Bioinspired Synthesis of Platensimycin from Natural ent-Kaurenoic Acids

The biomimetic formal synthesis of the antibiotic platensimycin for the treatment of infection by multidrug-resistant bacteria was accomplished starting from either ent-kaurenoic acid or grandiflorenic acid, each of which is a natural compound available in multigram scale from its natural source. Apart from the natural origin of the selected precursors, the keys of the described approach are the long-distance functionalization of ent-kaurenoic acid at C11 and the efficient protocol for the A-ring degradation of the diterpene framework.

S3 kaurenoic acid and trachilobanoic acid. Ratios were determined by integration of the 1 H spectrum of the mixture. Spectroscopic data match with those reported on bibliography. 1 The detailed procedure has been recently submitted to be patented.
Stevia lucida was collected in Páramo de la Negra, close to El Delgadito (Merida, Venezuela) in March 2019. Dried aerial parts (22 Kg) were milled to powder and extracted in a Soxhlet using EtOH. The resultant extract (5657 g) was redissolved on MeOH, adsorbed on silica gel and extracted with hexane. The solvent was evaporated in vacuo and concentrate (682 g), then solved in hot methanol (MeOH) and refrigerated 12 hours at 4ºC. The mixture was centrifugated and evaporated the supernatant, obtaining 613 g of extract which was absorbed on celite and purified in a silica gel chromatography column using increasing gradients of ethyl acetate (EtOAc) in hexane (H). Ent-kaurenoic acid (3) (6277 mg) and grandiflorenic acid (2) (5940 mg) were separated on silica gel/AgNO3 column. Spectroscopic data match with those reported on bibliography. 2 Grandiflorenic acid (2). White crystals. 1
Spectroscopic data match with those reported on bibliography. 3,7

Hydrogenation of ent-kaurenoic acid to give ent-kauran-19-oic acid (7).
To a stirred solution of ent-kaurenoic acid (3) (204 mg, 0.68 mmol) in dry methanol (9.5 mL) was added a catalytic amount of palladium in activated carbon. The flask was tightly closed and sealed. The mixture was stirred for 7h under a hydrogen balloon pressure. After that, the reaction mixture was diluted with MTBE (50 mL) and filtered through celite. The resultant crude was dried with anhydrous Na2SO4 and concentrated under reduced pressure. Purification by flash chromatography with H/MTBE, 4:1 provided 152 mg (75% yield) of a 3:1 mixture of epimers where 7 was the major one. The spectroscopic data of compound 7 match with those reported in the literature. 3,4 Ent-kauran-19-oic acid (7).

S6
To a dry, flamed and argon-filled flask containing 200 mg (0.66 mmol) of stirred 7 in anhydrous bencene (2 mL), 0.2 mL of distilled pyridine (Py) was added followed by Cu(OAc)2 (5 mg, 0.028 mmol) and lead tetracetate (LTA) (368 mg, 0.83 mmol) 5 . The mixture was refluxed for 2 h (oil bath) and then cooled to room temperature. The resulting mixture was diluted with MTBE (50 mL) and filtered through a short plug of silica gel, which was washed with ethyl acetate. The mixture was washed with 2N HCl, saturated NaHCO3 and brine, and dried with anhydrous Na2SO4. The solvent was removed, giving a residue, which was flash chromatographed (H/MTBE, 9:1) to give 70 mg (33% yield) of a mixture of epimers where 8 was the major one, and the mixture of isomers 9a-c in a 1.4:1:1 proportion (103 mg, 60% yield).
Ratios were determined by integration of the 1 H spectrum of the mixture.

Synthesis of 9c.
To a solution of the previous crude (227 mg) in 10 mL of benzene was added ptoluenesulfonic acid (TsOH) (30 mg, 0.18 mmol) under argon. The reaction was heated at 50ºC for 2.5 h (oil bath), and then diluted with 100mL of MTBE. The resulting mixture as washed with saturated Na2CO3 and brine, and dried with anhydrous Na2SO4. The solvent was removed under reduced pressure obtaining 202 mg of crude which was purified by flash chromatography with H/MTBE, 9:1 to afford 220 mg (97% yield) of a mixture of epimers where 9c was the major one.
Then, an O3 stream (500mg/h, 0,17 mmol/min) was bubbled in the stirring mixture 6 . After 1 hour, 3 mL of dimethyl sulfide was added and the resulting mixture was diluted with 50 mL of DCM. The organic layer was washed with brine and dried over Na2SO4. The crude was purified via flash chromatography (5:1 H/MTBE), to give 40 mg (82% yield) of a mixture of epimers where 10 was the major one.

Synthesis of 12.
A O3 stream (500 mg/h, 0.17 mmol/min) was bubbled into a solution of mixture of epimers

Synthesis of 13.
To a solution of a mixture of epimers where 12 was the major component (23 mg, 0.08 mmol) in 2.5 mL of EtOAc, PHSeCl (24 mg, 0.13 mmol) was added. The mixture was stirred for 22 h at room temperature under argon until the consumption of starting material. The solvent was then removed. The resulting crude was dissolved in 0.01 mL of pyridine and 2.5 mL of DCM. Finally, 0.01 mL of H2O2 was added at 0 ºC. The 0 ºC bath was then removed and the mixture refluxed S10 for 10 min. The mixture was purified via flash chromatography using H:EtOAc 6:1 as eluent to obtain 20 mg (83% yield) of a mixture of epimers where 13 was the major one.

Synthesis of 14.
To a solution of 2600 mg (8.22 mmol) of 3a in dry THF (100 mL), 2.3 mL of a 1M borane-THF solution was added dropwise at 0 ºC under inert gas (Ar). After 15 min, the 0 ºC bath was removed and the mixture was stirred for 1 h until consumption of the starting material. Then, the reaction was cooled (0 ºC) and EtOH (32 mL), 4N NaOH (22.5 mL) and 29 mL of H2O2 were added. The resulting mixture was stirred for 25 min and then diluted with 150 mL of EtOAc, washed with water and brine and dried over Na2SO4. The solvent was evaporated over reduced

Synthesis of 17.
To a solution of 15 (71 mg, 0.21 mmol) in 5 mL of dry acetonitrile, trimethylsilyl iodide (ITMS) (0.03 mL, 0.25 mmol) was added dropwise at 0 ºC under argon. After 15 minutes the ice bath was removed and the solution heated to 40 ºC (oil bath) for 45 min. The mixture was then cooled S12 to room temperature and Ac2O (0.36 mL) was added. After stirring for 10 min, the mixture was cooled to 0 ºC, diluted with 50 mL of MTBE and washed with 2N HCl, saturated NaCO3 and brine.

Synthesis of 24.
A solution of 23 (763 mg, 1.75 mmol) in 68 mL of 10% KOH/MeOH was stirred at room temperature for 10 min. After this time, the solvent was evaporated under reduced pressure and the crude was dissolved in MTBE (60 mL), washed with NH4Cl, water and brine and dried over Na2SO4. The solvent was evaporated under reduced pressure affording 24 (673 mg, 98% yield).

Synthesis of 25.
To a solution of 24 (120 mg, 0.21 mmol) in dry THF (10 mL) in a dry flamed flask, a solution of o-NO2PhSeCN (207 mg, 0.912 mmol) in dry THF (5 mL) was added dropwise. After that, 0.23 mL (0.91 mmol) of n-Bu3P were also added. The reaction mixture was stirred at room temperature for 2 h. After this time, the reaction was diluted with MTBE, washed with NH4Cl and brine and dried over Na2SO4. The solvent was evaporated under reduced pressure. The resulting crude was dissolved in dry THF (5.6 mL) at room temperature. Then, 0.16 mL (7.38 mmol) of H2O2 was added, and the mixture was heated for 10 minutes. The reaction was then diluted with MTBE 40 mL, washed with distilled water and brine and dried over Na2SO4. Flash chromatography (H/MTBE 9:1) afforded 25 (120 mg, 70% yield).

Synthesis of 5.
To a dry, flamed and argon-filled flask with 258 mg (0.81 mmol) of 6 in anhydrous benzene (4 mL) was added 0.25 mL of distilled pyridine, Cu(OAc)2 (11 mg, 0.065 mmol) and lead tetracetate (LTA) (934 mg, 2.1 mmol). The mixture was refluxed (oil bath) for 4 h and then cooled to room temperature, diluted in MTBE (50 mL) and filtered through a short plug of silica gel, which was washed with ethyl acetate. The resultant mixture was washed with 2N HCl, saturated NaHCO3 and brine, and dried over anhydrous Na2SO4. This reaction crude was treated with I2 (15 mg, 0.06 mmol) in benzene at room temperature and inert atmosphere (Ar). After stirring for 3 h, the reaction mixture was quenched with MTBE and washed with aqueous 10% Na2S2O3, brine and dried over anhydrous Na2SO4. The organic layer was concentrated in vacuo and purified by flash column chromatography (H/EtOAc 5:1)) to give 5 (198 mg, 78% yield).
The mixture was stirred for 30 min at room temperature. It was then diluted with MTBE, washed with brine and dried over Na2SO4. The crude was concentrated and purified by flash chromatography (H/MTBE, 2:1) to give 27 (31 mg, 69% yield).

Synthesis of compound 28.
A O3 stream (500mg/h, 0,17 mmol/min) was bubbled into a solution of methyl ketone 27 (31 mg, 0.11 mmol) in MeOH (7.2 mL) at 0 ºC. After 15 min, the reaction was heated to room temperature and stirred for further 45 min. Then, the solvent was evaporated and 2 mL of benzene/MeOH (4:1) was added to the mixture. To this solution, 0.06 mL of trimethylsilyldiazomethane (0.13 mmol) was added and the mixture was stirred for 15 min.

S22
To a solution of 28 (21mg, 0.07mmol) in EtOAc (2.5 mL), PHSeCl (21 mg, 0.11 mmol) was added. The solution was stirred for 22 h at room temperature under inert atmosphere. The solvent was then removed and the resulting crude was dissolved in 0.01 mL of Py and 2.5 mL of DCM. To this solution, 0.01 mL of H2O2 was added at 0 ºC. The 0 ºC bath was removed and the mixture refluxed (oil bath) for 10 min. The mixture was purified via flash chromatography using H/EtOAc (3:1) as eluent to give 4a (17 mg, 82% yield). Spectroscopic data match with those reported on literature. 12