Scalable total synthesis and comprehensive structure–activity relationship studies of the phytotoxin coronatine

Natural phytotoxins are valuable starting points for agrochemical design. Acting as a jasmonate agonist, coronatine represents an attractive herbicidal lead with novel mode of action, and has been an important synthetic target for agrochemical development. However, both restricted access to quantities of coronatine and a lack of a suitably scalable and flexible synthetic approach to its constituent natural product components, coronafacic and coronamic acids, has frustrated development of this target. Here, we report gram-scale production of coronafacic acid that allows a comprehensive structure–activity relationship study of this target. Biological assessment of a >120 member library combined with computational studies have revealed the key determinants of potency, rationalising hypotheses held for decades, and allowing future rational design of new herbicidal leads based on this template.

The reaction was stirred for 15 minutes at −78 °C before a solution of alcohol S1 (4.15 g, 26.24 mmol, 1 equiv.) in CH 2 Cl 2 (10 mL) was added dropwise. The reaction was stirred at −78 °C for a further 30 minutes before being quenched slowly with triethylamine (22 mL, 157.84 mmol, 5 equiv.). The reaction was allowed to warm to room temperature over 1 h. The pale orange suspension was then diluted with water (40 mL) and extracted with CH 2 Cl 2 (3 x 30 mL). The organics were combined, washed with brine (20 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a pale orange liquid. The crude material was loaded directly in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 10-20% EtOAc/petroleum ether to afford the title compound as a pale yellow liquid (3.26 g, 79%).

General Procedure B: Aldol addition.
For example, synthesis of compound syn-9.
To a three-necked flask at room temperature under an atmosphere of nitrogen was added ester 8 (2.72 mL, 17.12 mmol, 1.3 equiv.) in anhydrous CH 2 Cl 2 (50 mL) and DIPEA (3.44 mL, 19.75 mmol, 1.5 equiv.). Dibutylboryltrifluoromethanesulfonate solution (1 M in CH 2 Cl 2 ) (17.1 mL, 17.1 mmol, 1.3 equiv.) was added dropwise and the resulting solution stirred at room temperature for 30 minutes. A solution of aldehyde 7 (2.06 g, 13.16 mmol, 1 equiv.) in CH 2 Cl 2 (10 mL) was then added dropwise and the reaction stirred at room temperature for 1 h. The reaction was quenched with a potassium buffer solution (pH 7.4, 26 mL), MeOH (40 mL), and H 2 O 2 (30% solution, 13 mL) which were added sequentially. A small exotherm was observed on H 2 O 2 addition. The reaction was stirred vigorously at room temperature for 16 h, diluted with water (30 mL), and extracted with CH 2 Cl 2 (3 x 40 mL). The organics were combined, washed with brine (30 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a pale yellow oil. The crude material loaded directly in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 20% EtOAc/petroleum ether to afford the title compound as a colourless liquid. 68% yield, 83:17 syn:anti by 1 H NMR; 57%, 2.81 g of the syn-isomer. General Procedure C: Tandem dehydration/Diels-Alder followed by ester hydrolysis.
For example, synthesis of compound 11.
To a round bottom flask under an atmosphere of nitrogen was added compound syn-9 (2.00 g, 6.71 mmol, 1 equiv. (79% purity)), CuBr (96 mg, 0.67 mmol, 10 mol%), and anhydrous toluene (1.3 mL). DIC (1.56 mL, 10.07 mmol, 1.5 equiv.) was added in one portion and the resulting solution was brought to 110 °C for 16 h. The reaction was allowed to cool to room temperature and the crude solution was filtered through celite, eluting with EtOAc (30 mL). The organics were washed with water (30 mL), followed by brine (30 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a pale brown oil. The crude material was directly loaded in a solution of 10% EtOAc/petroleum ether and purified by flash silica column chromatography, eluent 10% EtOAc/petroleum ether to afford a pale yellow oil (S2) (1.49 g, 5.32 mmol) which was not characterised.
To the pale yellow oil was added EtOH (50 mL) and PTSA (mono-hydrate) (1.52 g, 7.99 mmol, 1.5 equiv.) and the resulting solution was brought to 75 °C for 5 h. The reaction was allowed to cool to room temperature and the solvent evaporated to afford an orange oil. The crude material was directly loaded in a solution of 20% EtOAc/petroleum ether and minimal CH 2 Cl 2 and purified by flash silica column chromatography, eluent 20% EtOAc/petroleum ether to afford the title compound as a colourless liquid (677 mg, 54% (2 steps)).

General Procedure D: DMP oxidation.
For examples, synthesis of compound S3.
To a round bottom flask charged with compound S2 (300 mg, 1.25 mmol, 1 equiv.) in anhydrous CH 2 Cl 2 (12 mL) was added DMP (794 mg, 1.86 mmol, 1.5 equiv.) in one portion under an atmosphere of nitrogen. The reaction was stirred at room temperature for 16 h before 2 M NaOH (10 mL) was added and the layers stirred vigorously for 10 minutes. The layers were separated and the aqueous further extracted with CH 2 Cl 2 (2 x 20 ml). The organics were combined, washed with brine (20 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a colourless oil. The crude material was loaded in a solution of 10% EtOAc/petroleum ether and purified by flash silica column chromatography, eluent 10% EtOAc/petroleum ether to afford the title compound as a colourless oil (245 mg, 83%).
To a round bottom flask was added compound S3 (1.10 g, 4.65 mmol) and 3 M HCl (150 mL). The reaction was brought to 100 °C and maintained at this temperature with stirring for 16 h. The reaction was allowed to cool to room temperature and extracted with EtOAc (3 x 30 mL). The organics were combined, washed with brine (30 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford an orange oil. The crude material was loaded directly in a solution of 30% EtOAc/petroleum ether and purified by flash silica column chromatography, eluent 30-60% EtOAc/petroleum ether to afford the title compound as a white solid (850 mg, 88%).
For example, synthesis of compound S4.
To a 2-dram vial was added (±)-CFA (2) (30 mg, 0.14 mmol, 1 equiv.) and HATU (66 mg, 0.17 mmol, 1.2 equiv.). DMF (0.7 mL) was added, followed by DIPEA (80 µL, 0.46 mmol, 3 equiv.) and the resulting solution stirred at room temperature for 5 minutes. Methyl L-isoleucinate hydrochloride (30 mg, 0.21 mmol, 1.5 equiv.) was then added in one portion and the vial capped with a screw top lid. The reaction was stirred for 16 h. The reaction was then diluted with H 2 O (10 mL) and the organics extracted with EtOAc (3 x 5 mL). The organics were combined, washed with brine (10 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a pale yellow oil. The crude material was loaded directly in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 30% EtOAc/CH 2 Cl 2 to afford the desired product as a colourless oil which solidified to a white solid on standing (35 mg, 76%).
Vinylmagnesium bromide (1 M in THF, 45 mL, 45.00 mmol, 1 equiv.) was added dropwise to a stirring solution of the isolated material in anhydrous THF (100 mL) at 0 °C in a three-necked flask under an atmosphere of nitrogen. The resulting solution was allowed to rise to room temperature and stirred for 1.5 h. The reaction was quenched by dropwise addition of acetic anhydride (8.5 mL, 90.09 mmol, 2 equiv.) at room temperature and stirred for a further 1.5 h. The yellow reaction mixture was diluted with water (30 mL) and extracted with EtOAc (3 x 30 mL). The organics were combined, washed with brine (20 mL), dried over Na 2 SO 4 , filtered and evaporated to afford a pale orange oil. The crude material was loaded directly in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 20% EtOAc/petroleum ether to afford the title compound as a colourless liquid (8.65 g, 63%). The spectral data were consistent with those previously reported in the literature. 9 Compound S1.
To a round bottom flask was added compound 6 (11.51 g, 47.51 mmol, 1 equiv.) and EtOH (170 mL). PPTS (1.15 g, 4.58 mmol, 0.1 equiv.) was added portionwise and the resulting solution heated to 65 °C and maintained at this temperature for 3 h. The reaction was allowed to cool to room temperature and was then evaporated onto silica gel and purified by flash silica column chromatography, eluent 40% EtOAc/petroleum ether to afford the title compound as a colourless liquid (5. The spectral data were consistent with those previously reported in the literature. 9

Compound S3.
Compound S3 was prepared according to General Procedure D using compound 11 (300 mg, 1.25 mmol, 1 equiv.), DMP (794 mg, 1.87 mmol, 1.5 equiv.) and CH 2 Cl 2 (12 mL). After 16 h the reaction was subjected to purification outlined in General Procedure D (silica gel, 10% EtOAc/petroleum ether) to afford the title compound as a colourless oil (245 mg, 83% (3:1 dr C 7a )). To a round bottom flask was added compound S3 (245 mg, 1.04 mmol) and 3 M HCl (36 mL) and the resulting suspension brought to 60 °C for 16 h. The reaction was allowed to cool to room temperature and the organics extracted with EtOAc (3 x 10 mL). The organics were combined, washed with brine (10 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a colourless oil. The crude material was loaded directly in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 10% EtOAc/petroleum ether to afford the title compound as a colourless oil (186 mg, 76%). The spectral data were consistent with those previously reported in the literature. 10 Synthesis of Compound S18.
To a round bottom flask charged with 1,5-pentane diol (S10) (31 g, 295.86 mmol, 5 equiv.) was added anhydrous aluminium trichloride (79 mg, 0.59 mmol, 1 mol%) followed by dropwise addition of DHP (5.42 mL, 59.41 mmol, 1 equiv.). The resulting mixture was warmed to 30 °C and maintained at this temperature for 1 h, before being allowed to cool to room temperature. The colourless, crude material was loaded directly in a solution of 30% EtOAc/petroleum ether and purified by flash silica column chromatography, eluent 30-60% EtOAc/petroleum ether to afford the title compound as a colourless liquid (9.90 g, 88%). The spectral data were consistent with those previously reported in the literature. 11 Compound S12.
Vinylmagnesium bromide (1 M in THF, 56.4 mL, 56.40 mmol, 1.1 equiv.) was added dropwise to a stirring solution of the aldehyde (9.55 g, 51.28 mmol) in THF (100 mL) at 0 °C in a three-necked flask under an atmosphere of nitrogen. The resulting solution was allowed to warm to room temperature and stirred for 2 h. The reaction was quenched by dropwise addition of acetic anhydride (9.7 mL, 102.62 mmol, 2 equiv.) at room temperature and stirred for a further 16 h. The yellow reaction was diluted with water (30 mL) and extracted with EtOAc (3 x 30 mL). The organics were combined, washed with brine (20 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a pale orange oil. The crude material was loaded in a solution of CH 2 Cl 2 , and purified by flash silica column chromatography, eluent 10% EtOAc/petroleum ether to afford the title compound (10.66 g, 79% (2 steps)) as a colourless liquid. Compound S15.
Compound S15 was prepared according to General Procedure C using compound S18 (1.11 g, 3.77 mmol, 1 equiv.), PTSA (mono-hydrate) (1.07 g, 5.63 mmol, 1.5 equiv.), and EtOH (35 mL). After 6 h the reaction was subjected to purification outlined in General Procedure C (silica gel, 20% EtOAc/petroleum ether) to afford the title compound as a colourless liquid (511 mg, 54% based on 79% purity of starting material (2 steps)). Isolated as a single diastereoisomer at C 1 , the stereochemistry of which was not determined. EtOH (2 mL). After 5 h the reaction was subjected to purification outlined in General Procedure C (silica gel, 30% EtOAc/petroleum ether) to afford the title compound as a colourless oil and as two separable diastereoisomers at C 1 (17 mg, 26% (combined yield)), the relative stereochemistry of which were not confirmed. The spectral data were consistent with those previously reported in the literature. 12 (Figure 3).

Compound S4.
Prepared according to General Procedure F using (±)-CFA (2)   The spectral data were consistent with those previously reported in the literature. 18

(±)-CMA (3).
To a round bottomed flask was added compound S61 (1.35 g, 4.11 mmol) and 3 M HCl (60 mL). The reaction was brought to 100 °C for 16 h. The reaction was allowed to cool to room temperature and concentrated in vacuo to afford a pale orange solid. The solid material was washed sparingly with acetone to afford the title compound as a beige solid (596 mg, 65%). The spectral data were consistent with those previously reported in the literature. 19 To a round bottom flask charged with compound S9 (50 mg, 0.21 mmol, 1 equiv.) in a solution was EtOH (1 mL) was added NaBH 4 (9 mg, 0.24 mmol, 1.2 equiv.) in one portion at room temperature under an atmosphere of nitrogen. The reaction was stirred for 30 minutes, quenched with water (5 mL) and extracted with EtOAc (3 x 10 mL). The organics were combined, washed with brine (10 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford the title compound as a colourless oil (50 mg, >99%).

Compound 38b.
To a round bottom flask charged with compound S4 (34 mg, 0.10 mmol, 1 equiv.) in a solution of EtOH (3 mL) was added NaBH 4 (6 mg, 0.16 mmol, 1.5 equiv.) in one portion under an atmosphere of nitrogen. The reaction was stirred at room temperature for 16 h, before being quenched with water (5 mL). The organics were extracted with EtOAc (3 x 10 mL) and the layers combined, washed with brine (10 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a colourless oil. The residue was suspended in 1:1 MeOH:H 2 O (5 mL) and LiOH (7 mg, 0.29 mmol, 3 equiv.) added. The resulting suspension was brought to 50 °C and maintained at this temperature for 16 h. The reaction was allowed to cool to room temperature, acidified with AcOH, and the organics extracted with EtOAc (3 x 10 mL). The organics were combined, dried over Na 2 SO 4 , filtered, and evaporated to afford a pale yellow oil. The crude material was taken up in diethyl ether and petroleum ether added until a white precipitate formed. The solvent was removed with a Pasteur pipette, and the residue dried under vacuum to afford the title compound as a white solid (8 mg, 24%).

Compound S68.
To a 2-dram vial was added (±)-CFA (2) (20 mg, 0.10 mmol, 1 equiv.) and HATU (44 mg, 0.12 mmol, 1.2 equiv.). DMF (0.5 mL) was added, followed by DIPEA (50 µL, 0.29 mmol, 3 equiv.) and the resulting solution stirred at room temperature for 5 minutes. Methyl L-isoleucinate hydrochloride (26 mg, 0.14 mmol, 1.5 equiv.) was then added in one portion and the vial capped with a screw top lid. The reaction was stirred for 16 h under air. The reaction was then diluted with H 2 O (10 mL) and the organics extracted with EtOAc (3 x 5 mL). The organics were combined, washed with brine (10 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a pale yellow oil. The crude material was loaded in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 30% EtOAc/CH 2 Cl 2 to afford compound S4 as a colourless oil. The residue which was taken up in EtOH (0.18 mL) and added to a stirring solution of O-methylhydroxylamine hydrochloride (13 mg, 0.16 mmol, 1.5 equiv.) and NaOAc (11 mg, 0.13 mmol, 1.25 equiv.) in H 2 O (0.55 mL). The reaction was stirred for 16 h before being diluted with H 2 O (5 mL) and extracted with EtOAc (3 x 5 mL). The organics were combined, washed with brine (5 mL), dried over Na 2 SO 4 , filtered, and evaporated to afford a colourless oil. The crude material was loaded in a solution of CH 2 Cl 2 and purified by flash silica column chromatography, eluent 10-20% EtOAc/CH 2 Cl 2 to afford the title compound as a colourless oil (23 mg, 59% (2 steps)). 7:3 oxime isomers.

Compound 41b.
Prepared according to General Procedure G using compound S70 (20 mg, 0.06 mmol, 1 equiv.), LiOH (5 mg, 0.21 mmol, 3 equiv.), and 1:1 MeOH:H 2 O (5 mL) and the resulting suspension brought to 50 °C for 16 h. The reaction was allowed to cool to room temperature, acidified with AcOH and the organics extracted with EtOAc (3 x 5 mL). The organics were combined, dried over Na 2 SO 4 , filtered, and evaporated to afford a colourless oil. The crude material was loaded in a solution of CH 2 Cl 2 and was purified by flash silica column chromatography, eluent 1% AcOH, 30% EtOAc/CH 2 Cl 2 to afford a colourless oil.
The material was washed with petroleum ether to afford the title compound as a colourless oil (12 mg, 63%).  The spectral data were consistent with those previously reported in the literature. 19 Compound S73.
To a round bottom flask charged with compound S72 (167 mg, 0.52 mmol) was added dioxane (1 mL), followed by dropwise addition of 6 M HCl (1 mL). The reaction was stirred at room temperature for 3 h, before the addition of further 6 M HCl (1 mL). The reaction was stirred at room temperature for a further 1 h, before being concentrated in vacuo to afford the title compound as a pale brown solid (132 mg, 99%). and was purified by flash silica column chromatography, eluent 1% AcOH, 30% EtOAc/CH 2 Cl 2 to afford a colourless oil.