Patent ID: 7250389

Claim:
A compound represented by formula (I) or a salt thereof: wherein R 1 represents isobutyryl, tigloyl, isovaleryl, or 2-methylbutanoyl; R 2 represents a hydrogen atom, a benzoyl group having a substituent, or a quinoxalinoyl group having a substituent, wherein the substituent is selected from the group consisting of hydroxyl, halogen atoms, nitro, amino, diC 1-6 alkylamino, formylamino, C 1-6 alkyl, C 1-6 alkoxy, benzyloxy, C 1-10 aliphatic acyloxy, benzoyloxy, C 1-4 alkyloxycarbonyloxy, (C 1-4 )alkyloxycarbonyl(C 1-4 )alkyloxy, p-nitrobenzyloxycarbonyl(C 1-4 )alkyloxy, C 1-6 alkylsulfonyloxy, di(C 1-6 )alkylphosphoryloxy, and diphenylphosphoryloxy; and R 3 represents a hydrogen atom.