The reaction of 5-(p-toluenesulfonyl)-3,4-disubstituted Δ3-pyrrolin-2-one with aldehydes in the presence of tributylphosphine and 1,8-diazabicyclo[5.3.0]undec-7-ene gave the corresponding 5-exomethylene compounds in good yields. A pyrromethenone derivative as a potential equivalent to C/D ring component of phytochromobilin dimethyl ester was effectively prepared by means of the present method.