PURPOSE: To obtain a compound useful as an inhibitor to serine protease or thiol protease by reacting a diazo methyl ketone of a specific formula with a metal chloride in the presence of an alcohol.

CONSTITUTION: A compound of formula I (R1 is amino-protecting group, N-end protected amino acid residue, etc.; R2 is H or 1-5C alkyl; R1 and R2 are bonded to form phthalyl group; R3 is H, 1-15C alkyl which may be replaced, 6-14C aryl which may be replaced, etc.; R2 and R3 are bonded to form ring) is reacted with a metal chloride (e.g. aluminum chloride) in the presence of an alcohol (e.g. methanol) at -5 to 40°C for 5 minutes to 1 hour to give the objective substance of formula II. The objective substance is obtained by an extremely simple operation and without reducing a yield even in the case of existence of functional group weak under an acidic condition.