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. 2016 Dec 21;8(2):445–451. doi: 10.1039/c6md00587j

Design, synthesis and cytotoxicity of bengamide analogues and their epimers† ‡

Thi Dao Phi a,b, Huong Doan Thi Mai a, Van Hieu Tran a, Bich Ngan Truong a, Tuan Anh Tran a,c, Van Loi Vu a, Van Minh Chau a, Van Cuong Pham a,✉
PMCID: PMC6072509  PMID: 30108762

graphic file with name c6md00587j-ga.jpgThe C-2′ of the lactam rings and the flexibility of polyketide chain should be critical for the cytotoxicity of bengamides.

Abstract

Starting from d-glycero-d-gulo-heptonic acid γ-lactone and amino acids, a number of diastereoisomeric bengamide analogues were synthesized. Optimization of the reaction conditions revealed that microwave irradiation assistance is a powerful method for the preparation of aminolactams, as well as for the coupling reactions of the lactone 5 with aminolactams. Cytotoxic activity evaluation against six cancer cell lines (KB, HepG2, LU1, MCF7, HL60, and Hela) demonstrated that the configuration of C-2′ seems to be critical for the cytotoxic activity of compounds 8b (2′R) and 8a (2′S). Additionally, comparison of cytotoxicity of the protected acetonide compounds with that of their corresponding deprotected bengamide analogues suggested that the flexibility of the ketide side chain should be required for their cytotoxic activity.

Introduction

Natural products from terrestrial sources play an important role in biomedical research, as the majority of antibacterial and cytotoxic anticancer drugs in clinical use are either such secondary metabolites or their derivatives.1–3 On the other hand, the marine environment represents an enormous additional reservoir for novel secondary metabolites.4,5 Marine sponges of the family Jaspidae have been proven to be an important source of bioactive secondary metabolites. The sponge-derived bengamides were firstly reported in 1986, and have unique molecular structures (Fig. 1).6 These compounds were found to have a broad spectrum of biological activities such as antitumor, antibiotic, and anthelmintic properties.7–9 Due to their interesting pharmacological applications, these molecules have attracted the attention of many organic chemists. The structural modification of bengamides has focused mainly on improving their water solubility, stability or biological activity. Accordingly, their synthetic analogues have been prepared by modification of the different stereocenters located in their polyketide chain,10–14 variation of the substituent located at the terminal olefinic position,15–17 or modification of the caprolactam unit.17–20 These modifications led to the acquisition of more potent bengamide derivatives. Modification of the side chain by replacement of isopropyl by a tert-butyl group has been proven successful. This modification simplified the synthesis of their analogues. Also, the introduction of the tert-butyl group at the terminal side chain led to the acquisition of more stable structures by avoiding olefin isomerization. A bengamide analogue, LAF389 with tert-butyl at the terminal side chain,21 has been used in a clinical trial. However, the poor pharmacokinetic properties and unclear side effects of LAF389, which appeared early in the trial, have prevented its further development.22 This prompted the search for new bengamide analogues with better therapeutic indices. To our knowledge, except for ent-bengamide,23 no synthesis of bengamide analogues with modification of chirality at C-2′ carbons has been reported. In this communication, we report the synthesis of bengamide analogues with replacement of isopropyl by tert-butyl and modification of C-2′ or C-5′ of the lactam ring, and evaluation of their cytotoxicity against six cancer cell lines.

Fig. 1. Representative bengamide structures.

Fig. 1

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Results and discussion

The synthetic approach to bengamide analogues is described in Schemes 1–4. The general strategy for the preparation of bengamide analogues is based on the lactone 5 and aminocaprolactam intermediates, via an amide coupling reaction.

Scheme 1. Main reported methods for synthesis of 3a from l-lysine.

Scheme 1

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Scheme 4. Reagents and conditions: (a) THF, MW at 100 W, 60–120 min, 6a (95%), 6b (76%), 7a (48%), 7b (44%) (93% overall yield for 7a + 7b), 7c (28%) and 7d (25%) (53% overall yield for 7c + 7d); (b) TFA, H2O, THF, 0 °C, 1 h, 8a (69%), 8b (58%), 9a (45%), 9b (63%), 9c (42%) and 9d (40%).

Scheme 4

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In the first step, the aminocaprolactams were synthesized from the corresponding amino acids. Several methods for synthesis of α-aminolactams from the corresponding amino acids have been reported.24–26 However, these methods involved the use of expensive reagents or the need for operating under high pressure, at high temperature and for long reaction time (methods a, b and c, Scheme 1). In this work, the preparation of α-aminolactams was examined under microwave irradiation. With the aid of microwave irradiation, the cyclization reactions proceed faster under mild conditions (Scheme 2). The α-aminolactams 3a and 3b were obtained from l-lysine and d-lysine in 79 and 82% yields, respectively. Similarly, the racemic 4a (2S*,5S*) and 4b (2R*,5S*) were also prepared from the corresponding racemic 2a and 2b (Scheme 3).

Scheme 2. Reagents and conditions: ethylene glycol, MW at 284 W, 60 min; 3a (79%), 3b (82%).

Scheme 2

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Scheme 3. Reagents and conditions: ethylene glycol, MW at 284 W, 60 min; rac-4a (51%), rac-4b (39%).

Scheme 3

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The coupling reaction of the lactone 5 (which was prepared according to the previously reported method)20,21 with each enantiomeric aminolactam, 3a and 3b, and two racemic mixtures, rac-4a and rac-4b, was optimized. An initial attempt for the coupling of 5 with these aminolactams was performed according to the previously described method by using sodium 2-ethylhexanoate as a base at room temperature for 15–20 h.21 The lactone ring opening by the aminolactams was then examined under microwave irradiation. Accordingly, treatment of 5 with the aminolactams 3a and 3b under microwave irradiation at 100 W for 1–2 h produced the corresponding compounds 6a and 6b in 95 and 76% yields, respectively. Thus, with microwave irradiation assistance, the coupling reaction occurred significantly faster with the yields comparable to those obtained using the previously described method. By using the same procedure with microwave irradiation assistance, the lactone 5 reacted with the racemic 4a (2S*,5S*), affording two diastereoisomers 7a and 7b, while the coupling reaction of 5 with the racemic 4b (2R*,5S*) provided the two diastereoisomers 7c and 7d. Finally, acetonide deprotection of compounds 6a and b and 7a–d was achieved by treatment with TFA and H2O in THF at 0 °C to afford the corresponding bengamide analogues 8a and b and 9a–d (Fig. 2).

Fig. 2. Structures of the synthetic bengamide analogues.

Fig. 2

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The absolute configuration of C-2′ and C-5′ of compounds 7a–d and 9a–d was established by comparison of their NMR data and optical rotation activity with those of the previously reported compound (in the case of 7c) or related structures. Accordingly, for the 2′,5′-trans-isomer series, 9a had a positive rotation activity {[α]25D = +7.0 (c 0.1, MeOH)}, while that for 9b was –20.0 (c 0.1, MeOH). Bengamide Y (2′S,5′S), a related structure to 9a and 9b, displayed a positive rotation activity {[α]25D = +14.0 (c 0.11, MeOH)}.9 This observation suggested the absolute configurations 2′S and 5′S for 9a, as well as for 7a. Thus, the configurations 2′R and 5′R were assigned for 9b and 7b. Similarly, for the 2′,5′-cis-isomers 7c and d and 9c and d, comparison of NMR data revealed that 7c had values identical to those reported for the 2′S,5′R-isomer.20 The configurations 2′S and 5′R were distributed for 7c and 9c. Therefore, 7d and 9d had the configurations 2′R and 5′S.

The synthetic bengamide analogues were evaluated for their cytotoxicity against six cancer cell lines, KB (mouth epidermal carcinoma cells), HepG2 (human liver hepatocellular carcinoma cells), LU (human lung adenocarcinoma cells), MCF7 (human breast cancer cells), HL60 (human promyelocytic leukemia cells), and Hela (human cervical carcinoma cells). Compound 8b (2′R) was the most active against the six tested cancer cell lines and exhibited slightly selective inhibition toward MCF7 cells (Table 1). Comparison of compound 8b (2′R) with its 2′-epimer (8a) which had the same stereochemistry at C-2′ as natural bengamides revealed that 8b (2′R) was more active than 8a (2′S) against almost all of the tested cancer cell lines. Additionally, in order to understand the flexibility effects of the ketide side chain on the cytotoxicity, the acetonide compounds were also tested for their activity against the six abovementioned cancer cell lines. However, all of them were much less active than their corresponding deprotected compounds. No inhibition was observed for the restricted compounds even at the concentration of 150 μM. As an example indicated in Table 1, the two compounds 6a and 6b with restricted C-3/C-4/C-5 bonds are significantly less active than 8a and 8b which had free rotational bonds for C-3/C-4/C-5. This suggested that the flexibility of the polyketide side chain of bengamides seems to be critical for their cytotoxicity. This observation is in agreement with the previous report that the hydroxyl groups at C-3, C-4 and C-5 of the polyketide chain are involved in the activity of bengamides by forming a complex with MetAps,27 enzymes playing important roles in cell proliferation and angiogenesis.28,29

Table 1. Cytotoxicity of the synthetic bengamide analogues (IC50 values are expressed in μM).

Compd KB LU1 HepG2 MCF7 HL60 Hela
8a 21.0 4.3 21.1 1.3 83.3 60.7
8b 1.1 0.5 1.1 0.2 5.3 2.1
9a 19.8 10.0 103.1 144.5 >150.0 >150.0
9b 5.1 17.2 21.3 164.9 12.1 15.4
9c 62.6 23.4 >150.0 23.4 >150.0 >150.0
9d 23.7 36.3 137.6 80.4 >150.0 >150.0
6a >150.0 >150.0 >150.0 >150.0 >150.0 >150.0
6b >150.0 >150.0 >150.0 >150.0 >150.0 >150.0
Ellipticine 1.2 1.6 1.2 2.4 2.0 1.6
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Conclusions

Six diastereoisomeric bengamide analogues were synthesized. Microwave irradiation assistance was found to be useful for the preparation of aminolactams, and especially for the coupling reactions of the lactone 5 with aminolactams. Cytotoxicity evaluation of these diastereoisomeric bengamide analogues revealed that the analogue 8b was remarkably more active than its 2′-epimer (8a) which represented a similar configuration at C-2′ to natural bengamides. Furthermore, since the acetonide compounds were much less active than their corresponding acetonide deprotected analogues, the flexibility of the ketide side chain should be required for their cytotoxic activity. This is consistent with the previous study on the mode of action of bengamides in which the hydroxyl groups at C-2, C-4 and C-5 are required for forming a complex with MetAps.

Experimental

General information

Optical rotations were recorded on a Polax-2L polarimeter in CHCl3 or MeOH. HRESIMS data were recorded on a VARIAN 910 spectrometer. NMR spectra were recorded on a Bruker AM500 FT-NMR spectrometer operating at 500.13 MHz and 125.76 MHz for 1H NMR and 13C NMR spectroscopy, respectively. 1H NMR chemical shifts were referenced to CHCl3 at 7.27 ppm, and 13C NMR chemical shifts to the central peak of CDCl3 at 77.0 ppm. HMBC measurements were optimized to 7.0 Hz long-range couplings, and NOESY experiments were run with 150 ms mixing time. All chemicals were purchased from Sigma-Aldrich and used without further purification.

Synthetic procedures

Synthesis of the aminolactams 3a and b, rac-4a and rac-4b

To a solution of each amino acid, l-lysine, d-lysine, rac-2a and rac-2b (0.3 mmol), in ethylene glycol (0.1 mL) was added pyridine (0.1 mL). The mixture was irradiated with microwaves at 284 W for 60 min. The corresponding aminolactam compounds were obtained by chromatographic purification on a silica gel (gradient acetone/MeOH). NMR data for 3a and 3b: 1H-NMR (500 MHz, DMSO-d6): 1.17 (1H, m), 1.32 (1H, m), 1.56 (1H, m), 1.68 (2H, m), 1.83 (1H, m), 3.05 (2H, m), 3.41 (1H, dd, J = 1.5 and 11.0 Hz), 7.56 (1H, br. s, NH). 13C-NMR (125 MHz, DMSO-d6): 27.9 (CH2), 29.1 (CH2), 34.1 (CH2), 40.5 (CH2), 52.8 (CH), 177.9 (C Created by potrace 1.16, written by Peter Selinger 2001-2019 O); NMR data for rac-4a: 1H-NMR (500 MHz, DMSO-d6): 1.36–1.67 (m, 3H), 1.95 (m, 1H), 2.90–3.50 (m, 4H), 4.85 (br. s, 1H), 7.55 (br. s, 1H). 13C-NMR (125 MHz, DMSO-d6): 32.5 (CH2), 36.9 (CH2), 47.4 (CH2), 52.6 (CH), 69.3 (CH), 177.8 (C Created by potrace 1.16, written by Peter Selinger 2001-2019 O); NMR data for rac-4b: 1H-NMR (500 MHz, DMSO-d6): 1.44 (m, 1H), 1.58–1.78 (m, 3H), 2.98–3.66 (m, 4H), 7.18 (br. s, NH). 13C-NMR (125 MHz, DMSO-d6): 28.3 (CH2), 34.4 (CH2), 45.2 (CH2), 52.9 (CH), 64.3 (CH), 177.6 (C Created by potrace 1.16, written by Peter Selinger 2001-2019 O).

General procedure for the coupling reaction of lactone 5 with aminolactams

To a solution of each amino lactam, 3a and b, rac-4a and rac-4b (0.5 mmol, 5.0 eq.), in 0.2 mL anhydrous THF were added the ketide 5 (1.0 eq.) and sodium 2-ethylhexanoate (1.2 eq.) under a nitrogen atmosphere. The solution mixture was irradiated with microwaves at 100 W for 60–120 min. The solvent was removed under vacuum. The residue was chromatographed on a silica gel column, eluting with a gradient mixture of CH2Cl2/MeOH to afford the corresponding protected bengamide analogues.

6a 

C21H36N2O6; white amorphous solids; [α]25D –7.0 (c 0.8, CHCl3); ESIMS: m/z 413 [M + H]+; 1H-NMR (500 MHz, CDCl3): 7.78 (1H, d, J = 6.0 Hz, NH-13), 5.91 (1H, br. s, NH-7′), 5.77 (1H, d, J = 16.0 Hz, H-7), 5.56 (1H, dd, J = 7.0 and 16.0 Hz, H-6), 4.58 (1H, ddd, J = 1.5, 6.0 and 10.5 Hz, H-2′), 4.26 (1H, br. d, J = 7.0 Hz, H-5), 4.07 (1H, br. d, J = 6.0 Hz, H-3), 3.88 (1H, d, J = 6.0 Hz, H-2), 3.54 (1H, br. d, J = 7.5 Hz, H-4), 3.51 (3H, OMe-12), 3.39 (1H, d, J = 7.5 Hz, OH), 3.28 (2H, m, CH2-6′), 2.06 (1H, m, Hb-4′), 2.01 (1H, m, Hb-3′), 1.85 (2H, m, Ha-4′, Hb-5′), 1.53 (1H, m, Ha-3′), 1.48 (3H, s, Me-15), 1.45 (3H, s, Me-16), 1.42 (1H, m, Hb-5′), 1.03 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 175.0 (C-1′), 169.5 (C-1), 145.3 (C-7), 121.7 (C-6), 99.7 (C-14), 81.6 (C-2), 74.7 (C-5), 73.3 (C-3), 66.3 (C-4), 59.6 (OMe-12), 51.9 (C-2′), 42.2 (C-6′), 33.1 (C-8), 31.7 (C-3′), 29.5 (Me-15), 29.4 (C-9, 10 and 11), 28.9 (C-5′), 27.9 (C-4′), 19.0 (C-16).

6b 

C21H36N2O6; white amorphous solids; [α]25D +8.8 (c 1.05, CHCl3); ESIMS: m/z 413 [M + H]+; 1H-NMR (500 MHz, CDCl3): 7.55 (1H, d, J = 6.0 Hz, NH-13), 6.05 (1H, t, J = 5.5 Hz, NH-7′), 5.77 (1H, d, J = 16.0 Hz, H-7), 5.52 (1H, dd, J = 6.5 and 16.0 Hz, H-6), 4.59 (1H, br. dd, J = 6.0 and 10.5 Hz, H-2′), 4.27 (1H, br. d, J = 6.5 Hz, H-5), 4.06 (1H, br. d, J = 7.5 Hz, H-3), 3.87 (1H, d, J = 7.5 Hz, H-2), 3.54 (1H, br. s, H-4), 3.48 (3H, s, OMe-12), 3.27 (2H, m, CH2-6′), 2.09 (1H, m, Hb-4′), 2.01 (1H, m, Hb-3′), 1.85 (2H, m, Ha-4′, Hb-5′), 1.65 (1H, m, Ha-3′), 1.46 (3H, s, Me-15), 1.45 (3H, s, Me-16), 1.44 (1H, m, Hb-5′), 1.02 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 175.2 (C-1′), 169.4 (C-1), 145.3 (C-7), 121.6 (C-6), 99.7 (C-14), 80.7 (C-2), 74.5 (C-5), 73.2 (C-3), 65.9 (C-4), 59.2 (OMe-12), 52.0 (C-2′), 42.1 (C-6′), 33.1 (C-8), 31.3 (C-3′), 29.6 (Me-15), 29.4 (C-9, 10 and 11), 28.9 (C-5′), 27.9 (C-6′), 19.1 (C-16).

7a 

C21H36N2O7; white amorphous; ESIMS: m/z 451 [M + Na]+; 1H-NMR (500 MHz, CDCl3): 7.77 (1H, d, J = 6.0 Hz, NH-13), 6.09 (1H, t, J = 6.0 Hz, NH-7′), 5.77 (1H, d, J = 16.0 Hz, H-7), 5.54 (1H, dd, J = 6.5 and 16.0 Hz, H-6), 4.61 (1H, m, H-2′), 4.26 (1H, br. d, J = 6.5 Hz, H-5), 4.05 (1H, br. d, J = 6.5 Hz, H-3), 3.88 (1H, d, J = 6.5 Hz, H-2), 3.62 (1H, m, H-5′), 3.53 (1H, br. s, H-4), 3.50 (3H, s, OMe-12), 3.33 (1H, m, Hb-6′), 3.29 (1H, m, Ha-6′), 2.21 (1H, m, Hb-4′), 2.10 (1H, m, Hb-3′), 1.89 (1H, m, Ha-4′), 1.65 (1H, m, Ha-3′), 1.47 (3H, s, Me-15), 1.45 (3H, s, Me-16), 1.02 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 173.6 (C-1′), 168.8 (C-1), 144.4 (C-7), 120.6 (C-6), 98.7 (C-14), 80.4 (C-2), 73.6 (C-5), 72.3 (C-3), 68.9 (C-5′), 65.2 (C-4), 58.6 (OMe-12), 50.6 (C-2′), 47.0 (C-6′), 35.9 (C-4′), 32.1 (C-8), 28.5 (C-3′), 28.3 (C-9, 10 and 11), 28.6 (C-15), 17.9 (C-16).

7b 

C21H36N2O7; white amorphous; [α]25D +23.4 (c 0.3, CHCl3); ESIMS: m/z 451 [M + Na]+; 1H-NMR (500 MHz, CDCl3): 7.57 (1H, d, J = 6.0 Hz, NH-13), 6.19 (1H, t, J = 6.0 Hz, NH-7′), 5.78 (1H, d, J = 16.0 Hz, H-7), 5.52 (1H, dd, J = 7.0 and 16.0 Hz, H-6), 4.62 (1H, br. dd, J = 6.0 and 9.5 Hz, H-2′), 4.28 (1H, br. d, J = 7.0 Hz, H-5), 4.06 (1H, br. d, J = 7.5 Hz, H-3), 3.88 (1H, d, J = 7.5 Hz, H-2), 3.61 (1H, m, H-5′), 3.53 (1H, br. s, J = 5.5 Hz, H-4), 3.48 (3H, s, OMe-12), 3.35 (1H, m, Hb-6′), 3.28 (1H, m, Ha-6′), 2.22 (1H, m, Hb-4′), 2.13 (1H, m, Hb-3′), 1.88 (1H, m, Ha-4′), 1.63 (1H, m, Ha-3′), 1.46 (3H, s, Me-15), 1.45 (3H, s, Me-16), 1.03 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 174.7 (C-1′), 169.7 (C-1), 145.4 (C-7), 121.5 (C-6), 99.7 (C-14), 80.7 (C-2), 74.4 (C-5), 73.2 (C-3), 69.9 (C-5′), 65.8 (C-4), 59.3 (OMe-12), 51.7 (C-2′), 48.1 (C-6′), 36.8 (C-4′), 33.1 (C-8), 29.6 (C-15), 29.4 (C-9, 10 and 11), 29.3 (C-3′), 19.1 (C-16).

7c 

C21H36N2O7; white amorphous; [α]25D +26.9 (c 0.45, CHCl3); ESIMS: m/z 451 [M + Na]+; 1H-NMR (500 MHz, CDCl3): 7.60 (1H, d, J = 7.0 Hz, NH-13), 6.27 (1H, t, J = 5.0 Hz, NH-7′), 5.78 (1H, d, J = 16.0 Hz, H-7), 5.53 (1H, dd, J = 7.0 and 16.0 Hz, H-6), 4.57 (1H, m, H-2′), 4.28 (1H, br. d, J = 7.0 Hz, H-5), 4.07 (1H, br. d, J = 7.0 Hz, H-3), 4.01 (1H, m, H-5′), 3.90 (1H, d, J = 7.0 Hz, H-2), 3.55 (1H, br. s, H-4), 3.50 (1H, m, Hb-6′), 3.48 (3H, s, OMe-12), 3.35 (1H, m, Ha-6′), 2.05 (1H, m, Hb-4′), 2.00 (1H, m, Ha-4′), 1.89 (1H, m, Hb-3′), 1.84 (1H, m, Ha-3′), 1.46 (3H, s, Me-16), 1.45 (3H, s, Me-15), 1.03 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 175.1 (C-1′), 169.7 (C-1), 145.4 (C-7), 121.5 (C-6), 99.7 (C-14), 80.6 (C-2), 74.5 (C-5), 73.2 (C-3), 65.8 (C-4), 64.7 (C-5′), 59.2 (OMe-12), 51.8 (C-2′), 46.0 (C-6′), 34.6 (C-4′), 33.1 (C-8), 29.6 (C-15), 29.3 (C-9, 10 and 11), 25.1 (C-3′), 19.1 (C-16).

7d 

C21H36N2O7; white amorphous; [α]25D –19.7 (c 0.3, CHCl3); ESIMS: m/z 451 [M + Na]+; 1H-NMR (500 MHz, CDCl3): 7.78 (1H, d, J = 6.0 Hz, NH-13), 6.10 (1H, br. s, H-7′), 5.78 (1H, d, J = 15.5 Hz, H-7), 5.55 (1H, dd, J = 7.0 and 15.5 Hz, H-6), 4.55 (1H, m, H-2′), 4.26 (1H, br. d, J = 7.0 Hz, H-5), 4.06 (1H, br. d, J = 6.5 Hz, H-3), 4.01 (1H, m, H-5′), 3.89 (1H, d, J = 6.5 Hz, H-2), 3.53 (1H, br. s, H-4), 3.51 (1H, m, Hb-6′), 3.50 (3H, s, OMe-12), 3.34 (1H, m, Ha-6′), 2.04 (1H, m, Hb-4′), 2.01 (1H, m, Ha-4′), 1.88 (1H, m, Hb-3′), 1.84 (1H, m, Ha-3′), 1.47 (3H, s, Me-15), 1.45 (3H, s, H-16), 1.03 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 175.0 (C-1′), 169.7 (C-1), 145.4 (C-7), 121.6 (C-6), 99.7 (C-14), 81.4 (C-2), 74.6 (C-5), 73.3 (C-3), 66.3 (C-4), 64.7 (C-5′), 59.6 (OMe-12), 51.7 (C-2′), 46.1 (C-6′), 34.6 (C-4′), 33.1 (C-8), 29.5 (C-15), 29.4 (C-9, 10 and 11), 25.4 (C-3′), 19.0 (C-16).

General procedure for synthesis of compounds 8a and b and 9a–d

A solution of each protected compound, 6a and b and 7a–d, (0.02 mmol) in THF (0.15 mL) was cooled down to 0 °C. To this solution, TFA (8 eq.) and H2O (30 μL) were successively added. The solution was stirred at 0 °C for 1 h, and passed through a Sephadex LH-20 column (eluting with MeOH). The solvent was removed under diminished pressure and the residue was purified by preparative TLC to give the corresponding title compounds.

8a 

C18H32N2O6; white amorphous solid; [α]25D +5.0 (c 0.4, CHCl3); HRESIMS: m/z 395.2135 [M + Na]+ (calcd 395.2158 for C18H32N2O6Na); 1H-NMR (500 MHz, CDCl3): 7.96 (1H, d, J = 6.5 Hz, NH-13), 6.13 (1H, br. s, NH-7′), 5.82 (1H, d, J = 15.5 Hz, H-7), 5.41 (1H, dd, J = 7.5 and 15.5 Hz, H-6), 4.55 (1H, br. dd, J = 6.5 and 10.5 Hz, H-2′), 4.22 (1H, dd, J = 5.0 and 7.5 Hz, H-5), 3.80 (1H, br. d, J = 7.0 Hz, H-3), 3.75 (1H, d, J = 7.0 Hz, H-2), 3.62 (1H, br. d, J = 5.0 Hz, H-4), 3.55 (3H, s, OMe-12), 3.28 (2H, CH2-6′), 2.03 (1H, m, Hb-4′), 2.01 (1H, m, Hb-3′), 1.87 (1H, m, Hb-5′), 1.82 (1H, m, Ha-4′), 1.56 (1H, m, Ha-3′), 1.42 (1H, m, Ha-5′), 1.02 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 174.8 (C-1′), 171.5 (C-1), 145.7 (C-7), 123.2 (C-6), 81.3 (C-2), 74.5 (C-5), 72.7 (C-3), 72.3 (C-4), 59.9 (OMe-12), 51.9 (C-2′), 42.1 (C-6′), 33.0 (C-8), 31.4 (C-3′), 29.4 (C-9, 10 and 11), 28.8 (C-5′), 27.9 (C-4′).

8b 

C18H32N2O6; white amorphous solid; [α]25D +37.7 (c 0.26, CHCl3); HRESIMS: m/z 395.2161 [M + Na]+ (calcd 395.2158 for C18H32N2O6Na); 1H-NMR (500 MHz, CDCl3): 7.88 (1H, br. s, NH-13), 5.84 (1H, d, J = 15.5 Hz, H-7), 5.39 (1H, dd, J = 7.5 and 15.5 Hz, H-6), 4.53 (1H, br. dd, J = 6.5 and 9.5 Hz, H-2′), 4.16 (1H, dd, J = 5.0 and 7.5 Hz, H-5), 3.93 (1H, m, H-3), 3.72 (1H, m, H-2), 3.67 (1H, m, H-4), 3.47 (3H, s, OMe-12), 3.30 (2H, CH2-6′), 2.03 (1H, m, Hb-4′), 2.02 (1H, m, Hb-3′), 1.85 (1H, m, Hb-5′), 1.78 (1H, m, Ha-4′), 1.67 (1H, m, Ha-3′), 1.44 (1H, m, Ha-5′), 1.01 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CDCl3): 175.1 (C-1′), 171.8 (C-1), 146.0 (C-7), 123.3 (C-6), 81.5 (C-2), 74.1 (C-5), 72.7 (C-4), 72.1 (C-3), 59.1 (OMe-12), 52.2 (C-2′), 41.9 (C-6′), 33.0 (C-8), 30.3 (C-3′), 29.4 (C-9, 10 and 11), 28.6 (C-5′), 27.9 (C-4′).

9a 

White amorphous solid; [α]25D +7.0 (c 0.1, MeOH); HRESIMS: m/z 411.2111 [M + Na]+ (calcd 411.2107 for C18H32N2O7Na); 1H-NMR (500 MHz, CD3OD): 5.82 (1H, d, J = 16.0 Hz, H-7), 5.43 (1H, dd, J = 8.0 and 16.0 Hz, H-6), 4.64 (1H, dd, J = 1.5 and 11.5 Hz, H-2′), 4.16 (1H, dd, J = 8.0 and 8.0 Hz, H-5), 3.83 (1H, d, J = 7.0 Hz, H-2), 3.74 (1H, dd, J = 1.5 and 7.0 Hz, H-3), 3.60 (1H, dd, J = 1.5 and 8.0 Hz, H-4), 3.51 (1H, m, H-5′), 3.43 (3H, s, OMe-12), 3.32 (1H, m, Hb-6′), 3.26 (1H, m, Ha-6′), 2.20 (1H, m, Hb-4′), 2.03 (1H, m, Hb-3′), 1.82 (1H, m, Ha-4′), 1.73 (1H, m, Ha-3′), 1.05 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CD3OD): 176.7 (C-1′), 173.1 (C-1), 146.0 (C-7), 125.4 (C-6), 83.6 (C-2), 75.4 (C-5), 74.3 (C-4), 72.7 (C-3), 70.5 (C-5′), 58.8 (OMe-12), 53.0 (C-2′), 48.6 (C-6′), 37.5 (C-4′), 33.7 (C-8), 30.2 (C-3′), 29.8 (C-9, 10 and 11).

9b 

White amorphous solid; [α]25D –20.0 (c 0.1, MeOH); HRESIMS: m/z 411.2103 [M + Na]+ (calcd 411.2107 for C18H32N2O7Na); 1H-NMR (500 MHz, CD3OD): 5.83 (1H, d, J = 16.0 Hz, H-7), 5.42 (1H, dd, J = 7.0 and 16.0 Hz, H-6), 4.58 (1H, dd, J = 6.5 and 11.0 Hz, H-2′), 4.23 (1H, dd, J = 7.0 and 7.0 Hz, H-5), 3.84 (1H, br. d, J = 6.5 Hz, H-3), 3.79 (1H, d, J = 6.5 Hz, H-2), 3.63 (1H, m, H-5′), 3.62 (1H, m, H-4), 3.53 (3H, s, OMe-12), 3.23 (2H, m, CH2-6′), 2.22 (1H, m, Hb-4′), 2.12 (1H, m, Hb-3′), 1.87 (1H, m, Ha-4′), 1.67 (1H, m, Ha-3′), 1.02 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CD3OD): 174.3 (C-1′), 172.2 (C-1), 145.8 (C-7), 123.2 (C-6), 81.4 (C-2), 74.5 (C-5), 72.7 (C-3), 72.5 (C-4), 69.8 (C-5′), 59.8 (OMe-12), 51.7 (C-2′), 48.0 (C-6′), 36.8 (C-4′), 33.0 (C-8), 29.4 (C-9, 10 and 11), 28.9 (C-3′).

9c 

White amorphous solid; [α]25D +16.0 (c 0.1, MeOH); HRESIMS: m/z 411.2108 [M + Na]+ (calcd 411.2107 for C18H32N2O7Na); 1H-NMR (500 MHz, CD3OD): 5.82 (1H, d, J = 15.5 Hz, H-7), 5.43 (1H, dd, J = 7.0 and 15.5 Hz, H-6), 4.59 (1H, br. d, J = 9.0 Hz, H-2′), 4.14 (1H, dd, J = 7.0 and 7.0 Hz, H-5), 3.83 (1H, overlap, H-2), 3.79 (1H, m, H-5′), 3.76 (1H, m, H-3), 3.59 (1H, m, H-4), 3.44 (3H, s, OMe-12), 3.03 (1H, m, Hb-6′), 2.80 (1H, m, Ha-6′), 1.96 (2H, m, CH2-3′), 1.61 (2H, m, CH2-4′), 1.06 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CD3OD): 174.7 (C-1′), 172.2 (C-1), 146.0 (C-7), 125.4 (C-6), 83.4 (C-2), 75.2 (C-5), 74.3 (C-4), 72.6 (C-3), 68.4 (C-5′), 58.7 (OMe-12), 53.1 (C-2′), 45.9 (C-6′), 35.4 (C-3′), 33.8 (C-8), 32.2 (C-4′), 29.9 (C-9, 10 and 11).

9d 

White amorphous solid; [α]25D –35.0 (c 0.1, MeOH); HRESIMS: m/z 411.2107 [M + Na]+ (calcd 411.2107 for C18H32N2O7Na); 1H-NMR (500 MHz, CD3OD): 5.80 (1H, d, J = 15.5 Hz, H-7), 5.42 (1H, dd, J = 7.5 and 15.5 Hz, H-6), 4.59 (1H, m, H-2′), 4.15 (1H, dd, J = 7.5 and 7.5 Hz, H-5), 3.92 (1H, m, H-5′), 3.83 (1H, d, J = 7.5 Hz, H-2), 3.74 (1H, dd, J = 1.7 and 7.5 Hz, H-3), 3.60 (1H, dd, J = 1.7 and 7.5 Hz, H-4), 3.51 (1H, br. d, J = 15.0 Hz, Hb-6′), 3.42 (3H, s, OMe-12), 3.26 (1H, dd, J = 6.5 and 15.0 Hz, Ha-6′), 1.99 (2H, m, CH2-4′), 1.94 (1H, m, Hb-3′), 1.77 (1H, m, Ha-3′), 1.04 (9H, s, Me-9, 10 and 11). 13C-NMR (125 MHz, CD3OD): 176.6 (C-1′), 173.1 (C-1), 146.1 (C-7), 125.4 (C-6), 83.5 (C-2), 75.5 (C-5), 74.2 (C-4), 72.8 (C-3), 65.7 (C-5′), 58.8 (OMe-12), 53.2 (C-2′), 46.5 (C-6′), 35.4 (C-4′), 33.8 (C-8), 29.8 (C-9, 10 and 11), 25.9 (C-3′).

Cytotoxic activity assay

Cytotoxicity assays were carried out in triplicate in 96-well microtiter plates against KB, HepG2, LU, MCF7, HL60, and Hela. Cells were maintained in Dulbecco's D-MEM medium, supplemented with 10% fetal calf serum, l-glutamine (2 mM), penicillin G (100 UI mL–1), streptomycin (100 μg mL–1) and gentamicin (10 μg mL–1). Stock solutions of the compounds were prepared in DMSO/H2O (1/9), and the cytotoxicity assays were carried out in 96-well microtiter plates against cancer or normal cells (3 × 103 cells per mL) using a modification of the published method.30 After 72 h of incubation at 37 °C in air/CO2 (95 : 5) with or without the test compounds, cell growth was estimated by colorimetric measurement of stained living cells using neutral red. Optical density was determined at 540 nm with a Titertek Multiscan photometer. The IC50 value was defined as the concentration of sample necessary to inhibit cell growth to 50% of the control. Ellipticine was used as a reference compound.

Supplementary Material

Click here for additional data file. (7.2MB, pdf)

Acknowledgments

The authors thank the National Foundation for Science and Technology (NAFOSTED) and the Ministry of Science and Technology of Vietnam (MOST) for financial support (NCCB-ĐHUD.2012-G/05).

Footnotes

†The authors declare no competing interests.

‡Electronic supplementary information (ESI) available: HRESIMS, 1D and 2D NMR spectra of compounds 3a and b, rac-4a and b, 6a and b, 7a–d, 8a and b, and 9a–d. See DOI: 10.1039/c6md00587j

References

  1. Newman D. J., Cragg G. M. J. Nat. Prod. 2012;75:311–335. doi: 10.1021/np200906s. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Bauer A., Bronstrup M. Nat. Prod. Rep. 2014;31:35–60. doi: 10.1039/c3np70058e. [DOI] [PubMed] [Google Scholar]
  3. Butler M. S., Robertson A. A., Cooper M. A. Nat. Prod. Rep. 2014;31:1612–1661. doi: 10.1039/c4np00064a. [DOI] [PubMed] [Google Scholar]
  4. Blunt J. W., Copp B. R., Keyzers R. A., Munro M. H., Prinsep M. R. Nat. Prod. Rep. 2015;32:116–211. doi: 10.1039/c4np00144c. [DOI] [PubMed] [Google Scholar]
  5. Molinski T. F., Dalisay D. S., Lievens S. L., Saludes J. P. Nat. Rev. Drug Discovery. 2009;8:69–85. doi: 10.1038/nrd2487. [DOI] [PubMed] [Google Scholar]
  6. Quinoa E., Adamczeski M., Crews P., Bakus G. J. J. Org. Chem. 1986;51:4494–4497. [Google Scholar]
  7. Thale Z., Kinder F. R., Bair K. W., Bontempo J., Czuchta A. M., Versace R. W., Phillips P. E., Sanders M. L., Wattanasin S., Crews P. J. Org. Chem. 2001;66:1733–1741. doi: 10.1021/jo001380+. [DOI] [PubMed] [Google Scholar]
  8. Weltchek S., Wood A., Zabludoff S. D., Phillips P. E. Proc. Am. Assoc. Cancer Res. 2000;41:600. [Google Scholar]
  9. Groweiss A., Newcomer J. J., O'Keefe B. R., Blackman A., Boyd M. R. J. Nat. Prod. 1999;62:1691–1693. [Google Scholar]
  10. Zhang W., Liang Q., Li H., Meng X., Li Z. Tetrahedron. 2013;69:664–672. [Google Scholar]
  11. Liu Q. J., Li H., Chen S. P., Zhou G. C. Chin. Chem. Lett. 2011;22:505–507. [Google Scholar]
  12. Sarabia F., Martín-Gálvez F., García-Ruiz C., Sánchez-Ruiz A., Vivar-García C. J. Org. Chem. 2013;78:5239–5253. doi: 10.1021/jo4003272. [DOI] [PubMed] [Google Scholar]
  13. Sarabia F., Sánchez-Ruiz A. J. Org. Chem. 2005;70:9514–9520. doi: 10.1021/jo0516032. [DOI] [PubMed] [Google Scholar]
  14. Sarabia F., Sánchez-Ruiz A. Tetrahedron Lett. 2005;46:1131–1135. [Google Scholar]
  15. Martín-Gálvez F., García-Ruiz C., Sánchez-Ruiz A., Valeriote F. A., Sarabia F. ChemMedChem. 2013;8:819–831. doi: 10.1002/cmdc.201300033. [DOI] [PubMed] [Google Scholar]
  16. Kim S., LaMontagne K., Sabio M., Sharma S., Versace R. W., Yusuff N., Phillips P. E. Cancer Res. 2004;64:2984–2987. doi: 10.1158/0008-5472.can-04-0019. [DOI] [PubMed] [Google Scholar]
  17. Liu G., Ma Y. M., Tai W. Y., Xie C. M., Li Y. L., Li J., Nan F. J. ChemMedChem. 2008;3:74–78. doi: 10.1002/cmdc.200700214. [DOI] [PubMed] [Google Scholar]
  18. Tai W. Y., Zhang R. T., Ma Y. M., Gu M., Liu G., Li J., Nan F. J. ChemMedChem. 2011;6:1555–1558. doi: 10.1002/cmdc.201100164. [DOI] [PubMed] [Google Scholar]
  19. Lu J. P., Yuan X. H., Yuan H., Wang W. L., Wan B., Franzblau S. G., Ye Q. Z. ChemMedChem. 2011;6:1041–1048. doi: 10.1002/cmdc.201100003. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Kinder Jr. F. R., Versace R. W., Bair K. W., Bontempo J. M., Cesarz D., Chen S., Crews P., Czuchta A. M., Jagoe C. T., Mou Y., Nemzek R., Phillips P. E., Tran L. D., Wang R. M., Weltchek S., Zabludoff S. J. Med. Chem. 2001;44:3692–3699. doi: 10.1021/jm010188c. [DOI] [PubMed] [Google Scholar]
  21. Xu D. D., Waykole L., Calienni J. V., Ciszewski L., Lee G. T., Liu W., Szewczyk J., Vargars K., Prasad K., Repic O., Blacklock T. J. Org. Process Res. Dev. 2003;7:856–865. [Google Scholar]
  22. Dumez H., Gall H., Capdeville R., Dutreix C., van Oosterom A. T., Giaccone G. Anti-Cancer Drugs. 2007;18:219–225. doi: 10.1097/CAD.0b013e328010ef5b. [DOI] [PubMed] [Google Scholar]
  23. Banwell M. G., McRae K. J. J. Org. Chem. 2001;66:6768–6774. doi: 10.1021/jo0159486. [DOI] [PubMed] [Google Scholar]
  24. Pellegata R., Pinza G., Pifferi G. Synthesis. 1978:614–616. [Google Scholar]
  25. Front B. Tetrahedron Lett. 1980;21:2443–2446. [Google Scholar]
  26. Matsumoto H., Kaiso K. Jpn. Kokai Tokkyo Koho. 2012:JP 2012162463. [Google Scholar]
  27. Xu W., Lu J. P., Ye Q. Z. J. Med. Chem. 2012;55:8021–8027. doi: 10.1021/jm3008695. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Griffith E. C., Su Z., Turk B. E., Chen S., Chang Y. H., Wu Z., Biemann K., Liu J. O. Chem. Biol. 1997;4:461–471. doi: 10.1016/s1074-5521(97)90198-8. [DOI] [PubMed] [Google Scholar]
  29. Hu X., Addlagatta A., Lu J., Matthews B. W., Liu J. O. Proc. Natl. Acad. Sci. U. S. A. 2006;103:18148–18153. doi: 10.1073/pnas.0608389103. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Mosmann T. J. Immunol. Methods. 1983;65:55–63. doi: 10.1016/0022-1759(83)90303-4. [DOI] [PubMed] [Google Scholar]

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Supplementary Materials

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