The free acid 24 was dissolved in 10 ml of dry tetrahydrofuran (THF), cooled to 0° C., and treated with 0.58 ml N-methyl-morpholine (5.3 mmol) and 0.69 ml of isobutylchloroformate. Dense white precipitate immediately formed, and after 30 minutes the reaction was filtered through a glass frit and transferred to a new flask with an additional 10 ml of THF. Meanwhile, diazomethane was prepared by reacting 1-methyl-3-nitro-1-nitrosoguanidine (2.3 g, 15.6 mmol) with 7.4 ml of 40% aqueous KOH and 25 ml diethyl ether for 45 minutes at 0° C. The yellow ether layer was then decanted into the reaction containing the mixed anhydride, and the reaction allowed to proceed while slowly warming to ambient temperature over a period of 165 minutes. The reaction was cooled to 8° C., and 1.5 ml of 4 N HCl in dioxane (6 mmol total) was added dropwise. This resulted in much bubbling, and the yellow solution became colorless. The reaction was allowed to proceed for two hours while gradually warming to ambient temperature and then quenched with 1 ml of glacial acetic acid. The solvent was removed under reduced pressure and the residue redissolved in 75 ml ethyl acetate, rinsed with 2×50 ml saturated sodium bicarbonate, 50 ml 5 M NaCl, dried over sodium sulfate, filtered, and evaporated to dryness before purification by flash chromatography using 90:10 chloroform:ethyl acetate to yield 0.747 g of 25 as a light yellow oil (2.2 mmol, 42% from 23). Expected MW=337.7, found 338 (M+1).
