HETEROCYCLES

Paper | Special issue | Vol. 90, No. 2, 2015, pp. 1094-1110
Received, 7th July, 2014, Accepted, 13th August, 2014, Published online, 21st August, 2014.
DOI: 10.3987/COM-14-S(K)78

■ [2+2+2] Cycloaddition of Sulfanylbenzene-Tethered Diynes with Alkynes for the Synthesis of Multi-Substituted Dibenzothiophene Derivatives

Yu-ki Tahara, Riku Matsubara, and Takanori Shibata*

Chemistry and Biochemistry, Advanced Research Center of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

Abstract

An intermolecular [2+2+2] cycloaddition of sulfanyl- and sulfonylbenzene-tethered 1,6-diynes with alkynes using rhodium catalysts gave dibenzothiophene derivatives in moderate to excellent yields. The consecutive reaction of tetraynes with an alkyne gave an axially chiral 1,1’-bi(dibenzothiophenyl) and its tetraoxide with up to excellent ee.

INTRODUCTION
The dibenzothiophene (DBT) skeleton is found in a lot of biologically active molecules, such as a potent inhibitor of DNA-dependent protein kinase1 and a selective estrogen receptor antagonist.2 In addition, DBT-containing derivatives have attracted much attention as organic electronics and materials in recent years, because they showed optoelectronic and redox properties, and electrochemical characteristics such as organic light-emitting diodes (OLED).3
In spite of the wide applicability of DBT derivatives, there is some limitations in the synthesis of DBT, especially in catalytic synthetic approaches to multi-substituted DBT derivatives. There are many methods for the synthesis of DBT skeleton; intramolecular cyclization,4 anionic cyclization,5 radical cyclization,6 photocyclization7 and C-S bond formations8 were successful examples. As for catalytic approaches, the protocols using cross-coupling,9 C-H bond activation,10 ring-closing metathesis11 and C-S bond formations12 were recently disclosed. Some of these reactions required harsh reaction conditions, such as high temperature, and/or basic or acidic condition, therefore, alternative efficient approaches, which can realize the milder reaction conditions, are strongly desired.
In contrast, transition-metal-catalyzed [2+2+2] cycloaddition is a reliable and atom-economical protocol for the construction of multicyclic six-membered ring systems.13 It was already used for the syntheses of dibenzoheteroles, such as carbazoles,14 dibenzofurans,15 and dibenzosiloles.16 However, to the best of our knowledge, it has never been used for the construction of DBT skeleton. We have comprehensively studied [2+2+2] cycloaddition of various substrates with alkyne motifs,17 and considered that [2+2+2] cycloaddition of sulfanylbenzene-tethered 1,6-diynes with alkynes can be a new approach to the synthesis of multi-substituted DBT derivatives.

RESULTS AND DISCUSSION
We chose 1,6-diyne 1a and dimethyl acetylenedicarboxylate (DMAD) (2a) as a model diyne and alkyne, and examined an intermolecular [2+2+2] cycloaddition using [Rh(cod)2]BF4 with several phosphine ligands. The desired reaction proceeded at room temperature, and tetra-substituted DBT 3aa was obtained (Table 1). Triphenylphosphine gave a poor result, but alkylene-tethered diphosphine, 1,3-bis(diphenylphosphino)propane (DPPP) achieved moderate yield (Entries 1 and 2). We further examined diphosphines possessing a biaryl scaffold, and BIPHEP gave the best results (Entry 4).18,19

Under the optimum conditions, various diynes were subjected to the intermolecular [2+2+2] cycloaddition (Table 2). The reaction of diyne 1b, which has phenyl groups on its alkyne termini (R1, R2), proceeded to give cycloadduct 3ba in moderate yield (Entry 1). The reaction of diynes 1c and 1d, which have an alkyl group and phenyl group on its alkyne termini, proceeded sluggishly, and a higher reaction temperature was needed (Entries 2 and 3). Diynes 1e and 1f, which have pentyl-substituted and unsubstituted alkyne terminus (R1), respectively, were very reactive, and the slow addition of the diynes to a dichloroethane solution of DMAD over 30 min by syringe pump was needed (Entries 4 and 5). These yields of the cross-cycloadducts 3ea and 3fa were still low due to the formation of self-cycloadducts of diynes 1e and 1f. The reaction of diynes 1g and 1h, which have bulky trimethylsilyl group on its alkyne terminus (R2), proceeded to give cycloadduct 3ga and 3ha in improved yields, due to the suppression of homo-cycloaddition of the diynes (Entries 6 and 7).20 While the reactivity of diynes 1i and 1j, which have trimethylsilyl groups and tert-butyl groups, respectively, on both of alkyne termini was poor, no reaction proceeded (Entries 8 and 9). Diynes 1k and 1l with unsubstituted alkyne terminus(i) were too reactive, and self-cycloadducts were major products (Entries 10 and 11). On the other hand, diynes 1m and 1n, which have phenyl- and pentyl-substituted terminus (R1), respectively, along with unsubstituted alkyne terminus, were good substrates (Entries 12 and 13). In particular, diyne 1n realized the best yield of 94%.
Next, we examined the reaction of diyne 1a with symmetrical or unsymmetrical alkynes 2b-2e (Table 3). Unprotected diol 2b could be used as a coupling partner and the corresponding DBT derivative 3ab was obtained in good yield (Entry 1). Diarylacetyelene was less reactive and the poly-arylated DBT 3ac was obtained, yet in low yield even at a higher temperature (Entry 2). The cycloaddition of unsymmetrical

alkynes also proceeded under the same reaction conditions. While the reaction of phenylacetylene (2d) gave a regioisomeric mixture of cycloadducts 3ad and 4ad, the reaction of methyl phenylpropiolate 2e regioselectively proceeded to give 3ae as a sole cycloadduct (Entries 3 and 4).

We next focused on the synthesis of dibenzothiophene-5,5-dioxides (DBT-dioxides), because they have attracted attention as electronic materials.21 For example, high binding affinities and selectivity to α7-nicotinic acetylcholine receptors (α7-nAChRs) was known.22 Two methods were examined for the preparation of DBT-dioxide (Scheme 1). Oxidation of the obtained DBT 3na readily proceeded using mCPBA at room temperature to give DBT-dioxide 6 (path A). As a more direct protocol, [2+2+2] cycloaddition of sulfonylbenzene-tethered diyne 5 with 2a was examined. The reaction smoothly proceeded under the same conditions as sulfanylbenzene-tethered diyne 1n, and DBT-dioxides 6 was obtained in moderate yield (path B).

We further examined consecutive [2+2+2] cycloaddition using tetraynes 7 possessing a 1,3-diyne moiety and chose tetrayne 7a, which has two sulfanylbenzene moieties, as a model substrate and submitted to the Rh-catalyzed reaction with 2a (Table 4). When an achiral diphosphine ligand, 2,2’-bis(diphenylphosphino)-1,1’-biphenyl (BIPHEP) was used, doubly-cyclized product 8 was obtained, yet in low yield (Entry 1). The generation of axial chirality in 8 was ascertained by the HPLC analysis using a chiral column. We next screened several chiral diphosphine ligands (Entries 2-5). BINAP and its derivatives achieved good enantioselectivity, but the yield was miserable, because many unidentified by-products were formed. When Me-DUPHOS was used, the yield was significantly improved, but ee was low (Entry 5). In contrast, the reaction of sulfonylbenzene-tethered tetrayne 7b using Rh-Me-DUPHOS catalyst realized excellent ee along with moderate yield (Entry 6). We ascertained the structural details of 1,1’-bi(dibenzothiophenyl)-5,5,5’,5’-tetraoxide 9 by X-ray crystallographic analysis and its absolute configuration was determined to be R (Figure 1).
We measured the UV-vis spectra of the obtained DBT derivatives 3na and 8, and the DBT-dioxide derivatives 6 and 9 (Table 5). The λmax of these compounds were observed at 294.5-330.5 nm. The DBT-dioxide derivatives were red-shifted compared with the DBT derivatives as expected (Entry 1 vs Entry 2 and Entry 3 vs Entry 4). In the case of DBT-dioxide derivatives, significant blue-shift of bi-DBT-dioxide was unexpectedly observed compared with mono-DBT-dioxide (Entry 2 vs Entry 4).

In addition, we examined the synthetic transformation of cycloadduct 3aa by iodine monochloride and obtained 4-iode-DBT 10 in good yield (Scheme 2).

In conclusion, we developed Rh-catalyzed [2+2+2] cycloaddition of sulfanylbenzene- and sulfonylbenzene-tethered 1,6-diynes with alkynes. The present reaction provides a new protocol for the synthesis of multi-substituted DBT derivatives. We will further synthesize various DBT derivatives and evaluate them as organic electronics and materials.

EXPERIMENTAL
General. All reactions were examined under an argon atmosphere in oven-dried glassware with a magnetic stirring bar. A hexane solution of n-butyllithium (1.58 M) was purchased from Kanto Chemical Co., Inc. Dehydrated dichloromethane and 1,2-dichloroethane were purchased from Wako Pure Chemical Industries Ltd. (Wako) and degassed by argon bubbling before use. Other reagents were purchased from Wako, Kanto, TCI, or Aldrich and were used without further purification. Flash column chromatography was performed with silica gel (Kanto Chemical Co., Inc. 60N). Preparative thin-layer chromatography (PTLC) was performed with silica gel-precoated glass plates (Merck 60 GF254) prepared in our laboratory. IR spectra were recorded with Horiba FT730 spectrophotometer. NMR spectra were measured with JEOL AL-400 (400 MHz), JEOL ECS400 (400 MHz), JEOL ECX500 (500 MHz), or JEOL Lambda 500 (500 MHz) using TMS as an internal standard and CDCl3 was used as a solvent. High-resolution mass spectra (HRMS) were measured on a JEOL JMS-SX102A with FAB (Fast Atomic Bombardment) method or JMS-T100CS with ESI (Electro Spray Ionization) method. Optical rotations were measured with Jasco DIP-1000 polarimeter. 2,3-Bis(methoxycarbonyl)dibenzothiophene (3ka) was a known compound and its spectra was accorded with those in the literature.23 Diynes 1 were prepared by Sonogashira coupling of the corresponding alkynes and (hept-1-ynyl)(2-iodophenyl)sulfane or (2-iodophenyl)(trimethylsilylethynyl)sulfane, which was prepared by the literature protocols.24,25 Tetrayne 7a was prepared by CuCl-TMEDA-mediated oxidative coupling26 of (2-ethynylphenyl)(hept-1-ynyl)sulfane (1n). The following oxidation using mCPBA27 gave tetrayne tetraoxide 7b.
[2-(Phenylethynyl)phenyl](trimethylsilylethynyl)sulfane (1a): a pale brown solid; mp 43 °C; IR (CH2Cl2) 2892, 2097, 879, 843, 753 cm-1; 1H NMR δ 0.27 (s, 9H), 7.20 (ddd, J = 1.1, 7.6, 7.6 Hz, 1H), 7.34-7.39 (m, 4H), 7.47 (dd, J = 1.3, 7.6 Hz, 1H), 7.55-7.57 (m, 2H), 7.73 (dd, J = 0.6, 8.0 Hz, 1H); 13C NMR δ -0.1, 85.4, 89.8, 96.8, 107.8, 120.3, 122.7, 125.4, 126.0, 128.4, 128.7, 129.2, 131.6, 132.0, 135.9; HRMS (ESI positive) m/z calcd for C19H19SSi ([M+H]+): 307.0971. Found: 307.0971.
(Phenylethynyl)[2-(phenylethynyl)phenyl]sulfane (1b): a brown oil; IR (CH2Cl2) 2850, 2170, 752, 688 cm-1; 1H NMR δ 7.20 (ddd, J = 1.1, 7.5, 7.5 Hz, 1H), 7.34-7.39 (m, 7H), 7.49 (dd, J = 1.2, 7.7 Hz, 1H), 7.53-7.55 (m, 2H), 7.58-7.60 (m, 2H), 7.77 (dd, J = 0.6, 8.1 Hz, 1H); 13C NMR δ 70.5, 85.5, 96.9, 99.3, 120.3, 122.7, 122.8, 125.5, 125.9, 128.4, 128.4, 128.7, 128.8, 129.2, 131.7, 131.9, 132.1, 136.5; HRMS (ESI positive) m/z calcd for C22H15S ([M+H]+): 311.0889. Found: 311.0889.
[2-(Phenylethynyl)phenyl](prop-1-ynyl)sulfane (1c):. a brown oil; IR (CH2Cl2) 2913, 1057, 752, 689 cm-1; 1H NMR δ 2.14 (s, 3H), 7.17 (ddd, J = 1.1, 7.5, 7.5 Hz, 1H), 7.32-7.36 (m, 4H), 7.45 (dd, J = 1.3, 7.7 Hz, 1H), 7.56-7.57 (m, 2H), 7.70 (d, J = 8.1 Hz, 1H); 13C NMR δ 5.3, 63.5, 85.6, 96.6, 96.8, 119.9, 122.8, 125.2, 125.5, 128.4, 128.6, 129.0, 131.6, 131.9, 137.3; HRMS (ESI positive) m/z calcd for C17H13S ([M+H]+): 249.0732. Found: 249.0773.
(3,3-Dimethylbut-1-ynyl)[2-(phenylethynyl)phenyl]sulfane (1d): a pale yellow oil; IR (CH2Cl2) 2865, 2215, 1058, 754, 689 cm-1; 1H NMR δ 1.35 (s, 9H), 7.16 (ddd, J = 1.2, 7.6, 7.6 Hz, 1H), 7.34-7.37 (m, 4H), 7.45 (dd, J = 1.3, 7.7 Hz, 1H), 7.55-7.57 (m, 2H), 7.66 (dd, J = 0.9, 8.1 Hz, 1H); 13C NMR δ 29.1, 30.9, 65.2, 85.6, 96.6, 109.2, 119.8, 122.8, 124.8, 125.4, 128.3, 128.6, 129.0, 131.6, 131.8, 137.5; HRMS (ESI positive) m/z calcd for C20H19S ([M+H]+): 291.1202. Found: 291.1202.
(Hept-1-ynyl)[2-(phenylethynyl)phenyl]sulfane (1e): a pale brown oil; IR (CH2Cl2) 2859, 2215, 1491, 753, 689 cm-1; 1H NMR δ 0.92 (t, J = 7.3 Hz, 3H), 1.32-1.47 (m, 4H), 1.63 (tt, J = 7.1, 7.1 Hz, 2H), 2.03 (t, J = 7.1 Hz, 2H), 7.16 (ddd, J = 1.1, 7.5, 7.5 Hz, 1H), 7.33-7.37 (m, 4H), 7.45 (dd, J = 1.1, 7.7 Hz, 1H), 7.56-7.57 (m, 2H), 7.70 (dd, J = 0.7, 8.1 Hz, 1H); 13C NMR δ 14.0, 20.3, 22.2, 28.3, 31.1, 64.2, 85.6, 96.6, 101.5, 119.8, 122.8, 125.1, 125.5, 128.4, 128.6, 129.0, 131.6, 131.9, 137.5; HRMS (ESI positive) m/z calcd for C21H21S ([M+H]+): 305.1358. Found: 305.1358.
Ethynyl[2-(phenylethynyl)phenyl]sulfane (1f): a pale brown oil; IR (CH2Cl2) 3287, 3057, 2219, 753, 688 cm-1; 1H NMR δ 3.35 (s, 1H), 7.20 (ddd, J = 1.0, 7.6, 7.6 Hz, 1H), 7.35-7.38 (m, 4H), 7.48 (dd, J = 1.1, 7.6 Hz, 1H), 7.56-7.57 (m, 2H), 7.73 (d, J = 8.1 Hz, 1H); 13C NMR δ 70.5, 85.3, 88.5, 96.9, 120.5, 122.6, 125.6, 126.1, 128.4, 128.8, 129.2, 131.6, 132.0, 135.0; HRMS (ESI positive) m/z calcd for C16H11S ([M+H]+): 235.0576. Found: 235.0578.
(Hept-1-ynyl)[2-(trimethylsilylethynyl)phenyl]sulfane (1g): pale yellow oil; IR (CH2Cl2) 2860, 2157, 1250, 864, 753 cm-1; 1H NMR δ 0.27 (s, 9H), 0.91 (t, J = 7.3 Hz, 3H), 1.33-1.45 (m, 4H), 1.62 (tt, J = 7.2, 7.2 Hz, 2H), 2.47 (t, J = 7.1 Hz, 2H), 7.10 (ddd, J = 1.1, 7.5, 7.5 Hz, 1H), 7.32 (ddd, J = 1.3, 7.6, 7.6 Hz, 1H), 7.38 (dd, J = 1.2, 7.6 Hz, 1H), 7.66 (dd, J = 0.7, 8.1 Hz, 1H); 13C NMR δ -0.2, 14.0, 20.3, 22.2, 28.3, 31.1, 64.3, 100.7, 101.6, 102.5, 119.6, 124.8, 125.2, 129.2, 132.2, 137.9; HRMS (ESI positive) m/z calcd for C18H25SSi ([M+H]+): 301.1441. Found: 301.1426.
Phenylethynyl[2-(trimethylsilylethynyl)phenyl]sulfane (1h): a yellow solid; mp 67 °C; IR (CH2Cl2) 2854, 2157, 862, 844 cm-1; 1H NMR δ 0.29 (s, 9H), 7.15 (ddd, J = 0.9, 7.5, 7.5 Hz, 1H), 7.33-7.36 (m, 4H), 7.41 (dd, J = 1.1, 7.6 Hz, 1H), 7.52-7.54 (m, 2H), 7.73 (d, J = 8.1 Hz, 1H); 13C NMR δ 0.9, 76.1, 100.4, 101.7, 103.8, 121.0, 123.9, 126.3, 126.8, 129.5, 129.8, 130.4, 132.8, 133.4, 138.0; HRMS (ESI positive) m/z calcd for C19H18NaSSi ([M+Na]+): 329.0791. Found: 329.0793.
(Trimethylsilylethynyl)[2-(trimethylsilylethynyl)phenyl]sulfane (1i): a pale brown oil; IR (CH2Cl2) 2898, 2098, 863, 842, 756 cm-1; 1H NMR δ 0.32 (s, 9H), 0.33 (s, 9H), 7.19 (ddd, J = 1.0, 7.5, 7.5 Hz, 1H), 7.41 (ddd, J = 1.3, 7.5, 7.5 Hz, 1H), 7.45 (dd, J = 1.0, 7.6 Hz, 1H), 7.71 (d, J = 8.1 Hz, 1H); 13C NMR δ -0.2, -0.1, 89.3, 100.6, 102.7, 107.9, 120.0, 125.1, 125.7, 129.3, 132.2, 136.3; HRMS (ESI positive) m/z calcd for C16H23SSi2 ([M+H]+): 303.1054. Found: 303.1054.
(3,3-Dimethylbut-1-ynyl)[2-(3,3-dimethylbut-1-ynyl)phenyl]sulfane (1j): a yellow oil; IR (CH2Cl2) 2863, 2240, 1033, 752 cm-1; 1H NMR δ 1.34 (s, 18H), 7.09 (ddd, J = 1.1, 7.5, 7.5 Hz, 1H), 7.27-7.30 (m, 2H), 7.59 (d, J = 8.1 Hz, 1H); 13C NMR δ 28.3, 29.1, 30.9, 30.9, 63.5, 75.5, 106.4, 109.0, 120.5, 124.4, 125.2, 128.2, 131.5, 137.2; HRMS (ESI positive) m/z calcd for C18H23S ([M+H]+): 271.1515. Found: 271.1515.
Ethynyl(2-ethynylphenyl)sulfane (1k): a brown oil; IR (CH2Cl2) 3288, 3059, 754 cm-1; 1H NMR δ 3.35 (s, 1H), 3.48 (s, 1H), 7.18 (ddd, J = 1.1, 7.6, 7.6 Hz, 1H), 7.38 (ddd, J = 1.4, 7.7, 7.7 Hz, 1H), 7.45 (dd, J = 1.3, 7.7 Hz, 1H), 7.71 (dd, J = 0.8, 8.1 Hz, 1H); 13C NMR δ 70.2, 79.5, 84.8, 88.7, 119.3, 125.8, 126.1, 129.8, 133.0, 135.4; HRMS (ESI positive) m/z calcd for C10H7S ([M+H]+): 159.0263. Found: 159.0264.
(2-Ethynylphenyl)(trimethylsilylethynyl)sulfane (1l): a brown oil; IR (CH2Cl2) 3291, 2898, 2097, 862, 754 cm-1; 1H NMR δ 0.27 (s, 9H), 3.47 (s, 1H), 7.17 (ddd, J = 1.1, 7.6, 7.6 Hz, 1H), 7.39 (ddd, J = 1.4, 7.6, 7.6 Hz, 1H), 7.45 (dd, J = 1.2, 7.8 Hz, 1H), 7.67 (dd, J = 0.6, 8.1 Hz, 1H); 13C NMR δ 0.9, 80.6, 85.6, 90.4, 109.1, 120.1, 126.5, 126.9, 130.8, 134.0, 137.3; HRMS (ESI positive) m/z calcd for C13H15SSi ([M+H]+): 231.0658. Found: 231.0656.
(2-Ethynylphenyl)(phenylethynyl)sulfane (1m): a pale brown oil; IR (CH2Cl2) 3287, 3020, 2170, 752, 688 cm-1; 1H NMR δ 3.51 (s, 1H), 7.18 (ddd, J = 1.1, 7.6, 7.6 Hz, 1H), 7.33-7.37 (m, 4H), 7.39 (ddd, J = 1.5, 7.6, 7.6 Hz, 1H), 7.52-7.55 (m, 2H), 7.76 (dd, J = 0.7, 8.1 Hz, 1H); 13C NMR δ 75.6, 80.7, 85.7, 100.5, 120.1, 123.8, 126.6, 126.9, 129.5, 129.9, 130.8, 132.9, 134.1, 137.9; HRMS (ESI positive) m/z calcd for C16H10S ([M]+): 234.0498. Found: 234.0498.
(2-Ethynylphenyl)(hept-1-ynyl)sulfane (1n): pale brown oil; IR (CH2Cl2) 3290, 2859, 1461, 753 cm-1; 1H NMR δ 0.92 (t, J = 7.2 Hz, 3H), 1.31-1.46 (m, 4H), 1.62 (tt, J = 7.1, 7.1 Hz, 2H), 2.47 (t, J = 7.1 Hz, 2H), 3.46 (s, 1H), 7.14 (ddd, J = 1.1, 7.6, 7.6 Hz, 1H), 7.36 (ddd, J = 1.4, 7.7, 7.7 Hz, 1H), 7.42 (dd, J = 1.2, 7.6 Hz, 1H), 7.68 (dd, J = 0.8, 8.1 Hz, 1H); 13C NMR δ 13.9, 20.3, 22.2, 28.3, 31.1, 63.4, 79.7, 84.4, 101.8, 118.6, 125.2, 125.4, 129.5, 132.8, 137.8; HRMS (ESI positive) m/z calcd for C15H17S ([M+H]+): 229.1046. Found: 229.1045.
(2-Ethynylphenyl)(hept-1-ynyl)sulfone (5): a brown oil; IR (CH2Cl2) 3267, 2200, 1163, 1126, 792 cm-1; 1H NMR δ 0.87 (t, J = 7.2 Hz, 3H), 1.25-1.38 (m, 4H), 1.55-1.61 (m, 2H), 2.40 (t, J = 7.1 Hz, 2H), 3.66 (s, 1H), 7.54 (ddd, J = 1.3, 7.8, 7.8 Hz, 1H), 7.61 (ddd, J = 1.3, 7.8, 7.8 Hz, 1H), 7.72 (dd, J = 1.2, 7.6 Hz, 1H), 8.10 (dd, J = 1.1, 7.9 Hz, 1H); 13C NMR δ 14.8, 20.0, 23.0, 27.7, 31.9, 78.2, 79.9, 88.3, 98.7, 122.3, 129.2, 130.0, 134.3, 136.6, 143.7; HRMS (ESI positive) m/z calcd for C15H16NaO2S ([M+Na]+): 283.0763. Found: 283.0762.
1,4-Bis[2-(hept-1-ynylsulfanyl)phenyl]buta-1,3-diyne (7a): a brown solid; mp 35 °C; IR (CH2Cl2) 2858, 2359, 1434, 752 cm-1; 1H NMR δ 0.92 (t, J = 7.4 Hz, 6H), 1.32-1.46 (m, 8H), 1.62 (tt, J = 7.3, 7.3 Hz, 4H), 2.48 (t, J = 7.0 Hz, 4H), 7.15 (ddd, J = 1.0, 7.4, 7.4 Hz, 2H), 7.38 (ddd, J = 1.1, 7.7, 7.7 Hz, 2H), 7.47 (dd, J = 1.3, 7.8 Hz, 2H), 7.68 (d, J = 8.2 Hz, 2H); 13C NMR δ 13.9, 20.3, 22.1, 28.2, 31.0, 63.6, 79.6, 80.3, 101.9, 118.2, 125.3, 125.5, 129.9, 133.3, 139.1; HRMS (ESI positive) m/z calcd for C30H30NaS2 ([M+Na]+): 477.1681. Found: 477.1682.
1,4-Bis[2-(hept-1-ynylsulfonyl)phenyl]buta-1,3-diyne (7b): a brown solid; mp 115 °C; IR (CH2Cl2) 2198, 1331, 1160, 766 cm-1; 1H NMR δ 0.83 (t, J = 7.3 Hz, 6H), 1.24-1.38 (m, 8H), 1.64 (tt, J = 7.1, 7.1 Hz, 4H), 2.52 (t, J = 7.2 Hz, 4H), 7.57 (ddd, J = 1.3, 7.8, 7.8 Hz, 2H), 7.64 (ddd, J = 1.3, 7.6, 7.6 Hz, 2H), 7.76 (dd, J = 1.0, 7.7 Hz, 2H), 8.12 (dd, J = 1.0, 7.9 Hz, 2H); 13C NMR δ 14.8, 20.0, 23.0, 27.8, 32.0, 77.9, 80.3, 83.8, 99.9, 121.7, 129.3, 130.5, 134.3, 136.7, 144.5; HRMS (ESI positive) m/z calcd for C30H30NaO4S2 ([M+Na]+): 541.1478. Found: 541.1478.
Typical Procedure for the Rh-catalyzed cycloaddition: [Rh(cod)2]BF4 (2.0 mg, 0.005 mmol) and BIPHEP (2.6 mg, 0.005 mmol) were placed in Schlenk tube, which was then evacuated and backfilled with argon (3 ×). To the reaction vessel was added CH2Cl2 (1.0 mL). Then it was filled with H2, and the mixture was stirred at r.t. for 30 min under H2. After removal of the solvent and H2 under reduced pressure, the reaction vessel was filled with argon. 1,2-Dichloroethane (0.2 mL) was added to the flask and the mixture was stirred to give a reddish solution. Then, a 1,2-dichloroethane solution (0.3 mL) of diyne (0.05 mmol) and DMAD (21.3 mg, 0.15 mmol) was added and the mixture was stirred at room temperature for 1 h. The volatiles were removed under reduced pressure, and the crude products were purified by PTLC.
2,3-Bis(methoxycarbonyl)-1-phenyl-4-(trimethylsilyl)dibenzothiophene (3aa): a brown solid; mp 135 °C; IR (CH2Cl2) 2849, 1734, 885, 841, 701 cm-1; 1H NMR δ 0.56 (s, 9H), 3.47 (s, 3H), 3.88 (s, 3H), 6.67 (d, J =8.4 Hz, 1H), 7.02 (ddd, J =1.0, 8.3, 8.3 Hz, 1H), 7.33-7.37 (m, 3H), 7.50-7.51 (m, 3H), 7.81 (d, J = 8.0 Hz, 1H); 13C NMR δ 0.8, 52.0, 52.6, 121.9, 124.0, 125.4, 126.9, 128.3, 128.7, 129.0, 129.9, 132.2, 133.5, 134.3, 135.4, 137.3, 137.8, 140.1, 148.0, 168.9, 169.8; HRMS (ESI positive) m/z calcd for C25H24NaO4SSi ([M+Na]+): 471.1057. Found: 471.1055.
2,3-Bis(methoxycarbonyl)-1,4-diphenyldibenzothiophene (3ba): a yellow solid; mp 178 °C; IR (CH2Cl2) 2849, 1739, 744, 701 cm-1; 1H NMR δ 3.52 (s, 3H), 3.56 (s, 3H), 6.72 (d, 8.4 Hz, 1H), 7.05 (ddd, J =1.1, 7.8, 7.8 Hz, 1H), 7.33 (ddd, J =1.1, 7.3, 7.3 Hz, 1H) 7.42-7.54 (m, 10H), 7.72 (d, J = 7.9 Hz, 1H); 13C NMR δ 52.2, 52.3, 122.5, 124.3, 125.6, 127.1, 128.4, 128.5, 128.6, 128.6, 128.7, 128.7, 129.3, 130.6, 134.7, 135.3, 135.4, 136.4, 137.6, 138.4, 141.2, 143.2, 168.1, 168.4; HRMS (ESI positive) m/z calcd for C28H20NaO4S ([M+Na]+): 475.0975. Found: 475.0974.
2,3-Bis(methoxycarbonyl)-4-methyl-1-phenyldibenzothiophene (3ca): a yellow solid; mp 150 °C; IR (CH2Cl2) 2850, 1732, 1216, 742, 701 cm-1; 1H NMR δ 2.74 (s, 3H), 3.50 (s, 3H), 3.93 (s, 3H), 6.68 (d, J = 8.3 Hz, 1H), 7.06 (ddd, J =1.1, 7.8, 7.8 Hz, 1H), 7.35-7.39 (m, 3H), 7.50-7.52 (m, 3H), 7.85 (d, J = 7.9 Hz, 1H); 13C NMR δ 19.9, 53.1, 53.7, 123.7, 125.4, 126.6, 128.0, 129.0, 129.3, 129.7, 130.4, 131.6, 131.9, 135.4, 136.2, 136.8, 138.8, 141.3, 143.9, 169.3, 169.7; HRMS (ESI positive) m/z calcd for C23H18NaO4SSi ([M+Na]+): 413.0818. Found: 413.0818.
4-(1,1-Dimethylethyl)-2,3-bis(methoxycarbonyl)-1-phenyldibenzothiophene (3da): a yellow solid; mp 170 °C; IR (CH2Cl2) 2951, 1734, 1216, 750, 708 cm-1; 1H NMR δ 1.30 (s, 9H), 3.91 (s, 3H), 4.04 (s, 3H), 5.74 (d, J = 8.5 Hz, 1H), 6.90 (ddd, J =1.2, 7.8, 7.8 Hz, 1H), 7.28 (ddd, J =1.1, 7.6, 7.6 Hz, 1H), 7.35-7.36 (m, 2H), 7.50-7.56 (m, 3H), 7.74 (d, J = 7.9 Hz, 1H); 13C NMR δ 35.1, 40.1, 53.7, 53.9, 123.1, 124.8, 125.8, 127.0, 127.6, 129.4, 129.8, 131.8, 134.2, 136.4, 138.0, 138.8, 142.0, 142.7, 143.4, 144.5, 168.8, 172.1; HRMS (ESI positive) m/z calcd for C26H24NaO4SSi ([M+Na]+): 455.1288. Found: 455.1285.
2,3-Bis(methoxycarbonyl)-4-pentyl-1-phenyldibenzothiophene (3ea): a yellow oil; IR (CH2Cl2) 2853, 1101, 741, 701 cm-1; 1H NMR δ 0.94 (t, J = 7.1 Hz, 3H), 1.39-1.50 (m, 4H), 1.79-1.86 (m, 2H), 2.89 (t, J = 8.1 Hz, 2H), 3.49 (s, 3H), 3.91 (s, 3H), 6.67 (d, J = 8.4 Hz, 1H), 7.04 (ddd, J =1.0, 8.3, 8.3 Hz, 1H), 7.35-7.38 (m, 3H), 7.50-7.51 (m, 3H), 7.83 (d, J = 7.9 Hz, 1H); 13C NMR δ 13.9, 22.3, 29.2, 32.1, 33.5, 52.0, 52.5, 122.5, 124.2, 125.4, 126.9, 127.7, 128.2, 128.5, 129.3, 130.6, 134.5, 135.2, 135.6, 135.7, 137.8, 140.1, 142.4, 168.3, 168.6; HRMS (ESI positive) m/z calcd for C27H26NaO4S ([M+Na]+): 469.1444. Found: 469.1443.
2,3-Bis(methoxycarbonyl)-1-phenyldibenzothiophene (3fa): a brown solid; mp 175 °C; IR (CH2Cl2) 2851, 1726, 770, 701 cm-1; 1H NMR δ 3.57 (s, 3H), 3.95 (s, 3H), 6.60 (d, J = 8.3 Hz, 1H), 7.70 (ddd, J =0.9, 7.7, 7.7 Hz, 1H), 7.37-7.40 (m, 3H), 7.51-7.53 (m, 3H), 7.83 (d, J = 8.0 Hz, 1H), 8.57 (s, 1H); 13C NMR δ 52.0, 52.6, 122.6, 124.1, 124.3, 124.6, 125.6, 127.4, 128.5, 128.6, 129.4, 132.5, 134.6, 136.5, 136.7, 136.8, 140.1, 141.3, 165.7, 168.8; HRMS (ESI positive) m/z calcd for C22H16NaO4S ([M+Na]+): 399.0662. Found: 399.0660.
2,3-Bis(methoxycarbonyl)-4-pentyl-1-(trimethylsilyl)dibenzothiophene (3ga): a yellow oil; IR (CH2Cl2) 2859, 1726, 1436, 842, 763 cm-1; 1H NMR δ 0.46 (s, 9H), 0.92 (t, J = 7.2 Hz, 3H), 1.36-1.47 (m, 4H), 1.72-1.79 (m, 2H), 2.94 (t, J = 8.2 Hz, 2H), 3.89 (s, 3H), 3.91 (s, 3H), 7.41 (ddd, J =1.1, 7.1, 7.1 Hz, 1H), 7.45 (ddd, J =1.1, 7.2, 7.2 Hz, 1H), 7.84 (d, J = 7.5 Hz, 1H), 8.32 (d, J = 7.6 Hz, 1H); 13C NMR δ 2.2, 14.0, 22.3, 29.1, 32.1, 33.6, 52.5, 52.5, 122.7, 123.3, 127.0, 127.3, 129.3, 134.8, 135.8, 136.5, 136.6, 140.0, 142.3, 142.4, 169.4, 170.0; HRMS (ESI positive) m/z calcd for C24H30NaO4SSi ([M+Na]+): 465.1526. Found: 465.1527.
2,3-Bis(methoxycarbonyl)-4-phenyl-1-(trimethylsilyl)dibenzothiophene (3ha): a white solid; mp 171 °C; IR (CH2Cl2) 2899, 1730, 882, 850, 847 cm-1; 1H NMR δ 0.52 (s, 9H), 3.58 (s, 3H), 3.91 (s, 3H), 7.43-7.50 (m, 7H), 7.75-7.77 (m, 1H), 8.36-8.38 (m, 1H); 13C NMR δ 2.1, 52.3, 52.6, 122.6, 123.4, 127.1, 127.4, 128.6, 128.6, 128.7, 129.7, 135.7, 136.2, 136.3, 136.4, 138.0, 141.1, 142.3, 143.2, 169.0, 169.8; HRMS (ESI positive) m/z calcd for C25H24NaO4SSi ([M+Na]+): 471.1057. Found: 471.1056.
2,3-Bis(methoxycarbonyl)-4-(trimethylsilyl)dibenzothiophene (3la): a brown oil; IR (CH2Cl2) 2850, 1727, 861, 843 cm-1; 1H NMR δ 0.53 (s, 9H), 3.96 (s, 3H), 3.96 (s, 3H), 7.49-7.54 (m, 2H), 7.88 (dd, J = 1.7, 7.7 Hz, 1H), 8.23 (dd, J = 1.7, 5.8 Hz, 1H), 8.78 (s, 1H); 13C NMR δ 0.0, 52.1, 52.1, 121.4, 121.8, 123.7, 123.8, 124.4, 127.2, 131.6, 133.6, 134.7, 138.2, 139.3, 149.7, 166.2, 170.0; HRMS (ESI positive) m/z calcd for C19H20NaO4SSi ([M+Na]+): 395.0744. Found: 395.0745.
2,3-Bis(methoxycarbonyl)-4-phenyldibenzothiophene (3ma): a yellow solid; mp 128 °C; IR (CH2Cl2) 2851, 1727, 741, 701 cm-1; 1H NMR δ 3.66 (s, 3H), 3.99 (s, 3H), 7.46-7.54 (m, 7H), 7.80 (dd, J = 2.6, 8.3 Hz, 1H), 8.27 (dd, J = 1.9, 6.1 Hz, 1H), 8.80 (s, 1H); 13C NMR δ 52.4, 52.7, 122.7, 122.5, 122.8, 124.8, 125.1, 127.8, 128.7, 128.8, 128.9, 132.4, 135.1, 135.2, 135.8, 137.2, 140.6, 145.5, 166.4, 168.9; HRMS (ESI positive) m/z calcd for C22H16NaO4S ([M+Na]+): 399.0662. Found: 399.0662.
2,3-Bis(methoxycarbonyl)-4-pentyldibenzothiophene (3na): a yellow oil; IR (CH2Cl2) 2860, 1725, 1110, 741 cm-1; 1H NMR δ 0.92 (t, J = 7.1 Hz, 3H), 1.36-1.45 (m, 4H), 1.74-1.80 (m, 2H), 2.89 (t, J = 8.3 Hz, 2H), 3.97 (s, 3H), 3.99 (s, 3H), 7.50-7.54 (m, 2H), 7.88-7.90 (m, 1H), 8.21-8.23 (m, 1H), 8.66 (s, 1H); 13C NMR δ 13.9, 22.2, 28.8, 32.0, 33.3, 52.5, 52.5, 121.3, 122.2, 122.8, 124.7, 124.9, 127.6, 131.9, 134.9, 135.2, 135.7, 139.7, 144.4, 166.4, 169.6; HRMS (ESI positive) m/z calcd for C21H22NaO4S ([M+Na]+): 393.1131. Found: 393.1131.
2,3-Bis(hydroxymethyl)-1-phenyl-4-(trimethylsilyl)dibenzothiophene (3ab): a white solid; mp 181 °C; IR (CH2Cl2) 3341, 2853, 840, 736, 703 cm-1; 1H NMR δ 0.67 (s, 9H), 2.96 (brs, 1H), 3.44 (brs, 1H), 4.65 (s, 2H), 5.08 (s, 2H), 6.39 (d, J = 8.3 Hz, 1H), 6.96 (ddd, J =1.1, 7.8, 7.8 Hz, 1H), 7.29 (ddd, J =1.1, 7.5, 7.5 Hz, 1H), 7.32-7.34 (m, 2H), 7.55-7.56 (m, 3H), 7.78 (d, J = 7.7 Hz, 1H); 13C NMR δ 3.1, 60.0, 62.8, 121.8, 123.6, 124.8, 125.9, 128.0, 129.1, 129.2, 132.8, 133.8, 134.8, 134.8, 139.4, 139.7, 140.6, 144.2, 146.4; HRMS (ESI positive) m/z calcd for C23H24NaO2SSi ([M+Na]+): 415.1158. Found: 415.1157.
2,3-Bis(4-methoxyphenyl)-1-phenyl-4-(trimethylsilyl)dibenzothiophene (3ac): a brown solid; mp 210 °C; IR (CH2Cl2) 2853, 1245, 756, 734 cm-1; 1H NMR δ 0.18 (s, 9H), 3.61 (s, 3H), 3.74 (s, 3H), 6.39 (d, J = 8.2 Hz, 2H), 6.49 (d, J = 8.2 Hz, 1H), 6.63 (ddd, J =0.6, 8.9, 8.9 Hz, 1H), 6.92 (d, J = 8.7 Hz, 2H), 6.96 (ddd, J =1.1, 7.2, 7.2 Hz, 1H), 7.12 (dd, J = 1.7, 7.9 Hz, 2H), 7.22-7.26 (m, 3H), 7.29 (ddd, J =0.9, 7.6, 7.6 Hz, 1H), 7.82 (d, J = 7.7 Hz, 1H); 13C NMR δ 2.6, 55.9, 56.2, 113.0, 113.4, 122.8, 124.5, 126.0, 126.7, 127.8, 129.3, 131.0, 133.0, 133.2, 133.3, 133.4, 136.4, 136.7, 139.6, 140.0, 140.7, 141.3, 146.4, 147.9, 157.9, 159.2 (a pair of peaks at the aromatic region was overlapped); HRMS (ESI positive) m/z calcd for C35H32NaO2SSi ([M+Na]+): 567.1784. Found: 567.1785.
1,2-Diphenyl-4-(trimethylsilyl)dibenzothiophene (3ad): NOESY correlation was observed between TMS and C-H on dibenzothiophene ring (Figure 2) (its NOESY spectrum was listed in supporting information); a white solid; mp 160 °C; IR (CH2Cl2) 2853, 884, 839, 743, 699 cm-1; 1H NMR δ 0.53 (s, 9H), 6.67 (d, J = 8.3 Hz, 1H), 6.99 (ddd, J =1.1, 7.7, 7.7 Hz, 1H), 7.13-7.18 (m, 5H), 7.21-7.23 (m, 2H), 7.29-7.34 (m, 4H) 7.60 (s, 1H), 7.82 (d, J =7.9 Hz, 1H); 13C NMR δ 0.0, 123.3, 124.7, 126.3, 126.9, 127.2, 128.4, 128.5, 129.5, 131.3, 133.7, 133.8, 135.3, 136.7, 138.9, 139.2, 140.6, 141.0, 142.6, 145.7 (a pair of peaks at the aromatic region was overlapped); HRMS (ESI positive) m/z calcd for C27H24NaSSi ([M+Na]+): 431.1260. Found: 431.1262.
1,3-Diphenyl-4-(trimethylsilyl)dibenzothiophene (4ad): NOESY correlation was observed between TMS and C-H on benzene ring (Figure 2) (its NOESY spectrum was listed in supporting information); a white solid; mp 214 °C; IR (CH2Cl2) 2850, 839, 742, 701, 699 cm-1; 1H NMR δ 0.21 (s, 9H), 7.05 (ddd, J =1.1, 7.6, 7.6 Hz, 1H), 7.14-7.16 (m, 2H), 7.32 (ddd, J =1.1, 7.6, 7.6 Hz, 1H), 7.33-7.37 (m, 5H), 7.48-7.49 (m, 5H), 7.83 (d, J = 7.9 Hz, 1H); 13C NMR δ 1.4, 121.9, 123.5, 124.8, 125.9, 127.2, 127.7, 128.6, 129.0, 129.4, 129.6, 130.6, 131.0, 134.7, 139.0, 139.4, 141.1, 144.5, 147.1, 147.5 (a pair of peaks at the aromatic region was overlapped); HRMS (ESI positive) m/z calcd for C27H25SSi ([M+H]+): 409.1441. Found: 409.1438.

2-(Methoxycarbonyl)-1,3-diphenyl-4-(trimethylsilyl)dibenzothiophene (3ae): NOESY correlation was observed between TMS and C-H on benzene ring, which is not observed in regioisomer 4ae (Figure 3) (its NOESY spectrum was listed in supporting information); a white solid; mp 174 °C; IR (CH2Cl2) 2852, 1733, 884, 840, 763, 701 cm-1; 1H NMR δ 0.17 (s, 9H), 3.09 (s, 3H), 6.60 (d, J = 8.3 Hz, 1H), 7.02 (ddd, J =1.1, 7.7, 7.7 Hz, 1H), 7.32-7.34 (m, 6H), 7.41-7.43 (m, 2H), 7.47-7.49 (m, 3H) 7.82 (d, J = 7.9 Hz, 1H); 13C NMR δ 1.2, 51.2, 121.9, 123.7, 124.9, 126.2, 127.4, 127.7, 128.2, 128.6, 129.3, 130.3, 131.3, 132.1, 133.3, 134.7, 135.7, 138.1, 139.6, 141.0, 143.3, 147.2, 169.3; HRMS (ESI positive) m/z calcd for C29H26NaO2SSi ([M+Na]+): 489.1315. Found: 489.1316.

2,3-Bis(methoxycarbonyl)-4-pentyldibenzothiophene-5,5-dioxide (6): a white solid; mp 140 °C; IR (CH2Cl2) 1733, 1151, 1101, 819 cm-1; 1H NMR δ 0.93 (t, J = 7.2 Hz, 3H), 1.36-1.49 (m, 4H), 1.76-1.82 (m, 2H), 3.00 (t, J = 8.5 Hz, 2H), 3.96 (s, 3H), 3.97 (s, 3H), 7.57 (dd, J = 7.6, 7.6 Hz, 1H), 7.66 (dd, J = 7.5, 7.5 Hz, 1H), 7.81 (d, J = 7.8 Hz, 1H), 7.83 (d, J = 7.9 Hz, 1H), 8.19 (s, 1H); 13C NMR δ 14.9, 23.2, 30.9, 32.0, 33.2, 53.9, 54.2, 121.6, 123.1, 123.1, 130.8, 132.2, 133.8, 134.3, 135.2, 138.5, 139.1, 140.4, 140.6, 165.9, 168.7; HRMS (ESI positive) m/z calcd for C21H22NaO6S ([M+Na]+): 425.1029. Found: 425.1028.
2,2’,3,3’-Tetrakis(methoxycarbonyl)-4,4’-dipentyl-1,1’-bidibenzothiophene (8): a brown solid; mp 115 ℃; IR (CH2Cl2) 2857, 1736, 1116, 739 cm-1; 1H NMR δ 0.97 (t, J = 7.2 Hz, 6H), 1.43-1.54 (m, 8H), 1.87-1.96 (m, 4H), 3.09-3.15 (m, 2H), 3.21-3.27 (m, 2H), 3.39 (s, 6H), 3.88 (s, 6H), 6.30 (d, J = 8.3 Hz, 2H), 6.88 (ddd, J =1.1, 7.8, 7.8 Hz, 2H), 7.25 (ddd, J =1.1, 7.6, 7.6 Hz, 2H), 7.75 (d, J = 7.9 Hz, 2H); 13C NMR δ 14.0, 22.4, 29.3, 31.9, 33.6, 52.0, 52.5, 122.4, 124.4, 124.8, 127.0, 129.2, 129.4, 131.5, 135.0, 135.2, 136.5, 139.9, 143.3, 167.2, 168.2; HRMS (ESI positive) m/z calcd for C42H42NaO8S2 ([M+Na]+): 761.2213. Found: 761.2205. [α]26D 6.3 (c 0.18, CHCl3, 82% ee). Ee was determined by HPLC analysis using a chiral column (Daicel Chiralpak IA: 4 x 250 mm, 254 nm UV detector, rt, eluent: 1% 2-propanol in hexane, flow rate: 1.0 mL/min, retention time: 26.3 min for major isomer and 22.1 min for minor isomer).
2,2’,3,3’-Tetrakis(methoxycarbonyl)-4,4’-dipentyl-1,1’-bidibenzothiophene-5,5,5’,5’-tetraoxide (9): a brown solid; mp 204 °C; IR (CH2Cl2) 1739, 1310, 1162, 739 cm-1; 1H NMR δ 0.96 (t, J = 7.2 Hz, 6H), 1.39-1.53 (m, 8H), 1.82-1.87 (m, 4H), 3.22-3.31 (m, 2H), 3.33-3.37 (m, 2H), 3.51 (s, 6H), 3.89 (s, 6H), 6.42 (d, J = 8.1 Hz, 2H), 7.31 (ddd, J =1.2, 7.8, 7.8 Hz, 2H), 7.64 (ddd, J =0.8, 7.6, 7.6 Hz, 2H), 7.79 (d, J = 7.1 Hz, 2H); 13C NMR δ 15.0, 23.3, 30.8, 32.3, 33.1, 53.9, 54.1, 123.2, 125.1, 130.1, 130.2, 132.3, 133.9, 135.7, 136.2, 139.1, 139.2, 140.1, 142.7, 166.4, 167.1; HRMS (ESI positive) m/z calcd for C42H42NaO12S2 ([M+Na]+): 825.2010. Found: 825.2002. [α]26D -52.1 (c 0.49, CHCl3, 97% ee). Ee was determined by HPLC analysis using a chiral column (Daicel Chiralpak IA: 4 x 250 mm, 254 nm UV detector, rt, eluent: 10% 2-propanol in hexane, flow rate: 1.0 mL/min, retention time: 17.7 min for major isomer and 21.7 min for minor isomer). The crystal data of 9: C42H42O12S2, M=802.91, triclinic, space group P1 (#1), a = 9.4846(3) Å, b = 11.7163(3) Å, c = 18.1232(5) Å, α = 82.456(2) °, β = 80.977(2) °, γ = 85.287(2) °, V = 1967.89(9) Å3, T=173 K, Z=2, µ(Cu Kα) 17.688 cm-1; number of reflections measured: total 23163 and unique 11998, R1=0.0552, wR2=0.1385, Flack parameter (Friedel pairs = 4945) 0.030(13). CCDC 1010885.
4-Iodo-2,3-bis(methoxycarbonyl)-1-phenyldibenzothiophene (10): a solution of 2,3-bis(methoxycarbonyl)-1-phenyl-4-(trimethylsilyl)dibenzothiophene (3aa) (22.4 mg, 0.05 mmol) and ICl (9.0 mg, 0.06 mmol) in CHCl3 (0.25 mL) was stirred under reflux for 6 h. The resulting mixture was concentrated under reduced pressure. Et2O was added and the solution was treated with sat. Na2S2O3. The almost colourless organic phase was separated. The solvent was removed in vacuo to afford crude products, which were purified by flash column chromatography on silica gel to give 4-iodo-2,3-bis(methoxycarbonyl)-1-phenyldibenzothiophene (10) (18 mg, 72%): a white solid; mp 172 °C; IR (CH2Cl2) 2850, 1735, 1206, 701 cm-1; 1H NMR δ 3.49 (s, 3H), 3.97 (s, 3H), 6.61 (d, J = 8.4 Hz, 1H), 7.07 (ddd, J =1.0, 7.7, 7.7 Hz, 1H), 7.34-7.36 (m, 2H), 7.40 (ddd, J =0.8, 7.2, 7.2 Hz, 1H), 7.49-7.55 (m, 3H), 7.84 (d, J = 8.0 Hz, 1H); 13C NMR δ 51.8, 52.5, 86.7, 122.0, 124.1, 125.8, 126.9, 128.1, 128.3, 128.5, 129.7, 133.2, 134.6, 135.8, 136.7, 137.0, 139.3, 149.4, 166.6, 167.3; HRMS (ESI positive) m/z calcd for C22H15INaO4S ([M+Na]+): 524.9628. Found: 524.9628.

ACKNOWLEDGEMENTS
This work was supported by ACT-C from JST (Japan) and Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 23655091).

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