Molecular-docking-guided design, palladium-catalyzed synthesis and anticancer activity of paclitaxel-benzoxazoles hybrids.

A series of new paclitaxel-benzoxazoles hybrids were designed based on both the molecular docking mode of beta-tubulin with paclitaxel derivatives (7a and 7g), and the activity-structure relationship of C-13 side chain in paclitaxel. Palladium-catalyzed direct Csp2-H arylation of benzoxazoles with different aryl-bromides was used as the key synthetic strategy for the aryl-benzoxazoles moieties in the hybrids. Twenty-six newly synthesized hybrids were screened for their antiproliferative activity against human cancer cell lines such as human breast cancer cells (MDA-MB-231) and liver hepatocellular cells (HepG2) by the MTT assay and results were compared with paclitaxel. Interestingly, most hybrids (7a-7e, 7i, 7k, 7l, 7A, 7B, 7D and 7E) showed significantly active against both cell lines at concentration of 50 µM, which indicated that the hybrid strategy is effective to get structural simplified paclitaxel analogues with high anti-tumor activity.

Introduction

Cancer, one of the most formidable common diseases, remains threat to human health and is responsible for increase in the mortality rate all over the world. It has been estimated that close to 550,000 deaths caused by this disease according to the epidemiological and clinical investigations[1-3]. Although many effective approaches including radiation, surgery and targeted therapy have been exploited to cure for cancer, natural products or its derivatives have been of particular interest as cancer chemotherapeutic agents in the past few decades. Up to date, more than 100 anticancer agents with varied mechanisms of action have been developed from diverse natural origin[4-6]. Paclitaxel (Taxol®), the naturally diterpenoid extract from the bark of Taxus brevifolia Nutt, and the semisynthetic analogues docetaxel (Taxotere®) and cabazitaxel (Jevana®) (Fig. 1), act as the momentous chemotherapeutics in current clinical treatment of breast cancer, non-small cell lung cancer, ovarian cancer and prostate cancer[7]. They accelerate the irreversible assembly of tubulin into microtubules and thus stimulate the apoptosis of tumor cells through disrupting mitosis to exert their therapeutic effect[8,9]. Since the poor solubility, undesirable side effects and drug resistance have immensely limited the clinical application of paclitaxel, and the highly complex structure have prevented large-scale producing paclitaxel as well[10,11]. Therefore, researchers have maintained a considerable interest in the discovery and development of novel anticancer drugs because of the aforementioned disadvantages of the paclitaxel.

Figure 1

Structures of paclitaxel, docetaxel, and cabazitaxel.

Hybrids are based on the principle of combining partial or whole structures in order to create new and possibly more safe and active molecular entities[12]. Some studies revealed that paclitaxel-natural-products based hybrids were potential agents which could be possible to extend and strengthen the medical utility of paclitaxel[13,14]. Numerous structure–activity- relationship studies (SARs) indicated that the C-13 side chain is an indispensable part for its antitumor activity[15-19]. In our previous studies, paclitaxel mimics possessing C-13 side chains showed moderate antitumor activities[19-21]. Literature data has been established that nitrogen containing heterocyclic moiety plays an important role in designing new class of structural entities for medicinal applications[22]. Therefore, we were encouraged to design and synthesize paclitaxel-nitrogen containing heterocyclic moiety hybrids in which the intricate baccatin-core is structurally replaced by simplified structures, to find new mimics with low side effects and high efficacy. Initially, docking studies of a series of paclitaxel-nitrogen containing heterocyclic moiety hybrids with beta-tubulin were carried out. Paclitaxel-benzoxazoles hybrids 7a and 7g were revealed to have good biding affinity with beta-tubulin, indicating this type of hybrids may be potential anticancer lead compounds. Moreover, some benzoxazole derivatives also are reported as anticancer agents[23,24]. Thus, as an ongoing part of our research on paclitaxel analogues[19-21], in current research, twenty-six paclitaxel-benzoxazoles hybrids were designed guided by molecular docking study. The aryl-benzoxazoles moieties of hybrids were synthesized via palladium-catalyzed direct Csp2–H arylation of benzoxazoles with different aryl-bromides. The biological activities of twenty-six hybrids were evaluated.

Results and discussion

Molecular docking study

It was believed that the simplified paclitaxel analogues with better antitumor activities could accelerate the polymerization of tubulin and stabilize the resultant microtubules to apoptosis through cell-signaling cascade either. In our continued work on finding paclitaxel analogues with better antitumor activities, the docking studies of a series of paclitaxel-nitrogen containing heterocyclic moiety hybrids with beta-tubulin were carried out. To our delight, the paclitaxel-benzoxazoles hybrids 7a and 7g showed good binding affinity with beta-tubulin. -Cdocker Interaction Energy values of compounds 7a and 7g with beta-tubulin were 52.9245 and 54.6571 kcal/mol, respectively, along with -Cdocker Energy values of them were 38.4268 and 38.2471 kcal/mol, respectively.

An overlay of the structures for compounds 7a, 7g and positive control paclitaxel was shown in Fig. 2. These three molecules embedded in 6I2I cavity and the binding region of compounds 7a and 7g was consistent with that of paclitaxel. The oxygen atom of the ester carbonyl in paclitaxel established the hydrogen bond interaction with residue Asp 226. However, Asp 226 didn’t interact with the other compounds, which could be caused by the greatly variation of structures.

Figure 2

An overlay of the structures for compounds 7a, 7g and paclitaxel. *Compounds 7a and 7g in purple carbon atoms; paclitaxel in yellow carbon atoms.

The binding model of 7a and 6I2I protein was depicted in Fig. 3. The docking result showed that eight amino acids Asn 101, Thr 145, Gln 11, Asp 179, Tyr 224, Cys 12, Leu 227 and Val 171 located in the binding pocket of protein played vital roles in the combination with compound 7a. The Pi-Pi stacking and Pi-alkyl bonds were formed between benzoxazoles of docking molecule and Tyr 224, Cys 12, respectively. The other three Pi-alkyl bonds were formed between the benzene ring locating at benzoxazoles and Cys 12, Leu 227 and Val 171. The portion of side chains deriving from paclitaxel made the hydrogen bonds with Asn 101 and Gln 11. On the other hand, a Pi-anion bond and van der waals as well formed between unique side chains and Asp 179, Thr 145, respectively.

Figure 3

The molecular docking result and pattern of compound 7a and beta-tubulin.

The molecular interactions for compound 7g within the 6I2I active site was showed in Fig. 4. The docking results suggested that 7g made hydrogen bonds with two amino acids Asn 18 and Asn 228. Besides, the molecule established the carbon hydrogen bonds interactions with amino acids Gln 15. Pyridine ring and one of benzene ring established Pi-alkyl bonds with Val 78 and Tyr 224. The above molecular docking result in molecular level foundation revealed that the paclitaxel-benzoxazoles hybrids could inhibit the cancer cells proliferation. Therefore, a series of paclitaxel-benzoxazoles hybrids were designed and synthesized using palladium-catalyzed direct coupling reaction to explore the potential inhibitors as therapy for cancer.

Figure 4

The molecular docking result and pattern of compound 7g and beta-tubulin.

Synthetic chemistry

The synthetic route to target paclitaxel-benzoxazoles hybrids 7a–7m and 7A–7M were illustrated in Scheme 1. The first task was to prepare the key benzoxazoles intermediates 3a–3m. Starting from the known compound 1, masking of the free hydroxy group as a benzyl ether or silyl ether provided precursors 2a or 2b in 87% or 89% yield, respectively. The key intermediates 3a–3m were obtained by palladium-catalyzed direct Csp2–H arylation of benzoxazoles 2a or 2b with different aryl-bromides. The coupling reaction was carried out under the catalytic system of Pd(OAc)2/Nixantphos/NaOtBu in DME at room temperature and furnished the desired compounds 3a–3m in 62–89% yields. Removement of benzyl ether or silyl ether of the benzoxazoles intermediates 3a–3m resulted in the formation of benzoxazoles 4a–4m with free hydroxy group in 72–92% yields. Next, we focused on synthesizing the paclitaxel-benzoxazoles hybrids. Esterification of 4a–4m with purchased oxazolidinecarboxylic acids 5a or 5b resulted in the formation of oxazolidinecarboxylate 6a–6m and 6A–6M (Scheme 1). Treatment of the intermediates 6a–6m and 6A–6M with p-TsOH yielded twenty-six paclitaxel-benzoxazoles hybrids 7a–7m and 7A–7M in 40–94% yields.

Scheme 1

Synthesis of the paclitaxel-benzoxazoles hybrids 7a–7m and 7A–7M.

Inhibitory effects on the proliferation of MDA-MB-231 and HepG2 for paclitaxel-benzoxazoles hybrids

At the beginning of the assessment, the inhibition of the synthesized paclitaxel-benzoxazoles hybrids (7a–7m and 7A–7M) on human breast cancer cells (MDA-MB-231) and liver hepatocellular cells (HepG2) was evaluated by MTT methods using paclitaxel as a control. The results revealed that most derivatives exhibited favorable antiproliferative activities against MDA-MB-231 and HepG2 cells at a concentration of 50 µM (Table 1). As shown in Table 1, Compounds 7a–7e, 7i, 7k, 7l, 7A, 7B, 7D and 7E showed better activities than positive control paclitaxel against both MDA-MB-231 and HepG2 cell lines at the high concentration. Among of them, compounds 7a, 7h, 7k, 7B, 7H and 7I showed antiproliferative effect of cell growth of MDA-MB-231 over 80%. Compounds 7a, 7d, 7h, 7k, 7B, 7E and 7I as well displayed superior inhibitory activity on HepG2 cells. Particularly, compared with paclitaxel (inhibition ratio of 27.4% and 69.8%, respectively), compound 7k showed a better inhibition ratio with 93.2% and 94.5% against MDA-MB-231 and HepG2, respectively. Furthermore, hybrids 7h and 7B exhibited better activities against HepG2 cells (97.1% and 92.3%, respectively) than MDA-MB-231 cells (82.1% and 83.7%, respectively).

Table 1

Antiproliferative ratio of paclitaxel-benzoxazoles hybrids 7a–7m and 7A–7M against two cancer cell lines at 50 µM.

Compd.	Antiproliferative ratio on MDA-MB-231 (%)	Antiproliferative ratio on HepG2 (%)	Compd.	Antiproliferative ratio on MDA-MB-231 (%)	Antiproliferative ratio on HepG2 (%)
7a	81.6	80.2	7A	73.7	71.6
7b	70.9	71.5	7B	83.7	92.3
7c	59.7	79.1	7C	38.4	50.2
7d	71.3	80.1	7D	77.1	75.8
7e	78.5	79.9	7E	65.9	84.0
7f.	66.7	58.8	7F	70.9	57.1
7g	13.0	8.0	7G	76.9	14.0
7 h	82.1	97.1	7H	85.4	5.8
7i	54.3	77.3	7I	82.1	5.7
7j	19.4	0.6	7J	70.7	27.2
7k	93.2	94.5	7k	41.0	68.1
7l	63.2	81.0	7L	42.1	56.9
7m	23.5	40.6	7M	29.5	70.6
Paclitaxel	27.4	69.8

According to the desirable effect of above synthetics on the 2 cancer cell lines, the compounds with antiproliferative activity over 70% at 50 µM were tested for their half maximal inhibitory concentration (IC50) (Fig. 5). The potential inhibitory activities expressed as IC50 values for all compounds were shown in Tables 2 and 3. It is regrettable that the IC50 value of all hybrids both against MDA-MB-231 and HepG2 were equally much higher than those of paclitaxel (IC50 values of 0.32 ± 0.08 and 0.78 ± 0.09 μM, respectively). According to the results, it is supposed that the antiproliferative activities of synthesized hybrids had dose-dependent effect with concentration. Nonetheless, IC50 values of compound 7A against MDA-MB-231 cells was 21.7 ± 0.8 µM, better than that of the others. And compounds 7a, 7e, 7k and 7E (IC50 values of 25.6 ± 0.9, 28.1 ± 1.9, 29.5 ± 1.1 and 27.8 ± 0.6 μM, respectively) exhibited moderate anticancer activity on MDA-MB-231. On the other hand, compounds 7c, 7i and 7k possessed the IC50 values of 27.2 ± 1.4, 23.6 ± 1.4 and 28.1 ± 2.1 μM, respectively to exhibit anti-proliferative activities on HepG2. Hybrid 7a (IC50 = 17.6 ± 0.8 μM) had good cytotoxic activities compared to other derivates.

Figure 5

Dose–response analysis of cell growth inhibition activity against MDA-MB-231 and HepG2.

Table 2

Antiproliferative activities of selected paclitaxel-benzoxazoles hybrids against MDA-MB-231.

Compd.	Percentage of antiproliferation against MDA-MB-231					IC50 (µM)
	3.125 µM	6.25 µM	12.5 µM	25 µM	50 µM
7a	6.8 ± 0.6	12.7 ± 0.3	23.1 ± 0.8	43.7 ± 0.5	80.2 ± 0.8	25.6 ± 0.9
7b	2.2 ± 0.4	6.1 ± 0.8	13.2 ± 0.4	20.7 ± 0.5	66.4 ± 0.6	38.8 ± 1.2
7c	6.9 ± 1.4	18.3 ± 1.0	22.9 ± 0.9	32.1 ± 0.5	55.4 ± 0.6	44.9 ± 1.7
7d	3.4 ± 0.4	6.3 ± 1.1	15.0 ± 0.6	25.8 ± 0.7	66.5 ± 0.5	37.4 ± 2.3
7e	8.4 ± 0.7	10.3 ± 0.9	21.2 ± 1.4	38.3 ± 2.3	78.4 ± 1.8	28.1 ± 1.9
7h	5.9 ± 0.8	7.8 ± 0.5	16.0 ± 0.6	32.6 ± 0.4	72.7 ± 0.5	35.5 ± 0.8
7i	5.8 ± 1.5	16.8 ± 1.0	21.8 ± 1.8	30.3 ± 1.9	54.8 ± 0.8	48.4 ± 2.4
7k	4.1 ± 0.6	10.0 ± 0.9	15.5 ± 0.6	32.5 ± 1.3	86.0 ± 1.0	29.5 ± 1.1
7l	15.7 ± 1.1	19.1 ± 0.4	30.5 ± 2.1	41.5 ± 1.2	56.3 ± 1.9	37.2 ± 1.6
7A	16.8 ± 0.9	18.1 ± 2.3	20.0 ± 1.8	62.5 ± 0.3	75.0 ± 2.1	21.7 ± 0.8
7B	12.5 ± 0.7	15.1 ± 1.2	18.6 ± 1.8	31.7 ± 1.5	70.5 ± 1.7	33.2 ± 1.3
7D	8.0 ± 1.1	12.6 ± 0.7	19.3 ± 1.4	41.0 ± 2.0	70.2 ± 0.9	31.2 ± 1.5
7E	17.7 ± 1.2	21.6 ± 0.5	31.0 ± 0.4	46.8 ± 0.5	65.4 ± 0.8	27.8 ± 0.6
Paclitaxel	–	–	–	–	–	0.32 ± 0.08

Table 3

Antiproliferative activities of selected paclitaxel-benzoxazoles hybrids against HepG2.

Compd.	Percentage of antiproliferation against HepG2					IC50 (µM)
	3.125 µM	6.25 µM	12.5 µM	25 µM	50 µM
7a	10.5 ± 0.7	15.7 ± 0.9	39.1 ± 0.3	63.3 ± 1.1	77.5 ± 0.6	17.6 ± 0.8
7b	3.2 ± 1.5	4.6 ± 1.1	8.5 ± 1.7	20.7 ± 1.0	62.9 ± 0.8	43.8 ± 1.3
7c	18.9 ± 0.6	28.3 ± 0.9	30.7 ± 1.8	42.4 ± 0.9	68.8 ± 1.1	27.2 ± 1.4
7d	8.4 ± 1.2	15.8 ± 1.5	23.9 ± 0.9	35.9 ± 0.8	74.6 ± 0.5	31.8 ± 1.6
7e	4.4 ± 1.3	8.3 ± 0.4	20.2 ± 0.5	30.9 ± 1.8	69.1 ± 1.2	37.0 ± 2.6
7h	4.9 ± 0.7	9.8 ± 0.8	18.1 ± 1.5	38.7 ± 1.6	81.7 ± 0.9	30.6 ± 2.0
7i	5.8 ± 2.1	16.9 ± 1.9	28.9 ± 1.7	47.3 ± 0.3	79.0 ± 0.6	23.6 ± 1.4
7k	10.1 ± 0.4	13.0 ± 0.7	24.5 ± 0.9	39.5 ± 1.5	82.0 ± 1.8	28.1 ± 2.1
7l	7.1 ± 2.6	11.1 ± 1.6	18.5 ± 0.4	30.5 ± 0.6	75.0 ± 0.9	34.5 ± 0.6
7A	14.9 ± 1.3	20.2 ± 1.5	25.1 ± 0.6	34.9 ± 1.4	71.9 ± 1.6	33.2 ± 1.9
7B	1.6 ± 1.9	3.9 ± 2.4	7.7 ± 2.5	22.7 ± 2.3	68.2 ± 1.7	41.8 ± 2.7
7D	3.3 ± 1.2	9.6 ± 0.9	16.2 ± 1.0	31.5 ± 0.5	72.4 ± 0.7	39.9 ± 0.5
7E	1.7 ± 1.4	9.65 ± 1.2	16.1 ± 2.1	42.8 ± 0.7	78.3 ± 1.1	32.0 ± 1.7
Paclitaxel	–	–	–	–	–	0.78 ± 0.09

Conclusion

In summary, twenty-six new paclitaxel-benzoxazoles hybrids bearing only a C-13 side chain of paclitaxel were designed on both the SAR of paclitaxel and the molecular docking study. The newly hybrids were rapidly prepared through five simple reactions. The key synthetic strategy for the aryl-benzoxazoles moieties in the hybrids was palladium-catalyzed direct Csp2–H arylation of benzoxazoles with different aryl-bromides. At the concentration of 50 µM, most compounds showed moderate to good antiproliferative activity against human breast cancer cells (MDA-MB-231) and liver hepatocellular cells (HepG2). Some of the compounds (7a–7e, 7i, 7k, 7l, 7A, 7B, 7D and 7E) at this concentration exhibited stronger activity against the two cell lines than paclitaxel. Unfortunately, the IC50 value of potential inhibitor were higher than those of paclitaxel. It is supposed that synthesized hybrids exhibited antiproliferative activities in a dose-dependent manner. Among of them, the optimal compound 7a showed the best therapeutic potential to inhibit HepG2 cell growth with the IC50 values of 17.6 ± 0.8 µM. Owing to the remarkable cytotoxicity, 7a needs an in-depth investigation to improve in terms of reducing dose-dependent effect, which might assist in the development of anticancer agents in the future. The results suggests that the use of baccatin-free hybrid component might be an effective strategy to establish paclitaxel-based hybrid library to find lead compounds against cancer.

Experimental

General experimental procedures

1H NMR spectra were recorded on a Bruker AV 400 or 600 nuclear magnetic resonance instrument (400 or 600 MHz). Chemical shifts were recorded in ppm relative to tetramethylsilane as the internal standard. 13C NMR data were collected on a Bruker AV 400 or 600 nuclear magnetic resonance instrument (100 or 150 MHz) with complete proton decoupling. Chemical shifts are reported in ppm with tetramethylsilane as the internal standard. HRESIMS were determined using a Waters Acquity UPLC/Xevo G2-S QTof mass spectrometer. Thin-layer chromatography silica gel GF254 plates and silica gel G (200–400 mesh) for column chromatography were purchased from Qingdao Ocean Chemical Plant (Qingdao, People's Republic of China). Unless otherwise specified, the reagents and solvents used in this work were all commercially available analytical or chemical grades, and used directly without any purification.

The crystallographic structures of tubulin (PDB ID: 6I2I) was chosen as the template for the modeling study of compounds. The pdb file about the crystal structure of refined 13pf Hela Cell tubulin microtubule (6I2I.pdb) was obtained from the Protein Date Bank at http://www.rcsb.org/. The structures of compound were drawn by ChemBioDraw software and converted to mol file. The ligands and bound water were removed from the protein and the polar hydrogen was added. The energy minimized structures of tubulin (6I2I.pdb) and the ligand were generated by using prepare protein module and prepare ligand module, respectively. The whole tubulin complex was defined as a receptor. The molecular docking procedure was performed by using CDOCKER protocol for receptor-ligand interactions section of Discovery Studio Client software. Then, the lowest energy configuration of docking molecule was docked into 20 different binding sites of the prepared protein molecule using the standard parameters of Discovery Studio throughout the simulation. The preferred site was determined calculating binding energy values, and types of interactions of the docked protein with ligand were analyzed after the end of molecular docking.

Preparation of compound 2a

To a solution of the known compound 1 (1.00 g, 5.58 mmol, 1.0 eq.) in dry DMF (20 mL) was added NaH (201 mg, 8.37Mmol, 1.5 eq.) at 0 °C. After 10 min, a solution of BnBr (0.80 mL, 6.70 mmol, 1.2 eq.) in DMF (5 mL) was added to the mixture dropwise. The whole mixture was stirred at 0 °C for 5 h, then quenched with saturated aqueous NaHCO3. After extraction with EtOAc (20 mL × 3), the combined organic layers were washed with H2O (20 mL × 3) and brine (30 mL), dried over Na2SO4, filtered and concentrated to dryness. Purification of the crude product via chromatography on silica gel (petroleum ether: EtOAc = 10:1) afforded compound 2a (1.31 g, 87% yield) as yellow oil.

Preparation of compound 2b

The known compound 1 (1.00 g, 5.58 mmol, 1.0 eq.) was dissolved in DCM (20 mL). Et3N (2.33mL, 16.74mmol, 3.0 eq.), TBSCl (1.56ML, 8.37Mmol, 1.5 eq.) and DMAP (341 mg, 2.79 mmol, 0.5 eq.) were added to the solution at ambient temperature. The reaction mixture was stirred at ambient temperature for 1.5 h, then quenched by H2O (10 mL). The layers were separated and the aqueous phase was extracted with DCM (5 mL × 2). The combined organic layers were washed with H2O (15 mL) and brine (10 mL), dried over Na2SO4, filtered, concentrated under reduced pressure led to a crude product. Flash column chromatography (petroleum ether: EtOAc = 50:1) gave the desired product 2b (1.46 g, 89% yield) as colorless oil.

General procedure for the synthesis of compounds 3a–3m

The mixture of Pd(OAc)2 (5 mol %) and NiXantPhos (7.5 mol %) in anhydrous DME (2.0 mL) was stirred at 25 °C under an argon atmosphere for 4 h to be a dark brown solution. Then, the dark brown solution was added to the compound 2a or 2b (0.50 mmol), arylbromides (0.60 mmol) and NaOtBu (1.20 mmol) in in anhydrous DME (1.0 mL) dropwise via syringe. The reaction mixture was stirred for 12 h at 25 °C under an argon atmosphere. Then, the reaction mixture was quenched by some drops of H2O, diluted with EtOAc (3.0 mL), dried over MgSO4, and filtered over a pad celite. The filtrate was concentrated in vacuo. The crude residue was purified by silica gel column chromatography (petroleum ether/EtOAc) to obtain the desired compounds 3a−3m as colorless oil, yield 62 − 89%.

Compound 3a, colorless oil; yield 72%; 1H NMR (400 MHz, CDCl3) δ 8.30 (2H, d, J = 8.3 Hz), 7.54–7.48 (3H, m), 7.42–7.32 (4H, m), 7.31 (1H, t, J = 8.0 Hz), 7.26–7.19 (2H, m), 6.88 (1H, d, J = 8.0 Hz), 4.68 (2H, s), 4.57–4.55 (2H, m), 3.99–3.96 (2H, m); HRESIMS (m/z): calcd. for C22H20NO3, 346.1443; found 346.1421 [M + H]+.

Compound 3b, colorless oil; yield 79%; 1H NMR (400 MHz, CDCl3) δ 8.25 (2H, d, J = 8.3 Hz), 7.55–7.49 (2H, m), 7.42–7.28 (5H, m), 7.25–7.16 (2H, m), 6.87 (1H, d, J = 8.0 Hz), 4.67 (2H, s), 4.56–4.54 (2H, m), 4.01–3.94 (2H, m), 1.37 (9H, s); HRESIMS (m/z): calcd. for C26H28NO3, 402.2069; found 406.2058 [M + H]+.

Compound 3c, colorless oil; yield 84%; 1H NMR (400 MHz, CDCl3) δ 8.21 (2H, d, J = 9.0 Hz), 7.24–7.14 (2H, m), 7.04–6.97 (2H, m), 6.86 (1H, dd, J = 7.9, 0.9 Hz), 4.40 (2H, t, J = 5.8 Hz), 4.10 (2H, t, J = 5.8 Hz), 3.88 (3H, s), 0.91 (9H, s), 0.12 (6H, s); HRESIMS (m/z): calcd. for C22H30NO4Si, 400.1944; found 400.1965 [M + H]+.

Compound 3d, colorless oil; yield 62%; 1H NMR (400 MHz, CDCl3) δ 8.23 (2H, d, J = 8.2 Hz), 7.41–7.38 (2H, m), 7.37–7.33 (2H, m), 7.31 (1H, d, J = 8.0 Hz), 7.26–7.17 (4H, m), 6.87 (1H, d, J = 8.0 Hz), 4.67 (2H, s), 4.56–4.52 (2H, m), 4.00–3.95 (2H, m); HRESIMS (m/z): calcd. for C22H18NO3F, 364.1349; found 364.1326 [M + H]+.

Compound 3e, colorless oil; yield 68%; 1H NMR (400 MHz, CDCl3) δ 8.39 (2H, d, J = 8.3 Hz), 7.77 (2H, d, J = 8.4 Hz), 7.41–7.28 (6H, m), 7.23 (1H, d, J = 8.0 Hz), 6.89 (1H, d, J = 8.0 Hz), 4.68 (2H, s), 4.57–4.53 (2H, m), 3.99–3.97 (2H, m); HRESIMS (m/z): calcd. for C23H19NO3F3, 414.1317; found 414.1329 [M + H]+.

Compound 3f, colorless oil; yield 74%; 1H NMR (400 MHz, CDCl3) δ 8.79 (1H, s), 8.35 (1H, dd, J = 8.6, 1.7 Hz), 7.97 (2H, t, J = 9.1 Hz), 7.92–7.87 (1H, m), 7.60–7.54 (2H, m), 7.42–7.33 (4H, m), 7.32–7.27 (2H, m), 7.26–7.23 (1H, m), 6.89 (1H, dd, J = 7.5, 1.5 Hz), 4.69 (2H, s), 4.60–4.56 (2H, m), 4.02–3.98 (2H, m); HRESIMS (m/z): calcd. for C26H22NO3, 396.1600; found 396.1614 [M + H]+.

Compound 3g, colorless oil; yield 81%; 1H NMR (400 MHz, CDCl3) δ 9.47 (1H, d, J = 1.6 Hz), 8.74 (1H, dd, J = 4.8, 1.6 Hz), 8.52 (1H, dt, J = 8.0, 1.9 Hz), 7.45 (1H, ddd, J = 8.0, 4.9, 0.6 Hz), 7.29 (1H, t, J = 8.1 Hz), 7.23–7.20 (1H, m), 6.90 (1H, d, J = 7.8 Hz), 4.42 (2H, t, J = 5.6 Hz), 4.11 (2H, t, J = 5.6 Hz,), 0.91 (9H, s), 0.12 (6H, s); HRESIMS (m/z): calcd. for C20H27N2O3Si, 371.1791; found 371.1804 [M + H]+.

Compound 3h, colorless oil; yield 89%; 1H NMR (400 MHz, CDCl3) δ 8.46 (1H, d, J = 2.7 Hz), 8.38 (1H, d, J = 8.7 Hz), 7.34 (1H, dd, J = 8.8, 2.9 Hz), 7.28 (1H, d, J = 8.2 Hz), 7.25 (1H, d, J = 6.1 Hz), 6.88 (1H, dd, J = 6.7, 2.3 Hz), 4.41 (2H, t, J = 5.7 Hz), 4.10 (2H, t, J = 5.7 Hz), 3.95 (3H, s), 0.91 (9H, s), 0.11 (6H, s); HRESIMS (m/z): calcd. for C21H29N2O4Si, 401.1897; found 401.1882 [M + H]+.

Compound 3i, colorless oil; yield 69%; 1H NMR (400 MHz, CDCl3) δ 9.04 (1H, s), 8.56 (1H, d, J = 8.3 Hz), 8.12 (1H, dd, J = 8.3, 2.0 Hz), 7.36 (1H, t, J = 8.1 Hz), 7.30 (1H, d, J = 7.7 Hz), 6.93 (1H, d, J = 7.9 Hz), 4.43 (2H, t, J = 5.6 Hz), 4.11 (2H, t, J = 5.6 Hz), 0.91 (9H, s), 0.11 (6H, s); HRESIMS (m/z): calcd. for C21H26N2O3F3Si, 439.1665; found 439.1682 [M + H]+.

Compound 3j, colorless oil; yield 73%; 1H NMR (400 MHz, CDCl3) δ 8.64 (1H, d, J = 5.0 Hz), 8.29 (1H, s), 7.33–7.28 (2H, m), 7.24 (1H, s), 6.90 (1H, dd, J = 7.4, 1.5 Hz), 4.43 (2H, t, J = 5.8 Hz), 4.10 (2H, t, J = 5.8 Hz), 2.47 (3H, s), 0.91 (9H, s), 0.11 (6H, s); HRESIMS (m/z): calcd. for C21H29N2O3Si, 385.1947; found 385.1958 [M + H]+.

Compound 3k, colorless oil; yield 81%; 1H NMR (400 MHz, CDCl3) δ 9.76 (1H, d, J = 2.1 Hz), 9.01 (1H, d, J = 1.9 Hz), 8.18 (1H, d, J = 8.4 Hz), 7.96 (1H, d, J = 7.9 Hz), 7.84–7.79 (1H, m), 7.66–7.62 (1H, m), 7.31 (1H, t, J = 8.1 Hz), 7.25–7.18 (1H, m), 6.93 (1H, d, J = 8.0 Hz), 4.44 (2H, t, J = 5.7 Hz), 4.13 (2H, t, J = 5.7 Hz), 0.92 (9H, s), 0.13 (6H, s); HRESIMS (m/z): calcd. for C24H28N2O3Si, 420.1869; found 420.1875 [M + H]+.

Compound 3l, colorless oil; yield 84%; 1H NMR (400 MHz, CDCl3) δ 7.91 (1H, dd, J = 3.7, 1.2 Hz), 7.52 (1H, dd, J = 5.0, 1.2 Hz), 7.23 (1H, t, J = 8.1 Hz), 7.18–7.13 (2H, m), 6.87 (1H, dd, J = 8.1, 0.6 Hz), 4.40 (2H, t, J = 5.8 Hz), 4.09 (2H, t, J = 5.8 Hz), 0.91 (9H, s), 0.11 (6H, s); HRESIMS (m/z): calcd. for C19H26NO3SSi, 376.1403; found 376.1423 [M + H]+.

Compound 3m, colorless oil; yield 79%; 1H NMR (400 MHz, CDCl3) δ 7.64 (1H, dd, J = 1.7, 0.7 Hz), 7.28 (1H, dd, J = 3.6, 0.7 Hz), 7.24 (1H, d, J = 8.1 Hz), 7.17 (1H, dd, J = 8.2, 0.8 Hz), 6.88 (1H, dd, J = 8.1, 0.7 Hz), 6.60 (1H, dd, J = 3.5, 1.8 Hz), 4.40 (2H, t, J = 5.8 Hz), 4.08 (2H, t, J = 5.8 Hz), 0.90 (9H, s), 0.10 (6H, s); HRESIMS (m/z): calcd. for C19H26NO4Si, 360.1631; found 360.1646 [M + H]+.

General procedure for the synthesis of compounds 4a, 4b and 4d-4f

A suspension of compound 4 (0.40 mmol) and Pd/C (10 mmol %) in EtOH (5.0 mL) was heated at ambient temperature under a hydrogen atmosphere for 5 h. The mixture was filtered through a pad of Celite®, which was rinsed with EtOH repeatedly. Concentration of the filtrate followed by flash column chromatography (petroleum ether/EtOAc) of the residue led to compounds 4a, 4b, and 4d–4f as colorless oil, yield 72–86%.

Compound 4a, colorless oil; yield 83%; 1H NMR (400 MHz, CDCl3) δ 8.27 (2H, d, J = 8.2 Hz), 7.54–7.48 (3H, m), 7.29–7.20 (2H, m), 6.87 (1H, d, J = 8.0 Hz), 4.43–4.41 (2H, m), 4.09–4.07 (2H, m); HRESIMS (m/z): calcd. for C15H14NO3, 256.0974; found 256.0983 [M + H]+.

Compound 4b, colorless oil; yield 86%; 1H NMR (400 MHz, CDCl3) δ 8.17 (2H, d, J = 8.0 Hz), 7.54–7.50 (2H, m), 7.28–7.19 (2H, m), 6.85 (1H, d, J = 8.0 Hz), 4.43–4.34 (2H, m), 4.08–4.06 (2H, m), 3.66 (1H, s), 1.36 (9H, s); HRESIMS (m/z): calcd. for C19H22NO3, 312.1600; found 312.1609 [M + H]+.

Compound 4d, colorless oil; yield 72%; 1H NMR (400 MHz, CDCl3) δ 8.32 (2H, d, J = 8.0 Hz), 7.27 (1H, t, J = 8.0 Hz), 7.25–7.14 (3H, m), 6.86 (1H, d, J = 7.9 Hz), 4.43–4.35 (2H, m), 4.09–4.04 (2H, m); HRESIMS (m/z): calcd. for C15H13NO3F, 274.0879; found 274.0862 [M + H]+.

Compound 4e, colorless oil; yield 79%; 1H NMR (400 MHz, CDCl3) δ 8.35 (2H, d, J = 8.2 Hz), 7.76 (2H, d, J = 8.3 Hz), 7.32 (1H, t, J = 8.1 Hz), 7.24 (1H, d, J = 8.2 Hz), 6.88 (1H, d, J = 8.0 Hz), 4.47–4.39 (2H, m), 4.10 (2H, d, J = 6.9 Hz), 3.14 (1H, s); HRESIMS (m/z): calcd. for C16H13NO3F3, 324.0848; found 324.0857 [M + H]+.

Compound 4f, colorless oil; yield 82%; 1H NMR (400 MHz, CDCl3) δ 8.75 (1H, s), 8.30 (1H, dd, J = 8.6, 1.7 Hz), 8.00–7.93 (2H, m), 7.89 (1H, dd, J = 6.1, 2.9 Hz), 7.62–7.53 (2H, m), 7.32–7.24 (2H, m), 6.87 (1H, dd, J = 7.4, 1.5 Hz), 4.47–4.41 (2H, m), 4.10 (2H, d, J = 3.6 Hz), 3.45 (1H, s); HRESIMS (m/z): calcd. for C19H16NO3, 306.1130; found 306.1149 [M + H]+.

General procedure for the synthesis of compounds 4c and 4g-4m

To a solution of compound 4 (0.40 mmol) in THF (3.0 mL) was added TBAF/THF (0.5 mL, 1 mol/L). After stirring for 5 min to 1 h, the reaction was quenched with saturated aqueous NH4Cl and extracted with EtOAc. The combined organic extracts were washed with brine, dried over anhydrous Na2SO4, and concentrated to give a residue. Purification by flash chromatography on silica gel (petroleum ether/EtOAc) provided compounds 4c and 4 g–4m as colorless oil, yield 80–92%.

Compound 4c, colorless oil; yield 92%; 1H NMR (400 MHz, CDCl3) δ 8.22–8.13 (2H, m), 7.25–7.17 (2H, m), 7.04–6.98 (2H, m), 6.84 (1H, dd, J = 7.5, 1.3 Hz), 4.42–4.36 (2H, m), 4.10–4.02 (2H, m), 3.89 (3H, s), 3.54 (1H, s); HRESIMS (m/z): calcd. for C16H16NO4, 286.1079; found 286.1083 [M + H]+.

Compound 4g, colorless oil; yield 84%; 1H NMR (400 MHz, CDCl3) δ 9.45 (1H, d, J = 1.5 Hz), 8.75 (1H, dd, J = 4.8, 1.6 Hz), 8.51 (1H, dt, J = 8.0, 1.9 Hz), 7.45 (1H, ddd, J = 8.0, 4.9, 0.7 Hz), 7.32 (1H, t, J = 8.1 Hz), 7.24 (1H, d, J = 0.8 Hz), 6.88 (1H, d, J = 8.0 Hz), 4.42 (2H, d, J = 4.6 Hz), 4.09 (2H, d, J = 4.0 Hz), 3.06 (1H, s); HRESIMS (m/z): calcd. for C14H13N2O3, 257.0926; found 257.0941[M + H]+.

Compound 4h, colorless oil; yield 85%; 1H NMR (400 MHz, CDCl3) δ 8.46 (1H, d, J = 2.8 Hz,), 8.30 (1H, d, J = 8.8 Hz), 7.34–7.27 (3H, m), 6.85 (1H, dd, J = 6.2, 2.8 Hz), 4.42–4.37 (2H, m), 4.07 (2H, d, J = 2.3 Hz), 3.94 (3H, s), 3.40 (1H, s); HRESIMS (m/z): calcd. for C15H15N2O4, 287.1032; found 287.1028 [M + H]+.

Compound 4i, colorless oil; yield 69%; 1H NMR (400 MHz, CDCl3) δ 9.05 (1H, s), 8.50 (1H, d, J = 8.3 Hz), 8.12 (1H, dd, J = 8.3, 2.0 Hz), 7.38 (1H, t, J = 8.1 Hz), 7.32 (1H, d, J = 8.2 Hz), 6.90 (1H, d, J = 7.9 Hz), 4.44–4.40 (2H, m), 4.10 (2H, d, J = 3.9 Hz), 2.90 (1H, s); HRESIMS (m/z): calcd. for C15H12N2O3F3, 325.0800; found 325.0820 [M + H]+.

Compound 4j, colorless oil; yield 80%; 1H NMR (400 MHz, CDCl3) δ 8.65 (1H, d, J = 5.0 Hz), 8.22 (1H, s), 7.31 (2H, dd, J = 10.7, 4.5 Hz), 7.29–7.23 (1H, m), 6.87 (1H, dd, J = 7.0, 2.0 Hz), 4.45–4.40 (2H, m), 4.11–4.06 (2H, m), 3.07 (1H, s), 2.47 (3H, s); HRESIMS (m/z): calcd. for C15H15N2O3, 271.1083; found 271.1075 [M + H]+.

Compound 4k, colorless oil; yield 87%; 1H NMR (400 MHz, CDCl3) δ 9.70 (1H, d, J = 2.1 Hz), 8.97 (1H, d, J = 1.9 Hz), 8.18 (1H, d, J = 8.5 Hz), 7.95 (1H, d, J = 8.1 Hz), 7.84–7.79 (1H, m), 7.64 (1H, t, J = 7.5 Hz), 7.31 (1H, t, J = 8.1 Hz), 7.25 (1H, s), 6.89 (1H, d, J = 7.9 Hz), 4.48–4.43 (2H, m), 4.13 (2H, d, J = 3.0 Hz), 3.09 (1H, s); HRESIMS (m/z): calcd. for C18H15N2O3, 307.1083; found 307.1096 [M + H]+.

Compound 4l, colorless oil; yield 84%; 1H NMR (400 MHz, CDCl3) δ 7.90 (1H, dd, J = 3.7, 1.2 Hz), 7.54 (1H, dd, J = 5.0, 1.2 Hz), 7.25 (2H, t, J = 8.1 Hz), 7.21–7.14 (2H, m), 6.84 (1H, dd, J = 8.0, 0.8 Hz), 4.41–4.36 (2H, m), 4.09–4.04 (2H, m), 3.46 (1H, s); HRESIMS (m/z): calcd. for C13H12NO3S, 262.0538; found 262.0547 [M + H]+.

Compound 4m, colorless oil; yield 89%; 1H NMR (400 MHz, CDCl3) δ 7.66 (1H, d, J = 1.6 Hz), 7.27 (2H, dd, J = 10.0, 6.3 Hz), 7.20 (1H, d, J = 8.1 Hz), 6.85 (1H, d, J = 8.0 Hz), 6.61 (1H, dd, J = 3.5, 1.7 Hz), 4.39–4.35 (2H, m), 4.08–4.03 (2H, m), 3.27 (1H, s); HRESIMS (m/z): calcd. for C13H12NO4, 246.0766; found 246.0749 [M + H]+.

General procedure for the synthesis of compounds 6a–6m and 6A–6M

A suspension of compound 4 (0.20 mmol), 5a or 5b (0.30 mmol), EDC (0.40 mmol) and DMAP (0.40 mmol) in DCM (5 mL) was stirred at ambient temperature for 5–14 h and then quenched with HCl (1 M). A saturated aqueous solution of NaHCO3 was added to the mixture to adjust the mixture to pH 7, then the mixture was extracted with EtOAc for three times. The organic layer was combined and treated with H2O and brine, then dried over Na2SO4. The residue was purified by silica gel column chromatography (petroleum ether/EtOAc) to afford the corresponding compound 6a–6m and 6A–6M as colorless oil, yield 56–87%.

Compound 6a, colorless oil; yield 79%; 1H NMR (400 MHz, CDCl3) δ 8.23 (2H, d, J = 8.3 Hz), 7.53–7.49 (3H, m), 7.27 (4H, d, J = 1.5 Hz), 7.26–7.18 (7H, m), 7.10 (2H, t, J = 7.7 Hz), 6.96–6.76 (4H, m), 5.44 (1H, s), 4.92 (1H, s), 4.74–4.65 (4H, m), 3.79 (3H, s); HRESIMS (m/z): calcd. for C39H33N2O7, 641.2288; found 641.2263 [M + H]+.

Compound 6b, colorless oil; yield 85%; 1H NMR (400 MHz, CDCl3) δ 8.17 (2H, d, J = 8.5 Hz), 7.51 (2H, d, J = 8.5 Hz), 7.50–7.33 (5H, m), 7.25–7.16 (7H, m), 7.11 (2H, t, J = 7.7 Hz), 6.85–6.78 (3H, m), 5.43 (1H, s), 4.92 (1H, s), 4.74–4.61 (4H, m), 3.80 (3H, s), 1.37 (9H, s); HRESIMS (m/z): calcd. for C43H41N2O7, 697.2914; found 697.2890 [M + H]+.

Compound 6c, colorless oil; yield 77%; 1H NMR (400 MHz, CDCl3) δ 8.22 (2H, d, J = 8.2 Hz), 7.36–7.27 (5H, m), 7.23–7.20 (6H, m), 7.11 (2H, t, J = 7.7 Hz), 7.03–6.98 (2H, m), 6.97–6.68 (4H, m), 5.44 (1H, s), 4.91 (1H, s), 4.73–4.62 (4H, m), 3.89 (3H, s), 3.80 (3H, s); HRESIMS (m/z): calcd. for C40H35N2O8, 671.2393; found 671.2405 [M + H]+.

Compound 6d, colorless oil; yield 87%; 1H NMR (400 MHz, CDCl3) δ 8.26 (2H, d, J = 8.2 Hz), 7.28–7.27 (5H, m), 7.25–7.20 (7H, m), 7.17 (2H, d, J = 8.7 Hz), 7.11 (2H, t, J = 7.7 Hz), 6.83–6.81 (3H, m), 5.43 (1H, s), 4.92 (1H, s), 4.72–4.69 (2H, m), 4.68–4.63 (2H, m), 3.80 (3H, s); HRESIMS (m/z): calcd. for C39H32FN2O7, 659.2194; found 659.2206 [M + H]+.

Compound 6e, colorless oil; yield 78%; 1H NMR (400 MHz, CDCl3) δ 8.35 (2H, d, J = 8.1 Hz), 7.74 (2H, d, J = 8.2 Hz), 7.52–7.27 (7H, m), 7.24–7.23 (4H, m), 7.11 (2H, t, J = 7.6 Hz), 6.85 (2H, d, J = 7.8 Hz), 6.80 (2H, d, J = 7.0 Hz), 5.44 (1H, s), 4.92 (1H, s), 4.71 (2H, d, J = 4.0 Hz), 4.66 (2H, d, J = 4.4 Hz), 3.79 (3H, s); HRESIMS (m/z): calcd. for C40H32F3N2O7, 709.2162; found 709.2196 [M + H]+.

Compound 6f, colorless oil; yield 66%; 1H NMR (400 MHz, CDCl3) δ 8.77 (1H, s), 8.32 (1H, dd, J = 8.6, 1.7 Hz), 7.99–7.88 (3H, m), 7.60–7.54 (2H, m), 7.44–7.27 (6H, m), 7.25–7.22 (5H, m), 7.11 (2H, t, J = 7.7 Hz), 6.95–6.75 (4H, m), 5.44 (1H, s), 4.93 (1H, s), 4.76–4.67 (4H, m), 3.77 (3H, s); HRESIMS (m/z): calcd. for C43H35N2O7, 691.2444; found 691.2417 [M + H]+.

Compound 6g, colorless oil; yield 74%; 1H NMR (400 MHz, CDCl3) δ 9.45 (1H, d, J = 2.0 Hz), 8.75 (1H, dd, J = 4.9, 1.6 Hz), 8.50 (1H, dt, J = 8.0, 1.8 Hz), 7.43 (1H, dd, J = 8.0, 4.9 Hz), 7.31–7.27 (7H, m), 7.25–7.21 (4H, m), 7.12 (2H, t, J = 7.7 Hz), 6.94–6.77 (4H, m), 5.44 (1H, s), 4.91 (1H, s), 4.71–4.65 (4H, m), 3.80 (3H, s); HRESIMS (m/z): calcd. for C38H32N3O7, 642.2240; found 642.2213 [M + H]+.

Compound 6h, colorless oil; yield 69%; 1H NMR (400 MHz, CDCl3) δ 8.46 (1H, d, J = 2.8 Hz), 8.32 (1H, d, J = 8.7 Hz), 7.53–7.26 (8H, m), 7.24–7.22 (4H, m), 7.11 (2H, t, J = 7.7 Hz), 7.01–6.71 (4H, m), 5.45 (1H, s), 4.91 (1H, s), 4.73–4.60 (4H, m), 3.94 (3H, s), 3.80 (3H, s); HRESIMS (m/z): calcd. for C39H34N3O8, 672.2346; found 672.2320 [M + H]+.

Compound 6i, colorless oil; yield 66%; 1H NMR (400 MHz, CDCl3) δ 9.02 (1H, s), 8.49 (1H, d, J = 8.3 Hz), 8.04 (1H, dd, J = 8.3, 2.1 Hz), 7.39–7.32 (3H, m), 7.30–7.19 (10H, m), 7.12 (2H, t, J = 7.7 Hz), 6.88 (2H, dd, J = 6.9, 2.0 Hz), 6.79 (2H, d, J = 8.1 Hz), 5.46 (1H, s), 4.92 (1H, s), 4.75–4.70 (2H, m), 4.66 (2H, dd, J = 9.5, 4.5 Hz), 3.78 (3H, s); HRESIMS (m/z): calcd. for C39H31F3N3O7, 710.2114; found 710.2116 [M + H]+.

Compound 6j, colorless oil; yield 83%; 1H NMR (400 MHz, CDCl3) δ 8.65 (1H, d, J = 5.0 Hz), 8.23 (1H, s), 7.30 (7H, m), 7.23 (5H, m), 7.11 (2H, t, J = 7.7 Hz), 6.99–6.72 (m, 4H), 5.44 (1H, s), 4.92 (1H, d, J = 4.3 Hz), 4.72–4.64 (4H, m), 3.79 (3H, s), 2.44 (3H, s); HRESIMS (m/z): calcd. for C39H34N3O7, 656.2397; found 656.2419 [M + H]+.

Compound 6k, colorless oil; yield 70%; 1H NMR (400 MHz, CDCl3) δ 9.72 (1H, d, J = 2.1 Hz), 9.00 (1H, d, J = 1.9 Hz), 8.19 (1H, d, J = 8.4 Hz), 7.95 (1H, d, J = 7.9 Hz), 7.84–7.79 (1H, m), 7.63 (1H, t, J = 7.5 Hz), 7.42–7.27 (7H, m), 7.25–7.23 (4H, m), 7.12 (2H, t, J = 7.7 Hz), 6.87 (2H, dd, J = 6.4, 2.6 Hz), 6.79 (2H, d, J = 8.1 Hz), 5.45 (1H, s), 4.92 (1H, s), 4.74–4.68 (4H, m), 3.77 (3H, s); HRESIMS (m/z): calcd. for C42H34N3O7, 692.2397; found 692.2390 [M + H]+.

Compound 6l, colorless oil; yield 72%; 1H NMR (400 MHz, CDCl3) δ 7.90 (1H, dd, J = 3.7, 1.1 Hz), 7.53 (1H, dd, J = 5.0, 1.1 Hz), 7.49–7.26 (6H, m), 7.25–7.19 (5H, m), 7.18–7.15 (1H, m), 7.12 (2H, t, J = 7.7 Hz), 7.00–6.74 (4H, m), 5.43 (1H, s), 4.91 (1H, s), 4.71–4.60 (4H, m), 3.80 (3H, s); HRESIMS (m/z): calcd. for C37H31N2O7S, 647.1852; found 647.1868 [M + H]+.

Compound 6m, colorless oil; yield 56%; 1H NMR (400 MHz, CDCl3) δ 7.64 (1H, d, J = 1.0 Hz), 7.50–7.26 (7H, m), 7.25–7.19 (5H, m), 7.12 (2H, t, J = 7.7 Hz), 6.83–6.81 (4H, m), 6.60 (1H, dd, J = 3.5, 1.8 Hz), 5.42 (1H, s), 4.91 (1H, s), 4.72–4.57 (4H, m), 3.80 (3H, s); HRESIMS (m/z): calcd. for C37H31N2O8, 631.2080; found 631.2050 [M + H]+.

Compound 6A, colorless oil; yield 80%; 1H NMR (400 MHz, CDCl3) δ 8.26 (2H, dd, J = 7.5, 2.0 Hz), 7.50 (3H, d, J = 7.1 Hz), 7.43–7.36 (4H, m), 7.31 (3H, dd, J = 14.5, 7.0 Hz), 7.22 (2H, d, J = 6.2 Hz), 6.87 (2H, d, J = 8.6 Hz), 6.77 (1H, dd, J = 6.7, 2.1 Hz), 6.40 (1H, s), 5.40 (1H, s), 4.62 (1H, d, J = 4.2 Hz), 4.59–4.42 (4H, m), 3.75 (3H, s), 1.04 (9H, s); HRESIMS (m/z): calcd. for C37H37N2O8, 637.2550; found 637.2561 [M + H]+.

Compound 6B, colorless oil; yield 83%; 1H NMR (400 MHz, CDCl3) δ 8.21 (2H, d, J = 8.0 Hz), 7.53–7.49 (2H, m), 7.42–7.36 (4H, m), 7.34–7.26 (3H, m), 7.25–7.21 (2H, m), 6.90–6.85 (2H, m), 6.79–6.74 (1H, m), 6.39 (1H, s), 5.41 (1H, s), 4.62 (1H, d, J = 4.3 Hz), 4.60–4.40 (4H, m), 3.75 (3H, s), 1.36 (9H, s), 1.03 (6H, s); HRESIMS (m/z): calcd. for C41H45N2O8, 693.3176; found 693.3164 [M + H]+.

Compound 6C, colorless oil; yield 76%; 1H NMR (400 MHz, CDCl3) δ 8.23 (2H, d, J = 8.3 Hz), 7.42–7.36 (4H, m), 7.32–7.29 (3H, m), 7.23–7.18 (2H, m), 7.00 (2H, dd, J = 9.3, 2.2 Hz), 6.87 (2H, dd, J = 9.1, 2.2 Hz), 6.79–6.72 (1H, m), 6.39 (1H, s), 5.41 (1H, s), 4.61 (1H, t, J = 4.2 Hz), 4.59–4.42 (4H, m), 3.88 (3H, s), 3.75 (3H, s), 1.04 (9H, s); HRESIMS (m/z): calcd. for C38H39N2O9, 667.2656; found 667.2584 [M + H]+.

Compound 6D, colorless oil; yield 63%; 1H NMR (400 MHz, CDCl3) δ 8.28–8.22 (2H, m), 7.41–7.36 (4H, m), 7.35–7.28 (3H, m), 7.25–7.21 (2H, m), 7.20–7.15 (2H, m), 6.89–6.85 (2H, m), 6.77 (1H, dd, J = 7.5, 1.4 Hz), 6.40 (1H, s), 5.41 (1H, s), 4.62 (1H, d, J = 4.3 Hz), 4.59–4.41 (4H, m), 3.76 (3H, s), 1.04 (9H, s); HRESIMS (m/z): calcd. for C37H36FN2O8, 655.2456; found 655.2468 [M + H]+.

Compound 6E, colorless oil; yield 71%; 1H NMR (400 MHz, CDCl3) δ 8.37 (1H, d, J = 8.1 Hz), 7.75 (1H, d, J = 8.3 Hz), 7.43–7.35 (2H, m), 7.35–7.27 (2H, m), 7.24 (1H, dd, J = 8.2, 1.1 Hz), 6.87 (1H, d, J = 8.7 Hz), 6.80 (1H, d, J = 7.8 Hz), 6.38 (1H, s), 5.41 (1H, s), 4.62 (1H, d, J = 4.3 Hz), 4.60–4.41 (4H, m), 3.76 (2H, s), 1.04 (9H, s); HRESIMS (m/z): calcd. for C38H36F3N2O8, 705.2424; found 705.2429 [M + H]+.

Compound 6F, colorless oil; yield 87%; 1H NMR (400 MHz, CDCl3) δ 8.78 (1H, s), 8.33 (1H, dd, J = 8.6, 1.7 Hz), 8.00–7.87 (3H, m), 7.60–7.53 (2H, m), 7.43–7.36 (4H, m), 7.36–7.26 (5H, m), 6.87 (2H, d, J = 8.7 Hz), 6.79 (1H, t, J = 4.5 Hz), 6.39 (1H, s), 5.42 (1H, s), 4.63 (1H, d, J = 4.2 Hz), 4.60–4.44 (4H, m), 3.75 (3H, s), 1.57 (9H, s); HRESIMS (m/z): calcd. for C41H39N2O8, 687.2706; found 687.2719 [M + H]+.

Compound 6G, colorless oil; yield 72%; 1H NMR (400 MHz, CDCl3) δ 9.51–9.44 (1H, m), 8.74 (1H, dd, J = 4.8, 1.7 Hz), 8.52 (1H, dt, J = 8.0, 1.9 Hz), 7.43 (1H, ddd, J = 8.0, 4.9, 0.7 Hz), 7.41–7.36 (4H, m), 7.34–7.27 (4H, m), 7.26–7.23 (1H, m), 6.89–6.84 (2H, m), 6.79 (1H, dd, J = 7.7, 1.1 Hz), 6.38 (1H, s), 5.41 (1H, s), 4.61 (1H, d, J = 4.2 Hz), 4.58–4.41 (4H, m), 3.76 (3H, s), 1.04 (9H, s); HRESIMS (m/z): calcd. for C36H36N3O8, 638.2502; found 638.2487 [M + H]+.

Compound 6H, colorless oil; yield 78%; 1H NMR (400 MHz, CDCl3) δ 8.46 (1H, d, J = 2.9 Hz), 8.34 (1H d, J = 8.8 Hz), 7.41–7.36 (4H, m), 7.34–7.27 (6H, m), 6.87 (2H, d, J = 8.6 Hz), 6.78 (1H, dd, J = 7.2, 1.6 Hz), 6.39 (1H, s), 5.42 (1H, s), 4.62 (1H, d, J = 4.2 Hz), 4.59–4.40 (4H, m), 3.94 (3H, s), 3.75 (3H, s), 1.03 (9H, s); HRESIMS (m/z): calcd. for C37H38N3O9, 668.2608; found 668.2581 [M + H]+.

Compound 6I, colorless oil; yield 67%; 1H NMR (400 MHz, CDCl3) δ 9.03 (1H, s), 8.52 (1H, d, J = 8.3 Hz), 8.08–8.05 (1H, m), 7.40–7.35 (5H, m), 7.34–7.29 (4H, m), 6.87 (2H, d, J = 8.7 Hz), 6.82 (1H, dd, J = 7.0, 1.8 Hz), 6.40 (1H, s), 5.42 (1H, s), 4.62 (1H, d, J = 4.3 Hz), 4.60–4.40 (4H, m), 3.76 (3H, s), 1.03 (9H, s); HRESIMS (m/z): calcd. for C37H35F3N3O8, 706.2376; found 706.2344 [M + H]+.

Compound 6J, colorless oil; yield 80%; 1H NMR (400 MHz, CDCl3) δ 8.64 (1H, d, J = 5.0 Hz), 8.25 (1H, s), 7.41–7.36 (4H, m), 7.34–7.28 (5H, m), 7.24 (1H, d, J = 0.9 Hz), 6.86 (2H, d, J = 8.7 Hz), 6.78 (1H, dd, J = 6.4, 2.6 Hz), 6.39 (1H, s), 5.41 (1H, s), 4.61 (1H, d, J = 4.2 Hz), 4.57–4.44 (4H, m), 3.74 (3H, s), 2.44 (3H, s), 1.03 (9H, s); HRESIMS (m/z): calcd. for C37H38N3O8, 652.2659; found 652.2653 [M + H]+.

Compound 6K, colorless oil; yield 77%; 1H NMR (400 MHz, CDCl3) δ 9.73 (1H, d, J = 2.1 Hz,), 9.01 (1H, d, J = 1.9 Hz), 8.18 (1H, d, J = 8.4 Hz), 7.95 (1H, d, J = 7.9 Hz), 7.82 (1H, m), 7.66–7.61 (1H, m), 7.42–7.36 (4H, m), 7.35–7.27 (5H, m), 6.91–6.85 (2H, m), 6.81 (1H, dd, J = 6.6, 2.3 Hz), 6.40 (1H, s), 5.42 (1H, s), 4.62 (1H, d, J = 4.2 Hz), 4.61–4.41 (4H, m), 3.76 (3H, s), 1.04 (9H, s); HRESIMS (m/z): calcd. for C40H38N3O8, 688.2659; found 688.2651 [M + H]+.

Compound 6L, colorless oil; yield 72%; 1H NMR (400 MHz, CDCl3) δ 7.90 (1H, dd, J = 3.7, 1.2 Hz), 7.52 (1H, dd, J = 5.0, 1.2 Hz), 7.39–7.37 (4H, m), 7.33 (2H, t, J = 7.2 Hz), 7.30–7.27 (1H, m), 7.25–7.14 (3H, m), 6.90–6.85 (2H, m), 6.76 (1H, dd, J = 7.7, 1.1 Hz), 6.39 (1H, s), 5.40 (1H, s), 4.61 (1H, d, J = 4.2 Hz), 4.59–4.40 (4H, m), 3.76 (3H, s), 1.05 (9H, s); HRESIMS (m/z): calcd. for C35H35N2O8S, 643.2114; found 643.2084 [M + H]+.

Compound 6M, colorless oil; yield 68%; 1H NMR (400 MHz, CDCl3) δ 7.64 (1H, d, J = 0.9 Hz), 7.38 (4H, d, J = 8.5 Hz), 7.32–7.31 (3H, m), 7.25–7.18 (3H, m), 6.87 (2H, d, J = 8.7 Hz), 6.77 (1H, d, J = 7.8 Hz), 6.59 (1H, dd, J = 3.5, 1.7 Hz), 5.40 (1H d, J = 2.4 Hz), 4.61 (1H, d, J = 4.2 Hz), 4.54–4.40 (4H, m), 3.76 (3H, s), 1.05 (9H, s); HRESIMS (m/z): calcd. for C35H35N2O9, 627.2343; found: 627.2318 [M + H]+.

General procedure for the synthesis of compounds 7a–7m and 7A–7M

Ester compound 6 (0.10 mmol) was added to MeOH (2.0 mL) and treated with p-TsOH (0.20 mmol). The reaction mixture was stirred at room temperature for 3 h and then diluted with ethyl acetate. The organic layer was washed with a saturated aqueous solution of NaHCO3 and brine, dried with Na2SO4, filtered and concentrated. The crude mixture was purified by silica gel column chromatography (petroleum ether/EtOAc) to afford the desired hybrids 7a–7m and 7A–7M as white solid, yield 40–94%.

Compound 7a, white solid; yield 52%; 1H NMR (400 MHz, CDCl3) δ 8.17 (2H, d, J = 8.5 Hz), 7.75 (2H, d, J = 7.3 Hz), 7.57–7.27 (10H, m), 7.23 (3H, t, J = 4.8 Hz), 6.83–6.81 (1H, m), 5.74–5.71 (1H, m), 4.77–4.75 (1H, m), 4.73–4.54 (4H, m), 3.86 (1H, d, J = 5.4 Hz); 13C NMR (100 MHz, CDCl3) δ 173.0, 167.5, 162.1, 152.4, 150.0, 138.0, 134.2, 131.9, 131.6, 131.5, 128.9, 128.7, 128.5, 128.0, 127.7, 127.3, 127.2, 127.0, 125.9, 108.4, 104.3, 73.5, 66.8, 64.4, 55.8; HRESIMS (m/z): calcd. for C31H27N2O6, 523.1869; found 523.1891 [M + H]+.

Compound 7b, white solid; yield 73%; 1H NMR (400 MHz, CDCl3) δ 8.03 (2H, d, J = 8.5 Hz), 7.84 (2H, d, J = 7.3 Hz), 7.62 (1H, d, J = 8.8 Hz), 7.49 (2H, d, J = 7.2 Hz), 7.43 (2H, d, J = 8.5 Hz), 7.38 (1H, t, J = 7.4 Hz), 7.34–7.27 (2H, m), 7.25–7.19 (4H, m), 6.81–6.80 (1H, m), 5.73–5.70 (1H, m), 4.77 (1H, d, J = 2.2 Hz), 4.69–4.60 (4H, m), 4.03 (1H, s), 1.34 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 167.5, 162.3, 155.1, 152.3, 149.9, 138.0, 134.1, 132.0, 131.6, 128.7, 128.5, 128.0, 127.5, 127.3, 127.2, 125.9, 125.6, 124.1, 108.3, 104.2, 73.5, 66.7, 64.3, 56.0, 35.1, 31.2; HRESIMS (m/z): calcd. for C35H35N2O6, 579.2495; found 579.2470 [M + H]+.

Compound 7c, white solid; yield 52%; 1H NMR (400 MHz, CD3OD) δ 8.08 (2H, d, J = 8.4 Hz), 7.73 (2H, d, J = 7.0 Hz), 7.49–7.41 (3H, m), 7.39–7.24 (7H, m), 7.05 (2H, d, J = 8.3 Hz), 6.85 (1H, d, J = 7.0 Hz), 5.62 (1H, s), 4.72–4.44 (5H, m), 3.87 (3H, s); 13C NMR (100 MHz, CD3OD) δ 173.6, 169.9, 164.1, 151.3, 151.3, 139.9, 135.4, 132.7, 132.6, 130.3, 130.3, 129.4, 129.4, 128.7, 128.4, 128.3, 126.7, 120.4, 115.5, 109.3, 104.7, 74.9, 68.1, 64.9, 57.8, 56.0; HRESIMS (m/z): calcd. for C32H29N2O7, 553.1975; found 553.1960 [M + H]+.

Compound 7d, white solid; yield 51%; 1H NMR (400 MHz, CDCl3) δ 8.11 (2H, d, J = 8.5 Hz), 7.73 (2H, d, J = 7.6 Hz), 7.48 (3H, t, J = 5.8 Hz), 7.38 (1H, t, J = 7.4 Hz), 7.34–7.27 (3H, m), 7.24–7.18 (3H, m), 7.11 (2H, t, J = 8.6 Hz), 6.82 (1H, d, J = 7.8 Hz), 5.73–5.72 (1H, m), 4.75 (1H, s), 4.72–4.50 (4H, m), 3.80 (1H, d, J = 4.4 Hz); 13C NMR (100 MHz, CDCl3) δ 173.0, 167.4, 166.1, 162.4 (d, J = 235.7 Hz), 161.2, 152.4, 150.0, 138.0, 134.2, 131.9, 131.7, 129.9 (d, J = 8.9 Hz), 128.8, 128.5, 128.1, 127.3, 127.2, 125.9, 123.3, 116.3, 116.1, 108.5, 104.2, 73.4, 66.8, 64.5, 55.8; HRESIMS (m/z): calcd. for C31H26FN2O6, 541.1775; found 541.1791 [M + H]+.

Compound 7e, white solid; yield 45%; 1H NMR (400 MHz, CDCl3) δ 8.22 (2H, d, J = 8.1 Hz), 7.76–7.65 (4H, m), 7.49 (2H, d, J = 7.0 Hz), 7.43–7.35 (2H, m), 7.30 (3H, dt, J = 5.2, 4.1 Hz), 7.26–7.20 (3H, m), 6.85 (1H, dd, J = 8.0, 0.7 Hz), 5.75 (1H, dd, J = 9.0, 2.6 Hz), 4.74 (1H, d, J = 3.2 Hz), 4.73–4.53 (4H, m), 3.72 (1H, d, J = 4.5 Hz); 13C NMR (100 MHz, CDCl3) δ 173.0, 167.3, 160.5, 152.5, 150.4, 138.0, 134.2, 133.0 (d, J = 32.8 Hz), 131.8, 131.7, 130.2, 130.0, 128.8, 128.5, 128.1, 127.9, 127.3, 127.1, 126.6, 125.9 (q, J = 3.6 Hz), 125.2, 122.5, 108.6, 104.3, 73.4, 66.9, 64.5, 55.7; HRESIMS (m/z): calcd. for C32H26F3N2O6, 591.1743; found 591.1744 [M + H]+.

Compound 7f, white solid; yield 52%; 1H NMR (400 MHz, CDCl3) δ 8.65 (1H, s), 8.17 (1H, d, J = 8.3 Hz), 7.91–7.84 (3H, m), 7.75 (2H, d, J = 7.6 Hz), 7.54–7.51 (5H, m), 7.40–7.28 (5H, m), 7.24–7.19 (2H, m), 6.84 (1H, dd, J = 6.5, 1.9 Hz), 5.75 (1H, d, J = 6.9 Hz), 4.78 (1H, s), 4.74–4.58 (4H, m), 3.80 (1H, s); 13C NMR (100 MHz, CDCl3) δ 173.1, 167.5, 162.3, 152.6, 150.1, 138.0, 134.8, 134.2, 133.0, 132.1, 131.7, 129.1, 128.8, 128.8, 128.5, 128.2, 128.1, 128.0, 127.9, 127.3, 127.2, 127.0, 125.9, 124.3, 124.1, 108.5, 104.3, 73.5, 66.9, 64.5, 55.9; HRESIMS (m/z): calcd. for C35H29N2O6, 573.2026; found 573.2045 [M + H]+.

Compound 7g, white solid; yield 48%; 1H NMR (400 MHz, CDCl3) δ 9.32 (1H, d, J = 1.6 Hz), 8.67 (1H, dd, J = 4.8, 1.5 Hz), 8.37–8.31 (1H, m), 7.71 (2H, d, J = 7.3 Hz), 7.48 (2H, d, J = 7.2 Hz), 7.44–7.39 (1H, m), 7.38–7.33 (2H, m), 7.32–7.27 (3H, m), 7.25–7.20 (3H, m), 6.84 (1H, d, J = 7.9 Hz), 5.75 (1H, dd, J = 9.0, 2.5 Hz), 4.74 (1H, s), 4.70–4.53 (4H, m), 4.04 (1H, d, J = 3.4 Hz); 13C NMR (100 MHz, CDCl3) δ 172.9, 167.3, 159.6, 152.4, 151.9, 150.2, 148.6, 138.1, 134.9, 134.1, 131.6, 131.6, 128.7, 128.5, 128.0, 127.2, 127.1, 126.5, 123.8, 123.4, 108.6, 104.3, 73.4, 66.9, 64.4, 55.6; HRESIMS (m/z): calcd. for C30H26N3O6, 524.1822; found 524.1838 [M + H]+.

Compound 7h, white solid; yield 50%; 1H NMR (600 MHz, CDCl3) δ 8.38 (1H, d, J = 2.8 Hz), 8.09 (1H, d, J = 8.7 Hz), 7.74–7.72 (2H, m), 7.56 (1H, d, J = 8.9 Hz), 7.51 (2H, d, J = 7.4 Hz), 7.36 (1H, t, J = 7.4 Hz,), 7.32–7.28 (2H, m), 7.27 (1H, d, J = 2.4 Hz), 7.25–7.18 (4H, m), 6.85–6.81 (1H, m), 5.74 (1H, dd, J = 8.9, 2.7 Hz), 4.78 (1H, s), 4.69–4.55 (4H, m), 4.01 (1H, d, J = 4.5 Hz), 3.91 (3H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 167.4, 160.6, 157.3, 152.6, 150.3, 138.4, 138.3, 138.2, 134.2, 131.9, 131.6, 128.7, 128.4, 128.0, 127.3, 127.2, 126.2, 124.7, 120.7, 108.8, 104.8, 73.5, 67.1, 64.4, 55.9, 55.8; HRESIMS (m/z): calcd. for C31H28N3O7, 554.1927; found 554.1928 [M + H]+.

Compound 7i, white solid; yield 40%; 1H NMR (400 MHz, CDCl3) δ 8.97 (1H, s), 8.28 (1H, d, J = 8.3 Hz), 7.99 (1H, dd, J = 8.3, 1.9 Hz), 7.72–7.67 (2H, m), 7.50 (2H, d, J = 7.2 Hz), 7.38–7.27 (6H, m), 7.21 (2H, t, J = 7.8 Hz), 6.88 (1H, dd, J = 7.7, 0.8 Hz), 5.76 (1H, dd, J = 9.1, 2.6 Hz), 4.79–4.75 (1H, m), 4.71–4.55 (4H, m), 3.73 (1H, d, J = 4.9 Hz); 13C NMR (150 MHz, CDCl3) δ 173.0, 167.3, 159.1, 152.8, 150.8, 148.8, 147.1 (q, J = 3.9 Hz), 138.1, 134.5 (q, J = 3.3 Hz), 134.1, 131.7, 128.7, 128.5, 128.1, 127.9 (d, J = 33.5 Hz), 127.7, 127.3, 127.1, 123.2, 123.1 (d, J = 272.8 Hz), 108.7, 104.9, 73.4, 67.0, 64.4, 55.6; HRESIMS (m/z): calcd. for C31H25F3N3O6, 592.1695; found 592.1684 [M + H]+.

Compound 7j, white solid; yield 52%; 1H NMR (600 MHz, CDCl3) δ 8.57 (1H, d, J = 5.0 Hz), 8.00 (1H, s), 7.73–7.71 (2H, m), 7.53–7.51 (3H, m), 7.37–7.33 (1H, m), 7.30–7.29 (4H, m), 7.22–7.18 (3H, m), 6.84 (1H, dd, J = 6.3, 2.6 Hz), 5.73 (1H, dd, J = 9.0, 2.7 Hz), 4.78 (1H, dd, J = 5.2, 2.8 Hz), 4.71–4.57 (4H, m), 3.98 (1H, d, J = 5.3 Hz), 2.33 (3H, s); 13C NMR (150 MHz, CDCl3) δ 173.0, 167.4, 160.7, 152.7, 150.5, 149.9, 148.7, 145.6, 138.1, 134.2, 131.8, 131.6, 128.7, 128.4, 128.0, 127.4, 127.1, 126.7, 126.6, 124.5, 108.6, 104.9, 73.4, 67.0, 64.4, 55.8, 21.1; HRESIMS (m/z): calcd. for C31H28N3O6, 538.1978; found 538.1969 [M + H]+.

Compound 7k, white solid; yield 42%; 1H NMR (400 MHz, CDCl3) δ 9.60 (1H, d, J = 2.1 Hz), 8.86 (1H, d, J = 1.8 Hz), 8.14 (1H, d, J = 8.5 Hz), 7.84 (1H, d, J = 8.1 Hz), 7.82–7.77 (1H, m), 7.74–7.70 (2H, m), 7.63–7.58 (1H, m), 7.51 (2H, d, J = 7.3 Hz), 7.41–7.35 (2H, m), 7.32 (2H, dd, J = 7.6, 2.4 Hz), 7.30–7.27 (2H, m), 7.26–7.21 (2H, m), 6.86 (1H, dd, J = 7.5, 1.4 Hz), 5.77 (1H, dd, J = 9.0, 2.5 Hz), 4.77 (1H, d, J = 2.2 Hz), 4.74–4.56 (4H, m), 3.82 (1H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 167.3, 160.0, 152.5, 150.3, 149.1, 148.5, 138.1, 135.5, 134.2, 131.8, 131.7, 131.4, 129.6, 128.8, 128.8, 128.5, 128.1, 127.8,127.3, 127.2, 126.5, 120.3, 108.7, 104.3, 73.5, 67.0, 64.5, 55.6; HRESIMS (m/z): calcd. for C34H28N3O6, 574.1978; found 574.1979 [M + H]+.

Compound 7l, white solid; yield 64%; 1H NMR (400 MHz, CDCl3) δ 7.79 (1H, dd, J = 3.7, 1.1 Hz), 7.77–7.72 (2H, m), 7.52–7.45 (4H, m), 7.38 (1H, t, J = 7.4 Hz), 7.34–7.29 (2H, m), 7.25 (3H, t, J = 10.3 Hz), 7.20–7.17 (1H, m), 7.12 (1H, dd, J = 5.0, 3.8 Hz), 6.81 (1H, d, J = 7.9 Hz), 5.72 (1H, dd, J = 8.9, 2.7 Hz), 4.75 (1H, d, J = 2.8 Hz), 4.70–4.53 (4H, m), 3.84 (1H, s); 13C NMR (100 MHz, CDCl3) δ 172.9, 167.4, 158.1, 152.0, 149.9, 138.1, 134.1, 131.8, 131.6, 130.3, 130.1, 129.3, 128.7, 128.5, 128.3, 128.0, 127.3, 127.2, 125.8, 108.5, 104.1, 73.6, 66.8, 64.3, 55.8; HRESIMS (m/z): calcd. for C29H25N2O6S, 529.1433; found 529.1421 [M + H]+.

Compound 7m, white solid; yield 42%; 1H NMR (600 MHz, CDCl3) δ 7.78 (2H, d, J = 7.3 Hz), 7.58 (1H, d, J = 0.9 Hz), 7.54 (1H, d, J = 8.9 Hz), 7.47 (2H, d, J = 7.5 Hz), 7.40 (1H, t, J = 7.4 Hz), 7.31–7.26 (4H, m), 7.26–7.22 (2H, dd, J = 7.7, 5.3 Hz), 7.20 (1H, d, J = 7.8 Hz), 7.17 (1H, d, J = 3.3 Hz), 6.82 (1H, d, J = 8.0 Hz), 6.56 (1H, dd, J = 3.4, 1.7 Hz), 5.71 (1H, dd, J = 8.9, 2.4 Hz), 4.77 (1H, d, J = 1.8 Hz), 4.68–4.62 (2H, m), 4.61–4.56 (1H, m), 4.55–4.49 (1H, m), 4.05 (1H, s); 13C NMR (150 MHz, CDCl3) δ 173.0, 167.5, 154.4, 151.8, 150.2, 145.8, 142.4, 138.2, 134.2, 131.7, 131.5, 128.7, 128.5, 127.9, 127.3, 127.2, 126.1, 114.5, 112.4, 108.9, 104.2, 73.5, 67.0, 64.3, 55.8; HRESIMS (m/z): calcd. for C29H25N2O7, 513.1662; found 513.1678 [M + H]+.

Compound 7A, white solid; yield 72%; 1H NMR (400 MHz, CDCl3) δ 8.24–8.20 (2H, m), 7.49 (3H, q, J = 6.4 Hz), 7.40 (2H, d, J = 7.4 Hz), 7.34–7.27 (3H, m), 7.26–7.22 (2H, m), 6.89 (1H, d, J = 7.6 Hz), 5.75 (1H, d, J = 8.8 Hz), 5.22 (1H, d, J = 8.6 Hz), 4.68–4.50 (5H, m), 3.47 (1H, s), 1.35 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 162.0, 155.5, 152.5, 150.2, 138.7, 132.0, 131.5, 128.9, 128.6, 127.8, 127.7, 127.1, 127.0, 125.8, 108.8, 104.2, 79.8, 73.6, 67.0, 64.4, 56.6, 28.3; HRESIMS (m/z): calcd. for C29H31N2O7, 519.2131; found 519.2121 [M + H]+.

Compound 7B, white solid; yield 70%; 1H NMR (400 MHz, CDCl3) δ 8.14 (2H, d, J = 8.4 Hz), 7.50 (2H, d, J = 8.5 Hz), 7.40 (2H, d, J = 7.3 Hz), 7.38–7.34 (1H, m), 7.31 (2H, t, J = 7.3 Hz), 7.29–7.26 (1H, m), 7.26–7.21 (2H, m), 6.89 (1H, d, J = 7.2 Hz), 5.74 (1H, d, J = 8.9 Hz), 5.22 (1H, d, J = 8.2 Hz), 4.71–4.57 (5H, m), 3.42 (1H, s), 1.37 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.1, 162.3, 155.5, 155.1, 152.5, 150.1, 132.2, 128.7, 127.8, 127.6, 127.1, 126.9, 125.9, 125.6, 124.3, 108.8, 104.2, 79.9, 73.7, 67.0, 64.4, 56.6, 35.2, 31.3, 28.3; HRESIMS (m/z): calcd. for C33H39N2O7, 575.2757; found 575.2738 [M + H]+.

Compound 7C, white solid; yield 68%; 1H NMR (400 MHz, CDCl3) δ 8.15 (2H, d, J = 8.7 Hz), 7.39 (2H, d, J = 7.3 Hz), 7.31 (2H, t, J = 7.4 Hz), 7.26 (1H, d, J = 7.1 Hz), 7.24–7.19 (2H, m), 6.97 (2H, d, J = 8.8 Hz), 6.87 (1H, d, J = 7.2 Hz), 5.76 (1H, d, J = 8.9 Hz), 5.22 (1H, d, J = 8.5 Hz), 4.68–4.58 (5H, m), 3.87 (3H, s), 3.49 (1H, s), 1.34 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 162.3, 162.2, 155.5, 152.4, 149.9, 138.7, 132.1, 129.5, 128.6, 127.8, 127.1, 125.3, 119.6, 114.4, 108.7, 104.1, 79.8, 73.6, 66.9, 64.4, 56.6, 55.6, 28.3; HRESIMS (m/z): calcd. for C30H33N2O8, 549.2237; found 549.2248 [M + H]+.

Compound 7D, white solid; yield 93%; 1H NMR (400 MHz, CDCl3) δ 8.20 (2H, dd, J = 8.6, 5.4 Hz), 7.39 (2H, d, J = 7.4 Hz), 7.30 (3H, m), 7.26–7.20 (2H, m), 7.15 (2H, t, J = 8.6 Hz), 6.89 (1H, d, J = 7.8 Hz), 5.73 (1H, d, J = 8.8 Hz), 5.22 (1H, d, J = 8.7 Hz), 4.70–4.60 (5H, m), 3.45 (1H, d, J = 4.2 Hz), 1.34 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 166.1, 162.4 (d, J = 243.6 Hz), 161.2, 155.5, 152.5, 150.2, 138.7, 132.0, 130.0 (d, J = 8.9 Hz), 128.6, 127.8, 127.1, 125.9, 123.4, 123.4, 116.3, 116.1, 108.8, 104.1, 79.8, 73.6, 67.0, 64.4, 56.6, 28.3; HRESIMS (m/z): calcd. for C29H30FN2O7, 537.2037; found 537.2015 [M + H]+.

Compound 7E, white solid; yield 57%; 1H NMR (400 MHz, CDCl3) δ 8.33 (2H, d, J = 8.1 Hz), 7.73 (2H, d, J = 8.3 Hz), 7.40 (2H, d, J = 7.3 Hz), 7.32–7.27 (3H, m), 7.29–7.23 (2H, m), 6.92 (1H, d, J = 8.0 Hz), 5.67 (1H, d, J = 8.7 Hz), 5.22 (1H, d, J = 8.4 Hz), 4.70–4.58 (5H, m), 3.34 (1H, s), 1.34 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 160.5, 155.4, 152.6, 150.5, 138.7, 133.0 (d, J = 32.7 Hz), 131.9, 130.4, 128.7, 128.0, 127.9, 127.1, 126.6, 125.9 (q, J = 3.8 Hz), 125.2, 122.5, 108.9, 104.3, 79.9, 73.6, 67.0, 64.4, 56.6, 28.3; HRESIMS (m/z): calcd. for C30H30F3N2O7, 587.2005; found 587.2017 [M + H]+.

Compound 7F, white solid; yield 57%; 1H NMR (600 MHz, CDCl3) δ 8.74 (1H, s), 8.28 (1H, d, J = 8.5 Hz), 7.92 (2H, d, J = 8.3 Hz), 7.88 (1H, d, J = 7.7 Hz), 7.58–7.53 (2H, m), 7.42 (2H, d, J = 7.2 Hz), 7.32–7.30 (3H, m), 7.29–7.27 (2H, m), 6.92 (1H, d, J = 7.4 Hz), 5.76 (1H, d, J = 8.9 Hz), 5.24 (1H, d, J = 8.9 Hz), 4.73–4.60 (5H, m), 3.40 (1H, d, J = 3.7 Hz), 1.35 (9H, s); 13C NMR (150 MHz, CDCl3) δ 173.1, 162.2, 155.5, 152.6, 150.3, 138.7, 134.8, 133.1, 132.2, 129.1, 128.8, 128.7, 128.2, 128.0, 127.9, 127.1, 127.0, 125.9, 124.4, 124.2, 108.7, 104.2, 79.9, 73.6, 67.0, 64.4, 56.6, 28.3; HRESIMS (m/z): calcd. for C33H33N2O7, 569.2288; found 569.2262 [M + H]+.

Compound 7G, white solid; yield 51%; 1H NMR (400 MHz, CDCl3) δ 9.40 (1H, d, J = 1.9 Hz), 8.70 (1H, dd, J = 4.8, 1.6 Hz), 8.45 (1H, d, J = 8.0 Hz), 7.42–7.36 (3H, m), 7.33–7.28 (3H, m), 7.26–7.22 (2H, m), 6.91 (1H, d, J = 7.9 Hz), 5.71 (1H, d, J = 8.9 Hz), 5.22 (1H, d, J = 8.8 Hz), 4.69–4.56 (5H, m), 3.65 (1H, s), 1.34 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 159.6, 155.4, 152.5, 152.0, 150.4, 148.7, 138.8, 134.9, 131.7, 128.6, 127.8, 127.0, 126.5, 123.7, 123.5, 109.0, 104.2, 79.9, 73.6, 67.0, 64.3, 56.6, 28.3; HRESIMS (m/z): calcd. for C28H30N3O7, 520.2084; found 520.2097 [M + H]+.

Compound 7H, white solid; yield 52%; 1H NMR (400 MHz, CDCl3) δ 8.43 (1H, d, J = 2.8 Hz), 8.25 (1H, d, J = 8.7 Hz), 7.40 (2H, d, J = 7.3 Hz), 7.39–7.26 (5H, m), 7.24 (1H, t, J = 4.8 Hz), 6.92–6.87 (1H, m), 5.73 (1H, d, J = 9.0 Hz), 5.22 (1H, d, J = 8.5 Hz), 4.68–4.57 (5H, m), 3.93 (3H, s), 3.58 (3H, s), 1.33 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 160.6, 157.4, 155.5, 152.7, 150.4, 138.4, 131.9, 128.6, 127.8, 127.1, 126.2, 124.8, 120.7, 109.0, 104.7, 79.9, 73.7, 67.2, 64.3, 56.6, 56.0, 28.3; HRESIMS (m/z): calcd. for C29H32N3O8, 550.2189; found 550.2189 [M + H]+.

Compound 7I, white solid; yield 90%; 1H NMR (400 MHz, CDCl3) δ 9.06–8.97 (1H, m), 8.42 (1H, d, J = 8.1 Hz), 8.04 (1H, d, J = 8.1 Hz), 7.40–7.38 (3H, m), 7.35–7.28 (3H, m), 7.27–7.22 (1H, m), 6.94 (1H, d, J = 7.8 Hz), 5.66 (1H, d, J = 9.0 Hz), 5.22 (1H, d, J = 8.8 Hz), 4.72–4.56 (5H, m), 3.43 (1H, d, J = 5.1 Hz), 1.32 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 159.1, 155.4, 152.9, 150.9, 149.0, 147.1 (dd, J = 7.9, 3.9 Hz), 138.7, 134.5 (q, J = 3.3 Hz), 131.8, 128.7, 128.0 (d, J = 33.5 Hz), 127.9, 127.6, 127.1, 123.3, 123.2 (d, J = 272.8 Hz), 109.1, 104.8, 79.9, 73.6, 67.1, 64.3, 56.6, 28.3; HRESIMS (m/z): calcd. for C29H29F3N3O7, 588.1958; found 588.1942 [M + H]+.

Compound 7J, white solid; yield 70%; 1H NMR (400 MHz, CDCl3) δ 8.58 (1H, d, J = 5.0 Hz), 8.12 (1H, s), 7.40 (2H, d, J = 7.2 Hz), 7.33–7.27 (4H, m), 7.22 (2H, dd, J = 12.6, 5.7 Hz), 6.89 (1H, d, J = 6.3 Hz), 5.78 (1H, d, J = 8.9 Hz), 5.21 (1H, d, J = 8.3 Hz), 4.69–4.55 (5H, m), 3.66 (1H, d, J = 3.9 Hz), 2.36 (3H, s), 1.30 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 160.7, 155.5, 152.8, 150.6, 150.0, 148.6, 145.8, 138.7, 131.9, 128.6, 127.8, 127.1, 126.7, 126.6, 124.5, 108.9, 104.8, 79.8, 73.6, 67.2, 64.3, 56.7, 28.3, 21.1; HRESIMS (m/z): calcd. for C29H32N3O7, 534.2240; found 534.2238 [M + H]+.

Compound 7K, white solid; yield 94%; 1H NMR (400 MHz, CDCl3) δ 9.69 (1H, d, J = 2.1 Hz), 8.95 (1H, s), 8.16 (1H, d, J = 8.5 Hz), 7.90 (1H, d, J = 8.1 Hz), 7.82–7.77 (1H, m), 7.65–7.59 (1H, m), 7.41 (2H, d, J = 7.4 Hz), 7.36–7.29 (4H, m), 7.28–7.23 (1H, m), 6.93 (1H, d, J = 7.6 Hz), 5.67 (1H, d, J = 9.0 Hz), 5.24 (1H, d, J = 9.0 Hz), 4.73–4.64 (4H, m), 4.59 (1H, s), 3.46 (1H, d, J = 5.2 Hz), 1.35 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.1, 160.0, 155.4, 152.6, 150.5, 149.1, 148.6, 138.8, 135.5 131.9, 131.4, 129.7, 128.8, 128.7, 127.9, 127.8, 127.3, 127.1, 126.5, 120.4, 109.0, 104.3, 79.9, 73.6, 67.0, 64.4, 56.6, 28.3; HRESIMS (m/z): calcd. for C32H32N3O7, 570.2240; found 570.2219 [M + H]+.

Compound 7L, white solid; yield 44%; 1H NMR (400 MHz, CDCl3) δ 7.63 (1H, d, J = 1.0 Hz), 7.38 (2H, d, J = 7.4 Hz), 7.34–7.27 (3H, m), 7.25 (2H, d, J = 3.2 Hz), 7.21 (1H, dd, J = 8.1, 0.7 Hz), 6.89 (1H, d, J = 7.9 Hz), 6.59 (1H, dd, J = 3.5, 1.7 Hz), 5.64 (1H, d, J = 9.1 Hz), 5.22 (1H, d, J = 8.5 Hz), 4.71–4.55 (5H, m), 3.45 (1H, d, J = 4.8 Hz), 1.37 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 155.4, 154.3, 151.9, 150.3, 145.7, 142.6, 138.9, 131.6, 128.6, 127.8, 127.0, 126.0, 114.4, 112.4, 109.2, 104.2, 79.9, 73.7, 67.2, 64.4, 56.5, 28.4; HRESIMS (m/z): calcd. for C27H29N2O8, 509.1924; found 509.1934 [M + H]+.

Compound 7M, white solid; yield 64%; 1H NMR (400 MHz, CDCl3) δ 7.88 (1H, dd, J = 3.6, 0.9 Hz), 7.52 (1H, dd, J = 5.0, 1.1 Hz), 7.39 (2H, d, J = 7.4 Hz), 7.31 (2H, t, J = 7.4 Hz), 7.28–7.22 (2H, m), 7.19 (1H, dd, J = 8.1, 0.8 Hz), 7.17–7.13 (1H, m), 6.88 (1H, d, J = 7.9 Hz), 5.69 (1H, d, J = 8.9 Hz), 5.22 (1H, d, J = 8.7 Hz), 4.69–4.53 (5H, m), 3.46 (1H, d, J = 4.7 Hz), 1.35 (9H, s); 13C NMR (100 MHz, CDCl3) δ 173.0, 158.1, 155.5, 152.1, 150.0, 138.8, 131.9, 130.2, 130.1, 129.5, 128.6, 128.3, 127.8, 127.0, 125.8, 109.0, 104.0, 79.9, 73.7, 67.0, 64.4, 56.6, 28.3; HRESIMS (m/z): calcd. for C27H29N2O7S, 525.1695; found 525.1707 [M + H]+.

In vitro antiproliferative assay

The twenty-six newly synthesized hybrids were evaluated in vitro for their antiproliferative activity against human tumor cell lines (MDA-MB-231 and HepG-2) by MTT assay. MTT (M2128) was purchased from Sigma-Aldrich (St. Louis, Mo, USA). The MDA-MB-231 and HepG-2 cells were purchased from American Type Culture Collection (ATCC, Manassas, VA, USA). Cells were cultured in DMEM with 10% fetal bovine serum and incubated with 5% CO2. Tested cell lines were seeded at a density of 5 × 103/well in a 96-well plate for 24 h, and then treated with different concentrations of compounds dissolved in 100% DMSO for 24 h, with the final DMSO concentrations lower than 0.1%. Control cells were treated with paclitaxel containing 0.1% DMSO. DMSO served as a negative control. Then, 10 μL MTT (5 mg/mL) were added into each well and incubated for another 4 h. The purple formazan crystals were solved in 100 μL DMSO and the absorbance was detected at 570 nm by a microplate reader (Thermo MK3, USA). The IC50 values were calculated according to the dose-dependent curves. All the tests were repeated in at least three independent experiments. Inhibition rate = (OD control − OD treated) / (OD control − OD vacuity) × 100%.

Supplementary Information.