Title of Invention

"A PROCESS FOR THE PREPARATION OF METHYL ETHERS FROM CURCUMINOIDS"

Abstract

The present invention relates to a process for preparation of methyl ethers from curcuminoids. Curcumin and its derivatives have been shown to possess a number of pharmacological effects, especially antioxidant, anti-carcinogenic and anti-inflammatory properties. Antioxidant properties of a number of ring substituted analogues of curcumin show that the highest antioxidant activity is obtained when the phenolic group is sterically hindered by the introduction of two methyl groups at the ortho position. Derivatives of curcumin and tetrahydrocurcumin are effective for the prevention and treatment of diseases causing increase of lipid peroxides. The active ingredients can be also added to food, beverage, and feed.

Full Text

The present invention relates to "a process for preparation of methyl ethers from curcuminoids". The main uses of the present invention are, i) preparation of methyl ethers of curcuminoids and ii) their application as colorants, anti-tumor drugs, pest-control agents, anti-inflammatory compounds and as antioxidants. Turmeric is used in Indian cuisine mainly for its colouring and flavouring attributes. Curcuminoids are the colouring principles of turmeric (Curcuma longa) of Zingiberaceae family viz., curcumin (1), demethoxycurcumin (2) and bis-demethoxycurcumin (3). Of these, curcumin is the major constituent. It is soluble in organic solvents like acetone and ethyl acetate and is practically insoluble in water at acidic or neutral pH. The pKa values for the dissociation of the three acidic protons in curcumin are 7.8, 8.5 and 9.0. Curcumin and its derivatives have been shown to possess a number of pharmacological effects, especially antioxidant, anti-carcinogenic and antiinflammatory properties (Srimal R C, Fitoterapia, 68, 483-493, 1997). The antioxidant activity of curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione), as determined by inhibition of controlled initiation of styrene oxidation, shows that synthetic non-phenolic curcuminoids exhibit lesser antioxidant activity. Therefore, curcumin is a classical phenolic chain-breaking antioxidant, donating H atoms from the phenolic groups and not the CH2 group as suggested earlier by Jovanovic et al. (J. Am. Chem. Soc. 1999, 121, 9677). The antioxidant activities of O-methoxyphenols are decreased in H bond accepting media (Barclay, L. et al., Org. Lett. 2(18), 2841-2843). Antioxidant properties of a number of ring substituted analogues of curcumin show that the highest antioxidant activity is obtained when the phenolic group is sterically hindered by the introduction of two methyl groups at the ortho position. This and several other compounds are more active than the standard antioxidants, a-tocopherol and "trolox" (Venkatesan, P.; Rao, M. N. A., Journal of Pharmacy and Pharmacology, 52(9), 1123-1128, 2000). 5-hydroxy-1,7-bis(3,4-dimethoxyphenyl)-1,4,6-heptatrien-3-one is one among several curcumin analogues prepared and evaluated as potential androgen receptor antagonists against two human prostate cancer cell lines, PC-3 and DU-145, in the presence of androgen receptor (AR) and androgen receptor coactivator, ARA70. They are found to be superior to hydroxyflutamide, which is the currently available anti-androgen for the treatment of prostate cancer. These compounds have been identified as a new class of antiandrogen agents, and along with their new synthetic analogues could be developed into clinical trial candidates to control androgen receptor-mediated prostate cancer growth (Ohtsu H, et al., Journal of Medicinal Chemistry, 45(23), 5037-5042, 2002). Derivatives of curcumin and tetrahydrocurcumin are effective for the prevention and treatment of diseases causing increase of lipid peroxides. The active ingredients can be also added to food, beverage, and feed. A tablet containing 20 % tetrahydrocurcumin is formulated for use as food supplement (Osawa, Toshihiko; Wanpuntragoon, Chantimar, Jpn. Kokai Tokkyo Koho JP 11246398 A2 14 Sep 1999 Heisei, 7 pp). A simple method has been devised for the efficient extraction and separation of curcuminoids, whose structures have been confirmed by 1H NMR spectroscopy and their mass fragmentation pattern. Curcumin, the major constituent has been converted to five alkyl ether derivatives. They have been tested along with the parent compounds and other extractives for insect growth inhibitory activity against Schistocerca gregaria and Dysdercus koenigii nymphs. At 20 mg per nymph, the benzene extract and dibutylcurcumin-l are the most active (60% inhibition) against S. gregaria, whereas at 50 mg per nymph, these substances exhibiting moderate growth-inhibitory activity (45%) against D koenigii nymphs. At these concentrations, turmeric oil causes 50-60% nymphal mortality in both test insects (Chowdhury, Hemanta; Walia, Suresh; Saxena, Vinod S. Pest Management, Science, 56(12), 1086-1092, 2000). Reference may be made to the synthetic method of Mohd. Ali et al., {Ind. J. Chem., 34 B, 884-888, 1995) wherein 1,7-bis(3,4-dimethoxyphenyl)-1,6-heptadiene-3,5-dione is synthesized form the reaction of curcumin with diazomethane. The main drawbacks of this procedure are that the diazomethane reagent is hazardous as it is prone to explosion, especially on coming into contact with rough and unpolished surfaces, its preparation involves the use of special apparatus along with protective care while handling and above all, the method is not amenable for scale-up. Reference may be made to the synthetic method of Nurfina et al., (Eur J Med Chem, 32, 321-328, 1997), wherein 1,7-bis(3,4-dimethoxyphenyl)-1,6-heptadiene-3,5-dione was synthesized from the condensation of 3,4-dimethoxybenzaldehyde with acetyl acetone in presence of excess of tributyl borate and boric anhydride followed by reaction with butylamine and subsequent acid treatment and work-up of the product. The drawbacks of the procedure are that it involves methylation of the starting material vanillin and it is a multi-step synthetic route involving many reagents with only a moderate overall yield of the product (40 %). Reference may be made to the synthetic method of Chowdhury H et al., (PestManag. Sci. 56, 1086-1092, 2000), wherein curcuminoids are reacted with alkyl iodides like methyl iodide in the presence of anhydrous potassium carbonate in dry acetone for the preparation of alkyl ethers. The drawback of this procedure is that the product is 1,7-bis(3,4-dimethoxyphenyl)-4-methylhepta-1,6-diene-3,5-dione which indicates that additional methylation occurs at the methylene carbon a to the carbonyl groups in this method. Reference may be made to the synthetic method of Pill-Hoon Bong (Bull. Korean Chem. Soc. 2000, Vol. 21, No. 1, 81-86), wherein curcumin or its analogues are taken in acetone and reacted with potassium hydroxide and sodium hydrogen sulphate in the presence of catalytic amounts of tetra-n-butyl ammonium iodide followed by reaction with methyl iodide. The drawback of this procedure is that it employs several reagents, it is slow (18 h) and affords the methyl ether in a moderate yield of 53 %. The main objective of the present invention is to provide a process for preparation of methyl ethers from curcuminoids, which obviates the aforesaid drawbacks of the processes described in the prior art. Accordingly, the present invention relates to a process for preparation of methyl ethers from curcuminoids, which comprises: reacting curcuminoids with dimethyl sulfate in a solvent medium such as acetone in the presence of a base selected from an alkali metal hydroxide like sodium hydroxides at a neutral pH in the presence of ultrasound in the frequency range of 40-50 kHz and RF power of 80 W at 25 to 55°C to obtain methyl ethers of curcuminoids. In an embodiment of the process the solvent medium is an organic solvent such as acetone. In an another embodiment of the process the base is selected from an alkali metal hydroxide like sodium hydroxide. In an another embodiment of the process the reaction is carried out at temperatures ranging 25 to 55° C. In yet an another embodiment of the process the reaction is carried out in the presence of ultrasound in the frequency range of 40-50 kHz and RF power of 80 W at 25°C to obtain methyl ethers of curcuminoids. The process constitutes reacting curcuminoids (1 part) with dimethyl sulphate (2-4 parts) in acetone (40 parts) as solvent in the presence of aqueous sodium hydroxide (2-4 parts) in the temperature range of 25-55°C. At the end of the reaction (4-24 h), the pH of the reaction mixture is adjusted to 7 with dilute HCI and the solvent is distilled off using a flash evaporator under reduced pressure. The residue is dissolved in dichloromethane and is washed with water to remove traces of dimethylsulfate. The organic layer is separated using a separating funnel and dried over anhydrous sodium sulphate. After the removal of the solvent using a flash evaporator under reduced pressure, the crude product is subjected to column chromatography using silica gel with chloroform as the eluant and the isolated product recrystallized from benzene. The novelty of the process is that it involves wet methylation as a single step of conversion of curcuminoids to the corresponding methyl ethers. The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention. EXAMPLE 1: To a solution of curcumin (1 g, 2.72 mmol) in acetone (40 ml), 11 mmol (1.36 g) of dimethyl sulphate was added while stirring at 25°C. Aqueous sodium hydroxide (11.0 mmol, 0.45 g in 2 ml) was then added drop-wise and the mixture was stirred under reflux at 55°C. The reaction was monitored by Thin Layer Chromatography (TLC) for the disappearance of curcumin. At the end of the reaction (4 h), the pH of the reaction mixture was adjusted to 7 using dilute HCI. The solvent acetone was distilled off. The residue was dissolved in dichloromethane (50 ml) and washed with water. The organic layer was separated and dried over anhydrous sodium sulphate (10 g). The solvent was distilled off and the crude product was subjected to column chromatography using silica gel (50 g) and chloroform as the eluant. Pure compound A (Table 1) (1,7-bis (3,4-dimethoxyphenyl)-1,6-heptadiene-3,5-dione, C23H24O6 ,Rf = 0.78 (eluant 2 % MeOH in CHCI3) was isolated and recrystallized from benzene ( 0.9 g, 84 % yield) 1H NMR : 3.86 (6H, s, 3-A1OCH3), 3.88(6H, s, 4-Ar-OCH3), 5.99 (1H, s, -C(OH)=CH-, enol form), 6.74 (2H, d, J = 16 Hz, 2,6-H, olefenic), 7.2 [6H, m, 2x6,2,5-ArH)], 7.61(2H, d, J = 16 Hz, 1,7-H, olefinic). EXAMPLE 2: To a solution of curcumin (0.5 g, 1.36 mmol) in acetone (20 ml), 2.7 mmol (0.34 g) of dimethyl sulphate was added while stirring at 25°C. Aqueous sodium hydroxide (2.7 mmol, 0.11 g in 2 ml) was then added drop- wise and the mixture was stirred under reflux at 55°C. The reaction was monitored by TLC for the disappearance of curcumin. At the end of the reaction (4 h), the pH of the reaction mixture was adjusted to 7 using dilute HCI. The solvent was distilled off and the residue was dissolved in dichloromethane (25ml). The organic layer was washed with water, separated using a separating funnel and was dried over anhydrous sodium sulphate (5 g). The solvent was distilled off and the crude product subjected to column chromatography using silica gel (40g) using chloroform as the eluant to yield two compounds which were recrystallized from benzene. A, 1,7-bis (3,4-Dimethoxyphenyl)-1,6-heptadiene-3,5-dione, C23H24O6,Rf = 0.78 (eluant 2 % MeOH in CHCI3), 0.35 g, 65 % yield. 1HNMR : 3.86 (6H, s, 3-ArOCH3), 3.88(6H, s, 4-Ar-OCH3), 5.99 (1H, s, -C(OH)=CH-, enol form), 6.74 [2H, d, J = 16 Hz, 2,6-H, alkene], 7.2 [6H, m, 2x6,2,5-ArH)], 7.61[2H, d, J = 16 Hz, 1,7-H, alkene]). B, 1 (4-hydroxy-3-methoxyphenyl]-7-(3,4-dimethoxyphenyl)-1,6-heptadiene-3,5- dione, C22H2206> Rf = 0.58 (eluant 2 % MeOH in CHCI3), 0.041 g, 7.62 % yield. 1HNMR : 3.86 (3H, s, 4-ArOCH3), 3.88(6H, s, 3-Ar-OCH3), 5.99 (1H, s, - C(OH)=CH-, enol form), 6.74 [2H, d, J = 16 Hz, 2,6-H, alkene], 7.2 [6H, m, 2x6,2,5-ArH], 7.61[2H, d, J = 16 Hz, 1,7-H, alkene]), 8.18 (1H, Ar-OH). EXAMPLE 3: To the solution of curcumin (1 g, 2.72 mmol) in acetone (40 ml), 5.5 mmol (0.68 g) of dimethyl sulphate was added while stirring at 25°C. Aqueous sodium hydroxide (5.5 mmol, 0.22 g in 2 ml) was then added drop wise and the mixture was stirred at 25° C. The reaction was monitored by TLC for the disappearance of curcumin. At the end of the 24 h, the pH of the reaction mixture was adjusted to 7 using dilute HCI. The solvent acetone was distilled off and the residue dissolved in dichloromethane (50 ml). The organic layer was washed with water and separated using separating funnel. Theo rganic layer was dried over anhydrous sodium sulphate (10 g). The solvent was distilled off and the crude product was subjected to column chromatography using silica (50g) using chloroform as the eluant to afford, apart from unreacted curcumin( 0.2 g, 20 %), two compounds, A & B, that were crystallized from benzene. A, 1,7-bis (3,4-Dimethoxyphenyl)-1,6-heptadiene-3,5-dione, C23H24O6, Rr = 0.78 (eluant 2 % MeOH in CHCI3), 0.6 g, 55 % yield). 1H NMR : 3.86 (6H, s, 3-ArOCH3), 3.88(6H, s, 4-Ar-OCH3), 5.99 (1H, s, -C(OH)=CH-, enol form), 6.74 (2H, d, J = 16 Hz, 2,6-H, oiefinic], 7.2 [6H, m, 2x6,2,5-ArH)], 7.61 (2H, d, J = 16 Hz, 1,7-H, oiefinic). B, [1 (4-hydroxy-3-methoxyphenyl]-7-(3,4-dimethoxyphenyl)-1,6-heptadiene-3,5- dione), C22H2206, Rf = 0.58 (eluant 2 % MeOH in CHCI3), 0.11 g, 9.2 % yield). 1H NMR : 3.86 (3H, s, 4-ArOCH3), 3.88(6H, s, 3-Ar-OCH3), 5.99 (1H, s, C(OH)=CH-, enol form), 6.74 [2H, d, J = 16 Hz, 2,6-H, olefenic], 7.2 [6H, m, 2x6,2,5-ArH], 7.61[2H, d, J = 16 Hz, 1,7-H, oiefinic), 8.18 (1H, Ar-OH). EXAMPLE 4: To the solution of curcumin (0.5 g, 1.36 mmol) in acetone (20 ml), 2.7 mmol (0.34 g) of dimethyl sulphate was added while sonicating the mixture at 25°C in an ultrasound bath having frequency range of 40-50 kHz and RF power of 80 W. Aqueous sodium hydroxide (2.7 mmol, 0.11 g in 2 ml) was then added drop-wise and the mixture sonicated at 25°C. The reaction was monitored by TLC for the disappearance of curcumin. At the end of the reaction (10 h), the pH of the reaction mixture was adjusted to 7 using dilute HCI. The solvent acetone was distilled off and the residue dissolved in dichloromethane (25 ml). The organic layer was washed with water and separated using a separating funnel. The organic layer was dried over sodium sulphate (5 g). The solvent was distilled off and the crude product was subjected to column chromatography using silica gel (40g) using chloroform as the eluant to afford apart from unreacted Curcumin (0.1 g, 20 %), pure 1,7-bis (3,4-dimethoxyphenyl)-1,6-heptadiene-3,5-dione (A), C23H24O6, Rf = 0.78 (eluant 2 % MeOH in CHCb) that recrystallized from benzene (0.3 g, 55 % yield). 1H NMR : 3.86 (6H, s, 3-ArOCH3), 3.88(6H, s, 4-Ar-OCH3), 5.99 (1H, s, -C(OH)=CH-, enol form), 6.74 (2H, d, J = 16 Hz, 2,6-H, olefenic), 7.2 (6H, m, 2x6,2,5-ArH), 7.61 (2H, d, J = 16 Hz, 1,7-H, olefinic). Also Mono-O-methylated curcumin ([1(4-hydroxy-3-methoxyphenyl]-7-(3,4-dimethoxyphenyl)-1,6-heptadiene-3,5-dione) C22H22O6, B, Rf = 0.58 (eluant 2 % MeOH in CHCI3) was isolated and recrystallized from benzene (0.04 g, 7.4 % yield). 1H NMR : 3.86 (3H, s, 4-ArOCH3), 3.88(6H, s, 3-Ar-OCH3), 5.99 (1H, s, C(OH)=CH-, enol form), 6.74 [2H, d, J = 16 Hz, 2,6-H, olefenic], 7.2 [6H, m, 2x6,2,5-ArH], 7.61[2H, d, J = 16 Hz, 1,7-H, olefinic), 8.18 (1H, Ar-OH) EXAMPLE 5: To a solution of demethoxycurcumin (0.2 g, 0.6 mmol) in acetone (10 ml), 2.37 mmol (0.30 g) of dimethyl sulphate was added while stirring at 25°C. Aqueous sodium hydroxide (2.37 mmol, 0.1g in 2 ml) was then added drop- wise and the mixture was stirred under reflux at 55°C. The reaction was monitored by TLC for the disappearance of demethoxycurcumin. At the end of the reaction (4 h), the pH of the reaction mixture was adjusted to 7 using dilute HCI. The solvent was distilled and the residue dissolved in dichloromethane (20 ml). The organic layer was washed with water and separated using a separating funnel. The organic layer was dried over sodium sulphate (4g). The solvent was distilled off and the crude product was subjected to column chromatography using silica gel (30g) using chloroform as the eluant. Pure compound (C), 1-(4-methoxyphenyl)-7-(3,4-dimethoxyphenyl)-1,6-heptadiene-3,5-dione, C22H2205l Rf = 0.74 (eluant 2 % MeOH in CHCI3), 0.17 g, 79 % yield) was obtained on recrystallization from benzene. 1H NMR : 3.86 (3H, s, 3-ArOCH3), 3.88(6H, s, 4-Ar-OCH3), 5.99 (1H, s, -C(OH)=CH-, enol form), 6.74 [2H, d, J = 16 Hz, 2,6-H, olefinic], 7.2 [6H, m, 2x6,2,5-ArH)], 7.61 [2H, d, J = 16 Hz, 1,7-H, olefinic]). EXAMPLE 6: To a solution of bis-demethoxycurcumin (0.1 g, 0.32 mmol) in acetone (10 ml), 1.3 mmol (0.16 g) of dimethyl sulphate was added while stirring at 25°C. Aqueous sodium hydroxide (1.5 mmol, 0.5 g in 1 ml) was then added drop-wise and the mixture was stirred under reflux at 55°C. The reaction was monitored by TLC for the disappearance of bis-demethoxycurcumin. At the end of the reaction (4 h), the pH of the reaction mixture was adjusted to 7 using dilute HCI. The solvent was distilled off and the residue dissolved in dichloromethane (20 ml). The organic layer was washed with water and separated using a separating funnel. The organic layer was dried over sodium sulphate (4g). The solvent was distilled off and the residue subjected to column chromatography using silica gel (30g) using chloroform as eluant. Pure compound D ([1-(4-methoxyphenyl)-7-(4-methoxyphenyl)-1,6-heptadiene-3,5-dione], C21H20O4, Rf = 0.28 (eluant 2 % MeOH in CHCI3) was isolated that recrystallized from benzene (0.07g, 64 % yield) 1H NMR : 3.88(6H, s, 4-Ar-OCH3), 5.99 (1H, s, -C(OH)=CH-, enol form), 6.74 [2H, d, J = 16 Hz, 2,6-H, olefinic], 7.2 [6H, m, 2x6,2,5-ArH)], 7.61 [2H, d, J = 16 Hz, 1,7-H, olefinic]. (Formula Removed) Table 1: Curcuminoids and their methyl ethers (Table Removed) Advantages: ♦ The reagent employed is simple, inexpensive and easy to handle. ♦ The reaction is efficient and the by-product formation is negligible. ♦ The reaction conditions are mild and the work-up procedure is simple. ♦ The process is amenable to facile scale-up. We claim: 1. A process for preparation of methyl ethers from curcuminoids, which comprises: reacting curcuminods with dimethyl sulfate in a solvent medium such as acetone in the presence of a base selected from an alkali metal hydroxide like sodium hydroxides at a neutral pH in the presence of ultrasound in the frequency range of 40-50 kHz and RF power of 80 W at 25 to 55°C to obtain methyl ethers of curcuminoids. 2. A process for preparation of methyl ethers from curcuminoids, substantially as herein described with reference to the examples accompanying the specification.

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