Electrochemical synthesis of biflavonoids

Abstract

Biologically active biflavonoids have consistently attracted the attention of researchers [1]. However, only a few of these compounds have a broad spectrum of biological activity. This has prompted scientists to prepare their semi-synthetic and synthetic analogs [2]. Thus, several approaches including metal-catalyzed cross coupling, oxidative coupling, Ulmann condensation with nucleophiles, etc. have been used to synthesize biflavonoids [2]. We hypothesized that it would be possible to use an electrochemical method to prepare novel compounds based on flavonoids [3–5]. The flavonols isorhamnetin and kaempferide that were isolated earlier by us from Alhagi pseudalhagi (M. Bieb.) Fisch. were used for the synthesis. The flavonol (1 mmol) was dissolved in CH3CN (100 mL). The electrolysis was carried out in the presence of LiClO4 (0.1 M) in a cell with a diaphragm and a Pt anode of working surface 2 cm 2 and current density 5 mA/cm2 for 3 h. When the electrolysis was finished, about 90% of the solvent was distilled off. The remainder was used to separate the products by column chromatography over silica gel. In both instances, the principal products were recrystallized from acetone to afford light-brown powders in 0.22 ± 0.05 g yields. Their physicochemical and spectral characteristics were determined by known methods. Compound 1. C32H22O14, mp 285 ± 3°C, [ ]D 20 +39.7° (c 0.57, MeOH). UV spectrum (MeOH, max, nm): 251, 325, 356. IR spectrum (KBr, max, cm –1): 3430, 3370 (OH), 1681, 1600 (C=O), 1607, 1570 (aromatic C=C). Mass spectrum (EI, 70 eV), m/z (Irel, %): 630 (M +, 100), 615 (13), 315 (17). PMR spectrum (400 MHz, CDCl3, , ppm): 3.96 (6H, d, 3 , 3 -OCH3), 6.83 (2H, s, H-6, 6 ), 7.23 (2H, s, H-5 , 5 ), 7.52 (4H, s, H-2 , 6 , 2 , 6 ), 9.35 (2H, s, OH-4 , 4 ), 10.12 (2H, s, OH-7, 7 ), 10.82 (2H, s, OH-3, 3 ), 12.10 (2H, s, OH-5, 5 ). Compound 2. C32H20O12, mp 291 ± 3°C, [ ]D 20 +38.4° (c 0.52, MeOH). UV spectrum (MeOH, max, nm): 210, 271, 368. IR spectrum (KBr, max, cm –1): 3423 (OH), 1590 (C=O), 1520, 1460 (aromatic C=C). Mass spectrum (EI, 70 eV), m/z (Irel, %): 596 (M +, 100), 581 (12), 298 (22). PMR spectrum (400 MHz, CDCl3, , ppm): 3.83 (6H, d, OCH3-4 , 4 ), 6.83 (2H, s, H-6, 6 ), 7.36 (4H, s, H-3 , 3 , 5 , 5 ), 7.50 (4H, s, H-2 , 6 , 2 , 6 ), 10.16 (2H, s, OH-7, 7 ), 10.76 (2H, s, OH-3, 3 ), 12.17 (2H, s, OH-5, 5 ).