English

• 1. INTRODUCTION

  In recent years, the widespread use of antibiotics, especially their clinical misuse, has resulted in a rapid increase of multidrug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), which represents a severe threat to human communities worldwide^[1]. However, the rate of development for new antibiotics is declining, far behind the rate of increase of multidrug-resistant strains^[2]. Especially, nowadays only a few antibiotics such as vancomycin, linezolid, daptomycin, and ceftaroline have been used clinically against infections caused by MRSA^[3]. Therefore, the discovery of new antibacterial agents with a novel mechanism of action has become increasingly important to the medical community. Natural products or semisynthetic derivatives of natural products play a vital role in developing new antibacterial agents^{[4, 5]}.

  Dehydroabietic acid (DAA, 1) is a natural diterpene resin acid abundant in Pinus rosin or commercial disproportionated rosin. DAA and its derivatives have exhibited a broad spectrum of biological activities such as antibacterial, antifungal, antitumor, antiprotozoal, antiviral, antidiabetic, anti-aging, and immunomodulatory activities^[6-13]. Especially, some DAA derivatives exhibited promising antibacterial activity against MRSA strains^[14-16], which indicated the potential for new class of antibacterial agents. On the other hand, nitrogen-containing moieties play a vital role in the field of medicinal chemistry because they can easily interact with biomolecules of living systems^[17]. Among them, benzimidazole has been considered as a prominent hetero-cyclic scaffold found in many natural and synthetic drugs which exhibit a wide range of therapeutic properties including antibacterial activity against MRSA^{[18, 19]}. Furthermore, sulfonamide is also a N-containing moiety readily present in antibacterial agents^{[20, 21]}. These findings suggested that the introduction of benzimidazole and sulfonamide moieties in the molecule of DAA might afford new derivatives with promising antibacterial activities. In continuation of our research on antibacterial derivatives of DAA^[22], a series of new N-(1H-benzo[d]imidazol-2-yl)benzenesulfonamide derivatives of DAA were synthesized. Their structures were characterized by spectroscopic techniques and the crystal structure of one title compound was detected by X-ray diffraction. In addition, their antibacterial activities were also presented here.

  2. EXPERIMENTAL

  2.1 Reagents and measurements

  The melting point was determined by means of an XT-4 apparatus (Taike Corp., Beijing, China) without correction. The HR-MS spectra were recorded on a high-resolution mass spectrometer equipped with electrospray (ESI) and nanospray sources, and a quadrupole-time of flight hybrid analyser (Q-TOF Premier/nanoAquity, Waters, Milford, MA). NMR spectra were accomplished in CDCl₃ on a Bruker DRX-600 spectrometer using TMS as the internal standard. Reactions were monitored by TLC which was carried out on TLC Silica gel 60 F₂₅₄ sheets from EMD Millipore Co., USA and visualized in UV light (254 and 365 nm). Silica gel (300~400 mesh) for column chromatography was purchased from Qingdao Marine Chemical Factory, China. The reagents and chemicals of AR grade were purchased from commercial suppliers and used without further purification.

  2.2 Synthesis of the title compounds (7a~7g)

  The title compounds (7a~7g) were synthesized from dehydroabietic acid (1) based on the following procedure (Scheme 1). The key intermediate 6 could be synthesized according to the method reported previously^[23]. Subsequently, compound 6 was treated as follows to afford compounds 7a~7g. To a solution of compound 6 (81 mg, 0.2 mmol) in 2 mL of pyridine was added 0.6 mmol of corresponding substituted benzenesulfonyl chloride. The mixture was stirred at 100 ℃ for 3 h, and the reaction was monitored by TLC. At the end of reaction, the mixture was poured into water (50 mL) and extracted with EtOAc for three times (3 × 50 mL). The organic phase was combined, washed with water, saturated NaHCO₃ solution and brine, dried over anhydrous Na₂SO₄ and concentrated in vacuo. The residue was purified by column chromatography on silica gel and eluted with petroleum ether-acetone (100:1~10:1, v/v) to afford compound 7a~7g. The solid of compound 7g was re-crystallized in methanol to yield yellow crystal suitable for X-ray diffraction analysis.

  Scheme 1

  

  Scheme 1.  Synthetic route of the title compounds 7a~7g

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  Compound 7a: yellow solid; 82 mg, 75%; m.p: 136~138 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.18 (s, 3H), 1.25 (s, 3H), 1.45 (td, J = 12.4, 3.2 Hz, 1H), 1.52 (dd, J = 13.1, 8.0 Hz, 1H), 1.62~1.85 (m, 5H), 2.18 (dd, J = 12.5, 1.8 Hz, 1H), 2.25 (d, J = 12.9 Hz, 1H), 2.80 (m, 1H), 2.97 (dd, J = 17.5, 6.7 Hz, 1H), 3.66 (s, 3H), 7.20 (s, 1H), 7.36 (t, J = 7.9 Hz, 2H), 7.46 (t, J = 7.4 Hz, 1H), 7.88 (d, J = 7.4 Hz, 2H), 9.67 (brs, 1H), 10.43 (brs, 1H); ¹³C-NMR (150 MHz, CDCl₃): δ 16.5, 18.5, 20.5, 24.5, 25.4, 36.7, 37.6, 38.3, 44.6, 47.6, 52.3, 100.5, 118.7, 122.3, 125.5, 126.0, 127.9, 128.9, 131.9, 143.2, 147.2, 150.7, 178.9; ESI-MS: m/z [M+H]⁺ calcd. for C₂₅H₂₉BrN₃O₄S: 546.1062; found: 546.1069.

  Compound 7b: light yellow solid; 72 mg, 64%; m.p: 217~219 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.19 (s, 3H), 1.26 (s, 3H), 1.46 (t, J = 12.3 Hz, 1H), 1.53 (dd, J = 12.8, 7.8 Hz, 1H), 1.65~1.85 (m, 5H), 2.19 (dd, J = 12.6, 2.0 Hz, 1H), 2.25 (d, J = 11.9 Hz, 1H), 2.37 (s, 3H), 2.79 (m, 1H), 2.94 (dd, J = 17.3, 6.6 Hz, 1H), 3.67 (s, 3H), 7.17 (d, J = 7.9 Hz, 2H), 7.20 (s, 1H), 7.77 (d, J = 8.0 Hz, 2H), 9.80 (brs, 1H), 10.44 (brs, 1H); ¹³C-NMR (150 MHz, CDCl₃): δ 16.6, 18.5, 20.6, 21.6, 24.5, 25.4, 36.7, 37.6, 38.4, 44.7, 47.6, 52.3, 100.5, 118.5, 122.3, 125.5, 126.1, 127.9, 129.5, 140.3, 142.6, 147.2, 150.6, 178.9; ESI-MS: m/z [M+H]⁺ calcd. for C₂₆H₃₁BrN₃O₄S: 560.1219; found: 560.1223.

  Compound 7c: light yellow solid; 81 mg, 70%; m.p: 242~244 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.19 (s, 3H), 1.26 (s, 3H), 1.46 (t, J = 11.6 Hz, 1H), 1.53 (dd, J = 12.5, 7.6 Hz, 1H), 1.65~1.85 (m, 5H), 2.18 (d, J = 12.4 Hz, 1H), 2.25 (d, J = 12.3 Hz, 1H), 2.80 (m, 1H), 2.97 (dd, J = 17.2, 6.3 Hz, 1H), 3.67 (s, 3H), 3.81 (s, 3H), 6.83 (d, J = 8.4 Hz, 2H), 7.19 (s, 1H), 7.80 (d, J = 8.2 Hz, 2H), 9.90 (brs, 1H), 10.51 (brs, 1H); ¹³C-NMR (150 MHz, CDCl₃): δ 16.6, 18.6, 20.6, 24.5, 25.4, 36.7, 37.6, 38.4, 44.7, 47.6, 52.3, 55.6, 100.4, 114.0, 118.6, 122.2, 125.5, 127.9, 128.0, 135.3, 147.2, 150.7, 162.4, 178.9; ESI-MS: m/z [M+H]⁺ calcd. for C₂₆H₃₁BrN₃O₅S: 576.1168; found: 576.1163.

  Compound 7d: light yellow solid; 70 mg, 62%; m.p: 128~130 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.20 (s, 3H), 1.27 (s, 3H), 1.46 (t, J = 12.4 Hz, 1H), 1.55 (dd, J = 13.1, 7.6 Hz, 1H), 1.65~1.85 (m, 5H), 2.19 (dd, J = 12.5, 1.2 Hz, 1H), 2.26 (d, J = 13.4 Hz, 1H), 2.80 (m, 1H), 2.92 (dd, J = 17.2, 6.6 Hz, 1H), 3.68 (s, 3H), 7.07 (t, J = 8.5 Hz, 2H), 7.22 (s, 1H), 7.91 (dd, J = 8.6, 5.1 Hz, 2H), 9.66 (brs, 1H), 10.24 (brs, 1H); ¹³C-NMR (150 MHz, CDCl₃): δ 16.6, 18.5, 20.6, 24.5, 25.4, 36.7, 37.6, 38.4, 44.7, 47.6, 52.3, 100.6, 116.0 (d, J = 22.4 Hz), 118.5, 122.5, 125.4, 127.7, 128.7 (d, J = 9.1 Hz), 139.4 (d, J = 3.1 Hz), 147.4, 150.4, 164.7 (d, J = 251.7 Hz), 178.9; ESI-MS: m/z [M+H]+ calcd. for C₂₅H₂₈BrFN₃O₄S: 564.0968; found: 564.0972.

  Compound 7e: light yellow solid; 88 mg, 76%; m.p: 229~230 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.19 (s, 3H), 1.26 (s, 3H), 1.44 (td, J = 12.6, 3.9 Hz, 1H), 1.52 (dd, J = 12.8, 7.9 Hz, 1H), 1.60~1.85 (m, 5H), 2.17 (dd, J = 12.1, 1.6 Hz, 1H), 2.25 (d, J = 13.1 Hz, 1H), 2.80 (m, 1H), 2.94 (dd, J = 17.4, 6.5 Hz, 1H), 3.67 (s, 3H), 7.21 (s, 1H), 7.31 (d, J = 8.6 Hz, 2H), 7.80 (d, J = 8.6 Hz, 2H), 10.01 (brs, 1H), 10.52 (brs, 1H); ¹³C-NMR (150 MHz, CDCl₃): δ 16.5, 18.5, 20.5, 24.5, 25.4, 36.7, 37.6, 38.3, 44.6, 47.6, 52.3, 100.6, 118.6, 122.5, 125.5, 127.5, 127.8, 129.1, 138.2, 141.8, 147.3, 150.5, 178.9; ESI-MS: m/z [M+H]⁺ calcd. for C₂₅H₂₈BrClN₃O₄S: 580.0672; found: 580.0668.

  Compound 7f: white solid, 66 mg, 58%; m.p: 162~164 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.19 (s, 3H), 1.27 (s, 3H), 1.43 (td, J = 11.9, 4.3 Hz, 1H), 1.53 (dd, J = 13.3, 7.0 Hz, 1H), 1.65~1.85 (m, 5H), 2.17 (dd, J = 12.4, 1.5 Hz, 1H), 2.26 (d, J = 12.8 Hz, 1H), 2.81 (m, 1H), 2.88 (dd, J = 17.1, 6.7 Hz, 1H), 3.67 (s, 3H), 7.23 (s, 1H), 7.69 (d, J = 8.3 Hz, 2H), 8.01 (d, J = 8.3 Hz, 2H), 9.75 (s, 1H, NH), 10.13 (s, 1H, NH); ¹³C-NMR (150 MHz, CDCl₃): δ 16.5, 18.5, 20.5, 24.6, 25.4, 36.7, 37.6, 38.3, 44.7, 47.6, 52.3, 100.7, 115.6, 117.7, 118.5, 122.7, 125.3, 126.8, 127.6, 132.7, 147.3, 147.6, 150.2, 179.0; ESI-MS: m/z [M+H]⁺ calcd. for C₂₆H₂₈BrN₄O₄S: 571.1015; found: 571.1010.

  Compound 7g: yellow solid; 84 mg, 71%; m.p: 155~157 ℃; ¹H-NMR (600 MHz, CDCl₃): δ 1.18 (s, 3H), 1.26 (s, 3H), 1.41 (td, J = 12.4, 3.8 Hz, 1H), 1.52 (dd, J = 14.2, 7.8 Hz, 1H), 1.67~1.85 (m, 5H), 2.16 (d, J = 10.0 Hz, 1H), 2.24 (d, J = 12.8 Hz, 1H), 2.82 (m, 1H), 2.89 (dd, J = 16.9, 5.8 Hz, 1H), 3.67 (s, 3H), 7.21 (s, 1H), 8.05 (d, J = 8.5 Hz, 2H), 8.17 (d, J = 8.5 Hz, 2H), 9.92 (s, 1H), 10.32 (brs, 1H); ¹³C-NMR (150 MHz, CDCl₃): δ 16.5, 18.5, 20.5, 24.6, 25.3, 36.7, 37.6, 38.3, 44.6, 47.5, 52.3, 100.7, 118.6, 122.7, 124.1, 125.3, 127.3, 127.7, 147.6, 148.8, 149.6, 150.2, 179.0; ESI-MS: m/z [M+H]⁺ calcd. for C₂₅H₂₈BrN₄O₆S: 591.0913; found: 591.0918.

  2.3 X-ray structure determination

  A yellow single crystal of compound 7g with dimensions of 0.20mm × 0.10mm × 0.10mm was mounted on the top of a glass fiber. X-ray diffraction data were collected using an Enraf-Nonius CAD-4 diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å) by using an ω-2θ scan mode in the range of 1.702≤θ≤25.386° (0≤h≤14, –11≤k≤11, –27≤l≤26) at 298(2) K. A total of 10339 reflections were collected, of which 9855 were independent (R_int = 0.0736) and 4098 were observed with I > 2σ(I). The structure was solved by direct methods using SHELXS-2014/7 and refined by full-matrix least-squares procedure on F² with SHELXL-2014/7. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were located by geometric calculations and refined by using a riding mode. The final refinement gave R = 0.0791, wR = 0.1330 (w = 1/[σ²(F_o²) + (0.0580P)²], where P = (F_o² + 2F_c²)/3, S = 1.000, (Δ/σ)_max = 0.000, (Δρ)_max = 0.353 and (Δρ)_min = –0.534 e/ų. The selected bond distances and bond angles are listed in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of the Title Compound

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  O(1)–C(18)	1.18(1)		S(1)–C(20)	1.719(9)		N(5)–C(26)	1.34(1)
  O(2)–C(18)	1.35(2)		Br(1)–C(3)	1.853(6)		N(5)–C(27)	1.39(1)
  O(2)–C(19)	1.47(2)		C(2)–C(15)	1.416(9)		N(6)–C(26)	1.351(9)
  O(3)–S(1)	1.469(9)		C(5)–C(6)	1.53(1)		N(6)–C(40)	1.42(1)
  O(4)–S(1)	1.438(7)		C(6)–C(7)	1.54(1)		N(7)–C(26)	1.34(1)
  O(5)–N(4)	1.21(3)		C(6)–C(11)	1.62(1)		N(7)–S(2)	1.561(7)
  O(6)–N(4)	1.15(4)		C(10)–C(11)	1.54(1)		N(8)–C(45)	1.50(4)
  N(1)–C(1)	1.33(2)		O(8)–C(43)	1.178(9)		S(2)–C(48)	1.804(8)
  N(1)–C(2)	1.38(1)		O(7)–C(43)	1.37(2)		Br(2)–C(28)	1.879(6)
  N(2)–C(1)	1.395(8)		O(7)–C(44)	1.45(1)		C(27)–C(40)	1.421(8)
  N(2)–C(15)	1.43(1)		O(9)–S(2)	1.463(8)		C(30)–C(31)	1.56(1)
  N(3)–C(1)	1.36(1)		O(10)–S(2)	1.43(1)		C(31)–C(32)	1.57(2)
  N(3)–S(1)	1.548(9)		O(11)–N(8)	1.17(3)		C(31)–C(36)	1.52(1)
  N(4)–C(23)	1.49(3)		O(12)–N(8)	1.23(3)		C(35)–C(36)	1.54(1)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–C(18)–O(2)	118(1)		C(6)–C(7)–C(8)	112.8(7)		N(7)–S(2)–C(48)	104.3(4)
  O(3)–S(1)–O(4)	117.2(5)		C(6)–C(11)–C(12)	107.8(5)		S(2)–N(7)–C(26)	120.8(6)
  O(5)–N(4)–O(6)	117(2)		C(9)–C(10)–C(11)	110.8(7)		C(26)–N(5)–C(27)	108.2(6)
  N(1)–C(1)–N(2)	109.5(7)		O(7)–C(43)–O(8)	114(1)		C(31)–C(32)–C(33)	111.9(9)
  N(3)–S(1)–C(20)	101.7(4)		O(9)–S(2)–O(10)	118.5(5)		C(31)–C(36)–C(37)	112.3(9)
  S(1)–N(3)–C(1)	126.0(6)		O(11)–N(8)–O(12)	129(2)		C(34)–C(35)–C(36)	107(1)
  C(1)–N(1)–C(2)	110.7(8)		N(5)–C(26)–N(6)	111.2(6)		C(15)–C(2)–C(3)–Br(1)	–176.8(13)
  N(1)–C(1)–N(3)–S(1)	179.4(7)		S(1)–C(20)–C(21)–C(22)	174.3(9)		N(7)–S(2)–C(48)–C(49)	75.2(8)
  N(3)–S(1)–C(20)–C(21)	116.9(8)		S(2)–N(7)–C(26)–N(5)	–161.6(6)		S(2)–C(48)–C(49)–C(50)	–175.9(8)

  2.4 Antibacterial activity

  The title compounds were screened for their in vitro antibacterial activity. Five bacterial strains were used as the test microorganisms: Bacillus subtilis (CGMCC1.1162), Escherichia coli (CGMCC1.1571), Pseudomonas fluorescens (CGMCC1.1828), Staphylococcus aureus (CGMCC1.1361), and MRSA (ATCC33591). The former four microbes were obtained from China General Microbiological Culture Collection Center (CGMCC), China and MRSA strain was bought from Fengshou Biotech., China. The antibacterial activity was assessed in terms of minimum inhibitory concentrations (MICs) by a modified microdilution method^[24]. Compounds were dissolved in DMSO and serial double dilutions of each compound (75 μL) were prepared in 96 well micro-trays. The same amount of test microorganism in Martin's broth (~10⁵ colony forming unit (CFU)/mL) was added to each well to give a final volume of 150 μL. After incubation at 37 ℃ for 24 h, the trays were examined for growth of the test microorganisms. The MIC was defined as the lowest concentrations of compound at which the microbial growth was inhibited. Chloromycin and norfloxacin were included as positive control, and DMSO was used as negative control. All assays were performed in duplicate.

  3. RESULTS AND DISCUSSION

  The structures of compounds 7a~7g were characterized on the basis of HR-MS, ¹H- and ¹³C-NMR spectroscopic data. For example, the molecular formula of compound 7g was determined as C₂₅H₂₇BrN₄O₆S through HR-ESI-MS spectrum (m/z [M+H]⁺ calcd. for C₂₅H₂₈BrN₄O₆S: 591.0913; found: 591.0918). In its ¹H-NMR spectrum, there are three singlets at δ 1.18, 1.26 and 3.67 ppm attributed to methyl protons at C(16), C(17) and the methyl ester group (C(19)), respectively. The signals in the range of δ 1.40~2.90 ppm are corresponding to the methylene and methine protons on rings A and B. As for aromatic hydrogens, the singlet at δ 7.21 ppm can be assigned to the aromatic hydrogen at C(4), while the two doublets at δ 8.05 and 8.17 ppm to protons at C(21)/C(25) and C(22)/C(24), respectively. In addition, two broad singlets at δ 9.92 and 10.32 ppm can be assigned to the two active hydrogens at N(1) and N(3). The ¹³C-NMR spectrum of compound 7g exhibits 23 well resolved resonances for 25 carbon atoms. Among them, the absorption peaks at δ 52.3 and 179.0 ppm are corresponding to the methyl carbon (C(19)) and the carbonyl carbon (C(18)), respectively, confirming the existence of methyl ester group. Six peaks at δ 100.7, 118.6, 122.7, 125.3, 127.7 and 147.6 ppm can be assigned to the carbons of benzene ring (C(2)~C(5), C(14), C(15)), and the peak at δ 150.2 ppm is the signal of C(1). In addition, four peaks at δ 124.1 (2C), 127.3 (2C), 148.8 and 149.6 ppm are attributed to the six carbons at the benzene ring (C(20)~C(25)). The assignments of the signals in the ¹H- and ¹³C-NMR spectra of 7g are in good accordance with its structure. Moreover, the structures of compounds 7a~7f can also be characterized by their spectral data in a similar manner.

  The title compound 7g crystallizes in the monoclinic space group P2₁ and the structure with corresponding atomic numbering scheme is shown in Fig. 1. Two crystallogra-phically independent molecules (molecules I and II) with variable conformations co-exist in the asymmetric unit. Distinctions can be observed among the bond lengths, bond angles, and torsion angles of the two molecules (Table 1), indicating a deviation between their conformations. Each molecule contains two chair-conformed cyclohexane rings (A/B and A'/B'), two phenyl rings (C/E and C'/E'), and one imidazole ring (D and D'). The dihedral angle between rings D and E in molecule I is 119.26°, while that in molecule II is 79.42°, exhibiting a trans and a cis arrangements of the former and latter molecules, respectively. With respect to molecule I, the cyclohexane fragment A has a classical chair conformation, while ring B is characterized as a cyclohexene half-chair with the out-of-plane distances of C(11) and C(12) atoms to be 0.615 and 0.235 Å, respectively. Two methyl groups (C(16) and C(17)) attached to the bis cyclohexane rings exist in the axial positions in molecule I, as well as those in molecule II. The aromatic rings of either C/D or C'/D' are approximately planar (dihedral angles: 2.05° and 6.09°, respectively) because of the conjugated structure. The C–N bond lengths of C(1)(sp²)–N(1)(sp³), C(1)(sp²)–N(2)(sp²), C(1)(sp²)–N(3)(sp³), C(2)(sp²)–N(1)(sp³) and C(15)(sp²)–N(2)(sp²) are all in the range of 1.33~1.43 Å, while the typical bond length of C(sp²)–N(sp³) is around 1.5 Å^[25], indicating a high p-π conjugation in the amino imidazole fragment^{[25, 26]}. Similar phenomenon could be found in molecule II, where p-π conjugation exists as well. In addition, bond lengths of N(3)–S(1) and N(7)–S(2) are close to each other, found as 1.548(9) and 1.561(7) Å, respectively. Interestingly, the C(20)–S(1) and C(48)–S(2) bonds are 1.719(9) and 1.804(8) Å, respectively, exhibiting a relatively larger variation of bond length (C–S) around 0.1 Å, which might be derived from the cis-tran conformation effect. Furthermore, the angles of N(3)–S(1)–C(20) and N(7)–S(2)–C(48) are similar to each other, shown as 101.7(4) and 104.3(4)°, respectively, although their conformations are completely in the opposite directions. With regard to the chirality of compound 7g, both molecules have three chiral centers that are derived from the natural diterpene dehydroabietic acid. In addition, due to the presence of heavy atom bromine in the molecule, the final refinement resulted in a small Flack parameter −0.042(18). Therefore, the absolute configurations are confirmed as 6S, 10R, 11R, and 31S, 35R, 36R for molecules I and II, respectively.

  Figure 1

  

  Figure 1.  Molecular diagram of the title compound with atomic labeling

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  Compounds 6 and 7a~7g were evaluated for their antibacterial activities against five bacterial strains via serial microdilution method. The minimal inhibitory concentration (MIC) values of test compounds against these test microorganisms are listed in Table 2. Also included are the MIC values of positive control chloromycin and norfloxacin. As shown in Table 2, compounds 6 and 7a~7c showed weak or no inhibition to most test strains (MIC > 100 μg/mL). Compounds 7d and 7e displayed moderate antibacterial activities against B. subtilis and S. aureus and weak activities against E. coli, P. fluorescens and MRSA. On the other hand, compounds 7f and 7g showed strong inhibitory activities against Gram-positive B. subtilis and S. aureus (MIC < 10 μg/mL). Notably, compound 7g exhibited the most potent activities against B. subtilis and S. aureus with the MIC values of 1.9 μg/mL. It also showed significant inhibition to MRSA with MIC to be 7.8 μg/mL, which was comparable to positive control norfloxacin. These results suggested that the existence of benzimidazole and benzenesulfonamide moieties were beneficial to the antibacterial activity of the derivatives. In addition, the introduction of electron-withdrawing groups on para-position of the benzene ring will substantially increase the antibacterial activity, while the electron-donating groups will decrease the activity.

  Table 2

  

  Table 2.  MIC Values (μg/mL) of Compounds against Test Microorganisms

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  Compound	Gram-negative bacteria		Gram-positive bacteria
  	E. coli	P. fluorescens	B. subtilis	S. aureus	MRSA
  6	> 100	> 100	> 100	62.5	> 100
  7a	> 100	> 100	62.5	62.5	> 100
  7b	> 100	> 100	> 100	> 100	> 100
  7c	> 100	> 100	> 100	> 100	> 100
  7d	> 100	62.5	15.6	15.6	62.5
  7e	> 100	62.5	15.6	31.2	> 100
  7f	62.5	31.2	3.9	7.8	31.2
  7g	31.2	15.6	1.9	1.9	7.8
  DMSO	> 100	> 100	> 100	> 100	> 100
  Chloromycin	31.2	31.2	15.6	7.8	15.6
  Norfloxacin	1.9	7.8	1.9	0.45	7.8

  4. CONCLUSION

  A series of new N-(1H-benzo[d]imidazol-2-yl)benzenesul-fonamide derivatives of dehydroabietic acid were designed, synthesized and characterized by spectroscopic methods. The crystal structure of compound 7g was determined by single-crystal X-ray diffraction. In addition, compound 7g exhibited significant in vitro antibacterial activity against B. subtilis, S. aureus and MRSA. These results afford a new scaffold for the investigation of potential antibacterial agents against drug-resistant bacterial pathogens. The study on further structure modifications, structure-activity relationships and antibacterial mechanisms will be carried out in the future.

  
  
• 1. [1]

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• 

  Scheme 1  Synthetic route of the title compounds 7a~7g

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  Figure 1  Molecular diagram of the title compound with atomic labeling

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of the Title Compound

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  O(1)–C(18)	1.18(1)		S(1)–C(20)	1.719(9)		N(5)–C(26)	1.34(1)
  O(2)–C(18)	1.35(2)		Br(1)–C(3)	1.853(6)		N(5)–C(27)	1.39(1)
  O(2)–C(19)	1.47(2)		C(2)–C(15)	1.416(9)		N(6)–C(26)	1.351(9)
  O(3)–S(1)	1.469(9)		C(5)–C(6)	1.53(1)		N(6)–C(40)	1.42(1)
  O(4)–S(1)	1.438(7)		C(6)–C(7)	1.54(1)		N(7)–C(26)	1.34(1)
  O(5)–N(4)	1.21(3)		C(6)–C(11)	1.62(1)		N(7)–S(2)	1.561(7)
  O(6)–N(4)	1.15(4)		C(10)–C(11)	1.54(1)		N(8)–C(45)	1.50(4)
  N(1)–C(1)	1.33(2)		O(8)–C(43)	1.178(9)		S(2)–C(48)	1.804(8)
  N(1)–C(2)	1.38(1)		O(7)–C(43)	1.37(2)		Br(2)–C(28)	1.879(6)
  N(2)–C(1)	1.395(8)		O(7)–C(44)	1.45(1)		C(27)–C(40)	1.421(8)
  N(2)–C(15)	1.43(1)		O(9)–S(2)	1.463(8)		C(30)–C(31)	1.56(1)
  N(3)–C(1)	1.36(1)		O(10)–S(2)	1.43(1)		C(31)–C(32)	1.57(2)
  N(3)–S(1)	1.548(9)		O(11)–N(8)	1.17(3)		C(31)–C(36)	1.52(1)
  N(4)–C(23)	1.49(3)		O(12)–N(8)	1.23(3)		C(35)–C(36)	1.54(1)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–C(18)–O(2)	118(1)		C(6)–C(7)–C(8)	112.8(7)		N(7)–S(2)–C(48)	104.3(4)
  O(3)–S(1)–O(4)	117.2(5)		C(6)–C(11)–C(12)	107.8(5)		S(2)–N(7)–C(26)	120.8(6)
  O(5)–N(4)–O(6)	117(2)		C(9)–C(10)–C(11)	110.8(7)		C(26)–N(5)–C(27)	108.2(6)
  N(1)–C(1)–N(2)	109.5(7)		O(7)–C(43)–O(8)	114(1)		C(31)–C(32)–C(33)	111.9(9)
  N(3)–S(1)–C(20)	101.7(4)		O(9)–S(2)–O(10)	118.5(5)		C(31)–C(36)–C(37)	112.3(9)
  S(1)–N(3)–C(1)	126.0(6)		O(11)–N(8)–O(12)	129(2)		C(34)–C(35)–C(36)	107(1)
  C(1)–N(1)–C(2)	110.7(8)		N(5)–C(26)–N(6)	111.2(6)		C(15)–C(2)–C(3)–Br(1)	–176.8(13)
  N(1)–C(1)–N(3)–S(1)	179.4(7)		S(1)–C(20)–C(21)–C(22)	174.3(9)		N(7)–S(2)–C(48)–C(49)	75.2(8)
  N(3)–S(1)–C(20)–C(21)	116.9(8)		S(2)–N(7)–C(26)–N(5)	–161.6(6)		S(2)–C(48)–C(49)–C(50)	–175.9(8)

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  Table 2.  MIC Values (μg/mL) of Compounds against Test Microorganisms

  Compound	Gram-negative bacteria		Gram-positive bacteria
  	E. coli	P. fluorescens	B. subtilis	S. aureus	MRSA
  6	> 100	> 100	> 100	62.5	> 100
  7a	> 100	> 100	62.5	62.5	> 100
  7b	> 100	> 100	> 100	> 100	> 100
  7c	> 100	> 100	> 100	> 100	> 100
  7d	> 100	62.5	15.6	15.6	62.5
  7e	> 100	62.5	15.6	31.2	> 100
  7f	62.5	31.2	3.9	7.8	31.2
  7g	31.2	15.6	1.9	1.9	7.8
  DMSO	> 100	> 100	> 100	> 100	> 100
  Chloromycin	31.2	31.2	15.6	7.8	15.6
  Norfloxacin	1.9	7.8	1.9	0.45	7.8

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