English

• 1. INTRODUCTION

  Over the last few decades, nucleoside analogues create a significant class of antimetabolic agents for the treatment of malignant tumors^[1], numerous viral diseases^[2] and bacterial infections^[3] with the corresponding mechanism of stopping cancer cell proliferation, inhibiting viral replication and preventing cell-wall biosynthesis. Sulfur is prevalent in biologically active natural products and commercial drugs for variable medical conditions including depression, arthritis, diabetes, cancer, and acquired immune deficiency syndrome (AIDS) because of its critical role in primary metabolism^[4], which also shows unique chemical reactivity in organic chemistry as the building blocks for the preparation of sulfur-containing heterocyclic rings or aliphatic chain, such as the β-lactam antibiotics penicillins^[5], sulfonamide antimicrobial agents^[6], cytotoxic thioamycolamides A–E^[7], and anti-infective glycolipid KRN7000^[8]. Thioglycosides are also common glycomimetics (e.g., S-linked galactosylceramides^[9]) and enhance hydrolytic and enzymatic degradation relative to the natural O-linked congeners while retaining similar conformational preferences^{[10, 11]}.

  Thioglycosides have attracted considerable interest owing to the synthetic challenges and their potential bioactivity, and extensive researches including cross-coupling reaction^{[12, 13]} and Ferrier-type reaction^{[14, 15]} were reported to the construction of these molecules in recent years. Inspired by such studies, we designed and synthesized the pivalyl protected β-thiogalactoside under the catalysis of palladium in this work (Scheme 1).

  Scheme 1

  

  Scheme 1.  Synthetic routes for the β-thiogalactoside

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  2. EXPERIMENTAL

  2.1 Reagents and physical measurements

  All chemical reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and used without further purification. Solvents were dried and purified using standard techniques. D-Galactal (1) was purchased from J & K Chemical Co., Ltd., China. Reactions were monitored by thin layer chromatography (TLC) on silica gel GF₂₅₄ pre-coated plates. 1D and 2D NMR spectra were recorded at a Bruker ultrashied^TM 400 MHz plus spectrometer. Chemical shifts (δ) were reported in ppm, using residual solvent as an internal standard. Melting points were tested with an uncorrected X-4 digital melting point apparatus. HR-ESI-MS was obtained using a Waters Q-TOF premier^TM mass spectrometer. Characterization data for known compounds were checked in comparison with literature for consistency and not presented in this report.

  2.2 Synthesis of the β-thiogalactoside

  D-Galactal (1, 1.46 g, 10 mmol) and imidazole (0.82 g, 12 mmol) were dissolved in anhydrous N, N΄-dimethylformamide (DMF, 40 mL), and tert-butyldiphenylchlorosilane (TBDPSCl, 2.90 g, 10.5 mmol) was slowly added to the mixture over 20 min. The mixture was stirred at room temperature for 6 hours until the completion of the reaction monitored by TLC. The reaction was quenched by 5% aqueous NaHCO₃ and the crude product was extracted by ethyl acetate (30 mL × 3). After removal of the solvent, pure (2R, 3R, 4R)-2-(((tert-butyldiphenylsilyl)oxy)methyl)-3, 4-dihydro-2H-pyran-3, 4-diol (2) was obtained by silica gel flash chromatography (eluent: petroleum ether/ethyl acetate = 1/1, v/v) as colorless syrup^[16] (3.41 g, yield 89%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.83~7.62 (m, 4H), 7.55~7.34 (m, 6H), 6.41 (dd, J = 6.2, 1.5 Hz, 1H), 4.75 (dt, J = 6.2, 1.9 Hz, 1H), 4.46~4.29 (m, 1H), 4.17 (dd, J = 6.3, 2.0 Hz, 1H), 4.01 (dd, J = 11.9, 6.9 Hz, 1H), 3.96~3.88 (m, 2H), 2.99 (d, J = 5.3 Hz, 1H), 2.60 (d, J = 10.2 Hz, 1H), 1.09 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 144.4, 135.6, 135.5, 132.6, 132.4, 130.0, 127.8, 127.8, 103.4, 65.8, 64.4, 63.8, 26.7, 19.1.

  The mixture of intermediate (2, 2.68 g, 7 mmol), N, N΄-carbonyldiimidazole (1.17 g, 7.2 mmol) and catalytic amount of imidazole (10 mg) was dissolved in anhydrous tetrahydrofuran (THF, 30 mL) under N₂ atmosphere. The solution was stirred at room temperature for 8 hours until completion of the acylating process by TLC monitoring. The mixture was quenched by ice water (50 mL), and crude product was extracted by ethyl acetate (30 mL × 3). After separating the organic layer and removal of the solvent, the oily crude was purified by silica gel flash chromatography (eluent: petroleum ether/ethyl acetate = 10/1, v/v) to give 1, 5-anhydro-6-O-(tert-butyldiphenylsilyl)-3, 4-O-carbonate-2-deoxy-D-lyxo-hex-1-enopyranose (3) as colorless syrup^[17] (2.61 g, yield 91%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.76~7.68 (m, 4H), 7.54~7.43 (m, 6H), 6.68 (d, J = 6.3 Hz, 1H), 5.25 (dd, J = 7.7, 3.1 Hz, 1H), 5.10 (d, J = 7.7 Hz, 1H), 4.99 (ddd, J = 6.2, 3.1, 1.1 Hz, 1H), 4.06 (d, J = 7.8 Hz, 1H), 4.03~3.99 (m, 2H), 1.13 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 154.2, 149.1, 135.5, 135.5, 134.8, 132.7, 132.5, 130.1, 130.0, 127.9, 127.9, 127.7, 98.0, 73.8, 72.9, 68.9, 61.7, 26.8, 26.6, 19.2.

  The mixture of D-galactal carbonate (3, 410 mg, 1.0 mmol), thiol (1.0 mmol), tris(dibenzylideneacetone)dipalladium (Pd₂(dba)₃, 9.1 mg, 0.01 mmol) and 9, 9-dimethyl-4, 5-bis(diphenylphosphino)xanthene (xantphos, 11.6 mg, 0.02 mmol) was dissolved in anhydrous dichloromethane (10 mL) under nitrogen atmosphere, which was stirred at room temperature for 12 hours until the completion of reaction (monitored by TLC). The mixture was quenched by ice water (5 mL), and crude product was extracted by dichloromethane (8 mL × 3). After removal of the solvent, the crude product was purified by silica gel flash chromatography (eluent: petroleum ether/ethyl acetate = 5/1, v/v) to give the target product (4).

  N-(4-(((2S, 5R, 6R)-6-(((tert-Butyldiphenylsilyl)oxy)methyl)-5-hydroxy-5, 6-dihydro-2H-pyran-2-yl)thio)phenyl)acetamide (4a). Colorless oil (458 mg, yield 86%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.76~7.63 (m, 4H), 7.49~7.34 (m, 10H), 6.04 (ddd, J = 10.0, 5.8, 2.0 Hz, 1H), 5.90 (dd, J = 10.0, 1.6 Hz, 1H), 5.41 (d, J = 1.9 Hz, 1H), 3.93~3.79 (m, 3H), 3.72 (ddd, J = 12.7, 6.3, 1.7 Hz, 1H), 2.15 (s, 3H), 1.05 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 138.3, 135.6, 135.1, 133.4, 133.3, 131.0, 129.7, 129.6, 127.7, 127.7, 126.4, 119.6, 81.8, 78.6, 63.3, 61.5, 26.8, 24.6, 19.2. HRMS (ESI) m/z: calcd. for C₃₀H₃₅NO₄SSiNa⁺ (M + Na)⁺ 556.1953; found 556.1950.

  (2R, 3R, 6S)-2-(((tert-Butyldiphenylsilyl)oxy)methyl)-6-((2-hydroxyethyl)thio)-3, 6-dihydro-2H-pyran-3-ol (4b). Colorless oil (359 mg, yield 81%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.79~7.71 (m, 4H), 7.49~7.42 (m, 6H), 6.26 (ddd, J = 10.0, 5.9, 2.2 Hz, 1H), 5.83 (dd, J = 10.0, 1.6 Hz, 1H), 5.35 (d, J = 1.9 Hz, 1H), 4.04~3.94 (m, 2H), 3.91~3.78 (m, 3H), 3.61 (ddd, J = 14.5, 9.7, 2.8 Hz, 1H), 3.53 (s, 1H), 3.08 (d, J = 8.4 Hz, 1H), 2.94 (ddd, J = 15.0, 9.7, 3.8 Hz, 1H), 2.75 (ddd, J = 15.0, 4.2, 2.9 Hz, 1H), 1.10 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 135.5, 133.3, 133.1, 130.7, 129.9, 129.7, 129.6, 127.6, 127.6, 79.4, 78.7, 63.1, 62.4, 61.4, 32.9, 26.8, 19.2. HRMS (ESI) m/z: calcd. for C₂₄H₃₂O₄SSiNa⁺ (M + Na)⁺ 467.1688; found 467.1684.

  Compound 3 (2.05 g, 5 mmol) was dissolved in anhydrous THF (20 mL) under N₂ atmosphere, and tetra-n-butylammonium fluoride trihydrate (TBAF, 1.6 g, 5.1 mmol) in THF (20 mL) was added with a syringe to the previous solution in an ice-water bath. The reaction mixture was stirred at 0 ^oC for 1.5 hours until the complete consumption of starting material (monitored by TLC). The reaction was quenched with water (20 mL) and the product was extracted by ethyl acetate (30 mL × 3). The organic phase was concentrated under reduced pressure and pure D-glalactal-3, 4-O-carbonate (5) was obtained by flash column chromatography (eluent: petroleum ether/ethyl acetate = 5/1, v/v) in silica gel as as colorless syrup^[16] (0.76 g, yield 88%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 6.74 (d, J = 6.3 Hz, 1H), 5.24 (dd, J = 7.7, 3.2 Hz, 1H), 5.01 (ddd, J = 6.3, 3.2, 1.2 Hz, 1H), 4.98~4.91 (m, 1H), 4.10~3.99 (m, 2H), 3.93 (d, J = 9.6 Hz, 1H), 2.24 (s, 1H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 154.0, 149.1, 98.1, 73.9, 73.1, 69.0, 61.5.

  D-glalactal-3, 4-O-carbonate (5, 0.69 g, 4 mmol) and triethylamine (Et₃N, 1.66 mL, 12 mmol) were dissolved in anhydrous CH₂Cl₂ (30 mL) under N₂ atmosphere, and pivaloyl chloride (Piv-Cl, 0.5 mL, 4.1 mmol) was added at 0 ℃. The mixture was stirred for 2 hours and monitored by TLC until the completion of the reaction. The reaction was quenched by 5% aqueous NaHCO₃ and the crude product was extracted by ethyl acetate (20 mL × 3). After removal of the solvent, the crude product obtained under reduced pressure was purified by silica gel flash chromatography (eluent: petroleum ether/ethyl acetate = 10/1, v/v) to give the ((3aR, 4R, 7aR)-2-oxo-4, 7a-dihydro-3aH-[1, 3]dioxolo[4, 5-c] pyran-4-yl)methyl pivalate (6) as yellow syrup (706 mg, yield 87%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 6.71 (d, J = 6.3 Hz, 1H), 5.20 (dd, J = 7.7, 3.2 Hz, 1H), 5.00 (ddd, J = 6.3, 3.2, 1.1 Hz, 1H), 4.87 (d, J = 7.7 Hz, 1H), 4.44 (dd, J = 11.6, 6.9 Hz, 1H), 4.34 (dd, J = 11.6, 5.8 Hz, 1H), 4.17 (dd, J = 11.3, 5.7 Hz, 1H), 1.23 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 177.9, 153.7, 149.1, 98.0, 73.0, 71.5, 68.7, 62.1, 38.7, 27.0.

  The mixture of D-galactal carbonate (6, 256 mg, 1.0 mmol), thiol (1.0 mmol), Pd₂(dba)₃ (9.1 mg, 0.01 mmol) and 9, 9-dimethyl-4, 5-bis(diphenylphosphino)xanthene (xantphos, 11.6 mg, 0.02 mmol) was dissolved in anhydrous dichloromethane (4 mL) under nitrogen atmosphere, and the reaction mixture was stirred at room temperature for 12 hours until the completion of reaction (TLC monitoring). The mixture was quenched by ice water (5 mL), obtaining the crude product extracted by dichloromethane (5 mL × 3). After removing the solvent, the crude product was purified by silica gel flash chromatography (eluent: petroleum ether/ethyl acetate = 5/1, v/v) to give the target product (7).

  ((2R, 3R, 6S)-3-Hydroxy-6-(naphthalen-2-ylthio)-3, 6-dihydro-2H-pyran-2-yl)methyl pivalate (7a). White solids (316 mg, yield 85%, m.p. 107~108 ℃). ¹H NMR (400 MHz, CDCl₃) δ 8.04 (d, J = 1.1 Hz, 1H), 7.83~7.77 (m, 3H), 7.64 (dd, J = 8.5, 1.8 Hz, 1H), 7.51~7.48 (m, 2H), 6.06 (ddd, J = 10.0, 5.5, 1.9 Hz, 1H), 6.00 (dd, J = 10.0, 1.4 Hz, 1H), 5.60 (d, J = 1.8 Hz, 1H), 4.37~4.28 (m, 2H), 3.84 (td, J = 6.3, 1.8 Hz, 1H), 3.81~3.76 (m, 1H), 1.18 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 178.3, 133.3, 132.7, 132.6, 131.0, 130.3, 129.5, 129.4, 128.3, 127.7, 127.6, 126.6, 82.2, 76.2, 63.4, 61.5, 38.7, 27.1. HRMS (ESI) m/z: calcd. for C₂₁H₂₄O₄SNa⁺ (M + Na)⁺ 395.1293; found 395.1291.

  ((2R, 3R, 6S)-6-(Benzylthio)-3-hydroxy-3, 6-dihydro-2H-pyran-2-yl)methyl pivalate (7b). Colorless oil (272 mg, yield 81%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.34~7.25 (m, 5H), 6.14 (ddd, J = 10.0, 5.7, 2.2 Hz, 1H), 5.86 (dd, J = 10.1, 1.4 Hz, 1H), 5.15 (dd, J = 3.7, 1.8 Hz, 1H), 4.37 (dd, J = 11.6, 5.7 Hz, 1H), 4.27 (dd, J = 11.6, 6.8 Hz, 1H), 4.00 (d, J = 13.3 Hz, 1H), 3.86~3.74 (m, 3H), 1.90 (d, J = 10.2 Hz, 1H), 1.24 (s, 9H).¹³C NMR (100 MHz, CDCl₃) δ (ppm): 178.5, 137.5, 130.8, 129.2, 129.1, 128.5, 127.2, 78.2, 76.1, 63.5, 61.9, 38.8, 33.9, 27.1. HRMS (ESI) m/z: calcd. for C₁₈H₂₄O₄SNa⁺ (M + Na)⁺ 359.1288; found 359.1287.

  2.3 Single-crystal structure determination

  Compound 7a was crystallized after slow evaporation from a saturated chloroform solution as colorless blocks with dimensions of 0.12mm × 0.10mm × 0.08mm in the monoclinic system. Diffraction data of the single crystal were collected at 120 K on a Bruker SMART APEX-II CCD diffractometer with graphite-monochromated CuKα radiation (λ = 1.54184 Å) using the Bruker Collect software. A total of 6645 reflections were collected in the range of 4.95≤θ≤73.79º by using an ω-scan mode, of which 3621 were unique with R_int = 0.0390 and 3326 were observed with I > 2σ(I). After the initial corrections and data reduction, intensities of reflections were used to solve (by direct methods) and refine the structures (on F²) using the WINGX program. A weighting scheme based upon P = (F_o² + 2F_c²)/3 was employed. All the hydrogen atoms were located from difference Fourier maps and included in the refinements as riding. Empirical absorption corrections were applied. The structures were solved by direct methods using SHELXS-97 programs^[18]. All of the non-hydrogen atoms were located from difference Fourier maps, and then refined anisotropically with SHELXL-97 via a full-matrix least-square procedure^[19]. The final R = 0.0478, wR = 0.1384, (Δ/σ)_max = 0.000, S = 1.040, (Δρ)_max = 0.345 and (Δρ)_min = –0.214 e/ų. The Flack parameter was 0.019(15) (Fig. 1).

  Figure 1

  

  Figure 1.  ORTEP drawing of compound 6a showing thermal ellipsoids at the 50% probability level

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  2.4 Cytotoxity activity evaluation

  Human gastric cancer HGC-27 cell lines and human gastric mucosa epithelial GES-1 cell lines were purchased from the Cell Bank of Type Culture Collection of Chinese Academy of Sciences, Shanghai Institute of Cell Biology, Chinese Academy of Sciences. Cell viability was assessed by 3-(4, 5)-dimethylthiazol-2-yl-2, 5-diphenyltetrazolium bromide (MTT) cell staining as previously described^[20]. The cell lines (1.0 × 10⁴ cells/well) were cultured in Dulbecco's modified eagle medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 100 units/mL penicillin at humidified 5% CO₂ atmosphere in a 96-well plate. The cell lines were exposed to the drugs with different concentrations (100.0, 50.0 and 10.0 μg/mL) for 48 hours, and paclitaxel (10.0 μg/mL) was as the positive control. MTT (50 μL of a 5 mg/mL in PBS; Sigma-Aldrich) was added to each well and the cell lines were incubated in a CO₂ incubator at 37 ℃ for 5 hours. Following media removal, the MTT-formazan formed by metabolically viable cells was dissolved in 200 μL of DMSO (Sigma-Aldrich) and the absorbance was measured in a plate reader at 492 nm. The survival (%) was calculated by the following formula: No. of viable cells (dye excludeed cells)/No. of the total.

  3. RESULTS AND DISCUSSION

  3.1 Synthesis

  Since the pivalate protected molecules could enhance the cytotoxicity as prodrugs^[21], the key intermediate D-galactal carbonate (6) was prepared from commercial D-galactal (1) through a four-step reaction with a total yield of 62% (Scheme 1). Conditions optimization for the S-glycosylation reaction was conducted by the palladium catalyst in the presence of phosphorus ligands (P-ligands) including 1, 4-bis(diphenylphosphino)butane (DPPB), 1, 1΄-bis(diphenylphosphino)ferrocene (DPPF), tricyclohexylphosphine (P(Cy)₃) and 4, 5-bis(diphnylphosphino)-9, 9-dimethylxanthene (xantphos) in various solvents (Table 1). The palladium(II) salt and DPPB ligand failed to catalyze the S-glycosylation reaction in THF (entries 1 and 2), and subsequently coor-dinated palladium catalysts seemed to promote the reaction, although it is in a poor yield (entry 3~5). Changing the polar solvent (DMF, DMSO) or non-polar toluene also resulted in failure (entry 6~8). To our surprise, less-polar solvent including dichloromethane and carbon tetrachloride showed some positive results (entry 9~11), and the optimal condition for the S-glycosylation reaction was finally confirmed as the 2.5 mol% Pd₂(dba)₃ catalyst and the 5 mol% xantphos in dichloromethane at 35 ℃ for 12 hours after the ligands and temperature screening (entry 12~15). Due to the potential steric effect and better cytotoxities for some aromatic substituents (phenyl, benzyl or naphthyl) of sugars^{[22, 23]}, three aromatic groups (4a, 7a and 7b) and one aliphatic group (4b) substituted thioglycosides were smoothly prepared under these conditions.

  Table 1

  

  Table 1.  Condition Optimization for the S-Glycosylation Reaction

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  Entrya	Catalyst	Ligand	Solvent	Yieldb
  1	PdCl2	DPPB	THF	0
  2	Pd(OAc)2	DPPB	THF	0
  3	Pd(PPh3)4	DPPB	THF	3
  4	Pd(acac)2	DPPB	THF	9
  5	Pd2(dba)3	DPPB	THF	15
  6	Pd2(dba)3	DPPB	DMF	0
  7	Pd2(dba)3	DPPB	DMSO	0
  8	Pd2(dba)3	DPPB	Toluene	0
  9	Pd2(dba)3	DPPB	CH3CN	0
  10	Pd2(dba)3	DPPB	CH2Cl2	30
  11	Pd2(dba)3	DPPB	CCl4	25
  12	Pd2(dba)3	DPPF	CH2Cl2	0
  13	Pd2(dba)3	PCy3	CH2Cl2	0
  14	Pd2(dba)3	Xantphos	CH2Cl2	68
  15c	Pd2(dba)3	Xantphos	CH2Cl2	85
  a Unless otherwise specified, all reactions were carried out with 0.2 mmol of intermediate 5, 0.2 mmol of 2-naphthalenethiol, 2.5 mol% Pd catalyst and 5 mol% P-ligand in 2 mL solvent, which were stirred at 20 ℃ for 12 hours under N2 atmosphere. b Isolated yield. cThe reaction was conducted under 35 ℃.

  3.2 Spectroscopy analysis

  The target thioglycosides (4a/4b and 7a/7b) were characterized by NMR and HR-ESI-MS. All of the proton and carbon signals were in accord with the characteristic functional groups of the sugar skeleton. For example, the alkenyl protons at C(12)=C(13) bonds were located at 5.86~6.14 ppm as two sets of multiplets (ddd and dd) in ¹H NMR spectroscopy, and the aliphatic carbon's signals were assigned to 61~83 ppm in the ¹³C NMR spectra. The protons of tert-butyl group were ascribable to 1.2 ppm approximately, and there was an additional carbon signal (33.9 ppm) of the methylene in benzyl group besides other aliphatic carbons'. The adduct ion [M + Na]⁺ in the HR-ESI-MS spectrum could be also observed for the target compound.

  3.3 Crystal structure description

  The selected bond lengths, bond angles and torsion angles of compound 7a are listed in Table 2, and all the bond lengths of C–C and C=C are in accordance with the standard compilations and the literature^[21]. The bond angles containing unsaturated bonds in C(3)–C(2)–C(1), C(13)–C(12)–C(11) and O(4)–C(17)–O(3) range from 120° to 124°. The torsion angle of naphthalen-2-ylthio group C(9)–S(1)–C(11)–O(1) was equal to –68.2(2)°, and the π-conjugated aliphatic backbone C(11)–C(12)–C(13)–C(14) and C(16)–O(3)–C(17)– O(4) showed a planar conformation with the corresponding torsion angles of 2.8(6)° and –6.2(6)°, respectively. The aliphatic 6-membered sugar ring O(1)–C(11)–C(12)–C(13)– C(14)–C(15) exhibited a twist-boat form (Φ = 328.7397°, θ = 51.90°, puckering amplitude (Q) = 0.5196°).

  Table 2

  

  Table 2.  Selected Bond Lengths (Å) and Bond Angles (°) for Compound 7a

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  C(1)–C(2)	1.425(5)		C(12)–C(13)	1.321(6)		O(1)–C(11)	1.421(4)
  C(2)–C(3)	1.363(6)		C(14)–C(13)	1.492(6)		O(2)–C(14)	1.426(4)
  C(10)–C(1)	1.418(5)		S(1)–C(9)	1.770(4)		O(3)–C(17)	1.324(5)
  C(11)–C(12)	1.495(5)		S(1)–C(11)	1.811(3)		O(4)–C(17)	1.205(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  C(3)–C(2)–C(1)	120.0(4)		O(1)–C(15)–C(14)	109.2(2)		C(9)–S(1)–C(11)–O(1)	–68.2(2)
  C(9)–S(1)–C(11)	100.90(14)		C(13)–C(12)–C(11)	121.7(3)		C(11)–C(12)–C(13)–C(14)	2.8(6)
  C(11)–O(1)–C(15)	110.6(2)		O(3)–C(17)–C(18)	111.8(3)		C(16)–O(3)–C(17)–O(4)	–6.2(6)
  O(1)–C(11)–S(1)	108.9(2)		O(4)–C(17)–O(3)	123.5(4)		C(13)–C(14)–C(15)–O(1)	–48.8(4)

  As listed in Table 3, intermolecular and intramolecular O–H···O and C–H···O interactions linked the molecules into a one-dimensional infinite chain running along the a axis, which helped to stabilize the supramolecular architecture (Fig. 2).

  Table 3

  

  Table 3.  Hydrogen Bonds for Compound 7a

  DownLoad: CSV

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠(DHA)
  O(2)–H(2)···O(3)a	0.82	2.29	3.0830(2)	162
  C(10)–H(10)···O(2)a	0.93	2.53	3.4321(2)	165
  C(16)–H(16B)···O(4)	0.97	2.27	2.6954(1)	105
  Symmetry code: (a) –1/2+x, 3/2–y, –z

  Figure 2

  

  Figure 2.  Packing diagram of compound 7a. The O–H···O interactions are shown as dashed lines

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  3.4 Evaluation of the bioactivity

  As our continuous interest in search of natural products-derived biological small molecules^[22-25], the target thioglycoside (4a, 4b, 7a and 7b) and its key synthetic intermediates (3, 5 and 6) were evaluated for in vitro anti-proliferative activities against human gastric carcinoma cell lines (HGC-27) for 48 hours at 37 ℃ by MTT assays. All data were presented as mean ± standard deviation and analyzed by SPSS^[26]. Paclitaxel was chosen as a positive control with the IC₅₀ of 4.16 μmol/L. The results indicated that both of the synthetic intermediates showed poor anti-proliferative effect against HGC-27 cell lines (IC₅₀ > 100 μmol/L, Table 4). However, four of the thioglycosides (4a, 4b, 7a and 7b) had better cytotoxic effect with the IC₅₀ value of 69~88 μmol/L, respectively. All of the tested compounds exhibited low toxicities against the normal human gastric mucosa epithelial GES-1 cell lines (IC₅₀ > 500 μmol/L). Further exploration of the substrate scope for this reaction was under way in our research group.

  Table 4

  

  Table 4.  Bioactivity Evaluation of the Compounds

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  Compound	Anti-proliferative results by MTT assays (IC50, μmol/L)
  	Human gastric cancer cell lines (HGC-27)	Human gastric epithelial cell lines (GES-1)
  3	> 100	> 500
  5	> 100	> 500
  6	> 100	> 500
  4a	78.6	> 500
  4b	81.8	> 500
  7a	69.5	> 500
  7b	88.2	> 500
  Paclitaxel a	4.16	> 500
  a Positive control

  4. CONCLUSION

  In summary, two β-thiogalactosides were prepared through the palladium-catalyzed one-pot Tsuji-Trost reaction. The absolute configuration of 7a was confirmed with a Flack parameter of 0.019(15) by X-ray crystallography. The in vitro cytotoxity evaluation indicated that thioglycosides showed better anti-proliferative effect than the D-galactal carbonate intermediates, which encouraged us to further investigate the reaction with the aim of systematically assessing the biological activity for these thioglycosides.

  
  
• 1. [1]

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• 

  Scheme 1  Synthetic routes for the β-thiogalactoside

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  Figure 1  ORTEP drawing of compound 6a showing thermal ellipsoids at the 50% probability level

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  Figure 2  Packing diagram of compound 7a. The O–H···O interactions are shown as dashed lines

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  Table 1.  Condition Optimization for the S-Glycosylation Reaction

  Entrya	Catalyst	Ligand	Solvent	Yieldb
  1	PdCl2	DPPB	THF	0
  2	Pd(OAc)2	DPPB	THF	0
  3	Pd(PPh3)4	DPPB	THF	3
  4	Pd(acac)2	DPPB	THF	9
  5	Pd2(dba)3	DPPB	THF	15
  6	Pd2(dba)3	DPPB	DMF	0
  7	Pd2(dba)3	DPPB	DMSO	0
  8	Pd2(dba)3	DPPB	Toluene	0
  9	Pd2(dba)3	DPPB	CH3CN	0
  10	Pd2(dba)3	DPPB	CH2Cl2	30
  11	Pd2(dba)3	DPPB	CCl4	25
  12	Pd2(dba)3	DPPF	CH2Cl2	0
  13	Pd2(dba)3	PCy3	CH2Cl2	0
  14	Pd2(dba)3	Xantphos	CH2Cl2	68
  15c	Pd2(dba)3	Xantphos	CH2Cl2	85
  a Unless otherwise specified, all reactions were carried out with 0.2 mmol of intermediate 5, 0.2 mmol of 2-naphthalenethiol, 2.5 mol% Pd catalyst and 5 mol% P-ligand in 2 mL solvent, which were stirred at 20 ℃ for 12 hours under N2 atmosphere. b Isolated yield. cThe reaction was conducted under 35 ℃.

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  Table 2.  Selected Bond Lengths (Å) and Bond Angles (°) for Compound 7a

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  C(1)–C(2)	1.425(5)		C(12)–C(13)	1.321(6)		O(1)–C(11)	1.421(4)
  C(2)–C(3)	1.363(6)		C(14)–C(13)	1.492(6)		O(2)–C(14)	1.426(4)
  C(10)–C(1)	1.418(5)		S(1)–C(9)	1.770(4)		O(3)–C(17)	1.324(5)
  C(11)–C(12)	1.495(5)		S(1)–C(11)	1.811(3)		O(4)–C(17)	1.205(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  C(3)–C(2)–C(1)	120.0(4)		O(1)–C(15)–C(14)	109.2(2)		C(9)–S(1)–C(11)–O(1)	–68.2(2)
  C(9)–S(1)–C(11)	100.90(14)		C(13)–C(12)–C(11)	121.7(3)		C(11)–C(12)–C(13)–C(14)	2.8(6)
  C(11)–O(1)–C(15)	110.6(2)		O(3)–C(17)–C(18)	111.8(3)		C(16)–O(3)–C(17)–O(4)	–6.2(6)
  O(1)–C(11)–S(1)	108.9(2)		O(4)–C(17)–O(3)	123.5(4)		C(13)–C(14)–C(15)–O(1)	–48.8(4)

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  Table 3.  Hydrogen Bonds for Compound 7a

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠(DHA)
  O(2)–H(2)···O(3)a	0.82	2.29	3.0830(2)	162
  C(10)–H(10)···O(2)a	0.93	2.53	3.4321(2)	165
  C(16)–H(16B)···O(4)	0.97	2.27	2.6954(1)	105
  Symmetry code: (a) –1/2+x, 3/2–y, –z

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  Table 4.  Bioactivity Evaluation of the Compounds

  Compound	Anti-proliferative results by MTT assays (IC50, μmol/L)
  	Human gastric cancer cell lines (HGC-27)	Human gastric epithelial cell lines (GES-1)
  3	> 100	> 500
  5	> 100	> 500
  6	> 100	> 500
  4a	78.6	> 500
  4b	81.8	> 500
  7a	69.5	> 500
  7b	88.2	> 500
  Paclitaxel a	4.16	> 500
  a Positive control

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•