A cross-metathesis approach to novel pantothenamide derivatives

Pantothenamides are known for their in vitro antimicrobial activity. Our group has previously reported a new stereoselective route to access derivatives modified at the geminal dimethyl moiety. This route however fails in the addition of large substituents. Here we report a new synthetic route that exploits the known allyl derivative, allowing for the installation of larger groups via cross-metathesis. The method was applied in the synthesis of a new pantothenamide with improved stability in human blood.


General information
Unless otherwise mentioned, all reagents and buffer components were LRMS was performed using a Finnigan LCQDUO mass spectrometer with ESI without fragmentation. Microwave reactions were performed using a Biotage microwave system.

Diethyl (R)-2-hydroxysuccinate (5)
Compound 5 was prepared as previously reported [1] or using the microwave conditions as described below. (R)-Malic acid (1.0 g, 7.5 mmol) was weighed into a microwave tube equipped with a stir bar before addition of anhydrous ethanol (5 mL). Thionyl chloride (0.25 mL, 3.4 mmol) was next added dropwise, and the tube was sealed. The resulting light yellow mixture was allowed to react in a microwave reactor for 30 min at S3 washed with a saturated solution of NaHCO 3 (20 mL). The organic layer was dried over anhydrous Na 2 SO 4 . The white solid was filtered off and the solution concentrated in vacuo by rotary evaporation. The crude product was obtained as a clear, light-yellow oil, which was used directly in the next step. Yield: 1.3 g, 90%. Characterization matched previous report of this known compound [1].

S4
Compound 9 was prepared from 7 via 8, as previously reported [1], in 98% yield over two steps. Characterization matched previous report of this known compound [1].
Diisopropylethylamine (14 mmol) was added and the reaction was stirred for 16 h at rt.
The mixture was then poured into a separatory funnel containing saturated NH 4 Cl solution (20 mL), and extracted in ethyl acetate (3 × 40 mL). The organic layers were dried over anhydrous Na 2 SO 4 , the solid filtered off, and the solution concentrated in vacuo by rotary evaporation. Flash chromatography using a gradient of 0-100% ethyl acetate in hexanes was used to purify the desired compounds.

5-yl)but-2-enoic acid (11a)
Grubbs' 2 nd generation catalyst 3 (12 mg, 5 mol %) was weighed into a flask in a N 2 atmosphere glovebox. The flask was capped, removed from the glovebox, and placed under a N 2 line. To this flask compound 9 (78 mg, 0.28 mmol) dissolved in dry DCM (4 mL) was added, followed by acrylic acid (20 mg, 0.28 mmol). The flask was attached to a reflux condenser and the reaction was stirred at 40 °C for 16 h under N 2 . Flash chromatography using a gradient of 0-100% ethyl acetate in hexanes was used to purify the desired compound, which was a brownish oil after evaporation of all solvents.

General procedure 2 for metathesis reactions in the microwave reactor
Hoveyda-Grubbs' 2 nd generation catalyst 4 (22 mg, 10 mol %) was weighed into a vial in a N 2 atmosphere glovebox. The vial was capped, removed from the glovebox, and placed under a N 2 line. To a microwave vial, compound 9 (100 mg, 0.36 mmol), dissolved in dry, degassed DCM (0.5 mL), was added. The desired coupling partner (0.18 mmol), which was dissolved in DCM (0.5 mL), was next added. The catalyst dissolved in DCM (1 mL) was finally added. The vial was then purged with N 2 and sealed. The reaction was run for 60 min at 60 °C in a microwave reactor. Flash chromatography using a gradient of 0-100% ethyl acetate in hexanes was used to purify the desired compounds.

S10
To a suspension of compound 12 (110 mg, 0.28 mmol) in a mixture of acetone and CH 2 Cl 2 (3:1, v/v, 12 mL) was added a freshly prepared solution of NaH 2 PO 4 (339 mg, 2.8 mmol) and NaClO 2 (160 mg, 1.4 mmol) in water (4 mL). The mixture was stirred at rt for 15 min. After evaporation of the solvent in vacuo, the residue was dissolved in a saturated solution of sodium sulfite (10 mL). The aqueous solution was extracted with ethyl acetate (3 × 25 mL). The combined organic layers were dried over magnesium sulfate and concentrated to give the crude acid 13 which was used without purification due to its poor stability.
A solution of the crude acid 13 in THF was added followed by DIPEA (0.49 mL, 2.8 mmol) and the mixture was stirred for 16 h at rt. The reaction mixture was poured into a separatory funnel containing an aqueous solution of saturated ammonium chloride, before extraction in ethyl acetate (3 × 40 mL). The combined organic layers were dried over magnesium sulfate and evaporated to give the crude product, which was purified by flash chromatography using a gradient of 0-100% ethyl acetate in

PanK enzyme activity
E. coli pantothenate kinase was expressed and purified as previously described [6].