Semi-synthesis, structural modification and biological evaluation of 5-arylbenzofuran neolignans

Abstract

5-Arylbenzofuran neolignans, a newfound class of natural products, were reported to possess several kinds of pharmacological activities. To solve the lack of natural sources and promote the research of 5-arylbenzofuran neolignans in all fields, an available semi-synthesis methodology of 5-arylbenzofuran neolignans was developed, and a detailed structural modification was conducted. In the meantime, a one-pot process of Waker-type cyclization and Wacker-type oxidation was developed. To explore the potential of 5-arylbenzofuran neolignans as bioactive substances, 5-arylbenzofuran neolignans and their derivatives were evaluated for their cytotoxicity. As a result, a preliminary structure-activity relationship was obtained. Most derivatives revealed low cytotoxic effects suggesting that they were relatively safer than the natural 5-arylbenzofuran neolignan. Several derivatives showed high cytotoxicities which were found to be closely associated with apoptosis-inducing. The selectivity assay for cytotoxicity showed tumor cells were more sensitive to the promising compounds than normal cells.

Fig. 1. The comparison of 5-arylbenzofuran neolignans with 2-arylbenzofuran neolignans, and the semi-synthesis methodology and structural modification of 5-arylbenzofuran neolignans.

Scheme 1. Reagents and conditions: (a) PdCl₂, NaOAc, O₂, DMA/H₂O, 60 °C; (b) H₂SO₄, DCE, 50 °C; (c) NaBH₄, NiCl₂·6H₂O, EtOH, r.t.; (d) Pd/C, H₂, MeOH, reflux; (e) CH₃I, KOH, THF, r.t.

Scheme 2. Reagents and conditions: (a) NBS, MeCN, r.t.; (b) HNO₃, HOAc, CH₂Cl₂, 0 °C; (c) NaNO₂, concentrated hydrochloric acid (36–38%), MeCN/H₂O, r.t.; (d) secondary amines, HCHO(37%), MeOH, 60 °C; (e) Zn, NH₄Cl, HOAc, EtOH/H₂O, r.t.; (f) Pd/C, H₂, MeOH, reflux.

Scheme 3. Reagents and conditions: (a) CH₃I, NaOAc, DMF, r.t.; (b) acetic anhydride, K₂CO₃, ethyl acetate, r.t.; (c) acryloyl chloride, K₂CO₃, CH₂Cl₂, r.t.; (d) chloroacetyl chloride or oxalyl chloride, DMAP, NEt₃, THF, r.t. to reflux; (e) benzaldehyde, acetic acid, EtOH, r.t.; (f) chloroacetyl chloride, K₂CO₃, CH₂Cl₂, r.t.; (g) secondary amines, NEt₃, THF, r.t.

Scheme 4. Reagents and conditions: (a) PdCl₂, O₂, DMA/H₂O, 60 °C; (b) R–NH₂, EtOH, r.t.; (c) NaBH₄, EtOH, r.t.

Fig. 2. (A) Cellular morphologies of HepG2 cells treated with different compounds (1, 23, 30) for 24 h. (B) Images of Hoechst 33258 stained HepG2 cells which were treated with 20 μM of different compounds (1, 23, 30) for 24 h. The arrows indicated the apoptotic cell. Original magnification is 400×.

Fig. 3. Apoptosis effect of compounds 1, 23 and 30 at 20 μM in HepG2 cells for 24 h. The cells were stained with Annexin V-FITC/P and followed by flow cytometry analysis.

Fig. 4. mRNA expression levels of (A) caspase-3, caspase-9, (B) Bax, Bcl-2, (C) p53 and p21 in HepG2 cells treated with 20 μM of different compounds (1, 23, 30) for 24 h. *P < 0.05, **P < 0.01.