Imidazopyrazine derivatives, process for preparation thereof, and their uses as luciferins

A composition comprising:

• (a) a compound of formula:
• wherein:
• R1 represents H;
• R2 represents a phenyl, optionally, comprising two substituents, each independently selected from —CH3, —F, —Cl, —OH, and —NH2;
• R3 represents a benzyl comprising one or two substituents, each independently selected from —F and —CF3;
• R4 represents H;
• Z represents a furanyl, optionally, comprising one substituent selected from —CH3 and —F;
• R5 represents —C(═O)Ra or —C(═O)ORa;
• Ra represents H, C1-C6 alkyl, C3-C7 cycloalkyl, C6-C10 aryl, or aralkyl;
• or
• (b) a corresponding imidazopyrazinone compound of the above that does not comprise an R5 moiety.