Under nitrogen atmosphere, 3-cyano-8-hydroxy-6,6-dimethyl-5,6-dihydro-benzo[b]carbazol-11-one (Compound A6, 85 mg, 0.28 mmol) and triphenylphosphine (150 mg, 2 eq.) were added with THF (2 ml), and then further added dropwise with 4-hydroxymethyl-piperidine-1-carboxylic acid tert-butyl ester (120 mg, 2 eq.) and 2.19 N toluene solution of diethyl azodicarboxylic acid (0.26 mL, 2 eq.). The resultant was stirred at room temperature for 12 hr under nitrogen atmosphere. The residues obtained after concentrating the reaction solution under reduced pressure were purified by silica gel column chromatography (ethyl acetate/dichloromethane) to obtain 4-(3-cyano-6,6-dimethyl-11-oxo-6,11-dihydro-5H-benzo[b]carbazol-8-yloxymethyl)-piperidine-1-carboxylic acid tert-butyl ester (white powder, 120 mg).
