A Concise Approach to Benzoic Acid Derivatives Bearing an α,β-Unsaturated Ketone Substituent: Synthesis of Methyl Taboganate

Abstract

A concise route to benzoic acid derivatives bearing an α,β-unsaturated ketone moiety and its application to the synthesis of the natural product methyl taboganate (1) is described. The methodology is based on the addition of a Grignard reagent to an appropriately substituted salicylaldehyde followed by oxidation of the resulting allylic alcohol to the corresponding α,β-unsaturated ketone. The new compounds were tested for their cytotoxic activity.

methyl taboganate - α,β-unsaturated ketones - cytotoxicity

References

<A NAME="RG33205ST-1">1</A> Parmar VS. Jain SC. Bisht KS. Jain R. Taneja P. Jha A. Tyagi OD. Prasad AK. Wengel J. Olsen CE. Boll PM. Phytochemistry  1997,  46:  597 

<A NAME="RG33205ST-2">2</A> Roussis V. Ampofo SA. Wiemer DF. Phytochemistry  1990,  29:  1787 

<A NAME="RG33205ST-3">3</A> Terreaux C. Gupta MP. Hostettmann K. Phytochemistry  1998,  49:  461 

<A NAME="RG33205ST-4">4</A> Seeram NP. Jacobs H. McLean S. Reynolds WF. Phytochemistry  1996,  43:  863 

<A NAME="RG33205ST-5">5</A> Lopez A. Ming DS. Neil Towers GH. J. Nat. Prod.  2002,  65:  62 

<A NAME="RG33205ST-6">6</A> Lago JHG. Ramos CS. Casanova DCC. de Morandim AA. Bergamo DCB. Cavalheiro AJ. da Bolzani VS. Furlan M. Guimaraes EF. Young MCM. Kato MJ. J. Nat. Prod.  2004,  67:  1783 

<A NAME="RG33205ST-7">7</A>

1-(2-Methoxy-5-carboxymethylphenyl)-1-trimethyl-silyloxyethylene ( 11). 3-Acetyl-4-methoxybenzoic acid methyl ester (10, 4.16 g, 0.02 mol) was dissolved in Et₃N (3.8 mL, 0.027 mol) followed by the addition of TMSCl (3.4 mL, 0.027 mol) and the resulting solution was heated at 35 °C. A solution of NaI (4.05 g, 0.027 mol) in MeCN (25 mL) was added at such a rate to keep the temperature at 35-40 °C without external heating. After completion of the addition, the mixture was stirred at r.t. for 2 h and was subsequently poured onto cold H₂O (350 mL) and extracted with Et₂O. The organic layer was dried (K₂CO₃) and the solvent was evaporated in vacuo to afford 1-(2-methoxy-5-carboxymethylphenyl)-1-trimethylsilyloxyethylene (11) as an oil (5.13 g, 92%). ¹H NMR (300 MHz, CDCl₃): δ = 8.18 (d, J = 2.14 Hz, 1 H), 7.92 (dd, J = 8.73, 2.17 Hz, 1 H), 6.89 (d, J = 8.71 Hz, 1 H), 4.98 (s, 1 H), 4.68 (s, 1 H), 3.90 (s, 3 H), 3.85 (s, 3 H), 0.25 (s, 9 H).
1-(2-Methoxy-5-carboxymethylphenyl)-3-hydroxy-3-methyl-1-butanone ( 12). TiCl₄ (2 mL, 0.018 mol) was added to ice-chilled CH₂Cl₂ (26 mL) followed by the dropwise addition of a solution of CH₃COCH₃ (1.5 mL, 0.021 mol) in CH₂Cl₂ (5.5 mL). Upon completion of the addition a solution of 1-(2-methoxy-5-carboxymethylphenyl)-1-trimethylsilyloxyethylene (11, 5.13 g, 0.018 mol) in CH₂Cl₂ (2.5 mL) was added and the resulting solution was stirred for 15 min. The reaction mixture was poured onto ice water with vigorous stirring and the organic layer was separated. The aqueous layer was extracted with CH₂Cl₂. The combined organic extracts were washed with a 1:1 mixture of sat. aq NaHCO₃ and H₂O, brine and were dried (Na₂SO₄). The solvent was evaporated in vacuo and the residue was purified by flash column chromatography (PE-EtOAc, 1:1) to afford 1-(2-methoxy-5-carboxymethylphenyl)-3-hydroxy-3-methyl-1-butanone (12) as a viscous oil (3.35 g, 69%). ¹H NMR (300 MHz, CDCl₃): δ = 8.31 (d, J = 2.14 Hz, 1 H), 8.15 (dd, J = 8.73, 2.18 Hz, 1 H), 6.99 (d, J = 8.71 Hz, 1 H), 3.96 (s, 3 H), 3.90 (s, 3 H), 3.15 (s, 2 H), 1.31 (s, 6 H).
Methyl 4-Methoxy-3-(3-methyl-2-butenoyl)benzoate ( 13). To an ice-cold solution of 1-(2-methoxy-5-carboxymethyl-phenyl)-3-hydroxy-3-methyl-1-butanone (12, 2.85 g, 0.012 mol) in Et₃N (5.3 mL, 0.038 mol) and CH₂Cl₂ (5.3 mL) was added dropwise SO₂Cl (1.60 mL, 0.022 mol) during 30 min. The mixture was then stirred for 2 h, poured onto 53 g of crushed ice and extracted with EtOAc. The combined organic extracts were washed with 1 M HCl, H₂O, sat. aq NaHCO₃, brine, then dried (Na₂SO₄), and concentrated under vacuum. The crude product was purified by flash column chromatography (PE-EtOAc, 85:15) as eluent to give the title compound (2.53 g, 90%) as an off-white solid; mp 47-48 °C. Anal. Calcd for C₁₄H₁₆O₄: C, 67.73; H, 6.50. Found: C, 68.01; H, 6.64. ¹H NMR (300 MHz, CDCl₃): δ = 8.19 (d, J = 2.20 Hz, 1 H), 8.08 (dd, J = 8.70, 2.20 Hz, 1 H), 6.95 (d, J = 8.75 Hz, 1 H), 6.53 (d, J = 2.50 Hz, 1 H), 3.91 (s, 3 H), 3.88 (s, 3 H), 2.21 (s, 3 H), 1.96 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 191.8, 166.3, 161.0, 156.1, 133.8, 131.8, 131.1, 124.9, 122.8, 111.3, 56.0, 51.9, 27.9, 21.3.

<A NAME="RG33205ST-8">8</A> Tsotinis A, Calogeropoulou T, Roussakis C, and Roussis V. inventors; PCT/EP99.02835,  WO99/54278.  ; Chem. Abstr. 1999, 131, 286260

<A NAME="RG33205ST-9">9</A> Carbonelle D. Jaquot C. Lanco X. le Dez G. Tomasoni C. Briand G. Tsotinis A. Calogeropoulou T. Roussakis C. Int. J. Cancer  2001,  92:  388 

<A NAME="RG33205ST-10">10</A> Lindoy LF. Meecham GV. Svenstrup N. Synthesis  1998,  1029 

<A NAME="RG33205ST-11">11</A> Diaz P. Michel S. Charpentier B. Bioorg. Med. Chem. Lett.  1997,  7:  2289 

<A NAME="RG33205ST-12A">12a</A> Shimizu H. Okamura H. Iwagawa T. Nakatami M. Tetrahedron  2001,  57:  1903 

<A NAME="RG33205ST-12B">12b</A>

General Procedure for the Synthesis of Allylic Alcohols 19-21.
To a 0.07 M solution of 3-formyl-4-hydroxy-benzoic acid methyl ester (15, 1 equiv) in THF was added dropwise a solution of the appropriate Grignard reagent 16-18 (0.5 M in THF, 5 equiv) at -78 °C over a period of 10 min. After stirring for 1 h, the reaction was quenched with sat. NH₄Cl at -78 °C, the mixture was extracted with EtOAc (3 × 15 mL), dried (Na₂SO₄) and the solvents were evaporated in vacuo. The yellowish oily crude product was used for the next reaction without purification. 4-Hydroxy-3-(1-hydroxy-3-methylbut-2-enyl)benzoic Acid Methyl Ester ( 19). ¹H NMR (300 MHz, CD₃COCD₃): δ = 7.91 (d, J = 2.40 Hz, 1 H), 7.79-7.76 (m, 1 H), 6.88 (d, J = 8.50 Hz, 1 H), 5.79 (d, J = 9.15 Hz, 1 H), 5.41 (d, J = 9.15 Hz, 1 H), 3.83 (s, 3 H), 1.81 (s, 3 H), 1.73 (s, 3 H). 4-Hydroxy-3-(1-hydroxybut-2-enyl)benzoic Acid Methyl Ester ( 20).
¹H NMR (300 MHz, CD₃COCD₃): δ = 7.96 (d, J = 2.40 Hz, 1 H), 7.78 (dd, J = 8.50, 2.40 Hz, 1 H), 6.89 (d, J = 8.50 Hz, 1 H), 5.92-5.83 (m, 1 H), 5.62-5.58 (m, 1 H), 3.84 (s, 3 H), 1.79 (d, J = 5.50 Hz, 3 H). 4-Hydroxy-3-(1-hydroxy-3-phenylallyl)benzoic Acid Methyl Ester ( 21). ¹H NMR (300 MHz, CD₃COCD₃): δ = 8.07 (br s, 1 H), 7.83-6.55 (m, 10 H), 5.98-5.76 (m, 1 H), 3.82 (br s, 3 H).

<A NAME="RG33205ST-13">13</A> Chen C. Layton ME. Shair MD. J. Am. Chem. Soc.  1998,  120:  10784 

<A NAME="RG33205ST-14">14</A> Alhuwalia VK. Jolly RS. Tehim AK. Tetrahedron  1982,  38:  3673 

<A NAME="RG33205ST-15">15</A> Diaz PPD. Arias TC. Joseph-Nathan P. Phytochemistry  1987,  26:  809 

<A NAME="RG33205ST-16">16</A> Talley JH. Synthesis  1983,  845 

<A NAME="RG33205ST-17">17</A> Cardillo G. Cricchio R. Merlini L. Nasini G. Gazz. Chim. Ital.  1969,  99:  612 

<A NAME="RG33205ST-18">18</A>

General Procedure for the Synthesis of α,β-Unsaturated Ketones. The crude allylic alcohols 19-21 were dissolved in dry MeCN (1.3% w/v), activated MnO₂ 85% (5-fold w/w) was added in one portion. After stirring the reaction mixture at r.t. overnight, it was filtered through Celite, which was washed with EtOAc (4 × 20 mL). Evaporation of the solvents followed by purification by flash column chromatography afforded pure α,β-unsaturated ketones 1, 22 and 23, respectively.
4-Hydroxy-3-(3-methylbut-2-enoyl)benzoic Acid Methyl Ester (Methyl Taboganate, 1).
According to the general procedure from 62 mg of crude allylic alcohol 19 (obtained from 50 mg, 0.28 mmol of aldehyde 15), 1 was obtained (25 mg, 40% overall yield, after purification by flash column chromatography using PE-EtOAc, 92:8) as a light yellow crystalline solid; mp 92-94 °C (lit. [2] mp 91-95 °C). ¹H NMR (300 MHz, CDCl₃): δ = 13.30 (s, 1 H), 8.46 (d, J = 1.83 Hz, 1 H), 8.04 (dd, J = 8.80, 2.10 Hz, 1 H), 6.94 (d, J = 8.50 Hz, 1 H), 6.84 (s, 1 H), 3.87 (s, 3 H), 2.21 (s, 3 H), 2.05 (s, 3 H).
3-But-2-enoyl-4-hydroxybenzoic Acid Methyl Ester ( 22).
Following the general procedure, from 220 mg of crude alcohol 20 (obtained from 150 mg, 0.83 mmol of aldehyde 15), 22 was obtained as a yellow solid [90 mg, 50% overall yield after purification by flash column chromatography (PE-EtOAc, 90:10)]. Anal. Calcd for C₁₂H₁₂O₄: C, 65.45; H, 5.49. Found: C, 65.42; H, 5.33. Ratio of cis:trans = 9:1. ¹H NMR (300 MHz, CDCl₃): δ = 13.18 (s, 0.9 H), 13.07 (s, 0.1 H), 8.55 (d, J = 1.83 Hz, 0.9 H), 8.50 (d, J = 2.43 Hz, 0.1 H), 8.10 (dd, J = 8.85, 2.13 Hz, 1 H), 7.33-7.11 (m, 1.8 H), 7.04-6.94 (m, 0.1 H), 6.98 (d, J = 8.55 Hz, 1 H), 6.61-6.52 (m, 0.1 H), 3.93 (s, 2.7 H), 3.90 (s, 0.3 H), 2.18 (dd, J = 7.32, 1.83 Hz, 0.3 H), 2.06 (d, J = 6.09 Hz, 2.7 H). ¹³C NMR (75 MHz, CDCl₃): δ = 193.8, 167.2, 166.1, 147.3, 136.9, 132.6, 132.4, 125.0, 124.9, 123.5, 120.8, 118.8, 118.7, 52.2, 18.8.
4-Hydroxy-3-(3-phenylacryloyl)benzoic Acid Methyl Ester ( 23).
According to the general procedure from 200 mg of crude allylic alcohol 21 (obtained from 60 mg, 0.33 mmol of aldehyde 15), the α,β-unsaturated ketone 23 was obtained as a bright yellow solid [38 mg, 41% overall yield after purification by flash column chromatography (PE-EtOAc, 95:5)]. Anal. Calcd for C₁₇H₁₄O₄: C, 72.33; H, 5.00. Found: C, 72.05; H, 5.21. Ratio of trans:cis = 3:1. ¹H NMR (300 MHz, CDCl₃): δ = 13.32 (s, 0.75 H), 12.68 (s, 0.25 H), 8.67 (d, J = 1.83 Hz, 0.75 H), 8.49 (d, J = 2.43 Hz, 0.25 H), 8.15 (dd, J = 8.55, 1.83 Hz, 0.75 H), 8.10 (dd, J = 8.55, 2.13 Hz, 0.25 H), 7.98 (d, J = 15.00 Hz, 0.75 H), 7.76 (d, J = 15.00 Hz, 0.75 H), 7.73-7.71 (m, 2 H), 7.47-7.45 (m, 2 H), 7.29-7.27 (m, 1 H), 7.13 (d, J = 12.81 Hz, 0.25 H), 7.05 (d, J = 8.52 Hz, 0.75 H), 7.01 (d, J = 8.55 Hz, 0.25 H), 6.73 (d, J = 12.81 Hz, 0.25 H), 3.94 (s, 2.25 H), 3.88 (s, 0.75 H). ¹³C NMR (75 MHz, CDCl₃): δ = 193.5, 167.2, 146.7, 166.1, 137.0, 134.3, 132.2, 131.3, 129.4, 129.1, 129.0, 128.4, 120.9, 119.4, 118.8, 52.2.

<A NAME="RG33205ST-19">19</A>

trans -4-Hydroxy-3-(3-phenylacryloyl)benzoic Acid Methyl Ester.
Separated from the trans-cis mixture 25, using HPLC (column DAICEL-CHIRACEL OD, 254 nm, 1.2 mL/min, hexane-2-PrOH, 80:20, t _R = 28 min). ¹H NMR (300 MHz, CDCl₃): δ = 13.31 (s, 1 H), 8.67 (d, J = 2.50 Hz, 1 H), 8.17 (dd, J = 9.15, 1.83 Hz, 1 H), 7.99 (d, J = 16.00 Hz, 1 H), 7.73 (d, J = 16.00 Hz, 1 H), 7.73-7.70 (m, 2 H), 7.48-7.45 (m, 3 H), 7.06 (d, J = 9.15 Hz, 1 H), 3.94 (s, 3 H).

<A NAME="RG33205ST-20">20</A>

Hydroxy-3-(3-methylbutyryl)benzoic Acid Methyl Ester ( 26).
To a solution of 1 (20 mg, 0.085 mmol) in EtOAc (2 mL) was added 10% Pd/C (2 mg). The suspension was stirred under 1 atm of H₂ for 1 h. The reaction mixture was filtered through Celite, which was washed with EtOAc, and the filtrate was concentrated in vacuo to afford compound 26 (17 mg, 85%) as an off-white solid; mp 43-44 °C. Anal. Calcd for C₁₃H₁₆O₄: C, 66.09; H, 6.83. Found: C, 66.12; H, 6.89. ¹H NMR (300 MHz, CDCl₃): δ = 12.88 (s, 1 H), 8.49 (d, J = 1.83 Hz, 1 H), 8.11 (dd, J = 8.83, 2.14 Hz, 1 H), 7.00 (d, J = 8.55 Hz, 1 H), 3.91 (s, 3 H), 2.91 (d, J = 6.69 Hz, 2 H), 2.34-2.27 (m, 1 H), 1.02 (d, J = 6.72 Hz, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 206.7, 166.2, 166.0, 136.9, 132.6, 120.9, 119.1, 118.7, 52.2, 30.9, 25.3, 22.7.

<A NAME="RG33205ST-21">21</A> Cytotoxicity Assay Protocol. Cell Lines and Culture. The NSCLC-N6-L16 cell line, derived from a human non-small-cell bronchopulmonary carcinoma (moderately differentiated, rarely keratinizing, classified as T2N0M0), and A549 obtained from ATCC collection reference CCL6185, was used for all experiments. Cells were seeded into 96-well plates (10⁵ cells/mL for NSCLC-N6, 2 × 10⁴ cells/mL for A549 and 3 × 10⁴ cells/mL for SRA) and exposed to various concentrations of the compounds for 72 h. The cytotoxicity was determined using the MTT dye reduction assay of Mosmann: Mosmann T. J. Immunol. Methods  1983,  65:  55