To 500 mg of diacid (Example 20, Step 4) in dichloromethane (39 mL) and acetonitrile (9.9 mL) at 0° C. was added 2-chloro-1-methylpyridinium iodide (315 mg). After 15 min triethylamine (170 μL) and 10 mL of a saturated solution of ammonia in ether were added. The solution was stirred 1 hr at room temperature then it was washed with acidified 25% ammonium acetate, extracted with ethyl acetate, dried over sodium sulfate, filtered and evaporated to dryness. Purification by chromatography afforded the title compound.
