Synthesis of alkyl quercetin derivatives

Abstract

Series of alkyl quercetin derivatives were synthesized from the reactions between quercetin (1) and the corresponding alkyl halides. Three methyl quercetin derivatives, 5,3′-dihydroxy-3,7,4′-trimethoxyflavone (2), 5-hydroxy-3,7,3′,4′-tetramethoxyflavone (3), 3,5,7,3′,4′-pentamethoxyflavone (4), three ethyl quercetin derivatives, 5,3′-dihydroxy-3,7,4′-triethoxyflavone (5), 5-hydroxy-3,7,3′,4′-tetraethoxyflavone (6), 3,5,7,3′,4′-pentaethoxyflavone (7), two propyl quercetin derivatives, 5,3′-dihydroxy-3,7,4′-tripropoxyflavone (8) and 5-hydroxy-3,7,3′,4′-tetrapropoxyflavone (9), two butyl quercetin derivatives, 5,3′-dihydroxy-3,7,4′-tributoxyflavone (10), 2-(3,4-dibutyoxybenzoyloxy)-4-butoxy-6-hydroxybenzoic acid (11), and two benzyl quercetin derivatives, 5,3′-dihydroxy-3,7,4′-tribenzoxyflavone (12), 5-hydroxy-3,7,3′,4′-tetrabenzoxyflavone (13), were purified by column chromatography. Among these, synthesis and physical properties of 5, 8, 9, 10, and 11 were reported for the first time. The chemical structures were determined by spectroscopy, including NMR, MS, and IR, and the physical properties of the quercetin derivatives were also characterized. Formation of 11 from the alkylation was of a great interest, and it was rationalized by the singlet oxygen-mediated product decomposition. Antioxidant and anti-bacterial activities of the synthesized alkyl quercetin derivatives were measured and found less active than 1.

Introduction

Quercetin (1) is a highly antioxidative flavonoid found in various fruits, nuts, herbs, and vegetables (Manach et al. 2004; Sinha et al. 2014). It is widely used in beverages and food supplements because it has diverse biological activities such as anti-inflammatory, anticancer, antiviral, and anti-melanogenic (Fujii and Saito 2009; Rogerio et al. 2007; Wang et al. 2009; Yu et al. 2007). Average people consume about 100 mg of quercetin, mainly as a glycosidic form. During the metabolism of quercetin, methylation of quercetin was observed along with sulfation and glucuronidation (Day et al. 2001). Due to the important health-promoting effects and low toxicity, various quercetin methyl ether derivatives were synthesized for the biological activity study. For examples, a few quercetin methyl ether derivatives were synthesized and the thrombin inhibition, anti-mutagenic, and anti-cancer activities were studied (Azuma et al. 2011; Shi et al. 2012; Yuan et al. 2012).

While we are continuing our research on the methylated flavones (Burapan et al. 2014; Hossain et al. 2012; Jakhar et al. 2014; Kim and Han 2013; Kim et al. 2014; Wongsrikaew et al. 2012), we have noticed that study on the synthesis and biological activity of other alkyl ether derivatives of quercetin are limited. Therefore, a series of alkyl quercetin derivatives were synthesized, and the physical properties were completely characterized in this report. Besides, antioxidant and antibacterial activities of quercetin alkyl ether derivatives were measured.

Materials and methods

A representative reaction is described here, and detailed experimental methods and characterizations are available from Supplementary Material.

Synthesis of 3,5,7,3′,4′-pentamethoxyflavone (4). To a solution of quercetin 1 (1.05 g, 3.0 mmol) in DMF (120 ml), K₂CO₃ (12.52 g, 90.1 mmol, 30 eq) and MeI (5.7 ml, 90.6 mmol, 30 eq) were added at room temperature. After 3 h stirring, the reaction mixture was poured into water (120 ml), and extracted with EtOAc (120 ml) three times. The combined organic extract was washed with brine (120 ml), dried over anhydrous MgSO₄, filtered, and concentrated under reduced pressure. The crude extract was purified by vacuum liquid chromatography (Wongsrikaew et al. 2011) using hexanes and EtOAc to yield 3,5,7,3′,4′-pentamethoxyflavone (4, 478 mg, 43 %) as white solid.

Results and discussion

Quercetin alkyl ether derivatives were prepared according to Williamson ether synthesis by reacting 1 with the corresponding alkyl halides in the presence of K₂CO₃ (Fig. 1). Three quercetin methyl ether derivatives of 5,3′-dihydroxy-3,7,4′-trimethoxyflavone (2), 5-hydroxy-3,7,3′,4′-tetramethoxyflavone (3), and 3,5,7,3′,4′-pentamethoxyflavone (4) were prepared from the reaction between 1 with 5 eq of MeI in DMF. When 30 eq of MeI were reacted, isolation yield of 4 increased from 8 to 43 %. Two quercetin ethyl ether derivatives of 5,3′-dihydroxy-3,7,4′-triethoxyflavone (5) and 5-hydroxy-3,7,3′,4′-tetraethoxyflavone (6) were produced similarly by reacting 1 with 5.5 eq of ethyl iodide. Isolable 3,5,7,3′,4′-pentaethoxyflavone (7) production (66%) was achieved only from the reaction between 1 and 30 eq of ethyl iodide. Two quercetin propyl ether derivatives of 5,3′-dihydroxy-3,7,4′-tripropoxyflavone (8) and 5-hydroxy-3,7,3′,4′-tetrapropoxyflavone (9) were isolated from the reaction between 1 and 30 eq of 1-bromopropane. Two benzyl quercetin derivatives of 5,3′-dihydroxy-3,7,4′-tribenzoxyflavone (12) and 5-hydroxy-3,7,3′,4′-tetrabenzoxyflavone (13) were synthesized by 6 eq of benzyl bromide (Table 1). Pentaalkylated propyl and butyl derivatives of 1 could not be isolated in pure form due to the low production yield, even with many different reaction conditions.

Fig. 1

Synthesis of quercetin (1) alkyl ether derivatives

Table 1 Synthesis and physical properties of quercetin alkyl ether derivatives

In case of butylation reaction, 5,3′-dihydroxy-3,7,4′-tributoxyflavone (10) and 2-(3,4-dibutyoxybenzoyloxy)-4-butoxy-6-hydroxybenzoic acid (11) were prepared by 5 eq of 1-bromobutane. Isolation of a depside 11 is of interest and unprecedented in the quercetin derivatization. Molecular structure of 11 was determined mainly by ¹³C NMR and ESI–MS spectra. The ¹H NMR peaks of 11 were similar to the other alkyl derivatives of 1 in the flavone skeleton, but one of butyl groups’ chemical shifts were noticeably off to the upfield region. Besides, ¹³C NMR spectrum of 11 was distinctively different (Supplementary Material). Apparently, it missed one carbon peak of flavone ring structure. Furthermore, the carbonyl stretching at 1743 cm^-1 in the IR was much higher than the others found at between 1655 and 1663 cm^-1 (Table 1). Molecular weight of 474 determined by ESI–MS was a key data, we expected the depside product from, because the butyl derivative was expected to show the molecular ion peak at 470 m/z (Fig. 2a). Combining these, we came up with molecular formula of C₂₆H₃₄O₈ and confirmed structure of 2-(3,4-dibutyoxybenzoyloxy)-4-butoxy-6-hydroxybenzoic acid (11). We hypothesized that oxygenation of 3,5-dihydroxy-7,3′,4′-tributoxyflavone A resulted in the endoperoxide intermediate B to produce 11 (Fig. 3). Although all the alkyl derivatives of 1 are vulnerable to oxygenation, free OH group at C3, available only from A, is required to initiate the reaction between alkyl derivatives of 1 and singlet oxygen at Fig. 3. Two geometric isomers were expected to form after transformation, and bulky aromatic groups of 11 would have slowed down the isomerization between 11 and 11′. In fact, the presence of two isomers was evident by two peaks of C4a and C8 from ¹³C NMR spectrum (Fig. 2b). The range of shielding effects by aromatic ring of 11 was believed to be limited to C4a and C8.

Fig. 2

ESI–MS spectrum (a) and partial ¹³C NMR spectrum of 11 (b). The inset of a shows MS² spectrum of the 473 m/z species

Fig. 3

Proposed reaction pathway of 11

The alkylation reactions of 1 showed regioselectivity preferring hydroxyl group at 7 > 4′ > 3 > 3′ > 5. Selective alkylation of quercetin was tried with various protective groups under different conditions (Zhou et al. 2010). But, the reported method did not requires protective groups and resulted in relatively high isolation yield of alkyl ether derivatives of 1 after simple silica gel column chromatography. Because compound 5, 8, 9, 10, and 11 were synthesized and characterized for the first time in this report, the structures were confirmed by ¹H NMR and ¹³C NMR spectra, and the data were reported (Supplementary Material). When UV spectra of the synthesized compounds were compared, alkyl substitutions were found to influence UV transitions (Table 1). For example, bathochromic shifts were observed in 5,3′-dihydroxy-3,7,4′-trialkoxyflavone derivatives (2, 5, and 8) and the λ _max in MeOH increased in the order of Me < Et < Pr. Since 1 is an excellent antioxidant, DPPH radical scavenging activity of alkyl ether derivatives was measured (Table 1). Overall, all the synthesized 1 derivatives showed less antioxidant activity, and the longer alkyl chain derivatives showed better activity. However, all tested quercetin alkyl ether derivatives didn’t show any significant anti-bacterial activity.

In summary, we have synthesized 12 quercetin (1) alkyl ether derivatives, including 5 new compounds, and the physical and biological properties were reported. Since alkyl derivatives of 1 are hydrophobic and stable to oxidation, new biological activities are expected to be discovered. Spontaneous oxygenation of 5,3′,4′-trialkylquercetin was also discovered, which may provide a new synthetic route to the depside polyphenols.