Process for producing coelenteramide or an analog thereof

A compound represented by formula (3):

• wherein,
• R1 represents an alkyl having 1 to 3 carbon atoms or an arylalkyl having 7 to 10 carbon atoms,
• R3 represents (i) an alkyl having 1 to 7 carbon atoms which may optionally be substituted with an alicyclic group, (ii) an alicyclic group or (iii) an arylalkyl having 7 to 10 carbon atoms, and
• R4 represents an alkyl having 1 to 3 carbon atoms or an arylalkyl having 7 to 10 carbon atoms.

A process for producing a blue fluorescent protein (BFP), which comprises the steps of:

• reacting a compound represented by formula (1):
• wherein,
• R1 represents an alkyl having 1 to 3 carbon atoms or an arylalkyl having 7 to 10 carbon atoms,
• R2 is amino, and
• R3 represents (i) an alkyl having 1 to 7 carbon atoms which may optionally be substituted with an alicyclic group, (ii) an alicyclic group or (iii) an arylalkyl having 7 to 10 carbon atoms,
• with a compound represented by formula (2):
• wherein,
• X is a splitting-off group, and
• R4 represents an alkyl having 1 to 3 carbon atoms or an arylalkyl having 7 to 10 carbon atoms,
• to produce a compound represented by formula (3):
• wherein R1, R3 and R4 are as defined above, and
• reacting the compound represented by formula (3) with the apoprotein of a calcium-binding photoprotein, in the presence of a calcium ion or a divalent or trivalent ion substitutable for the calcium ion.