A mixture of 50% potassium hydroxide in H2O (30 mL) and diethyl ether (100 mL), stirred at 0° C., was treated portion wise with solid N-nitroso-N-methyl urea (3.0 g, 29.1 mmol). The reaction mixture was stirred for 30 min then transferred into a separatory funnel, the aqueous layer was discarded and the yellow ethereal phase (diazomethane solution) was cooled to −78° C. in an Erlenmeyer flask. A suspension of the acid 230 (20 mg, 0.055 mmol) in DCM (2 mL) was treated with the diazomethane solution in diethyl ether (3 mL) and the yellow solution was stirred at room temperature for 3 h, concentrated and the residue was purified by flash chromatography (eluent: 2% isopropanol in DCM) to provide title compound 231 (19.7 mg, 94% yield).
