A High-Yielding Synthesis of the Naturally Occurring Antitumour Agent Irisquinone

Abstract

A short, high-yielding synthesis of the antitumour agent irisquinone (1) is de- scribed. The key steps are the palladium catalysed coupling reaction of dec-9-yn-1-ol with iodide (2) to form alkyne (3) and the Fremy’s salt oxidation of phenol (7).

Introduction

The seeds of Iris pallasi (Iridaceae) have been used in Chinese folk medicine for the treatment of various malignancies and for the treatment of metrorrhagia and vaginal discharge [1]. Irisquinone (1) has been isolated as an active principle [2] from the seed oil of I. pallasi and this 1,4-benzoquinone is effective against cervical carcinoma, lymphosarcoma, hepatoma and Ehrlich ascites carcinoma (EAC) in mice [2,3]. Irisquinone (1) damages nuclei of cells and inhibits [3] mitosis in cancer cells. Respira- tion of P388 cells is significantly inhibited by irisquinone (1) and mitochondrial damage to EAC cells has been noted [4]. Furthermore, levels of cyclic guanosine monophosphate have been shown to in- crease [5] in plasma of tumour bearing mice after treatment with this quinone (1). These results sug- gest that the antitumour action of irisquinone (1) is likely to be different from that of other cytotoxic agents. There are several syntheses of irisquinone (1) described [6,7,8,9] in the literature. However the best yield reported [6] was 25% over seven steps. In order to study its mechanism of action, our labo- ratories required a good supply of irisquinone (1). Herein is described a short, high-yielding synthesis of irisquinone (1).

Results and Discussion

Dec-9-yn-1-ol was prepared from commercially available dec-2-yn-1-ol by the acetylene Zipper re- action [10] and the aryl iodide (2) was prepared by a literature method [11]. Various palladium cata- lysed coupling reactions of iodide (2) with dec-9-yn-1-ol were tried to optimise the yield of alkyne (3) using different bases, co-catalysts, solvents and temperatures [12,13,14,15,16,17,18]. The method of Alami [19] us- ing 5 mol% Pd(PPh₃)₄, 10 mol% CuI and piperidine as both base and solvent afforded the alkyne (3) in 98% yield.

Reduction of this alkyne (3) with 1,4-cyclohexadiene and palladium on carbon using a transfer hy- drogenation process [20] afforded the fully saturated alcohol (4) in quantitative yield. Swern oxidation of alcohol (4) using activated DMSO afforded aldehyde (5) in only 21% yield. However, oxidation with tetrapropylammonium perruthenate (TPAP) [21] afforded the desired aldehyde (5), after column chromatography on silica in virtually quantitative yield after 10 minutes reaction. This aldehyde (5) proved to be unstable and was reacted as soon as it was prepared with n-heptyltriphenylphosphonium bromide. This reaction afforded exclusively the Z-olefin (6) in excellent yield. Removal of the meth- oxymethyl protecting group from olefin (6) using dilute hydrochloric acid afforded the phenol (7) in quantitative yield. Salcomine catalysed oxidation of phenols has previously been used [22] to prepare 1,4-benzoquinones. However, in our laboratory, salcomine oxidation of phenol (7) afforded the desired irisquinone (1) in varying yields of 30-60%. Also oxidation of phenol (7) with molecular oxygen in the presence of benzyltrimethylammonium hydroxide [23] failed to produce the desired benzoquinone (1) in satisfactory yield. Fremy’s salt oxidation [24] of this phenol (7), however, produced irisquinone (1) in excellent yield (87%)(Scheme 1).

The above synthesis of irisquinone (1) was attained from aryl iodide (2) in six steps in an overall yield of 70%. The general methodology described can be utilised to synthesise a range of alkyl or alkenyl substituted benzoquinones in excellent yield. The biological activities of irisquinone (1) and analogues will be reported in a subsequent paper.

Experimental

Synthetic intermediates were used as received from Aldrich Chemical Company or Lancaster Syn- thesis. NMR spectra were determined on a Bruker AC300 (at 300 MHz) NMR spectrometer in CDCl3 and are expressed in δ values relative to tetramethylsilane. Coupling constants (J) are measured in Hz. Infra-red spectra were recorded on a Perkin Elmer 1710 FT spectrometer and are expressed in cm-1. UV-Vis spectra were determined on a Varian Cary 1 UV-VIS spectrophotometer. Melting points are uncorrected. Electron impact mass spectra were determined using a VG Trio 2 or a Kratos MS25 mass spectrometer at an ionisation energy of 70 eV.

10-[3’-Methoxy-2’-(methoxymethoxy)phenyl]dec-9-yn-1-ol (3)

To a stirred solution of the aryl iodide (2) (108 mg, 0.367 mmol), copper (I) iodide (10 mol%, 10 mg, 55 μmol), tetrakis(triphenylphosphine) palladium (0) (5 mol%, 20 mg, 17 μmol) in piperidine (1 ml) was added dec-9-yn-1-ol (2.3 eq., 13l mg, 0.85 mmol) in piperidine (1ml) under an atmosphere of argon. The reaction mixture was shaken vigorously for 5 min, stirred for a further 25 min at room tem- perature, hydrolysed with saturated aqueous ammonium chloride (10 ml) and extracted with dichlo- romethane (3 x 10 ml). The combined organic extracts were washed with brine (3 x l0 ml), dried over anhydrous magnesium sulfate, filtered and concentrated to yield an orange/yellow oil (231 mg). The crude mixture was eluted on silica gel with petrol:ethyl acetate (7:3) to yield the title alkynol (3) (253 mg, 98%) as a pale orange oil. ν_max (film) 3407(OH). δ_H: 6.90 (2 H, m, Ar-H-4', H-5'); 6.80 (1 H, dd, J 6 and 4, Ar-H-6'); 5.20 (2 H, s, O-CH₂-O); 3.80 (3 H, s, Ar-OCH₃); 3.60 (3 H, s, -OCH₂OCH₃); 3.58 (2 H, t, J 7, H-l), 2.40 (2 H, t, J 8, H-8), 1.40 (12 H, m, Hs-2 - 7). m/z (El) 320 (M⁺, 25%), 277 (25), 262 (25), 183 (40), 81 (60), 69 (100), 55 (40). [Found: m/z 320.1988 (M⁺). C₁₉H₂₈O₄ requires 320.1988]

10-[3’-Methoxy-2’-(methoxymethoxy)phenyl]decan-l-ol (4)

To a stirred solution of the alkynol (3) (3.85 g, 12.0 mmol) in absolute ethanol (50 ml) was added 1,4-cyclohexadiene (20 eq., 23 ml, 240 mmol). The solution was cooled to 0°C and Pd/C 10% (1.2 eq., 3.85 g, 14 mmol) was added very carefully in small portions. The suspension was stirred under an at- mosphere of argon at room temperature for 1 h, filtered through Celite and the filtrate concentrated to yield the title alcohol (4) (3.88 g, 99.8%) as a colourless oil. ν_max (film) 3471 (OH). δ_H: 6.95 (1 H, t, J 8, Ar-H-5'); 6.83 (1 H, dd, J 8 and 2, Ar-H-4'); 6.75 (1 H, dd, J 8 and 2, Ar-H-6'); 5.10 (2 H, s, -OCH₂- O); 4.85 (1 H, s, -OH), 3.80 (3 H, s, Ar-OCH₃); 3.55 (3 H, s, -OCH₂OCH₃); 3.54 (2 H, t, J 7, H-l), 2.65 (2 H, t, J 8, H-10), 1.70-1.20 (16 H, bm, Hs-2 -9). m/z (EI) 324 (M⁺, 10%) 292 (20), 262 (45), 150 (65), 137 (100), 45(50).

10-[3'-Methoxy-2'-(methoxymethoxy)phenyl]decan-l-al (5)

To a stirred suspension of the alcohol (4) (859 mg, 2.65 mmol), 4-methylmorpholine-N-oxide (1.5 eq., 542 mg, 4.63 mmol) and activated powdered 4A molecular sieves (1.54 g, 500 mg/mmol) in anhy- drous dichloromethane (7 ml) was added, in one portion, tetrapropylammonium perruthenate (VII), (5 mol%, 54.2 mg, 0.154 mmol). The reaction mixture was stirred under argon for 10 min and filtered through a short pad of silica eluting with ethyl acetate to yield the title aldehyde (5) (830 mg, 97%) as a pale yellow oil. ν_max (film) 1725 (C=O). δ_H: 9.67 (1 H, t, J, 3, CHO); 6.95 (1 H, t, J 8, Ar-H-5'); 6.83 (1 H, dd, J 8 and 2, Ar-H-4'); 6.75 (l H, dd, J 8 and 2, Ar-H-6’); 5.10 (2 H, s, -OCH₂O); 3.80 (3 H, s, Ar-OCH₃); 3.55 (3 H, s, -OCH₂OCH₃); 2.65 (2 H, t, J 8, H-10); 2.35 (2 H, m, H-2), 1.60-1.20 (14 H, bm, Hs-3 - 9). m/z (EI) 322 (M⁺, 35%) 137 (45), 85 (50), 83 (75), 44 (100).

1-[(Z)Heptadec- l0'-enyl]-3-methoxy-2-(methoxymethoxy)benzene (6)

To a stirred suspension of n-heptyltriphenylphosphonium bromide (2 eq, 548 mg, 1.24 mmol) in 1,4-dioxane (2 ml) was added a solution of potassium t-butoxide (139 mg, 1.24 mmol) in 1,4-dioxane (l ml) under argon. After 30 min stirring at room temperature, a solution of aldehyde (5) (200 mg, 0.62 mmol) in 1,4-dioxane (0.5 ml) was added. After stirring for a further hour, the solution was quenched with saturated aqueous ammonium chloride (10 ml). The aqueous solution was extracted with diethyl ether (3 x 10 ml) and the combined ethereal extracts were washed with water (3 x 10 ml), brine (3 x 10 ml) and dried over anhydrous magnesium sulfate. The solvent was filtered and concentrated to yield a crude mixture as a brown oil (505 mg). This oil was eluted on silica with hexane:ether (9:1) to yield the title (Z)-alkene (6) (213 mg, 85%) as a pale brown syrup. δ_H: 6.95 (1 H, t, J 8, Ar-H-5’); 6.83 (1 H, dd, J 8 and 2, Ar-H-4’); 6.75 (1 H, dd, J 8 and 2, Ar-H-6’), 5.35 (2 H, t, J 4.7, H-10', H-11'); 5.10 (2 H, s, -OCH₂O); 3.80 (3 H, s, Ar-OCH₃); 3.60 (3 H, s, OCH₂OCH₃); 2.65 (2 H, t, J 8, H-1’); 2.00 (4 H, m, H-9', H- l 2'); 1.60 (2 H, m, H- l6'); 1.40-1.20 (20 H, bm, Hs-2'-8', H-13', H- 14', 11-15'); 0.90 (3 H, m, H-17'). m/z (EI) 404 (M⁺; 20%) 391 (20), 263 (100), 137 (40), 106 (35), 55 (40).

2-[(Z)Heptadec-10'-enyl]-6-methoxyphenol (7)

To a stirred solution of the alkene (6) (483 mg, 1.l9 mmol) in i-propanol/ THF (1:1, 10 ml) was added 2 M HCl (4 ml). The solution was stirred at room temperature for 1 h and was extracted with dichloromethane (3 x 25 ml). The combined organic extracts were washed with saturated aqueous so- dium bicarbonate (3 x 25 ml), brine (3 x 25 ml), dried over anhydrous magnesium sulfate, filtered and concentrated to yield the title phenol (7) (427 mg, 99.8%) as a colourless oil. ν_max (film) 3565 (OH). δ_H: 6.80 (3 H, m, Ar-H-3’, H-4’, H-5’); 5.70 (1 H, s, Ar-OH); 5.35 (2 H, t, J 4.7, H-10', H- 1l’); 3.90 (3 H, s, Ar-OCH₃); 2.65 (2 H, t, J 8, H-1'); 2.00 (4 H, m, H-9', H-12’); 1.60 (2 H, m, H-16’); 1.40-1.20 (20 H, m, Hs-2'-8', H13', H-14', H-15'); 0.90 (3 H, m, H-17'). m/z (EI) 360 (M⁺, 35%) 137 (l00).

2-[(Z)Heptadec-10'-enyl]-6-methoxybenzo-1,4-quinone, Irisquinone (1)

To a stirred mixture of NaH2PO4.H2O (7.4 eq., 644 mg, 4.67 mmol) in water (200 ml), containing Aliquat 336 (1.25 eq, 0.361 ml, 0.79 mmol), was added the phenol (7) (227 mg, 0.631 mmol) in di- chloromethane (13 ml). Potassium nitrosodisulfonate (Fremy’s salt) (2.5 eq, 423 mg, 1.58 mmol) was added and the mixture shaken vigorously until a colour change to yellow became permanent. The or- ganic layer was collected and the aqueous layer extracted with dichloromethane (3 x 5 ml). The com- bined organic extracts were washed with water (3 x 5 ml), brine (3 x 5 ml), dried over anhydrous mag- nesium sulfate, filtered and concentrated to yield the crude quinone (213 mg). The product was eluted on silica gel eluting using petrol:ethyl acetate (3:2) to yield the title quinone (1) (205 mg, 87%) as yellow crystals from EtOH; m.p. 42-42.5°C (lit. mp [25] 42.5-43.5°C). ν_max (film) 1685 (C=O), 1652 (C=O), 1598 (C=C). λ_max 267 nm (15,500) and 363 (980). δ_H: 6.50 (1 H, dt, J 2 and 1, H-3); 5.90 (1 H, d, J 2, H-5); 5.35 (2 H, t, J 4.7, H-10', H-11'); 3.90 (3 H, s, Ar-OCH₃); 2.40 (2 H, t, J 7, H-1'); 2.00 (4 H, m, H-9', H-l2'); 1.60 (2 H, m, H-16'); 1.40-1.20 (20 H, m, Hs-2' - 8', H-13', H-14', H-15'); 0.90 (3 H, m, H-17'). (Found: C, 76.94; H, 10.27. C₂₄H₃₈O₃ requires C, 76.96; H, 10.23%). m/z (EI) 374 (M⁺, 40% ) 153 (100), 109 (20).