Chinese Chemical Letters  2014, Vol.25 Issue (05):749-751   PDF    
First total synthesis of natural products cajanolactone A and cajanonic acid A
Wen-Zhang Chena, Ling-Ling Fana, Hai-Tao Xiaoa, Ying Zhoub, Wan Xiaoa, Jian-Ta Wanga, Lei Tanga     
a School of Pharmacy, Guiyang Medical University, Guiyang 550004, China;
b School of Pharmacy, Guizhou University, Guiyang 550025, China
Abstract: First total synthesis of cajanolactone A and cajanonic acid A has been achieved through steps of anion-anion condensations, cyclization, Williams etherification, selective demethylation, 1,3-sigmatropic rearrangement and hydrolysis. This work provides an efficient method for future cajanonic acid A derivatives synthesis.
Key words: Total synthesis     Cajanolactone A     Cajanonic acid A    

1. Introduction

Over the last few decades metabolic diseases such as type 2 diabetes have evolved into a global epidemic [1]. Insulin resistance (IR) is considered to be the main pathogenic factor leading to type 2 diabetes [2]. Antidiabetic insulin sensitizers thiazolidinediones (TZDs),including the widely used rosiglitazone (Avandia),showed good hypoglycemic effect by the activation of PPARg. However, mechanism-related side effects such as weight gain and other liabilities have come under scrutiny [3].

Two novel stilbenes,cajanonic acid A and cajanolactone A were isolated from the leaves of Pigeonpea [Cajanus cajan (L.) Millsp] by Sheng-Xiang Qiu et al. in 2008. Pharmacological studies showed that both compounds had excellent hypoglycemic activity in db/db mice,and cajanonic acid A could inhibit PTP-1B and PPARg with high potency,which might avoid the side-effects caused by classical thiazolidinediones PPARg activators and would be a good lead compound for modification [4]. As far as we know,the total synthesis of cajanolactone A and cajanonic acid A has not been reported. In this paper,we developed an efficient approach to synthesize cajanolactone A and cajanonic acid A. The synthetic pathways of cajanonic acid A (8) is outlined in Scheme 1.

2. Experimental

All reagents and solvents were of reagent grade or purified according to standard methods. Analytical thin-layer chromatography (TLC) and column chromatography were performed with silica gel 200-300 mesh (Qingdao Haiyang Chemical Co.,Ltd., Qingdao,China). Melting points were measured on a Beijing Tektronix X-4 melting point apparatus and were uncorrected. 1H NMR and 13C NMR were recorded on a Varian INOVA 400 MHz or a WIPM NMR 500 MHz instrument,using TMS as the internal standard. The ESI-MS or EI-MS was obtained on an Agilent 1100MSD or 5973 instrument,IR spectra were obtained on a Bruker IR instruments.

Preparation of ethyl 2,4-dihydroxy-6-methylbenzoate (2): Ethyl acetoacetate (1,13.0 g,100 mmol) was slowly added to a stirred suspension of sodium hydride (4.4 g,110 mmol,60% dispersion in mineral oil) in 200 mL of THF under nitrogen atmosphere. After stirring for 2 h,the mixture was cooled to-78 ℃ and 38 mL of n-butyl-lithium in hexane (2.5 mol/L,95 mmol) was added. After warming to room temperature the reaction solution was gently refluxed for 12 h. Cooled to room temperature,the mixture was quenched with 20% (v/v) cold hydrochloric acid and extracted with ethyl acetate. Evaporation of ethyl acetate under 30 ℃ gave an oil,which was treated with pH 9.2 buffer (100 mL) and allowed to stand for 12 h before acidification and re-extraction with ethyl acetate. Evaporation of the ethyl acetate gave a brown solid material,which was purified on a silica gel column (petroleum ether: ethyl acetate = 4:1,v/v) to afford ethyl orsellinate (2) (6.4 g,65%) as a white solid. Mp 131-132 ℃; 1H NMR (400 MHz,CDCl3): d 1.39 (t,3H,J = 7.2 Hz,CH2CH3),2.50 (s, 3H,6-CH3),4.37 (q,2H,J = 7.2 Hz,CH2CH3),6.22 (s,1H,3-Ar-H), 6.27 (s,1H,5-Ar-H),5.47 (s,1H,4-OH),11.84 (s,1H,2-OH); EI-MS: m/z 196 [M+],150,122,69.

Preparation of ethyl 2,4-dimethoxy-6-methylbenzoate (3): A mixture of compound 2 (6.4 g,32.7 mmol),potassium carbonate (26 g,188 mmol),and iodomethane (9 mL,146 mmol) in 120 mL of calcium chloride-dried acetone was heated at reflux for 24 h. After the completion of the reaction,the mixture was cooled down, diluted with water,neutralized with 20% (v/v) hydrochloric acid, and extracted three times with ethyl acetate. The combined organic extracts were washed with water and brine,dried over anhydrous Na2SO4,filtered,and concentrated in vacuo. Purification on silica gel column (petroleum ether: ethyl acetate = 10:1,v/v) provided compound 3 (6.95 g,95%) as colorless crystals. Mp 46- 47 ℃; 1H NMR (400 MHz,CDCl3): d 1.34 (t,3H,J = 7.2 Hz,CH3CH2), 2.29 (s,3H,6-CH3),3.79 (s,3H,4-OCH3),3.80 (s,3H,2-OCH3),4.33 (q,2H,J = 7.2 Hz,CH3CH2),6.31 (s,2H,3,5-Ar-H); EI-MS: m/z 224 [M+],179.

Preparation of ethyl 2,4-dimethoxy-6-(2-oxo-2-phenylethyl)- benzoate (4): Lithium diisopropylamide (LDA,2 mol/L,13.2 mL, 26.4 mmol) was added to anhydrous THF under nitrogen atmosphere at -78 ℃ and the mixture was allowed to stir for 10 min before compound 3 (5.6 g,25 mmol) in anhydrous THF (6 mL) was added. In the process,the temperature should be kept between -74 ℃ and -78 ℃. The red/orange solution was stirred further for 10 min before addition of N-methoxy-N-methylbenzamide (4.95 g, 30 mmol). The solution was stirred at -78 ℃ for 1.5 h,20% hydrochloric acid (35 mL) was added and allowed to warm to room temperature,then extracted with dichloromethane (80 mL×3). The combined organic extracts were dried over anhydrous Na2SO4, and the solvent removed under reduced pressure. The resulting yellow oil was purified by silica gel column chromatography (dichloromethane: petroleum ether: ethyl acetate = 5:15:2,v/v/v) to afford compound 4 (4.75 g,58%) as a light yellow solid. Mp 73- 76 ℃; 1H NMR (400 MHz,CDCl3): d 1.17 (t,3H,J = 6.8 Hz,CH3CH2), 3.78 (s,3H,4-OCH3),3.81 (s,3H,2-OCH3),4.20 (q,2H,J = 6.8 Hz, CH3CH2),4.33 (s,2H,10-2H),6.34 (d,1H,J = 2.4 Hz,3-Ar-H),6.41 (d, 1H,J = 2.0 Hz,5-Ar-H),7.44-7.58 (m,3H,300-500-Ar-H),7.99-8.01 (m,2H,200,600-Ar-H); ESI-MS: m/z 679 [2M+Na] +.

Preparation of ethyl 2-hydroxy-4-methoxy-6-(2-oxo-2-phenylethy) benzoate (5): Compound 4 (4.6 g,14 mmol) was dissolved in anhydrous CH2Cl2 (20 mL) and the solution was added dropwise to a well-stirred mixture prepared from anhydrous AlCl3 (8.4 g, 63 mmol) and anhydrous CH2Cl2 (45 mL). The reaction mixture was stirred at room temperature for 30 min. The solvent was removed under reduced pressure and the dry residue was decomposed by 10% (v/v) hydrochloric acid (40 mL),which was then extracted with dichloromethane (80 mL×2). Combined organic extracts were dried over anhydrous Na2SO4,concentrated in vacuo and purified by silica gel column chromatography (petroleum ether: ethyl acetate = 5:1,v/v) to give the compound 5 (3.94 g,90%) as a light yellow solid. Mp 120-122 8C; 1H NMR (400 MHz,CDCl3): d 0.87 (t,3H,J = 7.2 Hz,CH3CH2),3.81 (s,3H,4- OCH3),4.04 (q,2H,J = 7.2 Hz,CH3CH2),4.57 (s,2H,10-CH2),6.29 (d, 1H,J = 2.8 Hz,5-H),6.45 (d,1H,J = 2.4 Hz,3-H),7.49-7.62 (m,3H, 300-500-Ar-H),8.03-8.06 (m,2H,200,600-Ar-H),11.88 (s,1H,2-OH); ESI-MS: m/z 337 [M+Na]+.

Preparation of 6-methoxy-8-(3-methylbut-2-enyloxy)-3-phenyl- 1H-isochromen-1-one (6): A mixture of compound 5 (3.9 g, 12 mmol) and anhydrous K2CO3 (13.2 g,96 mmol) was stirred in anhydrous DMF (40 mL) at room temperature for 15 min,then 1- bromo-3-methyl-2-butene (1.7 mL,14.4 mmol) was slowly added dropwise. After stirred for 30 min at room temperature,the reaction mixture was diluted with water (80 mL) and extracted with dichloromethane (80 mL×3). Subsequently,the combined organic extracts were dried over anhydrous Na2SO4,concentrated in vacuo and purified by silica gel column chromatography (petroleum ether: ethyl acetate = 5:1,v/v) to give the pure 6 (3.25 g,78%) as a white solid. Mp 123-124 8C; 1H NMR (500 MHz, CDCl3): d 1.77 (s,3H,CH3),1.79 (s,3H,CH3),3.90 (s,3H,OCH3),4.70 (d,2H,J = 6.5 Hz,8-OCH2-CH55),5.58 (t,1H,J = 6.5 Hz,8-OCH2- CH55),6.45 (s,2H,5,7-Ar-H),6.77 (s,1H,4-Ar-H),7.40-7.45 (m,3H, 300-500-Ar-H),7.85-7.87 (m,2H,200,600-Ar-H); ESI-MS: m/z 359 [M+Na]+,695 [2M+Na]+.

Preparation of 8-hydroxy-6-methoxy-5-(3-methylbut-2-enyl)- 3-phenyl-1H-isochromen-1-one (7): To a stirred solution of compound 6 (2.69 g,8 mmol) in anhydrous CH2Cl2 (15 mL) was slowly added montmorillonite K10 (2.69 g×3) at 0 8C and the mixture was stirred for 1.5 h. After the completion of the reaction, the solid materials were filtrated off and washed with dichloromethane until no fluorescence existed. The volatiles were removed under vacuum the residue was purified by silica gel column chromatography (petroleum ether: ethyl acetate,10:1 or dichloromethane: petroleum ether = 5:3,v/v) to afford compound 7 (1.02 g,38%). Mp 127-129 8C; IR (KBr,cm-1): 3423,2975,1688, 1631,1612,1575,1499,1380; 1HNMR(400 MHz,CDCl3): d 1.69 (d, 3H,J = 1.2 Hz,500-CH3),1.85 (s,3H,400-CH3),3.45 (d,2H,J = 6.8 Hz, 100-CH2),3.90 (s,3H,OCH3),5.06 (m,1H,200-CH),6.53 (s,1H,7-Ar- H),7.04 (s,1H,4-Ar-H),7.43-7.49 (m,3H,30-50-Ar-H),7.82-7.84 (m,2H,20,60-Ar-H),11.29 (s,1H); 13C NMR (100 MHz,CDCl3): d 17.97 (C-500),23.60 (C-100),25.70 (C-400),55.93 (OCH3),98.16 (C-7), 99.57 (C-5),100.02 (C-4),116.00 (C-10),122.52 (C-200),125.19 (C-20,60),128.84 (C-30,50),129.94 (C-40),131.87 (C-300),131.96 (C-10),135.97 (C-9),152.49 (C-3),162.42 (C-6),164.28 (C-8), 166.36 (C-1); EI-MS: m/z 336 [M+],321,268,215,105.

Preparation of 6-hydroxy-4-methoxy-3-(3-methylbut-2-enyl)- 2-(2-oxo-2-phenylethyl)benzoic acid (8): A stirred solution of compound 7 (1 g,3 mmol) in ethanol (16 mL) was treated with 5% KOH (32 mL) and the mixture was refluxed for 8 h. The reaction mixture was cooled,cold water (5 mL) was added and the mixture was acidified with 20% hydrochloric acid. The solid was collected by filtration and dried under vacuum to afford 8 as a light reddish brown solid,which was recrystallized from ethanol to give pure compound 8 (0.86 g,82%) as a white solid. Mp 174-178 8C; IR (KBr, cm-1): 3291,2976,1641,1584,1464,1370; 1H NMR (400 MHz, DMSO-d6): d 1.49 (s,3H,5000-CH3),1.59 (s,3H,4000-CH3),3.15 (d,2H, J = 4.8 Hz,1000-CH2),3.17 (s,2H,10-CH2),3.83 (s,3H,OCH3),4.66 (m, 1H,2000-CH),4.84 (s,1H,CH = C-OH),6.50 (s,1H,3-Ar-H),7.44-7.64 (m,3H,300-500-Ar-H),8.03 (m,2H,200,600-Ar-H),12.03 (s,1H,OH), 13.49 (s,1H,COOH); 13CNMR(125 MHz,DMSO-d6): d 17.58 (C-5000), 24.37 (C-1000),25.39 (C-4000),41.15 (C-10),56.03 (OCH3),98.13 (C-3), 103.42 (C-5),121.39 (C-1),122.95 (C-2000),127.82 (C-200,600),128.68 (C-300,500),130.89 (C-3000),133.01 (C-400),136.69 (C-6),137.09 (C-100), 162.13 (C-4),163.58 (C-2),172.55 (COOH),196.69 (C-20); EI-MS: m/z 354 [M+],105,77.

3. Results and discussion

The intermediate ethyl 2,4-dihydroxy-6-methylbenzoate (2) was synthesized following the procedures reported in literature [5]. Ethyl 2,4-dimethoxy-6-methylbenzoate (3) was obtained in a yield of 95% by reacting 2 with CH3I instead of dimethyl sulfate that was reported in literature [6]. Compound 3 reacted with Nmethoxy- N-methylbenzamide to produce 4 in the presence of LDA in 58% yield [7].

Ethyl 2-hydroxy-4-methoxy-6-(2-oxo-2-phenylethyl)benzoate (5) is a unreported intermediate. We followed a selectively demethylation method reported by M.V. Paradkar [8]. Compound 5 was obtained successfully by demethylation of 4 with AlCl3 in anhydrous CH2Cl2. Its structure was confirmed by 1H NMR,in which the chemical shift of hydroxyl group of 2-desmethyl product was at 11.88 ppm,different from that of a similar 4-desmethyl product. Meanwhile we found that demethylation at both positions would occur once the reaction time is longer than 0.5 h.

6-Methoxy-8-(3-methylbut-2-enyloxy)-3-phenyl-1H-isochromen- 1-one (6) was synthesized by alkylation of 5 in a yield of 78% [9]. Cajanolactone A (7) was prepared via a 1,3-sigmatropic rearrangement of 6 [10]. However the yield of compound 7 was only 38%,which might be due to the generation of other isomers and de-prenylation by-product that we detected in the process. At last cajanonic acid A (8) was obtained by a ring-opening reaction of 7 in the presence of KOH in a yield of 82%.

4. Conclusion

In summary,we described an efficient synthetic route for the first total synthesis of cajanolactone A and cajanonic acid A. Considering cajanonic acid A possesses excellent hypoglycemic activity and novel mechanism of action,our work might provide an efficient synthetic route for its pharmacological derivatives.


This research was supported by the National Natural Science Foundation of China (Nos. 21102025 and 81360470) and Program for New Century Excellent Talents in University (No. NCET-12- 0656).

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