New Z -chalcones obtained from aloe-emodin: Synthesis,characterization and photophysical properties

Abstract

Three new Z-chalcone derivatives were synthesized under stereoselective conditions by condensation between an aldehyde derivative (prepared from aloe-emodin) and an acetophenone, using potassium hydroxide in dimethyl sulfoxide/H₂O at room temperature. The Z configuration of the chalcone derivatives was established by nuclear magnetic resonance (NMR) studies. Photophysical properties related to the UV-Vis absorption/emission spectra and molar extinction coefficients (ε) in different solvents were determined.

Introduction

Chalcone derivatives are recognized as compounds of enormous potential for their wide range of biological activities, such as antimicrobial, anticancer and antioxidant activities and inhibition of topoisomerase I, among others.^1–5 The C_α–C_β double bond in the enone moiety of chalcone can adopt a Z or E configuration. Thermodynamically, the E-isomer is more stable and consequently, most chalcone derivatives have been isolated or synthesized in this configuration. ² Different methods are reported for the synthesis of chalcones. Among them, the most commonly employed strategy is the base-mediated Claisen–Schmidt condensation between an aldehyde and a ketone in a polar solvent such as methanol, yielding in almost all cases the more stable E-isomer. ⁶ Furthermore, the synthetic methods to obtain the Z-chalcone are those associated with the photoisomerization of the corresponding E-isomer, which lead to lower yields and side products.^7,8 Recently, Z-chalcone derivatives were synthesized under stereoselective conditions from 1,3-diaryl-2-propynyl silyl ether. ⁹

Aloe-emodin (1), is a naturally occurring anthraquinone derivative present in the leaves and roots of a number of plants, such as Aloe barbadensis Miller (aloe vera) and Rheum palmatum L. ¹⁰ Plants containing 1 and structurally related anthraquinones have long been used in traditional medicine. They have been used therapeutically in gastro and liver diseases and have antifungal, laxative, antiviral, antibacterial, anticancer and hepatoprotective activities.^11–20 In vitro studies have provided evidence of the toxicity of 1 to carcinoma cells.^16–18 In addition, several investigators have demonstrated that ultraviolet (UV) and visible light potentiate the toxicity of 1 and structurally related anthraquinones.^19,20 To date, some synthetic approaches to derivatives of 1 have been reported.^21–27 Compound 1 has two phenolic hydroxyl groups at the C-1 and C-8 positions and one aliphatic hydroxyl group at the C-3 position. Previous workers have reported that introducing methyl groups at the 1 and 8 positions decreased the cytotoxicity of 1. ²¹

In this article, combining 1 and chalcones, we have designed and synthesized three new Z-chalcone-aloe-emodin hybrids by a Claisen–Schmidt condensation of a substituted acetophenone with an aromatic aldehyde derived from 1, using KOH in DMSO/H₂O as the solvent mixture. The presence of the chalcone and anthraquinone moieties in these new compounds is biologically relevant, due to the large number of biologically active compounds containing either of these groups. Moreover, the anthraquinone and chalcone groups in the condensation products may confer interesting attributes as a conjugated system on their chemical, physical and biological properties. As part of our studies, in this work, we investigated the UV-Vis absorption/emission properties of the Z-chalcones synthesized, in solvents of variable polarity in order to provide valuable information regarding the photophysical behaviour of these compounds.

Results and discussion

The methylation of 1 with dimethyl sulphate (see Safety warning in the ‘Experimental’ section) in the presence of K₂CO₃ was achieved according to the method reported by Wang et al. ²⁸ to give 3-hydroxymethyl-1,8-dimethoxyanthracene-9,10-dione 2 in a 97% yield, whose physical and spectroscopic properties were identical to those reported.^21,24,25,28 Subsequent oxidation of compound 2 with tetra-n-propylammonium perruthenate (TPAP) ²⁹ or pyridinium chlorochromate afforded the aldehyde 3 in good yield (Scheme 1). It is important to mention that the physical properties of this compound were not compatible with those reported for aldehyde 3 and we also found some discrepancies in the reported NMR data.^24,25,28 Since the ¹H and ¹³C chemical shifts of compounds 3 have not been reported, we carried out their complete assignment by means of ¹H and ¹³C 2D NMR single bond and multiple bond correlation studies. As a result, 17 peaks were observed in the ¹³C NMR spectrum and in the ¹H NMR spectrum we reassigned all the chemical shifts for all the aromatic protons. The most downfield-shifted aromatic peaks at 8.18 and 7.92 ppm were assigned to H-4 and H-2, respectively.

Scheme 1.

Reagents and conditions: (i) (CH₃)₂SO₄, K₂CO₃, acetone, reflux, 16 h (97%); (ii) TPAP, N-methylmorpholine N-oxide, CH₃CN/CH₂Cl₂ (77%), or pyridinium chlorochromate, CH₂Cl₂, r.t. (82%).

To synthesize chalcone derivatives, we first tried condensation of anthraquinone 3 by classical Claisen–Schmidt condensation using catalytic amounts of sodium hydroxide or potassium hydroxide in ethanol and acetophenones with electron-withdrawing and electron-donating substituents. However, these methods failed to provide condensation products, probably due to the insolubility of aldehyde 3 in protic solvents. In 2007, Vashchenko et al. ³⁰ reported an effective methodology for Claisen–Schmidt condensation of hindered cyclic ketones with aromatic aldehydes under highly basic conditions using a polar aprotic solvent and a strong base. To optimize the reaction condition for the synthesis of compounds 5, several different bases such as NaOH, KOH, LiOH and t-BuOK were utilized in DMSO as solvent (Table 1). As a result, we found the reaction of aldehyde 3 with acetophenone 4a gave a good yield in the presence of an aqueous solution of one equivalent of potassium hydroxide at room temperature (Table 1, entry 6). The reaction time was 2–4 h and no better results were observed when this was increased. Compounds 5b and 5c were also prepared by this method (Scheme 2).

Table 1.

Claisen–Schmidt condensation of 3 with 4a to form 5a under the catalytic influence of various metal hydroxides ^a .

Entry	Base (equiv. per acetophenone)	Temperature (°C)	Reaction time (h)	Yield (%)
1	LiOH (0.5)	25	2	No reaction
2	KOH (0.5)	0	1	No reaction
3	KOH b (0.5)	25	4	No reaction
4	KOH (0.5)	25	3	37
5	KOH (0.5)	60	1	18
6	KOH (1)	25	3	62
7	NaOH b (10)	25	2	No reaction
8	t-BuOK (2)	25	1	No reaction

a

The acetophenone 4a (0.336 mmol) was treated with the metal hydroxide (aq. solution 0.18 M) in DMSO (10 mL) (except for entries 3 and 7) for 10 min followed by 3 (0.168 mmol).

b

Addition of a powdered base to the mixture of acetophenone in dry DMSO at rt followed by 3.

Scheme 2.

Reagents and conditions: (i) KOH, DMSO, r.t, 2–4 h (44%–62%).

The structures of the new chalcone derivatives were studied by NMR spectroscopy in DMSO-d₆. Proton 1D, 2D-COSY45 and NOESY spectra were acquired and a complete assignment of the ¹H NMR signals was done. The NMR spectra suggest that only the Z-isomer is present. Protons H-15 and H-16 have a coupling constant of ca. 8 Hz, indicative of a Z-configuration. The relative configuration was further confirmed by NOESY experiments. There is a strong NOE correlation between protons H-15 and H-16 (Figure 1) (see also Electronic Supplementary Information). The isomerization of a number of Z-chalcones to the E-isomers catalysed by base in DMSO has been studied. ³¹ In our conditions, none of the products isomerized from the Z- to the E-isomer during reaction or during the purification steps.

Figure 1.

NOE effects based on NMR studies of compounds 5a–c.

Photophysical properties

The known photophysical behaviour associated with the anthraquinone and chalcone nucleus, now incorporated in a single molecular unit in the synthesized compounds, raises the idea of evaluating the absorption and emission properties of the new chromophore. The results obtained from the photophysical analysis of the synthesized compounds are presented in Table 2.

Table 2.

Photophysical properties of compounds 5a–c in different solvents.

Compound	Chloroform		Acetonitrile		Methanol
	λabs (nm)	ε (cm−1 M−1)	λabs (nm)	ε (cm−1 M−1)	λabs (nm)	ε (cm−1 M−1)
5a	391	10689	377	5324	389	15145
5b	395	3252	386	6668	395	3212
5c	390	6539	376	7997	390	6092

The molar extinction coefficients (ε) of compounds 5a–c were determined in chloroform, acetonitrile and methanol, resulting in values that indicate electronic transitions of type π-π*. The UV-Vis absorption spectra in the same solvents showed absorption maxima between 377 and 395 nm, associated with bands extending to the visible region (up to 550 nm). It is evident that the introduction of methyl (5b) and methoxy (5c) as donor groups causes a small chromic effect on their absorption in comparison with unsubstituted compound (5a). The interruption of the electronic flow between the two subunits of the molecule, due to the carbonyl of the chalcone unit, could explain the slight effect of the substituents on the absorption spectra.

The fluorescence emission spectra of 5a and 5b were also studied in chloroform, acetonitrile and methanol. None of the compounds showed measurable fluorescence emission signals in any solvent and therefore it was not possible to quantify the fluorescence quantum yields. This result is indicative that for the Z-chalcone derivatives synthesized in this work, the radiative deactivation pathways are not favoured in the solvent under study.

In conclusion, we have developed the stereoselective synthesis of three new Z-chalcone derivatives by condensation of the corresponding acetophenone with 1. Photophysical studies showed that Z-chalcones 5a–c have interesting characteristics as potential photoactive compounds, such as visible absorption, high molar extinction coefficients and insignificant fluorescence emission. All these results pave the way for complementary research related to the synthetic methods and the photochemical study of this type of structure.

Experimental

Aloe-emodin (>95%) was purchased from Waterstone Technology, Inc. and recrystallized before use. Melting points were determined on an Electrothermal MEL-TEMP 3.0 apparatus. The IR spectra were recorded on two Thermo Scientific spectrometers, models Nicolet iS5 (ATR) and Magna 560 (KBr pellets). ¹H NMR, ¹³C NMR, COSY and HMBC spectra were recorded in CDCl₃ or DMSO-d₆ on a Bruker Avance 300, Bruker Avance 500 UltraShield™ and Bruker Ascend™ 600 spectrometers with TMS and solvent signals as internal standards. The chemical shifts δ are reported in ppm, and the coupling constants (J) are reported in hertz. Mass spectra (MS) were obtained by Electrospray ionisation (ESI) on a TSQ Quantum mass spectrometer. Elemental analyses were performed using a Fisons EA-1108 instrument. Thin-layer chromatography (TLC) was performed on Analtech silica gel 60 G254 and the spots were observed either by exposure to iodine or by UV light. Reagent-grade chemicals and solvents were obtained from commercial suppliers and all solvents were distilled and dried before use.

3-Hydroxymethyl-1,8-dimethoxyanthracene-9,10-dione (2)

Safety warning: Dimethyl sulphate is carcinogenic, mutagenic and extremely hazardous. All work was carried out in an efficient fume cupboard.

Dimethyl sulphate (8.0 mL, 84 mmol) and K₂CO₃ (10.0 g, 72.5 mmol) were added to a solution of 1 (2.0 g, 7.4 mmol) in dry acetone (1500 mL), which was then stirred and refluxed for 16 h until the reaction was complete. The yielded mixture was then cooled at room temperature and filtered. The filtrate was distilled to afford a yellow solid and recrystallized from acetone. Yield 97%, m.p. 218 °C–220 °C (lit. m.p. 223 °C–226 °C).^21,24,25,28 IR (υ_max, cm⁻¹): 3466 (O–H), 2961 (C–H), 1637 (C=O), 1586 (C=C), 1329, 1239, 731. ¹H NMR (300 MHz, DMSO-d₆): δ 3.90 (s, 6H, 2×OCH₃), 4.63 (d, 2H, J = 5.7 Hz, H-15, CH₂OH), 5.54 (t, 1H, J = 5.7 Hz, OH), 7.44 (d, 1H, J = 1.0 Hz, H-2), 7.52 (dd, 1H, J_7-6 = 8.0 Hz, J_7-5 = 1.3 Hz, H-7), 7.66 (d, 1H, J = 1.0 Hz, H-4), 7.68 (dd, 1H, J_5-6 = 8.0 Hz, J_5-7 = 1.3 Hz, H-5), 7.74 (t, 1H, J = 8.0 Hz, H-6). ¹³C NMR (75.45 MHz, DMSO-d₆): δ 56.1 (OCH3-1), 56.2 (OCH3-8), 62.2 (C-15), 115.5 (C-13), 116.1 (C-12), 118.0 (C-4), 119.0 (C-5), 122.0 (C-2), 123.5 (C-7), 133.8 (C-11), 134.0 (C-14), 137.2 (C-6), 149.4 (C-3), 158.6 (C-8), 158.8 (C-1), 180.7 (C-10), 183.1 (C-9). MS (ESI, positive ions) m/z (%): 299.14 [M + H⁺] (100).

4,5-Dimethoxy-9,10-dioxo-9,10-dihydroanthracene-2-carbaldehyde (3)

Method A: Compound 2 (100 mg, 0.335 mmol) was dissolved in dichloromethane (80 mL) and oxidized with pyridinium chlorochromate (108.0 mg, 0.502 mmol). The mixture was stirred at room temperature for 3 h until the reaction was accomplished. After the completion of the reaction, the mixture was filtered through a Celite pad, and the filtrate was washed with brine. The dichloromethane layer was separated and the aqueous layer was extracted twice with dichloromethane (2 × 20 mL). The combined dichloromethane solutions were dried for 8 h and recrystallization from acetone/hexane gave compound 3 as orange needles. Yield 82%, m.p. 227 °C–230 °C (lit. m.p. 195 °C–197 °C).^24,25,28 IR (υ_max, cm⁻¹): 3068 (C–H), 2961 (C–H), 2831 (C–H, CHO), 1687 (C=O, CHO), 1663 (C=O), 1583 (C=C), 1446 (C–H), 1237, 883. ¹H NMR (300 MHz, DMSO-d₆): δ 3.93 (s, 3H, OCH3-1), 4.00 (s, 3H, OCH3-8), 7.55 (dd, 1H, J_7-6 = 8.0 Hz, J_7-5 = 1.3 Hz, H-7), 7.70 (dd, 1H, J_5-6 = 8.0 Hz, J_5-7 = 1.3 Hz, H-5), 7.78 (t, 1H, J = 8.0 Hz, H-6), 7.92 (d, 1H, J = 1.3 Hz, H-2), 8.18 (d, 1H, J = 1.3 Hz, H-4), 10.15 (s, 1H, CHO). ¹³C NMR (75.45 MHz, DMSO-d₆): δ 56.2 (OCH3-1), 56.5 (OCH3-8), 117.4 (C-13), 118.1 (C-12), 118.9 (C4), 119.1 (C-5), 128.3 (C-2), 131.1 (C-7), 133.8 (C-11), 134.2 (C-14), 134.7 (C-6), 139.4 (C-3), 158.7 (C-1), 159.2 (C-8), 180.4 (C-9), 182.3 (C-10), 191.9 (C-15). MS (ESI, positive ions) m/z (%): 297.14 [M + H⁺] (100).

Method B: Tetra-n-TPAP (6.0 mg, 0.017 mmol) was added in one portion to a stirred mixture of compound 2 (100.0 mg, 0.335 mmol), N-methylmorpholine N-oxide (60.0 mg, 0.512 mmol) and powdered 4 Å molecular sieves (170.1 mg) in dry 25% acetonitrile/dichloromethane (5.0 mL) at room temperature under nitrogen. After 4 h of reaction, the solvent mixture was removed by vacuum evaporation and the solid was filtered through a short pad of silica, eluting with dichloromethane. The filtrate was distilled to afford an orange solid and recrystallized from acetone/hexane. Yield 77%, m.p. 227 °C–230 °C (lit. m.p. 195 °C–197 °C).^24,27

General procedure for the synthesis of Z-chalcones 5a–c

A solution of KOH (2.0 mL, 0.18 M) was added to a solution of the corresponding p-substituted acetophenone 4a–c (0.336 mmol) in DMSO (10 mL). The resulting mixture was stirred for 10 min at room temperature before to be added to a solution of aldehyde 3 (50.0 mg, 0.168 mmol) in DMSO (10 mL). This mixture was stirred till the reaction was accomplished (2–4 h). After completion of the reaction, the yielded mixture was cautiously acidified with 2 M HCl and extracted with CHCl₃ (2 × 20 mL). The combined organic layers were dried and chromatographed over silica gel. Elution with acetone: CHCl₃ (3:1) afforded pure compounds 5a–c.

Compound 5a

Yield 62%, m.p. 268 °C–270 °C. IR (υ_max, cm⁻¹): 3133 (C–H Ar), 3006 (C–H Ar), 2912 (Csp3-H), 2839 (Csp3-H), 1665 (C=O), 1587 (C=C Ar), 1399 (C–H), 1237 (C–O, Ar-O-CH₃), 967. ¹H NMR (300 MHz, DMSO-d₆): δ 3.56 (s, 3H, OCH3-1), 3.87 (s, 3H, OCH3-8), 6.95 (d, 1H, J = 1.3 Hz, H-2), 6.99–7.04 (m, 2H, H-19 and H-23), 7.07 (d, 1H, J = 7.7 Hz, H-16), 7.11–7.16 (m, 1H, H-21), 7.20 (d, 1H, J = 1.3 Hz, H-4), 7.47 (d, 1H, J = 7.7 Hz, H-7), 7.50 (d, 1H, J = 7.7 Hz, H-15), 7.50–7.52 (m, 2H, H-20 and H-22), 7.57 (d, 1H, J = 7.7 Hz, H-5), 7.68 (t, 1H, J = 7.7 Hz, H-6). ¹³C NMR (75.45 MHz, DMSO-d₆): δ 55.5 (OCH3-1), 56.2 (OCH3-8), 118.0 (C-5), 118.8 (C-13), 119.0 (C-7), 119.9 (C-2), 120.8 (C-4), 123.4 (C-11), 127.3 (C-19 and C-23), 127.3 (C-16), 128.7 (C-15), 129.1 (C-20 and C-22), 130.4 (C-21), 132.0 (C-18), 133.9 (C-6), 134.0 (C-12), 141.0 (C-3), 145.6 (C-14), 157.3 (C-1), 158.6 (C-8), 180.8 (C-9), 183.3 (C-10), 193.6 (C-17). MS (ESI, positive ions) m/z (%): 399.18 [M + 1]⁺ (100). Anal. calcd for C₂₅H₁₈O₅: C, 75.37; H, 4.55. Found: C, 75.10; H, 4.53%.

Compound 5b

Yield 51%, m.p. 216 °C–218 °C. IR (υ_max, cm⁻¹): 3138 (C–H Ar), 3010 (C–H Ar), 2925 (Csp3-H), 2843 (Csp3-H), 1669 (C=O), 1587 (C=C Ar), 1400 (C–H), 1238 (C–O, Ar-O-CH₃), 957. ¹H NMR (500 MHz, DMSO-d₆): δ 2.02 (s, 3H, CH3-24), 3.56 (s, 3H, OCH3-1), 3.87 (s, 3H, OCH3-8), 6.82–6.84 (m, 2H, H-19 and H-23), 6.84 (d, 1H, J = 8.0 Hz, H-16), 6.92 (d, 1H, J = 1.2 Hz, H-2), 7.11 (d, 1H, J = 1.2 Hz, H-4), 7.40 (d, 1H, J = 8.0 Hz, H-15), 7.40–7.44 (m, 2H, H-20 and H-22), 7.47 (d, 1H, J = 7.9 Hz, H-7), 7.57 (dd, 1H, J = 7.5 Hz, H-5), 7.70 (t, 1H, J = 7.9 Hz, H-6). ¹³C NMR (125 MHz, DMSO-d₆): δ 20.7 (C-24), 55.5 (OCH3-1), 56.2 (OCH3-8), 117.9 (C-5), 118.7 (C-13), 118.8 (C-7), 119.7 (C-2), 120.7 (C-4), 123.5 (C-11), 127.4 (C-19), 127.8 (C-16), 127.9 (C-23), 128.5 (C-20), 128.9 (C-15), 129.4 (C-22), 132.1 (C-18), 133.9 (C-6), 134.2 (C-12), 138.4 (C-21), 140.2 (C-3), 145.7 (C-14), 157.2 (C-1), 158.5 (C-8), 180.9 (C-9), 183.4 (C-10), 193.4 (C-17). MS (ESI, positive ions) m/z (%): 413.19 [M + 1]⁺ (100). Anal. calcd for C₂₆H₂₀O₅: C, 75.72; H, 4.89. Found: C, 75.43; H, 4.88%.

Compound 5c

Yield 44%, m.p. 245 °C–247 °C. IR (υ_max, cm⁻¹): 3168 (C–H Ar), 3011 (C–H Ar), 2933 (Csp3-H), 2833 (Csp3-H), 1667 (C=O), 1587 (C=C Ar), 1400 (C–H), 1241 (C–O, Ar-O-CH₃), 958. ¹H NMR (300 MHz, DMSO-d₆): δ 3.50 (s, 3H, OCH3-21), 3.53 (s, 3H, OCH3-1), 3.83 (s, 3H, OCH3-8), 6.54 (bs, 1H, H-22), 6.56 (d, 1H, J = 8.8 Hz, H-16), 6.57 (bs, 1H, H-20), 6.90 (d, 1H, J = 1.5 Hz, H-2), 7.13 (d, 1H, J = 1.5 Hz, H-4), 7.41–7.44 (m, 2H, H-19 and H-23), 7.44 (d, 1H, J = 8.8 Hz, H-15), 7.50 (dd, 1H, J = 7.9 Hz, H-7), 7.54 (dd, 1H, J = 7.8 Hz, J = 1.1 Hz, H-5), 7.65 (t, 1H, J = 7.8 Hz, H-6). ¹³C NMR (75.45 MHz, DMSO-d₆): δ 55.4 (OCH3-21), 56.0 (OCH3-1), 56.6 (OCH3-8), 113.0 (C-22), 114.3 (C-20), 118.6 (C-5), 118.9 (C-13), 119.3 (C-7), 120.1 (C-2), 121.2 (C-4), 123.6 (C-11), 127.9 (C-16), 129.2 (C-15), 131.4 (C-18), 131.9 (C-19), 132.5 (C-23), 134.0 (C-6), 134.4 (C-12), 134.8 (C-3), 146.3 (C-14), 157.8 (C-1), 159.0 (C-8), 161.7 (C-21), 182.1 (C-9), 184.1 (C-10), 193.9 (C-17). Anal. calcd. for C₂₆H₂₀O₆: C, 72.89; H, 4.71. Found: C, 72.60; H, 4.70.

Supplemental Material

1805787ESI_070319 – Supplemental material for New Z-chalcones obtained from aloe-emodin: Synthesis, characterization and photophysical properties

Supplemental material, 1805787ESI_070319 for New Z-chalcones obtained from aloe-emodin: Synthesis, characterization and photophysical properties by Nieves Canudas, Manuel Moreno, Sara Pekerar, Carlos Gámez, Estefania Sucre and José E Villamizar in Journal of Chemical Research