Under an argon atmosphere, 1,1-diethoxy-3-(4-ethylphenyl)propan-2-one (43) (301 mg, 1.20 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (238 mg, 858 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (17 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→10/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give et-coelenterazine (44, et-CTZ) as a yellow powder (240 mg, 551 μmol, 64.2%).
