Under an argon atmosphere, 1,1-diethoxy-3-(4-methylphenyl)propan-2-one (33) (216 mg, 914 μmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (196 mg, 707 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (14 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate═1/2→ethyl acetate→ethyl acetate/methanol═50/1→20/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give me-coelenterazine (34, me-CTZ) as a yellow powder (180 mg, 427 μmol, 60.4%).
