English

• 1.   Introduction

  The synthesis of tetrazole has become one of the most important research topics in heterocyclic chemistry due to their important biological activities displayed in medicinal chemistry such as antituberculosis, ^[1] anti-allergy, ^[2] antiinflammatory, ^[3] antitumor^[4] and so on. Furthermore, as the isostere of the carboxyl acid group, ^[5] tetrazole derivatives have attracted increasing attention and are often used as testing materials, explosives, metal coordination compounds, chemical catalysts and other materials.^[6] By far, the addition of azides to isothiocyanates^[7] or the [3+2] cycloaddition of an azide to a nitrile by using a Lewis acid as the catalyst have been recognized as the powerful approaches for constructing tetrazole derivatives.^[8] For instance, Hantzsch and Vagt^[8a] disclosed the first example of Lewis acid catalyzed addition reaction of hydrazoic acid to the cyanide group to synthesis of 5-substituted tetrazole derivatives. In 2001, Sharpless and co-workers^[8b] developed this method to synthesize a variety of 5-substituted 1H-tetrazoles from sodium azide to nitriles with water as solvent. However, to our surprise, the synthesis of the polycyclic skeleton 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazine is quite rare, only two published example are known.^{[8c, 9]} Recently, a silver-catalyzed cascade bicyclization of o-alkynylphenyl isothiocyanates with sodium azide for the synthesis of 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazines has been reported by our group (Scheme 1).^[9] However, with the urgent demand for green and sustainable chemistry, developing a new catalytic system in place of the noble catalyst like silver or palladium would be highly desired. Of the available cheap metals, copper salt is the representative example. It is inexpensive, low toxic and environmentally friendly. The catalytic activities of copper salts are more remarkable, even comparable to those of noble metals such as palladium catalysts in cross coupling reactions.^[10] Numerous catalytic systems have been developed for C—O, ^[11] C—N, ^[12] C—S^[13] and C—C^[14] bond formation. Meanwhile, the applications of copper catalysts in heterocycle synthesis have also been explored by many organic chemists. In continuation of our interest in the synthesis of biologically relevant heterocyclic compounds by using copper salts as the catalyst, ^[15] we envisioned that o-alkenylphenyl isothiocyanate could also be applied in the reaction with sodium azide by using tandem bicylization strategy.^[16] Herein, we wish to introduce our recent work for the generation of 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}-thiazines via copper catalyzed cascade bicyclization of o-alkenylphenyl isothiocyanate with sodium azide.

  Scheme 1

  

  Scheme 1.  Comparison between the previous and present work

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  2.   Results and discussion

  The starting material o-alkenylphenyl isothiocyanates were easily obtained from Heck coupling of 2-iodoanilines with acrylates, acrylonitrile or N, N-dimethyl acrylamide, ^[17] followed by the treatment with thiophosgene according to the literature procedure.^[18] In order to find an optimal reaction condition, methyl (E)-3-(2-isothiocyanatophenyl)-acrylate (1a) and sodium azide (2) were used as the model substrates (Table 1). Reaction of 1a (0.2 mmol) with sodium azide 2 (2.0 equiv.) using 10 mol% CuI and NaHCO₃ (2.0 equiv.) in 2.0 mL of MeCN at 80 ℃ for 20 h afforded the desired product 3a in 25% yield (Table 1, Entry 1). Other copper salts used as catalysts in the reaction were then screened (Table 1, Entries 2~7). CuCl was found to be more effective and provided the desired product 3a in 54% yield (Table 1, Entry 3). A screen of different protonic acid effect on the reaction indicated that CH₃CO₂H was the best choice, leading to the desired product 3a in 89% yield (Table 1, Entry 9). Lower yields were observed when other protonic acids such as HCl, TsOH, PivOH were used as the additive (Table 1, Entries 8~11). It is worth noting that only 15% yield of product was obtained in the absence of acid (Table 1, Entry 12). When different solvents such as N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), toluene, tetrahydrofuran (THF), dichloromethane (DCM) and 1, 2-dichloroethane (DCE) were examined, it was observed that no better result emerged (Table 1, Entries 13~18). The temperature also has great impact on the reaction, and the yield of 3a was decreased to 45% when the temperature was lowered to 60 ℃ (Table 1, Entry 19). Rising the reaction temperature to 100 ℃ also resulted in a diminished yield (Table 1, Entry 20).

  Table 1

  

  Table 1.  Initial studies for the tandem reaction of o-alkenyl- phenyl isothiocyanate (1a) with sodium azide 2^a

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  Entry	Catalyst	Additive	Solvent	Temp./℃	Yieldb/%
  1	CuI	NaHCO3	MeCN	80	25
  2	CuBr	NaHCO3	MeCN	80	28
  3	CuCl	NaHCO3	MeCN	80	54
  4	Cu(OAc)2	NaHCO3	MeCN	80	53
  5	CuO	NaHCO3	MeCN	80	52
  6	CuCl2	NaHCO3	MeCN	80	35
  7	Cu(OTf) 2	NaHCO3	MeCN	80	26
  8	CuCl	TsOH	MeCN	80	25
  9	CuCl	CH3CO2H	MeCN	80	89
  10	CuCl	PivOH	MeCN	80	45
  11	CuCl	HCl	MeCN	80	80
  12	CuCl	—	MeCN	80	23
  13	CuCl	CH3CO2H	DMF	80	NR
  14	CuCl	CH3CO2H	DMSO	80	NR
  15	CuCl	CH3CO2H	Toluene	80	35
  16	CuCl	CH3CO2H	THF	80	60
  17	CuCl	CH3CO2H	DCM	80	27
  18	CuCl	CH3CO2H	DCE	80	29
  19	CuCl	CH3CO2H	MeCN	60	45
  20	CuCl	CH3CO2H	MeCN	100	82
  aReaction was performed with 1a (0.2 mmol), 2 (0.4 mmol), catalyst (0.02 mmol), acid (0.4 mmol) in solvent (2 mL) for 20 h. b Isolated yield based on o- alkenylphenyl isothiocyanate (1a).

  With the optimized conditions in hand, the reaction scope of different o-alkenylphenyl isothiocyanates was in- vestigated. The results are summarized in Table 2. All reactions proceeded smoothly, leading to the desired 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazines in moderate to good yields. As shown in Table 2, substrates 1 bearing different ester groups including COOMe, COOEt, COOⁿBu, COOⁱBu, COO^tBu were all tolerated in the reaction to afford the corresponding products 3a~3e in good yields. Electronic properties and substitution position on the benzene ring of substrate 1 did not hamper the reaction process. With both of electron withdrawing groups, such as F, Cl, Br, CF₃, and electron-donating group CH₃ on the benzene ring, the reactions could afford the desired products in moderate yields 3f~3l. To further demonstrate the substrate scope, the reactions of (E)-3-(2-isothiocyanato- phenyl)acrylonitrile (1m) and (E)-3-(2-isothiocyanato- phenyl)-N, N-dimethylacrylamides (1p) with sodium azide 2 were next investigated. As shown in Table 2, the reactions proceeded well under the optimized conditions to afford the desired products 3m and 3p in 80% and 84% yield, respectively. Compared with the electron-donating group (CH₃) on the benzene ring (3o and 3r), the reaction of (E)-3-(2-isothiocyanatophenyl)acrylonitriles or (E)-3-(2-isothiocyanatophenyl)-N, N-dimethylacrylamides bearing electron-withdrawing group (Cl or F) could lead to the desired products (3n and 3q) in lower yields.

  Table 2

  

  Table 2.  Synthesis of 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazines via the copper(I)-promoted tandem reaction of o-alkenylphenyl isothiocyanates ^a

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  aReaction was performed with o-alkenylphenyl isothiocyanates 1 (0.2 mmol), sodium azide 2 (0.4 mmol), CH3COOH (0.4 mmol), CuCl (0.02 mmol) in MeCN (2 mL) under 80 ℃ for 20 h.

  The structure of 3a was corroborated by X-ray diffraction analysis of the crystal structure of 3a, and its ORTEP diagram is displayed in Figure 1.

  Figure 1

  

  Figure 1.  Single-crystal X-ray diffraction structure of 3a

  The thermal ellipsoids are at a 30% probability level, the CCDC number is 1959528

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  Interestingly, when DMF was used as the solvent and trimethylchlorosilane as the additive, the reaction between 1a and sodium azide 2 still could happen at room temperature and a sulfur-containing five-membered ring 4 (the structure of 4 was corroborated by X-ray diffraction analysis) can be obtained in 97% yield. Then intermediate 4 were treated with iodine in the presence of sodium hydrogencarbonate to give methyl 1-(1, 2, 3, 4-thiatriazol-5-yl)-1H-indole-2-carboxylate 5 in 45% yield (Scheme 2).

  Scheme 2

  

  Scheme 2.  Synthesis of indole from o-alkenylphenyl isothiocyanate

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  A possible mechanism for this transformation was proposed in Scheme 3. Firstly, a [3+2] cycloaddition of azido anion 2 to an isothiocyanate moiety in substrate 1 would occur to produce the intermediate A. Then, intermediate A could undergo isomerization to afford intermediate B. Next, copper-catalyzed intramolecular Michael addition occurred to give the intermediate C. Finally, the protonation of intermediate C would happen to give the target product 3.

  Scheme 3

  

  Scheme 3.  Proposed mechanistic pathway

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  3.   Conclusions

  In summary, a new method for the synthesis of various valuable 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]} thiazines from easily available o-alkenylphenyl isothiocyanates and sodium azide in one pot has been developed. Mechanistically, stepwise [3+2] cycloaddition-Michael addition-proto- nation sequence, is proposed. This present cascade bicyclization strategy provided an effective route for the generation of small molecular N-, and S-heterocycles.

  4.   Experimental section

  4.1   Instruments and reagents

  ¹H NMR and ¹³C NMR spectra were recorded in 400 MHz apparatus and the frequencies for ¹H NMR and ¹³C NMR test are 400 MHz and 100 MHz, respectively. The chemical shifts were reported with TMS as internal standard. Melting points were tested in an X-4A instrument without correcting temperature and the HRMS were obtained under ESI model in a mass spectrometer equipped with TOF analyzer. All other chemicals and solvents used in the experiments were obtained from commercial sources and used directly without further treatment. o-Alkenyl-phenyl isothiocyanates 1 were synthesized following literature processes.^[17-18]

  4.2   General procedure for the synthesis of product 3

  A mixture of o-alkenylphenyl isothiocyanate 1 (0.20 mmol), sodium azide 2 (0.4 mmol), CuCl (10 mol%) and CH₃COOH (0.40 mmol) was added into a tube. Subsequently MeCN (2 mL) was added. Then, the sealed tube was heated at 80 ℃ for 20 h. After completion of reaction as indicated by thin-layer chromatography (TLC), the mixture was concentrated and directly purified by flash column chromatography (EtOAc/petroleum ether, V:V＝1:2) to give the desired product 3.

  Methyl 2-(5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetate (3a): Yellow solid, 46.7 mg, 89% yield. m.p. 140.1~141.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.00 (d, J＝8.0 Hz, 1H), 7.47~7.52 (m, 1H), 7.40 (d, J＝4.0 Hz, 2H), 4.71 (d, J＝8.4 Hz, 1H), 3.60 (s, 3H), 2.61~2.75 (dq, J＝8.8, 16.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.5, 148.9, 130.9, 130.2, 129.4, 128.0, 124.9, 119.2, 52.3, 42.2, 39.8; HRMS calcd for C₁₁H₁₁N₄O₂S (M+H⁺) 263.0597, found 263.0600.

  Ethyl 2-(5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetate (3b): Yellow solid, 48.6 mg, 88% yield. m.p. 156.6~158.4 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.07 (d, J＝8.0 Hz, 1H), 7.56~7.60 (m, 1H), 7.49 (d, J＝3.6 Hz, 2H), 4.81 (dd, J＝6.6, 8.4 Hz, 1H), 4.15 (dq, J＝2.4, 7.2 Hz, 2H), 2.75 (dq, J＝8.8, 16.8 Hz, 2H), 1.22 (t, J＝6.8 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.0, 149.0, 130.9, 130.2, 129.4, 128.1, 125.0, 119.2, 61.5, 42.5, 42.2, 14.1; HRMS calcd for C₁₂H₁₃N₄O₂S (M+H⁺) 277.0754, found 277.0761.

  Butyl 2-(5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetate (3c): Yellow solid, 54.8 mg, 90% yield. m.p. 98.4~99.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.08 (d, J＝8.0 Hz, 1H), 7.56~7.60 (m, 1H), 7.49 (d, J＝4.0 Hz, 2H), 4.80 (dd, J＝6.8, 8.8 Hz, 1H), 4.09 (t, J＝6.8 Hz, 2H), 2.75 (dq, J＝8.8, 16.8 Hz, 2H), 1.53~1.60 (m, 2H), 1.26-1.36 (m, 2H), 0.90 (t, J＝7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.1, 149.0, 130.9, 130.2, 129.4, 128.1, 125.0, 119.2, 65.3, 42.5, 39.8, 30.4, 19.0, 13.6; HRMS calcd for C₁₄H₁₇N₄O₂S (M+H⁺) 305.1067, found 305.1068.

  Isobutyl 2-(5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetate (3d): Yellow solid, 55.4 mg, 91% yield. m.p. 102.6~103.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.99 (d, J＝8.0 Hz, 1H), 7.45~7.51 (m, 1H), 7.39 (d, J＝4.0 Hz, 2H), 4.72 (dd, J＝6.8, 8.8 Hz, 1H), 3.79 (dq, J＝2.4, 6.8 Hz, 2H), 2.69 (dq, J＝8.8, 16.4 Hz, 2H), 1.74~1.84 (m, 1H), 0.79 (dd, J＝0.8, 6.8 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.1, 148.9, 130.9, 130.2, 129.4, 128.1, 125.0, 119.2, 71.5, 42.4, 39.8, 27.5, 19.0; HRMS calcd for C₁₄H₁₇N₄O₂S (M+H⁺) 305.1067, found 305.1061.

  tert-Butyl 2-(5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetate (3e): Yellow solid, 55.4 mg, 91% yield. m.p. 110.2~111.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.07 (d, J＝8.0 Hz, 1H), 7.54~7.60 (m, 1H), 7.48 (d, J＝4.0 Hz, 2H), 4.75 (dd, J＝6.8, 8.8 Hz, 1H), 2.66 (dq, J＝8.4, 16.4 Hz, 2H), 1.42 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ: 168.2, 149.1, 130.9, 130.1, 129.3, 128.1, 125.1, 119.1, 82.4, 43.5, 40.1, 28.0; HRMS calcd for C₁₄H₁₇N₄O₂S (M+H⁺) 305.1067, found 305.1077.

  Methyl 2-(7-fluoro-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}-thiazin-5-yl)acetate (3f): Yellow solid, 33.6 mg, 60% yield. m.p. 100.6~100.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.04 (d, J＝8.4 Hz, 1H), 7.56 (dd, J＝2.0, 8.8 Hz, 1H), 7.48 (d, J＝2.0 Hz, 1H), 4.73 (dd, J＝6.8, 8.4 Hz, 1H), 3.71 (s, 3H), 2.76 (dq, J＝8.4, 16.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.3, 162.1 (d, ¹J_CF＝250 Hz), 148.5, 139.1, 127.2, 121.4 (d, ³J_CF＝9 Hz), 117.4 (d, ²J_CF＝23 Hz), 115.3, (d, ²J_CF＝24 Hz), 52.4, 41.9, 39.4; HRMS calcd for C₁₁H₁₀FN₄O₂S (M+H⁺) 281.0503, found 281.0517.

  Methyl 2-(7-chloro-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}- thiazin-5-yl)acetate (3g): Yellow solid, 45.1 mg, 76% yield. m.p. 111.3~112.5 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.03 (q, J＝8.4 Hz, 1H), 7.55 (dd, J＝2.0, 8.6 Hz, 1H), 7.49 (d, J＝2.0 Hz, 1H), 4.72~4.76 (m, 1H), 3.71 (s, 3H), 2.76 (dq, J＝8.0, 16.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.3, 148.7, 135.1, 130.4, 129.5, 128.1, 126.4, 120.6, 52.4, 42.0, 39.4; HRMS calcd for C₁₁H₁₀ClN₄O₂S (M+H⁺) 297.0208, found 297.0207.

  Methyl 2-(8-chloro-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}- thiazin-5-yl)acetate (3h): Yellow solid, 44.5 mg, 75% yield. m.p. 89.4~89.6 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.09 (s, 1H), 7.46 (d, J＝8.0 Hz, 2H), 4.80 (dd, J＝6.4, 8.0 Hz, 1H), 3.69 (s, 3H), 2.76 (dq, J＝8.4, 16.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.3, 149.1, 136.2, 131.6, 129.5, 129.4, 123.2, 119.5, 52.4, 42.2, 39.4; HRMS calcd for C₁₁H₁₀ClN₄O₂S (M+H⁺) 297.0208, found 297.0202.

  Methyl 2-(7-bromo-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}- thiazin-5-yl)acetate (3i): Yellow solid, 51.9 mg, 76% yield. m.p. 121.0~121.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.97 (d, J＝8.4 Hz, 1H), 7.71 (d, J＝8.8 Hz, 1H), 7.65 (s, 1H), 4.74 (t, J＝8.0 Hz, 1H), 3.71 (s, 3H), 2.76 (dq, J＝8.8, 16.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.3, 148.8, 133.4, 131.0, 129.9, 126.6, 122.9, 120.8, 52.5, 42.1, 39.3; HRMS calcd for C₁₁H₁₀BrN₄O₂S (M+H⁺) 340.9702, found 340.9691.

  Methyl2-(7-(trifluoromethyl)-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetate (3j): Yellow solid, 33.0 mg, 50% yield. m.p. 148.6~149.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.24 (d, J＝8.4 Hz, 1H), 7.86 (d, J＝7.6 Hz, 1H), 7.78 (s, 1H), 4.86 (t, J＝8.0 Hz, 1H), 3.70 (s, 3H), 2.80 (dq, J＝8.4, 16.8 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.1, 149.4, 133.3, 131.5 (q, J_CF3＝33 Hz), 127.4 (d, ⁴J_CF＝4 Hz), 125.6 (d, ⁴J_CF＝3 Hz), 125.5, 124.4, 119.9, 52.5, 42.1, 39.5; HRMS calcd for C₁₂H₁₀F₃N₄O₂S (M+H⁺) 331.0471, found 331.0480.

  Methyl 2-(7-methyl-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}- thiazin-5-yl)acetate (3k): Yellow solid, 42.0 mg, 76% yield. m.p. 105.6~105.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.95 (d, J＝8.4 Hz, 1H), 7.36 (d, J＝8.0 Hz, 1H), 7.26 (s, 1H), 4.72 (dd, J＝6.4, 8.8 Hz, 1H), 3.69 (s, 3H), 2.73 (dq, J＝8.8, 16.8 Hz, 2H), 2.44 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.6, 148.5, 139.9, 130.8, 128.7, 128.3, 124.8, 119.1, 52.3, 42.3, 39.9, 21.2; HRMS calcd for C₁₂H₁₃N₄O₂S (M+H⁺) 277.0754, found 277.0751.

  Methyl 2-(8-methyl-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}- thiazin-5-yl)acetate (3l): Yellow solid, 40.9 mg, 74% yield. m.p. 193.6~193.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.88 (s, 1H), 7.36 (d, J＝8.0 Hz, 1H), 7.28 (d, J＝8.0 Hz, 1H), 4.76 (t, J＝8.0 Hz, 1H), 3.68 (s, 3H), 2.68~32.82 (m, 2H), 2.48 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 169.5, 148.9, 140.8, 130.7, 130.1, 127.8, 122.0, 119.5, 52.2, 42.4, 39.6, 21.3; HRMS calcd for C₁₂H₁₃N₄O₂S (M+H⁺) 277.0754, found 277.0757.

  2-(5H-Benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)aceto- nitrile (3m): Yellow solid, 36.7 mg, 80% yield. m.p. 178.6~179.8 ℃; ¹H NMR(400 MHz, CDCl₃) δ: 8.00 (d, J＝8.0 Hz, 1H), 7.62 (t, J＝7.6 Hz, 2H), 7.54 (t, J＝7.6 Hz, 1H), 5.17 (t, J＝6.4 Hz, 1H), 3.16 (dq, J＝7.2, 17.2 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.5, 131.1, 130.8, 129.9, 129.6, 124.3, 119.0, 117.7, 39.1, 27.1; HRMS calcd for C₁₀H₈N₅S (M+H⁺) 230.0495, found 230.0505.

  2-(7-Chloro-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetonitrile (3n): Yellow solid, 27.4 mg, 52% yield. m.p. 188.6~189.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.10 (d, J＝8.8 Hz, 1H), 7.64 (dd, J＝2.0, 8.8 Hz, 1H), 7.54 (d, J＝2.0 Hz, 1H), 4.56 (t, J＝7.6 Hz, 1H), 2.85 (dq, J＝6.4, 17.2 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.9, 135.6, 131.4, 128.4, 124.2 121.0, 119.5, 114.9, 39.5, 27.3; HRMS calcd for C₁₀H₇ClN₅S (M+H⁺) 264.0105, found 264.0114.

  2-(7-Methyl-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetonitrile (3o): Yellow solid, 40.9 mg, 84% yield. m.p. 168.0~168.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.95 (d, J＝8.8 Hz, 1H), 7.50 (d, J＝2.8 Hz, 2H), 5.17 (d, J＝6.8 Hz, 1H), 3.21 (dq, J＝6.8, 17.2 Hz, 2H), 2.41 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 148.1, 139.8, 131.6, 129.7, 128.6, 124.2, 118.9, 117.7, 27.2, 21.2; HRMS calcd for C₁₁H₁₀N₅S (M+H⁺) 244.0651, found 244.0656.

  2-(5H-Benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)-N, N- dimethylacetamide (3p): Yellow solid, 46.3 mg, 84% yield. m.p. 140.6~140.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.98 (d, J＝8.0 Hz, 1H), 7.38~7.50 (m, 3H), 4.91 (q, J＝4.8 Hz, 1H), 2.87 (s, 3H), 2.80 (dd, J＝9.2, 16.4 Hz, 1H), 2.73 (s, 3H), 2.59 (dd, J＝4.8, 16.4 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 167.9, 149.6, 131.0, 129.9, 129.4, 128.4, 125.8, 119.0, 41.0, 40.2, 36.9, 35.6; HRMS calcd for C₁₂H₁₄N₅OS (M+H⁺) 276.0914, found 276.0918.

  2-(7-Fluoro-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)-N, N-dimethylacetamide (3q): Yellow solid, 38.7 mg, 66% yield. m.p. 182.3~182.5 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.98 (q, J＝4.8 Hz, 1H), 7.17~7.24 (m, 2H), 4.90 (dd, J＝5.6, 8.4 Hz, 1H), 2.88 (s, 3H), 2.81 (dd, J＝8.8, 16.8 Hz, 1H), 2.76 (s, 3H), 2.62 (dd, J＝5.2, 16.4 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 167.7, 162.1 (d, ¹J_CF＝250 Hz), 149.2, 128.2 (d, ³J_CF＝8 Hz), 127.4 (d, ⁴J_CF＝3 Hz), 121.1 (d, ³J_CF＝9 Hz), 117.1 (d, ²J_CF＝23 Hz), 115.6 (d, ²J_CF＝24 Hz), 41.2, 39.8, 36.9, 35.7; HRMS calcd for C₁₂H₁₃FN₅OS (M+H⁺) 294.0819, found 294.0814.

  N, N-Dimethyl-2-(7-methyl-5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazin-5-yl)acetamide (3r): Yellow solid, 50.9 mg, 88% yield. m.p. 174.7~174.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.87 (q, J＝8.4 Hz, 1H), 7.45 (s, 1H), 7.40 (d, J＝8.4 Hz, 1H), 5.00 (dd, J＝4.4, 10.0 Hz, 1H), 3.03 (dd, J=10.0, 16.8 Hz, 1H), 2.81 (s, 3H), 2.77 (s, 3H), 2.75 (dd, J＝4.8, 16.8 Hz, 1H), 2.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 168.6, 149.4, 139.6, 130.7, 129.6, 128.8, 126.0, 118.5, 41.2, 36.7, 35.4, 21.2; HRMS calcd for C₁₃H₁₆N₅OS (M+H⁺) 290.1070, found 290.1084.

  4.3   General procedure for the synthesis of product 4

  A mixture of o-alkenylphenyl isothiocyanate 1 (0.20 mmol), sodium azide 2 (0.4 mmol), CuCl (10 mol%), (CH₃)₃SiCl (3.0 equiv.) was added into a tube. Subsequently DMF (2 mL) was added. Then, the reaction was carried out at room temperture for 16 h. After completion of reaction as indicated by TLC, the mixture was concentrated and directly purified by flash column chromatography (EtOAc/petroleum ether, V:V＝1:2) to give the desired product methyl (E)-3-(2-((1, 2, 3, 4-thiatriazol-5-yl)- amino)phenyl)acrylate (4): Yellow solid, 50.9 mg, 97% yield. m.p. 155.0~157.0 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 10.47 (s, 1H), 7.90 (d, J＝15.6 Hz, 1H), 7.43~7.61 (m, 3H), 7.28 (s, 1H), 6.81 (s, 1H), 3.87 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 178.5, 167.0, 139.4, 138.7, 131.8, 128.9, 128.1, 127.6, 122.4, 121.3, 51.9; HRMS calcd for C₁₃H₁₆N₅OS (M+H⁺) 263.0597, found 263.0610.

  4.4   General procedure for the synthesis of product 5

  A mixture of product 4 (0.20 mmol), I₂ (3.0 equiv.) and NaHCO₃ (2.0 equiv.) was added into a tube. Subsequently DMF (2 mL) was added. Then, the reaction was cooled at 0 ℃ for 6 h. After completion of reaction as indicated by TLC, the mixture was concentrated and directly purified by flash column chromatography (EtOAc/petroleum ether, V:V＝1:2) to give the desired product methyl 1-(1, 2, 3, 4-thiatriazol-5-yl)-1H-indole-2-carboxylate (5): Yellow solid, 23.4 mg, 45% yield. m.p. 177.6~178.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.37 (d, J＝8.0 Hz, 1H), 7.91 (d, J＝8.0 Hz, 1H), 7.75 (t, J＝7.6 Hz, 1H), 7.55 (t, J＝7.6 Hz, 1H), 6.81 (s, 1H), 3.87 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 166.1, 148.1, 141.1, 133.4, 130.9, 129.3, 126.1, 119.5, 118.9, 113.0, 52.2; HRMS calcd for C₁₃H₁₆N₅OS (M+H⁺) 261.0441, found 261.0442.

  Supporting Information ¹H NMR and ¹³C NMR spectra for compounds 3a~3r, 4, 5. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

  
  
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• 

  Scheme 1  Comparison between the previous and present work

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  Figure 1  Single-crystal X-ray diffraction structure of 3a

  The thermal ellipsoids are at a 30% probability level, the CCDC number is 1959528

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  Scheme 2  Synthesis of indole from o-alkenylphenyl isothiocyanate

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  Scheme 3  Proposed mechanistic pathway

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  Table 1.  Initial studies for the tandem reaction of o-alkenyl- phenyl isothiocyanate (1a) with sodium azide 2^a

  Entry	Catalyst	Additive	Solvent	Temp./℃	Yieldb/%
  1	CuI	NaHCO3	MeCN	80	25
  2	CuBr	NaHCO3	MeCN	80	28
  3	CuCl	NaHCO3	MeCN	80	54
  4	Cu(OAc)2	NaHCO3	MeCN	80	53
  5	CuO	NaHCO3	MeCN	80	52
  6	CuCl2	NaHCO3	MeCN	80	35
  7	Cu(OTf) 2	NaHCO3	MeCN	80	26
  8	CuCl	TsOH	MeCN	80	25
  9	CuCl	CH3CO2H	MeCN	80	89
  10	CuCl	PivOH	MeCN	80	45
  11	CuCl	HCl	MeCN	80	80
  12	CuCl	—	MeCN	80	23
  13	CuCl	CH3CO2H	DMF	80	NR
  14	CuCl	CH3CO2H	DMSO	80	NR
  15	CuCl	CH3CO2H	Toluene	80	35
  16	CuCl	CH3CO2H	THF	80	60
  17	CuCl	CH3CO2H	DCM	80	27
  18	CuCl	CH3CO2H	DCE	80	29
  19	CuCl	CH3CO2H	MeCN	60	45
  20	CuCl	CH3CO2H	MeCN	100	82
  aReaction was performed with 1a (0.2 mmol), 2 (0.4 mmol), catalyst (0.02 mmol), acid (0.4 mmol) in solvent (2 mL) for 20 h. b Isolated yield based on o- alkenylphenyl isothiocyanate (1a).

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  Table 2.  Synthesis of 5H-benzo[d]tetrazolo[5, 1-b]^{[1, 3]}thiazines via the copper(I)-promoted tandem reaction of o-alkenylphenyl isothiocyanates ^a

  aReaction was performed with o-alkenylphenyl isothiocyanates 1 (0.2 mmol), sodium azide 2 (0.4 mmol), CH3COOH (0.4 mmol), CuCl (0.02 mmol) in MeCN (2 mL) under 80 ℃ for 20 h.

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•