, 118.6, 123.5, 124.5, 128.8, 135.7, 138.7, 170.0. MS (EI): m/z (%) 254 (M+, 13), 212 (43), 211 (15), 184 (11), 171 (12), 170 (100), 169 (33), 168 (46), 167 (20), 156 (13). HRMS (EI): m/z Calcd for C14H14N4O: 254.1168; Found: 254.1161.

3-Azido-4-(4-methoxybenzyl)-1,2,3,4-tetrahydrocyclopenta[b]indole (2p)
Brown oil. IR (CHCl3): 3007, 2936, 2864, 2094, 1512, 1464, 1246 cm-1. 1H NMR (400 MHz, CDCl3) δ: 2.62 (1H, ddt, J = 2.8, 7.6, 16.4 Hz, CH2CH2CHN3), 2.82 (1H, ddd, J = 3.6, 8.8, 14.0 Hz, CH2CH2CHN3), 2.95 (1H, ddt, J = 6.0, 8.0, 13.8 Hz, CH2CH2CHN3), 3.02-3.15 (1H, m, CH2CH2CHN3), 3.77 (3H, s, OCH3), 4.64 (1H, dt, J = 2.8, 9.2 Hz, CHN3), 5.19 (1H, d, J = 16.1 Hz, NCH2Ar), 5.34 (1H, d, J = 15.6 Hz, NCH2Ar), 6.77-6.89 (2H, m, Ar-H), 7.03-7.08 (2H, m, Ar-H), 7.10 (1H, td, J = 1.0, 8.0 Hz, Ar-H), 7.17 (1H, td, J = 1.6, 8.4 Hz, Ar-H), 7.25 (1H, td, J = 1.2, 8.4 Hz, Ar-H), 7.52 (1H, dt, J = 0.8, 7.6 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): 23.2, 36.7, 47.6, 55.2, 60.0, 110.5, 114.1, 119.6, 119.8, 121.9, 122.2, 123.5, 127.9, 129.6, 141.2, 142.0, 159.0. MS (EI): m/z (%) 318 (M+, 12), 290 (13), 276 (29), 275 (23), 121 (100). HRMS (EI): m/z Calcd for C19H18N4O: 318.1481; Found: 318.1479.

3-Azido-4-(tert-butoxycarbonyl)-1,2,3,4-tetrahydrocyclopenta[b]indole (2q)
Yellowish green oil. IR (CHCl3): 2980, 2936, 2099, 1730, 1368, 1319 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.69 (9H, s, tBu), 2.56 (1H, ddd, J = 0.8, 6.0, 9.9 Hz, CH2CH2CHN3), 2.70-2.92 (2H, m, CH2CH2CHN3), 2.98 (1H, ddt, J = 1.5, 2.1, 12.3 Hz, CH2CH2CHN3), 5.10 (1H, d, J = 6.6 Hz, CHN3), 7.25 (1H, dt, J = 1.2, 7.5 Hz, Ar-H), 7.33 (1H, dt, J = 1.2, 7.2 Hz, Ar-H), 7.46 (1H, dd, J = 0.6, 5.7 Hz, Ar-H), 8.20 (1H, d, J = 8.1 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 22.8, 28.2, 36.6, 62.0, 84.1, 116.2, 119.8, 123.0, 124.9, 125.5, 129.1, 139.8, 140.8, 149.3. MS (EI): m/z (%) 298 (M+, 26), 256 (20), 242 (11), 201 (13), 200 (97), 169 (21), 157 (12), 156 (100), 155 (47), 57 (43). HRMS (EI): m/z Calcd for C16H18N4O2: 298.1430; Found: 298.1424.

1-Azido-4-(tert-butoxycarbonyl)-1,2,3,4-tetrahydrocyclopenta[b]indole (7g)
Brown oil. IR (CHCl3): 3007, 2980, 2936, 2093, 1730, 1369, 1321 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.66 (9H, s, tBu), 2.49 (1H, ddt, J = 3.0, 8.4, 14.4 Hz, CHN3CH2CH2), 2.81-2.98 (1H, m, CHN3CH2CH2), 3.06 (1H, ddd, J = 3.6, 8.7, 17.1 Hz, CHN3CH2CH2), 3.27 (1H, dddd, J = 1.8, 5,1, 7.8, 16.8 Hz, CHN3CH2CH2), 4.96 (1H, dt, J = 2.1, 7.8 Hz, CHN3), 7.27 (1H, ddd, J = 1.8, 7.2, 19.2 Hz, Ar-H), 7.27 (1H, t, J = 2.1 Hz, Ar-H), 7.52 (1H, dd, J = 2.7, 6.9 Hz, Ar-H), 8.18 (1H, dd, J = 1.8, 6.6 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 27.9, 28.2, 35.6, 60.7, 83.9, 115.9, 118.7, 122.5, 123.2, 123.8, 125.1, 140.3, 146.3, 149.6. MS (EI): m/z (%) 298 (M+, 14), 256 (22), 201 (13), 200 (100), 199 (16), 169 (11), 156 (56), 155 (22), 154 (12), 57 (41). HRMS (EI): m/z C16H18N4O2: 298.1430; Found: 298.1424.

6-Azido-5-(4-methoxybenzyl)-5,6,7,8,9,10-hexahydrocyclohepta[b]indole (2r)
Brown oil. IR (CHCl3): 2930, 2100, 1512, 1464 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.50-1.75 (1H, m, CCH2CH2), 1.83 (1H, ddt, J = 3.0, 12.0, 16.8 Hz, CH2CHN3), 1.90-2.12 (3H, m, CCH2CH2CH2), 2.20 (1H, ddt, J = 2.4, 5.1, 13.2 Hz, CH2CHN3), 2.86 (1H, ddd, J = 2.7, 11.7, 14.7 Hz, CCH2), 3.10 (1H, ddd, J = 2.4, 6.0, 15.9 Hz, CCH2), 3.76 (3H, s, OCH3), 4.76 (1H, dd, J = 2.7, 5.4 Hz, CHN3), 5.32 (1H, d, J = 17.1 Hz, NCH2Ar), 5.41 (1H, d, J = 17.1 Hz, NCH2Ar), 6.76-6.85 (2H, m, Ar-H), 6.85-6.95 (2H, m, Ar-H), 7.14 (1H, ddd, J = 1.5, 6.9, 8.1 Hz, Ar-H), 7.21 (1H, dt, J = 1.2, 6.6 Hz, Ar-H), 7.27 (1H, d, J = 7.2 Hz, Ar-H), 7.61 (1H, d, J = 6.9 Hz, Ar-H). 13C NMR (75 MHz, CDCl3): δ 23.7, 24.8, 27.9, 32.3, 46.0, 55.2, 57.1, 109.6, 114.2, 117.5, 119.0, 119.4, 122.6, 126.9, 127.3, 129.8, 132.9, 136.4, 158.9. MS (EI): m/z (%) 346 (M+, 3), 304 (21), 303 (57), 121 (100). HRMS (EI): m/z Calcd for C21H22N4O: 346.1794; Found: 346.1791.

5-(4-Methoxybenzyl)-5,6,7,8-tetrahydrocyclohepta[b]indole (11)
Green oil. IR (CHCl3): 2932, 1612, 1512, 1468, 1248 cm-1. 1H NMR (300 MHz, CDCl3): δ 2.02 (2H, quint, J = 5.7 Hz, CH2CH2CH2), 2.46 (2H, dd, J = 5.1, 10.8 Hz, CHCH2CH­2), 2.95 (2H, t, J = 5.7 Hz, CCH2CH­2), 3.74 (3H, s, OCH3), 5.24 (2H, s, NCH2Ar), 5.74 (1H, dt, J = 5.7, 11.4 Hz, CHCHCH2), 6.68 (1H, dt, J = 1.5, 11.4 Hz, CHCHCH2), 6.73-6.84 (2H, m, Ar-H), 6.85-7.00 (2H, m, Ar-H), 7.08-7.18 (2H, m, Ar-H), 7.18-7.22 (1H, m, Ar-H), 7.60-7.70 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 23.3, 28.3, 30.6, 45.9, 55.2, 109.0, 110.8, 114.2, 117.6, 119.6, 120.0, 121.3, 125.3, 127.2, 127.5, 129.7, 136.3, 138.6, 158.8. MS (EI): m/z (%) 303 (M+, 44), 121 (100). HRMS (EI): m/z Calcd for C21H21NO: 303.1623; Found: 303.1622.

6-Azido-5-(tert-butoxycarbonyl)-5,6,7,8,9,10-hexahydrocyclohepta[b]indole (2s)
Dark green oil. IR (CHCl3): 2932, 2104, 1722, 1454, 1371, 1360, 1315, 1308 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.71 (9H, s, tBu), 1.45-1.81 (1H, m, CHN3CH2CH2CH2), 1.81-1.98 (2H, m, CHN3CH2CH2), 1.98-2.13 (2H, m, CHN3CH2CH2CH2), 2.13-2.23 (1H, m, CHN3CH2), 2.75-2.97 (2H, m, CCH2), 5.92 (1H, dd, J = 2.1, 6.3 Hz, CHN3), 7.24 (1H, dt, J = 1.2, 7.5 Hz, Ar-H), 7.31 (1H, dt, J = 1.5, 7.2 Hz, Ar-H), 7.50 (1H, dt, J = 0.6, 7.5 Hz, Ar-H), 8.04 (1H, dt, J = 1.2-7.5 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 23.1, 24.4, 26.6, 28.3, 31.5, 57.4, 84.5, 115.8, 118.7, 122.6, 124.8, 124.9, 129.3, 134.4, 135.5, 150.6. MS (EI): m/z (%) 326 (M+, 20), 228 (34), 227 (30), 226 (16), 185 (15), 184 (100), 183 (30), 182 (32), 180 (30), 169 (15), 168 (14), 57 (42). HRMS (EI): m/z Calcd for C18H22N4O2: 326.1743; Found: 326.1746.

10-Azido-5-(tert-butoxycarbonyl)-5,6,7,8,9,10-hexahydrocyclohepta[b]indole (7i)
Dark green oil. IR (CHCl3): 2981, 2932, 2102, 1726, 1456, 1371, 1354, 1312 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.69 (9H, s, tBu), 1.60-1.80 (1H, m, CHN3CH2CH2CH2), 1.82-1.98 (2H, m, CHN3CH2CH2), 1.98-2.16 (2H, m, CHN3CH2CH2CH2), 2.16-2.36 (1H, m, CHN3CH2), 3.15 (1H, ddd, J = 2.4, 9.9, 17.1 Hz, CCH2), 3.45 (1H, ddd, J = 2.4, 8.1, 17.1 Hz, CCH2), 5.10 (1H, dd, J = 1.8, 5.7 Hz, CHN3), 7.20-7.30 (2H, m, Ar-H), 7.44-7.60 (1H, m, Ar-H), 7.94-8.07 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 24.4, 26.5, 27.3, 28.3, 31.5, 56.5, 84.2, 115.2, 117.2, 117.5, 122.8, 123.8, 129.3, 135.2, 142.3, 150.6. MS (EI): m/z (%) 326 (M+, 19), 284 (17), 283 (25), 229 (15), 228 (100), 227 (89), 185 (10), 184 (76), 183 (57), 182 (51), 180 (12), 169 (11), 168 (31), 167 (18), 57 (66). HRMS (EI): m/z Calcd for C18H22N4O2: 326.1743; Found: 326.1742.

2-(Azidomethyl)-1-(4-methoxybenzyl)-3-methyl-1H-indole (2t)
Brown oil. IR (CHCl3): 3007, 2936, 2108, 1512, 1248 cm-1. 1H NMR (300 MHz, CDCl3): δ 2.40 (3H, s, CCH3), 3.75 (3H, s, OCH3), 4.40 (2H, s, CH2N3), 5.33 (2H, s, NCH2Ar), 6.74-6.83 (2H, m, Ar-H), 6.86-6.95 (2H, m, Ar-H), 7.14 (1H, ddd, J = 1.5, 6.3, 7.5 Hz, Ar-H), 7.22 (1H, dt, J = 1.5, 8.1 Hz, Ar-H), 7.24-7.29 (1H, m, Ar-H), 7.61 (1H, dt, J = 1.2, 7.8 Hz, Ar-H). 13C NMR (75 MHz, CDCl3): δ 8.9, 44.2, 46.3, 55.2, 109.6, 111.9, 114.2, 119.3, 119.4, 122.8, 127.1, 127.8, 128.9, 129.8, 137.1, 158.9. MS (EI): m/z (%) 306 (M+, 23), 278 (28), 264, (21), 157 (15), 121 (100). HRMS (EI): m/z Calcd for C18H18N4O: 306.1480; Found: 306.1478.

3-(Azidomethyl)-1-(4-methoxybenzyl)-2-methyl-1H-indole (7j)
Brown oil. IR (CHCl3): 3007, 2936, 2106, 1512, 1248 cm-1. 1H NMR (300 MHz, CDCl3): δ 2.39 (3H, s, CCH3), 3.75 (3H, s, OCH3), 4.55 (2H, s, CH2N3), 5.28 (2H, s, NCH2Ar), 6.76-6.82 (2H, m, Ar-H), 6.86-6.93 (2H, m, Ar-H), 7.11-7.20 (2H, m, Ar-H), 7.22-7.28 (1H, m, Ar-H), 7.59-7.68 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 10.4, 45.4, 46.2, 55.3, 105.8, 109.4, 114.2, 117.9, 120.0, 121.7, 127.1, 127.5, 129.4, 136.0, 136.5, 158.9. MS (EI): m/z (%) 306 (M+, 12), 278 (16), 264 (27), 121 (100). HRMS (EI): m/z Calcd for C18H18N4O: 306.1481; Found: 306.1478.

2-(Azidomethyl)-1-(tert-butoxycarbonyl)-3-methyl-1H-indole (2u)
Yellowish green oil. IR (CHCl3): 2982, 2928, 2102, 1724, 1454, 1357, 1339, 1329 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.71 (9H, s, tBu), 2.32 (3H, s, CH3), 4.78 (2H, s, CH2N3), 7.27 (1H, dt, J = 1.2, 9.6 Hz, Ar-H), 7.35 (1H, dt, J = 1.5, 7.2 Hz, Ar-H), 7.52 (1H, dd, J = 0.9, 7.5 Hz, Ar-H), 8.12 (1H, d, J = 9.6 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 8.8, 28.2, 45.6, 84.4, 115.9, 119.06, 119.11, 122.7, 125.3, 129.4, 129.7, 136.1, 150.2. MS (EI): m/z (%) 286 (M+, 34), 230 (13), 188 (28), 186 (26), 159 (16), 158 (32), 157 (29), 145 (13), 144 (100), 143 (24), 130 (25), 57 (80), 41 (11). HRMS (EI): m/z Calcd for C15H18N4O2: 286.1430; Found: 286.1428.

3-(Azidomethyl)-1-(tert-butoxycarbonyl)-2-methyl-1H-indole (7k)
Yellowish green oil. IR (CHCl3): 2982, 2934, 2108, 1730, 1458, 1358 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.70 (9H, s, tBu), 2.63 (3H, s, CH3), 4.46 (2H, s, CH2N3), 7.21-7.35 (2H, m, Ar-H), 7.53 (1H, ddd, J = 3.9, 5.2, 10.1 Hz, Ar-H), 8.12 (1H, ddd, J = 4.5, 5.4, 11.4 Hz, Ar-H). 13C NMR (75 MHz, CDCl3): δ 14.0, 28.2, 44.5, 84.1, 112.3, 115.5, 117.7, 122.9, 124.0, 128.8, 135.6, 136.6, 150.5. MS (EI): m/z (%) 286 (M+, 47), 230 (39), 213 (11), 188 (76), 158 (25), 157 (23), 145 (12), 144 (100), 143 (17), 130 (14), 57 (89), 41 (12). HRMS (EI): m/z Calcd for C15H18N4O2: 286.1430; Found: 286.1427.

2-(1-Azidoethyl)-1-(4-methoxybenzyl)-3-methyl-1H-indole (2v)
Yellow oil. IR (CHCl3) 3007, 2932, 2106, 1512, 1466, 1246 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.47 (3H, d, J = 7.2 Hz, CHN3CH3), 2.43 (3H, s, CCH3), 3.75 (3H, s, OCH3), 5.05 (1H, q, J = 7.2 Hz, CHN3), 5.37 (1H, d, J = 17.4 Hz, NCH2Ar), 5.46 (1H, d, J = 17.4 Hz, NCH2Ar), 6.75-6.83 (2H, m, Ar-H), 6.84-6.93 (2H, m, Ar-H), 7.10-7.18 (3H, m, Ar-H), 7.59-7.63 (1H, m, Ar-H). 13C NMR (75 MHz, CDCl3): δ 9.0, 20.5, 46.8, 53.8, 55.2, 109.7, 114.1, 119.0, 119.3, 122.5, 126.9, 128.2, 130.0, 133.2, 137.0, 144.7, 158.9. MS (EI): m/z (%) 320 (M+, 24), 278 (33), 277 (29), 122 (12), 121 (100). HRMS (EI): m/z Calcd for C19H20N4O: 320.1637; Found: 320.1635.

3-(Azidomethyl)-2-ethyl-1-(4-methoxybenzyl)-1H-indole (7l)
Brown oil. IR (CHCl3): 3007, 2970, 2936, 2108, 1512, 1466, 1248 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.17 (3H, t, J = 7.5 Hz, CH2CH3), 2.79 (2H, q, J = 7.5 Hz, CH2CH3), 3.73 (3H, s, OCH3), 4.54 (2H, s, CH2N3), 5.29 (2H, s, NCH2Ar), 6.74-6.81 (2H, m, Ar-H), 6.83-6.90 (2H, m, Ar-H), 7.10-7.22 (3H, m, Ar-H), 7.60-7.68 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 15.4, 17.9, 45.5, 46.1, 55.2, 105.1, 109.8, 114.2, 118.1, 120.1, 121.8, 127.0, 127.7, 129.6, 136.5, 141.9, 158.9. MS (EI): m/z (%) 320 (M+, 14), 278 (31), 121 (100). HRMS (EI): m/z Calcd for C19H20N4O: 320.1637; Found: 320.1638.

3-(1-Azidomethyl)-1-(tert-butoxycarbonyl)-2-ethyl-1H-indole (7m)
Yellow oil. IR (CHCl3): 2982, 2108, 1730, 1458, 1371, 1360, 1329 cm-1. 1H NMR (300 MHz, CDCl3): δ 1.27 (3H, t, J = 7.2 Hz, CH2CH3), 1.70 (9H, s, tBu), 3.09 (2H, q, J = 7.2 Hz, CH2CH3), 4.45 (2H, s, CH2N3), 7.25 (1H, dt, J = 2.4, 7.8 Hz, Ar-H), 7.29 (1H, dt, J = 2.4, 7.2 Hz, Ar-H), 7.50-7.58 (1H, m, Ar-H), 8.12 (1H, dd, J = 1.8, 6.6 Hz, Ar-H). 13C NMR (100 MHz, CDCl3): δ 15.4, 20.1, 28.2, 44.7, 84.2, 111.9, 115.7, 118.0, 123.0, 124.1, 128.8, 135.9, 142.5, 150.2. MS (EI) m/z (%): 300 (M+, 44), 244 (36), 216 (13), 203 (12), 202 (89), 172 (32), 171 (35), 159 (12), 158 (100), 157 (21), 156 (24), 155 (12), 144 (13), 143 (12), 57 (86), 41 (12). HRMS (EI): m/z Calcd for C16H20N4O2: 300.1586; Found: 300.1583.

3-Ethyl-1-(4-methoxybenzyl)-2-propyl-1H-indole (1n)
To a suspension of NaH (128 mg, 60% in mineral oil, 3.21 mmol) in dry DMF (5.0 mL) was added 3-ethyl-2-propylindole16 (400 mg, 214 mmol) at 0 °C. After stirring at room temperature for 10 min, the reaction mixture was added tetrabutylammonium iodide (79 mg, 0.214 mmol) and p-methoxybenzyl chroride (260 µL, 2.57 mmol) and stirred for 30 min. The reaction was quenched by the addition of saturated aqueous NH4Cl (10 mL) and extracted with Et2O (30 mL, 3 times). The organic layer was dried over MgSO4 and filtrate was concentrated. The residue was purified by silica gel chromatography (AcOEt/n-hexane = 1/5) to afford 1n (504 mg, 77%).
Yellow oil. IR (CHCl3): 3005, 2963, 2932, 2870, 1612, 1512, 1468, 1246 cm-1. 1H NMR (400 MHz, CDCl3): δ 0.93 (3H, t, J = 7.2 Hz, CH2CH2CH3), 1.23 (3H, t, J = 7.2 Hz, CCH2CH3), 1.50 (2H, ddd, J = 7.2, 15.2, 15.2 Hz, CH2CH2CH3), 2.66 (2H, t, J = 8.0 Hz, CH2CH2CH3), 2.76 (2H, q, J = 7.2 Hz, CCH2CH3), 3.71 (3H, s, OCH3), 5.22 (2H, s, NCH2Ar), 6.70-6.82 (2H, m, Ar-H), 6.82-6.92 (2H, m, Ar-H), 7.02-7.10 (2H, m, Ar-H), 7.11-7.22 (1H, m, Ar-H), 7.54-7.61 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 15.6, 17.5, 19.4, 25.3, 28.1, 47.5, 56.7, 110.9, 115.5 (2C), 119.8, 120.3, 122.2, 128.5, 129.3, 132.0, 137.8, 138.0, 160.2. MS (EI): m/z (%) 308 ([M++1], 12), 307 (M+, 52), 121 (100). HRMS (EI): m/z Calcd for C21H25NO: 307.1936; Found: 307.1936.

3-(1-Azidoethyl)-1-(4-methoxybenzyl)-2-propyl-1H-indole (7n)
Yellowish green oil. IR (CHCl3): 3007, 2961, 2934, 2872, 2104, 1612, 1512, 1466, 1248, 1223 cm-1. 1H NMR (300 MHz, CDCl3): δ 0.95 (3H, t, J = 7.2 Hz, CH2CH2CH3), 1.42-1.62 (2H, m, CH2CH2CH3), 1.69 (3H, d, J = 6.9 Hz, CHN3CH3), 2.71 (2H, t, J = 7.8 Hz, CCH2), 3.73 (3H, s, OCH3), 5.04 (1H, q, J = 6.9 Hz, CCHN3), 5.25 (2H, s, NCH2Ar), 6.74-6.82 (2H, m, Ar-H), 6.82-6.90 (2H, m, Ar-H), 7.06-7.21 (3H, m, Ar-H), 7.75-7.83 (1H, m, Ar-H). 13C NMR (100 MHz, CDCl3): δ 14.0, 21.5, 24.0, 26.6, 46.0, 55.2, 55.5, 109.8, 110.9, 114.1, 119.56, 119.64, 121.4, 125.7, 126.9, 129.6, 136.8, 138.2, 158.8. MS (EI): m/z (%) 348 (M+, 1), 306 (14), 305 (44), 122 (10), 121 (100). HRMS (EI): m/z Calcd for C21H24N4O: 348.1950; Found: 348.1946.

ACKNOWLEDGEMENTS
We thank N. Eguchi, T. Koseki, and S. Yamada at the Analytical Center of our university for performing microanalysis, NMR and mass spectrometry measurements.

References

1. Y. Kita, G. F. Koser, M. Ochiai, H. Tohma, A. Varvoglis, T. Wirth, and V. V. Zhdankin, ‘Top. Curr. Chem.: Hypervalent Iodine Chemistry, Modern Developments in Organic Synthesis’ Vol. 224, ed. by T. Wirth, Springer, Berlin, 2003, pp. 1-248; A. Varvoglis, Tetrahedron, 1997, 53, 1179; CrossRef R. M. Moriarty and O. Prakash, Org. React., 1999, 54, 273; V. V. Zhdankin and P. J. Stang, Chem. Rev., 2008, 108, 5299; CrossRef L. F. Silva, Jr. and B. Olofsson, Nat. Prod. Rep., 2011, 28, 1722. CrossRef
2. T. Dohi, M. Ito, N. Yamaoka, K. Morimoto, H. Fujioka, and Y. Kita, Tetrahedron, 2009, 65, 10797. CrossRef
3. T. Dohi, K. Morimoto, N. Takenaga, A. Goto, A. Maruyama, Y. Kiyono, H. Tohma, and Y. Kita, J. Org. Chem., 2007, 72, 109. CrossRef
4. J. A. Campbell, C. A. Broka, L. Gong, K. A. M. Walker, and J.-H. Wang, Tetrahedron Lett., 2004, 45, 4073; CrossRef D. Lubriks, I. Sokolovs, and E. Suna, J. Am. Chem. Soc., 2012, 134, 15436; CrossRef D. Tu, L. Ma, X. Tong, X. Deng, and C. Xia, Org. Lett., 2012, 14, 4830; CrossRef Q. Liu, Q. Y. Zhao, J. Liu, P. Wu, H. Yi, and A. Lei, Chem. Commun., 2012, 48, 3239. CrossRef
5. N. A. Braun, J. D. Bray, and M. A. Ciufolini, Tetrahedron Lett., 1999, 40, 4985; CrossRef X. Qi, H. Bao, and U. K. Tambar, J. Am. Chem. Soc., 2011, 133, 10050. CrossRef
6. H. Zaimoku, T. Hatta, T. Taniguchi, and H. Ishibashi, Org. Lett., 2012, 14, 6088. CrossRef
7. K. W. Fiori and J. Du Bois, J. Am. Chem. Soc., 2007, 129, 562. CrossRef
8. K. Higuchi, M. Tayu, and T. Kawasaki, Chem. Commun., 2011, 47, 6728; CrossRef M. Tayu, K. Higuchi, M. Inaba, and T. Kawasaki, Org. Biomol. Chem., 2013, 11, 496. CrossRef
9. G. Büchi and R. E. Manning, J. Am. Chem. Soc., 1966, 88, 2532; CrossRef H. Sakakibara and T. Kobayashi, Tetrahedron, 1966, 22, 2475; CrossRef M. Ikeda, F. Tabusa, Y. Nishimura, S. Kwon, and Y. Tamura, Tetrahedron Lett., 1976, 27, 2347; CrossRef A. S. Bailey, J. B. Haxby, A. N. Hilton, J. M. Peach, and M. H. Vandrevala, J. Chem. Soc., Perkin Trans. 1, 1981, 382; CrossRef Y. Murakami and H. Ishii, Chem. Pharm. Bull., 1981, 29, 699; CrossRef Y. Tamura, M. Kirihara, M. Sasho, S. Akai, J. Sekihachi, R. Okunaka, and Y. Kita, Chem. Commun., 1987, 1474; CrossRef P. He and S. Zhu, Tetrahedron, 2005, 61, 12398; CrossRef Y. Nakano and D. W. Lupton, Chem. Commun., 2014, 1757. CrossRef
10. R. Di Fabio, R. Giovannini, B. Bertani, M. Borriello, A. Bozzoli, D. Donati, A. Falchi, D. Ghirlanda, C. P. Leslie, A. Pecunioso, G. Rumboldt, and S. Spada, Bioorg. Med. Chem. Lett., 2006, 16, 1749; CrossRef K. S. Gudmundsson, P. R. Sebahar, L. D. Richardson, J. G. Catalano, S. D. Boggs, A. Spaltenstein, P. B. Sethna, K. W. Brown, R. Harvey, and K. R. Romines, Bioorg. Med. Chem. Lett., 2009, 19, 3489; CrossRef L. Li, C. Beaulieu, M.-C. Carriere, D. Denis, G. Greig, D. Guay, G. O’Neill, R. Zamboni, and Z. Wang, Bioorg. Med. Chem. Lett., 2010, 20, 7462. CrossRef
11. O. Neiland and B. Karele, J. Org. Chem., USSR (Engl. Transl.), 1970, 6, 889; G. F. Koser, A. G. Relenyi, A. N. Kalos, L. Rebrovic, and R. H. Wettach, J. Org. Chem., 1982, 47, 2487. CrossRef
12. E. Zbiral and G. Nestler, Tetrahedron, 1970, 26, 2945; CrossRef F. Cech and E. Zbiral, Tetrahedron, 1975, 31, 605. CrossRef
13. P. Magnus, J. Lacour, and W. Weber, J. Am. Chem. Soc., 1993, 115, 9347; CrossRef F. Fontana, F. Minisci, Y.-M. Yan, and L. Zhao, Tetrahedron Lett., 1993, 34, 2517; CrossRef P. Magnus, J. Lacour, P. A. Evans, M. B. Roe, and C. Hulme, J. Am. Chem. Soc., 1996, 118, 3406; CrossRef C. M. Pedersen, L. G. Marinescu, and M. Bols, Org. Biomol. Chem., 2005, 3, 816. CrossRef
14. R. M. Moriarty and J. S. Khosrowshahi, Tetrahedron Lett., 1986, 27, 2809; CrossRef M. Arimoto, H. Yamaguchi, E. Fujita, M. Ochiai, and Y. Nagao, Tetrahedron Lett., 1987, 28, 6289; CrossRef P. Magnus and J. Lacour, J. Am. Chem. Soc., 1992, 114, 767; CrossRef P. Magnus, C. Hulme, and W. Weber, J. Am. Chem. Soc., 1994, 116, 4501; CrossRef Y. Kita, H. Tohma, K. Hatanaka, T. Takada, S. Fujita, S. Mitoh, H. Sakurai, and S. Oka, J. Am. Chem. Soc., 1994, 116, 3684; CrossRef Y. Kita, H. Tohma, T. Takada, S. Mitoh, S. Fujita, and M. Gyoten, Synlett, 1994, 427. CrossRef
15. T. Kitawaki, Y. Hayashi, A. Ueno, and N. Chida, Tetrahedron, 2006, 62, 6792. CrossRef
16. K. Matsumoto, A. Tanaka, I. Yukio, N. Hayashi, M. Toda, and R. A. Bulman, Heterocycl. Commun., 2011, 9, 9.